Sensory and Motor Pathways

INTRODUCTION Sensory systems, also called modalities (singular, modality), share many features� All sensory systems begin with receptors, sometimes free nerve endings and others that are highly specialized, such as those in the skin for touch and vibration sense and the hair cells in the cochlea for hearing, as well as the rods and cones in the retina� These receptors activate the peripheral sensory fibers appropriate for that sensory system� The peripheral nerves have their cell bodies in sensory ganglia that belong to the peripheral nervous system (PNS)� For the body (neck down), these are the dorsal root ganglia, located in the intervertebral spaces (see Figure  1�12)� The trigeminal system, which is the sensory nerve for the face and head, has its ganglion inside the skull� The central process of these peripheral neurons enters the CNS and synapses in the nucleus appropriate for that sensory system (this is hard-wired)�

Generally speaking, the older systems both peripherally and centrally involve axons of small diameter that are thinly myelinated or unmyelinated, with a slow rate of conduction� In general, these pathways consist of fibers-synapsesfibers, with collaterals, creating a multisynaptic chain with many opportunities for spreading the information, but thereby making transmission slow and quite insecure�

The various forms of sensation in this category include pain and temperature, as well as related sensations of “itch” and perhaps also sensations of a “pleasurable” or sexual nature�

• A clinician examines the pain pathways by using the tip of a (clean, sterile) pin-gently, sometimes using the dull side to check whether the patient is reporting accurately�

• Temperature is more difficult to check clinically and involves the use of objects that are either cold (e�g�, metal) or warm (e�g�, water)�

The newer pathways that have evolved have larger axons that are more thickly myelinated and therefore

conduct more rapidly� These form rather direct connections with few, if any, collaterals� The latter type of pathway transfers information more securely and is more specialized functionally�

The various forms of sensation in this category include the following�

• Discriminative Touch is defined as the ability to sense fine touch such as a cloth or tissue (with the eyes closed)� This should not be confused with cortical analysis of sensation such as stereognosis, two-point discrimination or graphesthesia�

• Stereognosis: the recognition of an object using only tactile information (e�g�, a coin such as a 10 cent denomination in the U�S� or Canada)�

• Two-point discrimination: the perception of being touched by 2 points simultaneously (finer in the hand where there are more receptors than on the back)�

• Graphesthesia: the recognition of letters or numbers “written” on the skin (with a blunt object) which would again be better where there are more receptors�

• Joint position is tested by moving the bones about a joint and asking the patient to report the direction of the movement (again with the eyes closed)�

• Vibration is tested by placing a 256 Hz tuning fork that has been set into motion onto a joint space (e�g�, the interphalangeal joint of hands or feet, wrist, the ankle)� These sensory receptors in the joint capsule are quite specialized; the fibers carrying the afferents to the CNS are rapidly conducting large diameter and thickly myelinated axons, meaning that the information is carried quickly and with a high degree of fidelity�

Because of the upright posture of humans, the sensory systems go upward or ascend to the cortex-the ascending systems� The sensory information is “processed” by various nuclei along the pathway� Three pathways are

concerned with carrying sensory information from the skin (and the joints), two from the body region, and one (with subparts) from the head:

• The dorsal column-medial lemniscus pathway, a newer pathway for the somatosensory sensory modalities of discriminative touch, joint position, and “vibration�”

• The anterolateral system, an older system that carries pain and temperature and some less discriminative forms of touch sensations; formerly called the lateral spino-thalamic and ventral (anterior) spino-thalamic tracts, respectively�

• The trigeminal pathway, carrying sensations from the face and head area (including discriminative touch, pain, and temperature) and involving both newer and older types of sensation�

All the sensory pathways including the special senses, except for olfaction, relay in the thalamus before going on to the cerebral cortex (see Figure 5�5 and Figure 6�13); the olfactory system (smell) is considered with the limbic system in Section 4�

The aim of the sensory examination is to establish whether the deficit, if there is any, involves the peripheral nerves supplying a region or whether there is a level (spinal or brainstem) below which some or all or some sensory sensations are not perceived� Both sides need to be tested�

The sensory systems from the periphery (including the head region) have traditionally been described as a system of neurons:

• First order neurons-these are the neuronal cell bodies of the peripheral nerve fibers themselves, located in the dorsal root ganglia (DRG, part of the peripheral nervous system, the PNS) for all the somatosensory systems from the body� For the trigeminal afferents (CN V from the face and head region), the sensory ganglion is located inside the skull� The central processes of these neurons enter the central nervous system (CNS), the spinal cord and brainstem respectively� Once within the CNS, the oligodendrocyte becomes the glial cell responsible for myelin, whereas the Schwann cell is responsible for myelination in the PNS�

• Second order neurons-these neurons are located in the central nervous system and are the ones that project their axons upward� In the dorsal column-medial lemniscal pathwaythey are located in the lowermost brainstem (see Figure 5�2)� In the anterolateral pathway, they are found in the spinal cord (see Figure 5�3)� In the trigeminal system, they are located either in the pons or in the lower brainstem (see Figure 5�4)� It is important to note that there may be synaptic relays preceding the activation of the second order neuron� In all of these systems, the axons of the second order neurons cross the midline-decussate-and ascend to the thalamus� As will be shown, the crossing (decussation) occurs at different levels for each of the pathways�

• Third order neurons-these are located in the thalamic relay nuclei and project to the cortex (see Figure 4�3 and Figure 6�13)�

This analysis of the sensory systems is more difficult to apply to the special senses and it is probably best to consider each of these-vision, audition, olfaction (smell)—separately�

This is a representation of a spinal cord cross-section, at the cervical level (see Figure 3�9 and Figure 4�1), with a focus on the sensory afferent side� All levels of the spinal cord have the same sensory organization, although the size of the nuclei varies with the number of afferents�

The dorsal horn of the spinal cord has a number of nuclei related to sensory afferents, particularly pain and temperature, as well as crude touch (see upper illustration)� The first nucleus encountered is the posteromarginal, where some sensory afferents terminate� The next and most prominent nucleus is the substantia gelatinosa, composed of small cells, where many of the pain afferents terminate� Medial to this is the proper sensory nucleus, which is a relay site for these fibers; neurons in this nucleus project across the midline and give rise to a tract-the anterolateral tract (see later and Figure 5�3)�

A small local tract that carries pain and temperature afferents up and down the spinal cord for a few segments is called the dorsolateral fasciculus (of Lissauer)�

The dorsal nucleus (of Clarke) is a relay nucleus for cerebellar afferents (see Figure 5�15 and Figure 6�10)�

The illustrations show the difference at the entry level between the two sensory pathways-the dorsal column tracts and the anterolateral system� The cell bodies for both these sensory nerves are located in the dorsal root ganglion (DRG) (see Figure 1�12)�

On the left side, the afferent fibers carrying discriminative touch, position sense, and vibration enter the dorsal horn and immediately turn upward� The fibers may give off local collaterals (e�g�, to the intermediate gray), but the information from these rapidly conducting heavily myelinated fibers is carried upward in the two tracts that lie between the dorsal horns, called collectively the dorsal columns (see Figure 6�10)� The first synapse in this pathway occurs at the level of the lower medulla (see Figure 5�2)�

On the same side, the afferents carrying the pathways for pain, temperature, and crude touch enter and synapse in the nuclei of the dorsal horn� The nerves conveying this sensory input into the spinal cord are thinly myelinated or unmyelinated and conduct slowly� After several synapses, these fibers cross the midline (decussate) in the white matter in front of the commissural gray matter (the gray matter joining the two sides), called the ventral (anterior) white commissure (see upper illustration)� The fibers then ascend as the spino-thalamic tracts, called collectively the anterolateral system (see Figure 5�3 and Figure 6�10)�

A lesion of one side of the spinal cord therefore affects the two sensory systems differently because of this arrangement� The sensory modalities of the dorsal column system are disrupted on the same side (ipsilateral)� The pain and temperature pathway, having crossed, leads to a loss of these modalities on the opposite side� This is known as a dissociated sensory deficit�

This pathway carries the modalities discriminative touch, joint position, and the somewhat artificial “sense” of vibration from the body� Receptors for these modalities are generally specialized endings in the skin and joint capsule�

The axons enter the spinal cord and turn upward, with no synapse (see also Figure  5�1)� Those fibers entering below spinal cord level T6 form the fasciculus gracilis, the gracile tract; those entering above T6, particularly those from the upper limb, form the fasciculus cuneatus, the cuneate tract, which is situated more laterally� These tracts ascend the spinal cord between the two dorsal horns and form the dorsal column (see Figure 6�10)�

The first synapse in this pathway is found in two nuclei located in the lowermost part of the medulla, in the nuclei gracilis and cuneatus (see Appendix Figure A�10; also Figure 1�9 and Figure 3�3)� Topographical representation, also called somatotopic organization, is maintained in these nuclei, meaning that there are distinct populations of neurons that are activated by areas of the periphery that were stimulated�

After neurophysiological processing, axons emanate from these two nuclei that cross the midline (decussate)� This stream of fibers, called the internal arcuate fibers, can be recognized in suitably stained sections of the lower medulla (see Figure 6�11 and Appendix Figure A�10)� The fibers then group together to form the medial lemniscus, which ascends through the brainstem� This pathway does not give off collaterals to the reticular formation in the brainstem� This pathway changes orientation and position as it ascends through the pons and midbrain (see Figure 6�11, Appendix Figure A�6, and Appendix Figure A�10)�

The medial lemniscus terminates (i�e�, synapses) in the ventral posterolateral nucleus of the thalamus (VPL) (see Figure 4�3, Figure 5�5, and Figure 6�13)� The fibers then enter the internal capsule, its posterior limb (see Figure 4�4), and travel to the somatosensory cortex, to terminate along the post-central gyrus areas 1, 2, and 3

(see Figure 1�3, Figure 4�5, and Figure 6�13)� The representation of the body on this gyrus is not proportional to the size of the area being represented; for example, the fingers, particularly the thumb, are given a much larger area of cortical representation than the trunk; this is called the sensory “homunculus” (see Figure 4�5)� The lower limb, represented on the medial aspect of the hemisphere, has little cortical representation�

In the spinal cord, the pathways are found between the two dorsal horns as a well-myelinated bundle of fibers called the dorsal column (see Figure 4�1 and Figure 6�10)� The tracts have a topographical organization, with the lower body and lower limb represented in the medially placed gracile tract (thoracic level) and the upper body and upper limb in the laterally placed cuneate tract (cervical level)� After synapsing in their respective nuclei (see Figure 1�9 and Figure 3�3) and the crossing of the fibers in the lower medulla (internal arcuate fibers, see Appendix Figure A�10), the medial lemniscus tract is formed� This heavily myelinated tract, which is easily seen in myelin-stained sections of the brainstem, is located initially between the inferior olivary nuclei and is oriented in the dorsoventral position� The tract moves more posteriorly, shifts laterally, and also changes orientation as it ascends� The fibers are topographically organized, with the leg represented laterally and the upper limb medially� The medial lemniscus is joined by the anterolateral system and trigeminal pathway in the upper pons (see Figure 5�5 and Figure 6�11)�

Lesions involving this tract result in the loss of the sensory modalities carried in this pathway� A lesion of the dorsal column in the spinal cord causes a loss on the same side; note that this area is supplied by the posterior spinal arteries (see Figure 8�7)� After the crossing in the lower brainstem, any lesion of the medial lemniscus results in the deficit on the opposite side of the body� Lesions occurring in the midbrain and internal capsule usually involve the fibers of the anterolateral pathway, as well as the modalities carried in the medial lemniscus and trigeminothalamic pathways (to be discussed with Figure 5�3 and Figure 5�4)� With cortical lesions, the area of the body affected is determined by the area of the post-central gyrus involved (see Figure 4�5)�

This pathway carries the modalities of pain and temperature and a form of touch sensation called crude or light touch� The sensations of itch and tickle, and other forms of sensation (e�g�, pleasurable/“sexual”) are likely carried in this system� In the periphery the receptors are usually simply free nerve endings, without any specialization�

These incoming fibers (sometimes called the first order neuron) enter the spinal cord and synapse in the dorsal horn (see Figure 4�1 and Figure 5�1)� Many collaterals within the spinal cord are the basis of several protective reflexes (see Figure 5�7)� The number of synapses formed is variable, but eventually a neuron is reached that will project its axon up the spinal cord (sometimes referred to as the second order neuron)� This axon crosses the midline, decussates, in the ventral (anterior) white commissure, usually within two to three segments above the level of entry of the peripheral fibers�

These axons now form the anterolateral tract, located in that portion of the white matter of the spinal cord� It was traditional to speak of two pathways, one for pain and temperature, the lateral spino-thalamic tract, and another for light (crude) touch, the anterior (ventral) spino-thalamic tract� Both are now considered together under one name�

The tract ascends in the same position through the spinal cord (see Figure 6�10)� As fibers are added from the upper regions of the body, they are positioned medially, pushing the fibers from the lower body more laterally� Thus, there is a topographic organization to this pathway in the spinal cord� The axons of this pathway are either unmyelinated or thinly myelinated� In the brainstem, collaterals are given off to the reticular formation (see Figure 3�6B) that are thought to be quite significant functionally� Some of the ascending fibers terminate in the ventral posterolateral (VPL) nucleus of the thalamus (sometimes referred to as the third order neuron in a sensory pathway), and some terminate in the non-specific intralaminar nuclei (see Figure 4�3, Figure 5�5, Figure 6�11, and Figure 6�13)� There is likely some processing of sensory information in these nuclei of the thalamus, not simply a relay, including some aspects of a “crude” touch and particularly pain�

There is a general consensus that acute pain sensation has two components� The older (also called the paleospino-thalamic) pathway involves the reported sensation of an ache, or diffuse pain that is poorly localized� The fibers underlying this pain system are likely unmyelinated

both peripherally and centrally, and the central connections are probably very diffuse; most likely these fibers terminate in the non-specific thalamic nuclei and influence the cortex widely� The newer pathway, sometimes called the neospino-thalamic system, involves thinly myelinated fibers in the peripheral nervous system and the central nervous system, likely ascends to the VPL nucleus of the thalamus, and from there is relayed to the postcentral (sensory) gyrus� Therefore, the sensory information in this pathway can be well localized� The common example for these different pathways is a paper cut-immediately one knows exactly where the cut has occurred; this is followed several seconds later by a diffuse, poorly localized aching sensation�

In the spinal cord, this pathway is found among the various pathways in the anterolateral region of the white matter (see Figure 4�1 and Figure 6�10), hence its name� Its two parts cannot be distinguished from each other or from the other pathways in that region� In the brainstem, the tract is small and cannot usually be seen as a distinct bundle of fibers� In the medulla, it is situated dorsal to the inferior olivary nucleus; in the uppermost pons and certainly in the midbrain, the fibers join the medial lemniscus (see Figure 5�5 and Figure 6�11)�

Lesions of the anterolateral pathway from the point of crossing in the spinal cord upward result in a loss of the modalities of pain and temperature and crude touch on the opposite side of the body� The exact level of the lesion can be quite accurately ascertained because the sensation of pain can be quite simply tested at the bedside by using the end of a pin� The best instrument is a new unused safety pin, which can then be discarded after use on a single patient� (The tester should be aware that this is quite uncomfortable and/or unpleasant for the patient being tested�)

Any lesion that disrupts just the crossing pain and temperature fibers at the segmental level leads to a loss of pain and temperature of just the levels affected� There is an uncommon disease, called syringomyelia, that involves a pathological cystic enlargement of the central canal, called a syrinx (pipe in Greek)� The cause of this disease is largely unknown, but sometimes it may be related to a previous traumatic injury� The enlargement of the central canal interrupts the pain and temperature fibers in their crossing anteriorly in the anterior white commissure� Usually, this occurs in the cervical region, and patients complain of the loss of these modalities in the upper limbs and hand, in what is called a cape-like distribution� The enlargement can be visualized with magnetic resonance imaging (MRI)�

The sensory input carried in the trigeminal nerve comes from the face, particularly from the lips, all the mucous membranes inside the mouth including the tongue, the teeth, the mucous membranes of the paranasal (air) sinuses, and, the conjunctiva of the eye� The sensory fibers include the modalities of discriminative touch, as well as pain and temperature� The fiber sizes and degree of myelination are similar to the sensory inputs below the neck� The cell bodies of these fibers are found in the trigeminal ganglion inside the skull�

The fibers enter the brainstem in the pontine region (along the course of the middle cerebellar peduncle; see Figure 1�8 and Figure 3�4)� Within the central nervous system there is a differential handling of the modalities, comparable to the previously described pathways in the spinal cord�

Those fibers carrying the sensations of discriminative touch synapse in the principal (main) nucleus of cranial nerve (CN) V, in the mid-pons, below the level of entry of the nerve (see Appendix Figure A�6)� The fibers then cross the midline and join the medial lemniscus, to terminate in the ventral posteromedial (VPM) nucleus of the thalamus (see Figure  4�3)� They are then relayed via the posterior limb of the internal capsule to the postcentral gyrus, where the face area is represented on the dorsolateral surface (see Figure 4�5); the lips and tongue are very well represented on the sensory homunculus�

Those fibers carrying the modalities of pain and temperature descend within the brainstem on the same side� They form a tract that starts at the mid-pontine level, descends through the medulla, and reaches the upper level of the spinal cord (see Appendix Figure A�8, Appendix Figure A�9, and Appendix Figure A�10) called the descending or spinal tract of V (see Figure 3�4), and also called the spinal trigeminal tract� Immediately medial to this tract is a nucleus with the same name, the spinal (descending) nucleus of CN V� The fibers terminate in this nucleus and, after synapsing, cross to the other side and ascend (see Figure 6�11)� Therefore, these fibers decussate over a wide

region and do not form a compact bundle of crossing fibers; they also send collaterals to the reticular formation� These trigeminal fibers join with those carrying touch to form the trigeminal pathway in the mid-pons� They terminate in the VPM and other thalamic nuclei, similar to those of the anterolateral system (see Figure 5�5)� The trigeminal pathway joins the medial lemniscus in the upper pons, as does the anterolateral pathway (se Figure 6�11)�

The principal nucleus of CN V is seen at the mid-pontine level� The descending trigeminal tract is found in the lateral aspect of the medulla, with the nucleus situated immediately medially� The crossing pain and temperature fibers join the medial lemniscus over a wide area and are thought to have completely crossed by the lower pontine region� The collaterals of these fibers to the reticular formation are shown�

Trigeminal neuralgia is an affliction of the trigeminal nerve of uncertain origin that causes severe “lightning” pain in one of the branches of CN V; often there is a trigger such as moving the jaw or an area of skin� The shooting pains may occur in paroxysms lasting several minutes� An older name for this affliction is tic douloureux. There is often a history of trauma to the CN V such as dental work or facial trauma; in some cases a vascular loop from the basilar artery can impinge on the CN V� Treatment of these cases, which cause enormous pain and suffering, is difficult� There are surgical options for treatment based on the underlying cause� Most patients can be managed with medical therapy�

An ischemic infarct of the lateral medulla disrupts the descending pain and temperature fibers and results in a loss of these sensations on the same side of the face while leaving the fibers for discriminative touch sensation from the face intact� This lesion, known as the lateral medullary syndrome (of Wallenberg), includes other deficits (see Figure 6�11 and discussed with Appendix Figure A�9)� A lesion of the medial lemniscus above the pontine level involves all trigeminal sensations on the opposite side� Internal capsule and cortical lesions cause a loss of trigeminal sensations from the opposite side of the face, as well as involving other pathways�

This diagram presents all the somatosensory pathways, the dorsal column-medial lemniscus, the anterolateral pathway, and the trigeminal pathway as they pass through the midbrain region into the thalamus and onto the cortex� The view is a dorsal perspective (as in Figure 1�9 and Figure 3�3)�

The pathway that carries discriminative touch sensation and information about joint position (as well as vibration) from the body is the medial lemniscus� The equivalent pathway for the face comes from the principal nucleus of the trigeminal nerve, which is located at the mid-pontine level� The anterolateral pathway conveying pain and temperature from the body has joined up with the medial lemniscus by this level (see also Figure 6�11)� The trigeminal pain and temperature fibers have similarly joined up with the other trigeminal fibers�

The various sensory pathways are all grouped together at the level of the midbrain (see the inset crosssection; note the dorsal perspective)� At the level of the lower midbrain, these pathways are located near the surface, dorsal to the substantia nigra; as they ascend they are found deeper within the midbrain, dorsal to the red nucleus (shown in Appendix Figure A�3 and Appendix Figure A�4)�

The two pathways carrying the modalities of fine touch and position sense (and vibration) terminate in different specific relay nuclei of the thalamus (see Figure 4�3 and Figure 6�13):

• The medial lemniscus in the ventral posterolateral nucleus (VPL)�

• The trigeminal pathways in the ventral posteromedial nucleus (VPM)�

Sensory modality and topographic information is retained in these nuclei� There is physiological processing of the sensory information, and some type of sensory “perception” likely occurs at the thalamic level�

After the synaptic relay, the pathways continue as the (superior) thalamo-cortical radiation through the posterior limb of the internal capsule, between the thalamus and lentiform nucleus (see Figure  2�10A, Figure  2�10B, and Figure 4�4)� The fibers are then found within the white matter of the hemispheres� The somatosensory information is distributed to the cortex along the postcentral gyrus (see the small diagrams of the brain), also called S1� Precise localization and two-point discrimination are cortical functions�

The information from the face and hand is topographically located on the dorsolateral aspect of the hemispheres (see Figure 1�3 and Figure 4�5)� The information from the lower limb is localized along the continuation of this gyrus on the medial aspect of the hemispheres (see Figure 1�7 and Figure 4�5)� This cortical representation is called the sensory “homunculus,” a distorted representation of the body and face with the trunk and lower limbs having very little area, whereas the face and fingers receive considerable representation (similar to the motor homunculus, see Figure 4�5)�

Further elaboration of the sensory information occurs in the parietal association areas adjacent to the postcentral gyrus (known as areas 5 and 7; see Figure  6�13)� This allows us to learn to recognize objects by tactile sensations, called stereognosis (e�g�, the denomination of various coins in the hand)�

The pathways carrying pain and temperature from the body (the anterolateral system) and the face (spinal trigeminal system) terminate in part in the specific relay nuclei, VPL and VPM, but mainly in the intralaminar nuclei� These latter terminations may well be involved with the emotional correlates that accompany many sensory experiences (e�g�, pleasant or unpleasant)�

The fibers that have relayed pain information project from these nuclei to several cortical areas, including the post-central gyrus, SI, and area SII (a secondary sensory area), which is located in the lower portion of the parietal lobe, as well as other cortical regions� The output from the intralaminar nuclei of the thalamus goes to widespread cortical areas�

Knowledge of the exact location of the various pathways in the brainstem assists with the localization of various lesions (including vascular) affecting that region (see the Appendix)�

Lesions of the thalamus may sometimes give rise to pain syndromes (also discussed with Figure 5�3)�

There are three principal thalamic syndromes� The lateral thalamic syndrome causes deficits to the contralateral sensation (refer to Figure 4�3)� The anterior thalamic syndrome causes clinical effects similar to Korsakoff’s syndrome (psychosis) due to the disconnection of the thalamus to the hippocampus via the fornices (see Figure 10�1A and Figure 10�2)� The medial thalamic syndrome results in a anhedonic syndrome (see definition below) similar to a frontal lobe syndrome (discussed with Figure 10�1B)�

On occasion, language can be affected with thalamic lesions�

Note to the Learner: The dictionary definition of anhedonia is an “insensitiveness to pleasure,” and also an “incapacity for experiencing happiness�”

Pain from physical sources is recognized by the nervous system at multiple levels� Localization of pain, knowing which part of the limbs and body wall is involved, requires the cortex of the postcentral gyrus (SI); SII is likely also involved in the perception of pain (discussed with Figure 5�5)� There is good evidence that some “conscious” perception of pain occurs at the thalamic level�

We have a built-in system for dampening the influences of pain from the spinal cord level-the descending pain modulation pathway� This system apparently functions in the following way:

The neurons of the periaqueductal gray (see Figure 3�6B and Appendix Figure A�3) can be activated in a number of ways� It is known that many ascending fibers from the anterolateral system and trigeminal system activate neurons in this area (only the anterolateral fibers are shown in this illustration) either as collaterals or as direct endings of these fibers in the midbrain� This area is also known to be rich in opiate receptors, and it seems that neurons of this region can be activated by circulating endorphins� Experimentally, one can activate these neurons by direct stimulation or by a local injection of morphine� In addition, descending cortical fibers (cortico-bulbar; see Figure 5�10) may activate these neurons�

The axons of some of the neurons of the periaqueductal gray descend and terminate in one of the serotonincontaining raphe nuclei in the upper medulla, the nucleus raphe magnus (see Figure 3�6B)� From here, there is a descending, crossed, pathway that is located in the dorsolateral white matter (funiculus) of the spinal cord� The serotonergic fibers terminate in the substantia gelatinosa of the spinal cord, a nuclear area of the dorsal horn of the spinal cord where the pain afferents synapse (see Figure 3�9 and Figure  5�1)� The descending serotonergic fibers are thought to terminate on small interneurons that contain enkephalin� There is evidence that these enkephalincontaining spinal neurons inhibit the transmission of the pain afferents entering the spinal cord from peripheral pain receptors� Thus, descending influences are thought to modulate a local circuit�

There is a proposed mechanism that these same interneurons in the spinal cord can be activated by stimulation of other sensory afferents, particularly those from the mechanoreceptors in the skin and joints; these are

anatomically large, well-myelinated peripheral nerve fibers� This is the physiological basis for the gate control theory of pain� In this model, the same circuit is activated at a segmental level�

It is useful to think about multiple gates for pain transmission� We know that mental states and cognitive processes can affect, positively and negatively, the experience of pain and our reaction to pain� The role of the limbic system and the “emotional reaction” to pain are discussed in Section 4�

In our daily experience with local pain, a bump or small cut, the common response is to rub the limb or the affected region vigorously� What we may be doing is activating the local spinal segmental circuits via the mechanoreceptors to decrease the pain sensation�

Some of the current treatments for pain are based on the structures and neurotransmitters discussed here� The gate control theory underlies the use of transcutaneous stimulation, one of the current therapies offered for the relief of pain� More controversial and certainly less certain is the postulated mechanism for the use of acupuncture in the treatment of pain�

Most discussions concerning pain refer to acute pain, or short-term pain caused by an injury or dental procedure� Chronic pain should be regarded from a somewhat different perspective� Living with pain on a daily basis that is caused, for example, by arthritis or cancer or diabetic neuropathy, including post-herpetic neuralgia (see later), is an unfortunately tragic state of being for many people� Those involved with pain therapy and research on pain have proposed that the central nervous system actually re-wires itself in reaction to chronic pain and may in fact become more sensitized to pain the longer the pain pathways remain active; some of this may occur at the receptor level� Many of these people are now being referred to pain clinics, where a team of physicians and other health professionals (e�g�, anesthetists, neurologists, psychologists) try to assist people, by using a variety of therapies, to alleviate their disabling condition�

Note to the Learner: Herpes zoster is a not uncommon affliction affecting primarily older individuals and particularly persons with reduced immunity (e�g�, those undergoing chemotherapy for cancer)� Often these individuals are afflicted with recurrent (persistent) pain following the acute phase, called post-herpetic neuralgia� There is  now a specific vaccine available-as a preventive-to boost the immunity for this vulnerable population�

INTRODUCTION There are multiple areas involved in motor control, and this is the reason for the title motor systems (plural)� The parts of the central nervous system that regulate the movement of our muscles include: motor areas of the cerebral cortex; the basal ganglia (including the substantia nigra and the subthalamus); the cerebellum (with its functional subdivisions); nuclei of the brainstem including the red nucleus, the reticular formation, and vestibular nuclei; and finally, the output motor neurons of the cranial nerve motor nuclei and the motor neurons of the spinal cord, anatomically the anterior horn cell in the ventral horn of the spinal cord�

The anterior horn cell is the neuron that completes the pathway from command to action by connecting to the muscle; clinically, it is called the lower motor neuron� This neuron reacts to stretch of the muscle it supplies by contracting, known simply as the stretch reflex, also called the myotatic reflex, and is clinically examined as the deep tendon reflex (DTR, see Figure 5�7)� The examination of the degree of reflex reactivity is one of the most important aspects of the neurological examination� It is graded clinically (discussed in The Integrated Nervous System)� Most important, this reflex circuit is monosynaptic, involving only one synapse between the afferent fiber from the stretch receptors in the muscle and its anterior horn cell (further discussed with Figure  5�7)� This reflex circuit is also involved in the control of muscle tone, which is another important aspect of the clinical neurological assessment�

Voluntary motor activity can be separated into two systems-a newer system (from the evolutionary perspective) for the control of fine motor movements of the fingers and hand and another, “older” system for the control of the proximal (large) joints and postural (axial) musculature that is partly under voluntary control� It is important to recognize that almost all voluntary motor movements involve some postural adjustments�

The motor pathways (tracts) are called descending because they commence in the cortex or brainstem and influence motor cells lower down in the neuraxis, either in the brainstem or the spinal cord� Several pathways “descend” through the spinal cord; only some of them cross (decussate), whereas others are uncrossed or have fibers that descend on both sides�

VOLUNTARY MOTOR SYSTEM Voluntary motor control involves both direct and indirect pathways:

The direct voluntary pathway, for the control of fine motor movements, is controlled from the cerebral cortex� Areas of the frontal lobe are involved in motor

planning, which precedes the activation of the motor commands that occurs in the motor strip� The descending pathways include the cortico-bulbar fibers to cranial nerve nuclei and the cortico-spinal tract to spinal cord motor neurons� In this atlas, this pathway may be considered the neo-motor system because it is controlled directly from the (neo)cortex� In the clinical milieu, the cortical neurons giving rise to this direct pathway are known as the upper motor neuron� The cortical areas involved in this motor control are modulated by the basal ganglia and also by the evolutionary newer areas of the cerebellum (to be discussed later in this chapter under Motor Modulation)�

The descending tracts or pathways in the neo/direct category include:

• Cortico-spinal tract-This pathway originates in motor areas of the cerebral cortex� The cortico-spinal tract, from cortex to spinal cord, is a relatively new (from an evolutionary perspective) tract and the most important for  voluntary movements in humans, particularly of the hand and digits-the direct voluntary motor pathway� As explained later, it is also known as the pyramidal tract because it courses through the medulla in the structure called the pyramid� Because of the location of this pathway in the spinal cord, it continues under the name the lateral cortical-spinal tract� There is also a smaller anterior (ventral) portion of this pathway, the anterior cortico-spinal tract, which may have a different functional contribution�

Note to the Learner: This pathway synapses sometimes directly on the anterior horn cells of the spinal cord, the lower motor neuron, and sometimes via an interneuron in the spinal cord�

• Cortico-bulbar fibers-This is a descriptive term that is poorly defined and includes all fibers that go to the brainstem, both cranial nerve nuclei and other brainstem nuclei� The fibers that go to the reticular formation include those that form part of the indirect voluntary motor pathway (discussed next)� The corticopontine fibers are described with the cerebellum�

The indirect voluntary pathways are functionally an older system (from the evolutionary perspective) for the control of proximal joint movements and axial (postural) musculature� The motor cortex is controlling movements via the reticular formation and other nuclei of the brainstem� This functional part of motor control could be considered the paleo-motor system� It is most important to

reiterate that almost all voluntary motor movements involve some postural adjustments� The various nuclei of the brainstem (the red nucleus, the vestibular nuclei, and the reticular formation) are also regulated by the functionally older parts of the cerebellum�

The descending tracts or pathways involved in the paleo/indirect category include:

• Rubro-spinal tract-The red nucleus of the midbrain gives rise to the rubro-spinal tract� Its connections are such that it may play a role in voluntary and non-voluntary motor activity; this may be the case in higher primates, but its precise role in humans is not clear� Nevertheless, the red nucleus region appears to be of clinical importance in the localization of lesions causing comatose states and abnormal reflex posturing of the limbs-named decorticate and decerebrate rigidity or posturing (see Figure 5�13)�

• Reticulo-spinal tracts-These tracts are involved both in the indirect voluntary pathways and in non-voluntary motor regulation (see later), as well as in the underlying regulation of muscle tone and reflex responsiveness� Two tracts descend from the reticular formation-one from the pontine region, the medial reticulo-spinal tract, and one from the medulla, the lateral reticulo-spinal tract�

Note to the Learner: This older paleo/indirect system controls the reactivity of the lower motor neuron and hence influences the reactivity of the stretch reflex (the tendon reflex) and muscle tone, most important both functionally and clinically�

NON-VOLUNTARY MOTOR SYSTEM This very old system (from the evolutionary perspective) serves for adjustment of the body to vestibular rotational and gravitational changes, via the vestibular apparatus in the inner ear� It also responds to visual input, motor responses that do not involve volitional actions but are essential for our daily activities� These would constitute the archi-motor system�

The descending tracts or pathways involved in the archi-motor category include:

• Lateral and medial vestibulo-spinal tractsThe lateral vestibular nucleus of the pons gives

rise to the lateral vestibulo-spinal tract� It is under the control of the oldest parts of the cerebellum, not the cerebral cortex� The medial vestibular nucleus sends some of its fibers downward in the medial vestibulo-spinal tract�

• Reticulo-spinal tracts-These tracts are involved both in the indirect voluntary pathways (see earlier) and in non-voluntary motor regulation�

The next part of this chapter considers the motor areas of the cerebral cortex, and the nuclei of the brainstem and reticular formation involved in motor regulation� The same standardized diagram of the nervous system is used as with the sensory systems, as well as the select crosssections of the spinal cord and brainstem�

The last part of this chapter will discuss Motor Modulation by the basal ganglia and cerebellum� Both subsystems of motor control, acting via the thalamus, provide feedback to the motor cortex and modify motor movements� There are additional areas of the cerebral cortex involved in other aspects of motor activity� Broca’s area for the motor control of speech is situated on the dominant side for language (usually the left hemisphere), on the dorsolateral surface, a little anterior to the lower portions of the motor areas (see Figure 4�5)� The frontal eye field, in front of the premotor area, controls voluntary eye movements (see Figure  4�5 and discussed with Figure 6�8)�

The conceptual approach to the motor system as comprising an upper motor neuron and a lower motor neuron is most important for clinical neurology� Injuries and diseases (all called lesions) in humans rarely if ever affect a single component of these pathways� A typical human lesion of the brain (e�g�, vascular, trauma, tumor) usually affects cortical and/or subcortical areas, involving both the direct and indirect motor systems� The end results are alterations in several of the descending systems that lead to a mixture of deficits of movement, changes of the stretch reflexes (hyperreflexia, hyporeflexia, or absent reflexes), and a change in muscle tone (flaccidity or spasticity)�

The plantar reflex and its abnormal response, now called the extensor plantar response-no longer using the terminology of a positive or negative Babinski signare extremely important clinically and are discussed with Figure 5�9�

The motor regions of the spinal cord in the ventral horn are shown in this diagram� The lateral motor nuclei supply the distal musculature (e�g�, the hand), and, as would be expected, this area is largest in the region of the limb plexuses (brachial and lumbosacral; see Figure  3�9 and Figure  4�1)� The medial group of neurons supplies the axial musculature�

In the spinal cord, the neurons that are located in the ventral or anterior horn and are (histologically) the anterior horn cells, are usually called the lower motor neuron� Physiologists call these neurons the alpha motor neurons� In the brainstem, these neurons include the motor neurons of the cranial nerves (see Figure 3�5)� Because all the descending influences converge on the lower motor neurons, these neurons have also been called, in a functional sense, the final common pathway�

The axons of these neurons leave the spinal cord though a series of rootlets (see Figure 1�11, Figure 1�12, and Figure 8�7)� The lower motor neuron and its axon and the muscle fibers that it activates are collectively called the motor unit� The intactness of the motor unit determines the reflex reactivity, muscle strength, and muscle function� The ventral and dorsal roots unite outside the vertebral canal to form the mixed spinal nerve (see Figure 1�12 and 7�3)�

The myotatic reflex is elicited by stretching a muscle (e�g�, by tapping on its tendon), and this causes a contraction of the same muscle that was stretched; the receptor for this stretch is the muscle spindle� Thus, the reflex is also known as the stretch reflex, the deep tendon reflex, often simply called the DTR� In this reflex arc (shown on the left side), the information from the muscle spindle (afferent) ends directly on the anterior horn cell (efferent); there is only one synapse (i�e�, a monosynaptic reflex)�

Note to the Learner: This reflex is discussed in The Integrated Nervous System, and an animation of the reflex is shown on the Web site of that text�

All other reflexes, even a simple withdrawal reflex (e�g�, touching a hot surface), involves some central processing (more than one synapse, multisynaptic) in the

spinal cord, before the response (shown on the right side)� All these reflexes involve hard-wired circuits of the spinal cord, but they can be and are influenced by information descending from higher levels of the nervous system�

Studies indicate that complex motor patterns are present in the spinal cord, such as stepping movements with alternating movements of the limbs, and that influences from higher centers provide the organization for these built-in patterns of activity�

The deep tendon reflex is a monosynaptic reflex and perhaps the most important for a neurological examination� The degree of reactivity of the lower motor neuron is influenced by higher centers, also called descending influences, particularly by the reticular formation (discussed with Figure 5�12B)� An increase in this reflex responsiveness is called hyperreflexia, and a decrease is hyporeflexia� The state of activity of the lower motor neuron also influences muscle tone-the “feel” of a muscle at rest and the way in which the muscles react to passive stretch (by the examiner); again, there may be hypertonia or hypotonia�

Disease or destruction of the anterior horn cells results in weakness or paralysis of the muscles supplied by those neurons� The extent of the weakness depends on the extent of the neuronal loss and is rated on a clinical scale, called the MRC (Medical Research Council)� There is also a decrease in muscle tone, as well as a decrease in reflex responsiveness (hyporeflexia) of the affected segments; the plantar response is normal�

The clinical usefulness of this information is discussed in The Integrated Nervous System�

The specific disease that affects the lower motor neurons is poliomyelitis (usually called simply polio), a infectious disease usually affecting children carried in fecally contaminated water� This disease entity has almost been totally eradicated in the industrialized world by immunization of all children�

In adults, the disease that affects both the upper and lower motor neurons specifically (including cranial nerve motor neurons) is amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig disease� In this progressive degenerative disease, there is a loss of the motor neurons in the cerebral cortex (the upper motor neuron) and the anterior horn cells (the lower motor neuron)� The clinical picture depends on the degree of loss of the neurons at both levels� People afflicted with this devastating disease suffer a continuous march of loss of function, including swallowing and respiratory function, that leads to their death� Their intellectual functions are not affected� This exacts a heavy emotional toll on the person afflicted and on family and friends� Researchers are actively seeking ways to arrest the degenerative loss of these neurons�

There are three areas of the cerebral cortex directly involved in motor control (see Figure 1�3 and Figure 4�5):

• The motor cortex is otherwise known as the precentral gyrus, anatomically area 4 (also called the motor strip), in which the various portions of the body are functionally represented; the fingers and particularly the thumb, as well as the tongue and lips, are heavily represented on the dorsolateral surface, with the lower limb on the medial surface of the hemisphere� This motor “homunculus” is not unlike the sensory homunculus (see Figure 4�5)� The large neurons of the motor strip (in the deeper cortical layers) send their axons as projection fibers to form the cortico-bulbar and corticospinal tracts� It is this cortical strip that contributes most to voluntary movements�

• Anterior to this is another wedge-shaped cortical area, the premotor cortex, area 6, with a less definite body representation� This cortical area sends its axons to the motor cortex, as well as to the cortico-spinal tract, and likely its function has more to do with proximal joint control and postural adjustments needed for movements�

• The supplementary motor cortex is located on the dorsolateral surface and mostly on the medial surface of the hemisphere, anterior to the motor areas� This is an organizing area for movements and its axons are sent to the premotor and motor cortex�

These motor areas of the cerebral cortex are regulated by the basal ganglia and (newer) parts of the cerebellum� These two important large areas of the brain are “working behind the scenes” to adjust and calibrate, to modulate, the neuronal circuits of the cerebral cortex involved in motor control (to be discussed under Motor Modulation in this chapter)� All these areas also receive input from other parts of the cerebral cortex, particularly from the sensory postcentral gyrus, as well as from the parietal lobe�

The cortico-spinal tract, a direct pathway linking the cortex with the spinal cord, is the most important pathway for precise fine voluntary motor movements in humans�

This pathway originates mostly from the motor areas of the cerebral cortex, areas 4 and 6 (see Figure 1�3, Figure 4�5, and Figure 5�8; also discussed earlier with Motor SystemsIntroduction)� The well-myelinated axons descend through the white matter of the hemispheres, through the posterior limb of the internal capsule (see Figure  4�4), continue through the midbrain (see Appendix Figure A�3) and pons (as disbursed fibers; see Appendix Figure A�6), and are then found within the medullary pyramids (see Figure  1�8, Figure 3�1, Appendix Figure A�8, Appendix Figure A�9, and Appendix Figure A�10)� Hence, the cortico-spinal pathway is often called the pyramidal tract, and clinicians may sometimes refer to this pathway as the pyramidal system� At the lowermost part of the medulla, most (90%) of the cortico-spinal fibers decussate (cross) in the pyramidal decussation (see Figure 6�12) and form the lateral corticospinal tract in the spinal cord (see Figure 6�10)�

Many of these fibers end directly on the lower motor neuron, particularly in the cervical spinal cord� This pathway is involved with controlling the individualized movements, especially of our fingers and hands (i�e�, the distal limb musculature)� Experimental work with monkeys has shown, after a lesion is placed in the medullary pyramid, muscle weakness and a loss of ability to perform fine movements of the fingers and hand (on the opposite side); the animals were still capable of voluntary gross motor movements of the limb� There was no change in the deep tendon reflexes, and a decrease in muscle tone was reported� The innervation for the lower extremity is similar, but it clearly involves less voluntary activity�

Those fibers that do not cross in the pyramidal decussation form the anterior (or ventral) cortico-spinal tract� Many of these axons cross before terminating, whereas others supply motor neurons on both sides� The ventral pathway is concerned with movements of the proximal limb joints and axial movements, similar to other pathways of the indirect voluntary motor system�

Other areas of cortex contribute to the cortico-spinal pathway-these include the sensory cortical areas, the postcentral gyrus (also discussed with Figure 5�10)�

After emerging from the internal capsule, the corticospinal tract is found in the mid-portion of the cerebral

peduncles in the midbrain (see Figure 1�8 and Figure 5�10)� The cortico-spinal fibers are then dispersed in the pontine region and are seen as bundles of axons among the pontine nuclei� The fibers collect again in the medulla as a single tract, in the pyramids on each side of the midline� At the lowermost level of the medulla, 90% of the fibers decussate and form the lateral cortico-spinal tract, situated in the lateral aspect of the spinal cord (see Figure 6�10)� The ventral (anterior) cortico-spinal tract is found in the anterior portion of the white matter of the spinal cord�

Lesions involving the cortico-spinal pathway in humans are quite devastating because they rob the individual of voluntary motor control, particularly the fine skilled motor movements� This pathway is quite commonly involved in strokes, as a result of vascular lesions of the cerebral arteries or of the deep arteries to the internal capsule (reviewed with Figure  8�4, Figure  8�5, and Figure 8�6)� This lesion results in an upper motor neuron pattern of weakness (paresis) or paralysis of the muscles on the opposite side� The clinical signs in humans reflect the additional loss of cortical input to the brainstem nuclei, particularly to the reticular formation�

Damage to the tract in the spinal cord is seen after traumatic injuries (e�g�, automobile and diving accidents)� In this case, other pathways would be involved, and the clinical signs reflect this damage, with the loss of the indirect voluntary and non-voluntary tracts (discussed with Figure 5�12B)� If one-half of the spinal cord is damaged, the loss of function is ipsilateral to the lesion�

One abnormal reflex indicates, in humans, that there has been a lesion interrupting the cortico-spinal pathway-at any level (cortex, white matter, internal capsule, brainstem, spinal cord)� The reflex involves stroking the lateral aspect of the bottom of the foot (a most uncomfortable sensation for most people)� Normally, the response involves flexion of the toes (note especially the big toe)—the plantar reflex, and often an attempt to withdraw the limb� Testing this same reflex after a lesion interrupting the cortico-spinal pathway results in an upward movement of the big toe (extension) and a fanning apart of the other toes� The abnormal response is called an extensor plantar response (this terminology will be used in the atlas), and it can be elicited almost immediately after any lesion that interrupts any part of the cortico-spinal pathway, from cortex through to spinal cord (except in the period immediately after an acute transaction of the spinal cord, called spinal shock; discussed with Figure 6�10)�

Note to the Learner: The term positive Babinski sign-not reflex-is no longer being used because generations of students often misused the terminology of positive and negative Babinski (see also Clinical Cases)�

The word “bulb” (i�e�, bulbar) is descriptive and refers to the brainstem� The cortico-bulbar fibers do not form a single pathway� The fibers end in a wide variety of nuclei of the brainstem; those fibers ending in the pontine nuclei to be relayed to the cerebellum are considered separately (see Figure 5�15)�

Wide areas of the cortex send fibers to the brainstem as projection fibers� These axons course via the internal capsule and continue into the cerebral peduncles of the midbrain (see Figure  1�8, Appendix Figure A�3, and Appendix Figure A�4)� The fibers involved with motor control occupy the middle third of the cerebral peduncle along with the cortico-spinal tract (described with Figure 5�9) supplying the motor cranial nerve nuclei of the brainstem (see Figure  3�5), the reticular formation (see Figure 3�6A and Figure 3�6B), and other motor-associated nuclei of the brainstem�

• Cranial nerve nuclei-The motor neurons of the cranial nerves of the brainstem are lower motor neurons (see Figure  3�5); the cortical motor cells are the upper motor neuron� These motor nuclei are generally innervated by fibers from both sides (i�e�, each nucleus receives input from both hemispheres)� The major exception to this rule, which is very important in the clinical setting, involves the cortical input to the facial nucleus� The portion of the facial nucleus supplying the upper facial muscles (the forehead) is supplied from both hemispheres, whereas the part of the nucleus supplying the lower facial muscles (around the mouth) is innervated only by the opposite hemisphere (crossed)� The significance of this clinically is discussed under Clinical Aspect�

• Brainstem motor control nuclei-Cortical fibers influence all the brainstem motor nuclei, including the red nucleus (see Figure 5�11) and the substantia nigra (see Figure 5�14), and particularly the reticular formation (see Figure 5�12A and Figure 5�12B), but not the lateral vestibular nucleus (see Figure  5�13, Figure  5�16, and Figure  5�17)� The cortico-reticular fibers are extremely important for voluntary movements of the proximal joints (indirect voluntary pathway) and for the regulation of muscle tone�

• Other brainstem nuclei-The cortical input to the sensory nuclei of the brainstem is

consistent with cortical input to all relay nuclei; this includes the somatosensory nuclei, the nuclei cuneatus and gracilis (see Figure  5�2)� There is also cortical input to the periaqueductal gray, as part of the pain modulation system (see Figure 5�6)�

Loss of cortical innervation to the cranial nerve motor nuclei is usually associated with weakness, not paralysis, of the muscles supplied� For example, a lesion on one side may result in difficulty in swallowing or phonation; often these problems dissipate in time�

Facial Movements

A lesion of the facial area of the cortex or of the corticobulbar fibers affects the muscles of the face differentially� Due to bilateral innervation by the upper motor neurons supplying the upper facial muscles (including the frontalis muscles of the forehead), a patient with such a lesion is able to wrinkle his or her forehead normally on both sides when asked to look up� She/he will not be able to show the teeth or smile symmetrically on the side opposite the lesion because the innervation to the part of the motor nucleus of CN VII which supplies the lower facial muscles is only from the opposite side� Because of the marked weakness of the muscles of the lower face, there is a drooping of the lower face on the side opposite the lesion� This also affects the muscle of the cheek (the buccinator muscle) and may cause some difficulties with drinking, eating, and chewing (the food becomes stuck in the cheek and often has to be manually removed); sometimes there is also drooling�

This clinical situation must be distinguished from a lesion of the facial nerve itself, a lower motor neuron lesion, most often seen with Bell’s palsy (a lesion of the facial nerve as it emerges from the skull)� In this case, the movements of the muscles of the upper face, midface, and lower face are lost on one (ipsilateral) side�

Tongue Movements

In the clinical setting, movement of the tongue is assessed by asking the patient to “stick out the tongue” and move it from side to side� A defect in movement of the tongue rarely occurs in isolation� A lesion affecting the hypoglossal nucleus or nerve is a lower motor lesion of one half of the tongue (on the same side) and leads to atrophy and paralysis of the side affected� When the patient is asked to “stick out your tongue,” the tongue deviates to the side that has the lesion�

A cortical lesion or a white matter lesion affecting the cortico-bulbar system is an upper motor lesion� In this case, there is weakness on the opposite side and perhaps mild atrophy, and the tongue protrusion deviates to the side opposite the lesion�

The red nucleus is a prominent nucleus of the midbrain (see Figure  3�3)� It takes its name from a reddish color seen in fresh dissections of the brain, presumably the result of its high vascularity� The nucleus (see Figure 4�2C and Appendix Figure A�3) has two portions, a smallcelled upper division and a portion with large neurons more ventrally located� The rubro-spinal pathway originates, at least in humans, from the larger cells�

The red nucleus receives its input from the motor areas of the cerebral cortex (cortico-bulbar) and from the cerebellum (see Figure 5�17)� The cortical input is directly onto the projecting cells, thus forming a potential twostep pathway from motor cortex to spinal cord�

The rubro-spinal tract is also a crossed pathway, with the decussation occurring in the ventral part of the midbrain (see Figure  6�9 and Figure  6�12)� The tract descends within the central part of the brainstem (the tegmentum) and is not clearly distinguishable from other fiber systems� The fibers then course in the lateral portion of the white matter of the spinal cord, just anterior to and intermingled with the lateral cortico-spinal tract (see Figure 6�10)�

The rubro-spinal tract is a well-developed pathway in some animals� In monkeys, it seems to be involved in flexion movements of the limbs� Stimulation of this tract in cats produces an increase in tone of the flexor muscles�

The extent of this tract and its function in humans are discussed in the following paragraphs�

The location of this tract within the brainstem is shown at cross-sectional levels of the upper midbrain, the midpons, the mid-medulla, and spinal cord level C8� The tract is said to continue throughout the length of the spinal cord in primates but probably extends only into the cervical spinal cord in humans (as shown)�

The fibers of CN III (oculomotor) exit through the medial aspect of this nucleus at the level of the upper midbrain (see Appendix Figure A�3)�

The functional significance of this pathway in humans is not well known� The number of large cells in the red nucleus in the human is significantly less than in the monkey� Motor deficits associated with a lesion involving only the red nucleus or only the rubro-spinal tract have not been adequately described� Although the rubro-spinal pathway may play a role in some flexion movements, it seems that the cortico-spinal tract predominates in the human�

Animal (cat) studies have been done with lesions either above or below the level of the red nucleus to try to understand the resultant motor deficits�

In humans, massive lesions above or below the level of the red nucleus region usually (if the person survives) lead to a comatose state accompanied by abnormal reflex posturing of the limbs-named decorticate and decerebrate rigidity (further discussed with Figure 5�13)�

FIGURE 5.12-RETICULAR FORMATION: INTRODUCTION Interspersed with the consideration of the functional systems is the reticular formation, located in the core of the brainstem (see Figure  3�6A and Figure  3�6B and the Appendix)� This group of nuclei comprises a rather old system with multiple functions-some generalized and some involving the sensory or the motor systems� Some sensory pathways have collaterals to the reticular formation, and some do not� Its nuclei participate in a number of functions, some quite general (e�g�, “arousal”) and others more specific (e�g�, respiratory control)�

In addition, some motor pathways originate in the reticular formation� These nuclei of the reticular formation are part of the indirect voluntary motor pathway, as well as non-voluntary motor regulation (see Motor SystemsIntroduction)� The indirect voluntary pathway, the corticoreticulo-spinal pathway, is thought to be an older pathway for the control of movements, particularly of proximal joints and the axial musculature�

Muscle tone and reflex responsiveness are greatly influenced by activity in the reticular formation as part of the non-voluntary motor system (discussed with Figure 5�12B)�

The reticular formation receives input from many sources, including most sensory pathways (anterolateral, trigeminal, auditory, and visual)� At this point, the focus is on the input from the cerebral cortex, from both hemispheres� These axons form part of the so-called corticobulbar system of fibers (see Figure 5�10)�

Note to the Learner: Understanding the complexity of the various parts of the motor system and the role of the reticular formation in particular is not easy� One approach is to start with the basic myotatic (stretch) reflex-the reticular formation assumes a significant role in the modification of this response (i�e�, hyperreflexia or hyporeflexia), as well as muscle tone� The next step would be the role of the reticular formation in motor control, particularly for axial musculature, as part of the indirect voluntary motor system� In addition, there is the role of the reticular formation and other motor brainstem nuclei in the non-voluntary response of the organism to gravitational changes� It now

becomes important to understand that the cortex has an important role in controlling this system�

There are two pathways from the reticular formation to  the spinal cord-one originates in the pontine region (see below-Figure 5�12A) and one in the medullary region (see Figure 5�12B)�

This tract originates in the pontine reticular formation from two nuclei� The upper nucleus is called the oral portion of the pontine reticular nuclei (nucleus reticularis pontis oralis), and the lower part is called the caudal portion (see Figure  3�6B)� The tract descends to the spinal cord and is located in the medial region of the white matter (see Figure 6�10); this pathway is therefore called the medial reticulo-spinal tract�

Functionally, this pathway exerts its action on the extensor muscles, both movements and tone� The area in the pons is known as the reticular extensor facilitatory area� The fibers terminate on the anterior horn cells controlling the axial muscles, likely via interneurons� This system is complementary to that from the lateral vestibular nucleus (see Figure 5�13)�

The location of the reticular formation is shown in the brainstem� The tract begins in the pons and descends through the medulla and is found at cross-sectional levels of the spinal cord levels C8 and L3� The tract is intermingled with others in the white matter of the spinal cord in the anterior funiculus (see Figure 3�9 and Figure 6�10)�

Lesions involving the cortico-bulbar fibers including the cortico-reticular fibers are discussed with the medullary reticular formation (see Figure 5�12B)�

This tract originates in the medullary reticular formation, mainly from the nucleus gigantocellularis (meaning very large cells; see Figure 3�6B)� The tract descends more laterally in the spinal cord than the pontine pathway and is thus named the lateral reticulo-spinal tract (see Figure 6�10); some of the fibers cross and descend� The tract lies beside the (lateral) vestibulo-spinal pathway�

The pathway also has its greatest influence on axial musculature� This part of the reticular formation is functionally the reticular extensor inhibitory area, opposite to that of the pontine reticular formation� This area depends for its normal activity on influences coming from the cerebral cortex�

The location of the reticular formation is shown in the brainstem� The tract begins in the medulla and is found at cross-sectional levels of the spinal cord levels C8 and L3� The tract is intermingled with others in the white matter of the spinal cord�

A lesion destroying the cortico-bulbar fibers, an upper motor neuron lesion, results in an increase in the tone of the extensor/anti-gravity muscles that develops over a period of days� This increase in tone, called spasticity, is tested by passive flexion and extension of a limb; this test is velocity dependent, meaning that the joint of the limb has to be moved quickly� The anti-gravity muscles are affected in spasticity; in humans, for reasons that are difficult to explain, these muscles are the flexors of the upper limb and the extensors of the lower limb� There is also an increase in responsiveness of the stretch reflex, called hyperreflexia, as tested using the deep tendon reflex (DTR) discussed in Motor Systems-Introduction, which also develops over a period of a several days�

There are two hypotheses for the increase in the stretch (monosynaptic) reflex responsiveness:

• Denervation supersensitivity: One possibility is a change of the level of responsivity of the

neurotransmitter receptors of the motor neurons themselves caused by the loss of the descending input that leads to an increase in excitability�

• Collateral sprouting: Another possibility is that axons adjacent to an area that has lost synaptic input will sprout branches and occupy the vacated synaptic sites of the lost descending fibers� In this case, the sprouting is thought to be of the incoming muscle afferents (called 1A afferents, from the muscle spindles)�

There is experimental evidence (in animals) for both mechanisms� Spasticity and hyperreflexia usually occur in the same patient�

Another feature accompanying hyperreflexia is clonus� This can be elicited by grasping the foot and jerking the ankle upward; in a person with hyperreflexia, the response is a short burst of flexion-extension responses of the ankle, which the tester can feel and that can also be seen�

Lesions involving parts of the motor areas of the cerebral cortex, large lesions of the white matter of the hemispheres or of the posterior limb of the internal capsule, and certain lesions of the upper brainstem all may lead to a similar clinical state in which a patient is paralyzed or has marked weakness, with spasticity and hyperreflexia (with or without clonus) on the contralateral side some days after the time of the damage� The cortico-spinal tract would also be involved in most of these lesions, with loss of voluntary motor control and with the appearance of the extensor plantar response in most cases immediately after the lesion (see the Clinical Aspect discussion with Figure 5�9)�

A similar situation can and does occur following large lesions of the spinal cord in which all the descending motor pathways are disrupted, both voluntary and non-voluntary� Destruction of the whole cord would lead to paralysis below the level of the lesion (paraplegia), bilateral spasticity, and hyperreflexia (with or without clonus), a severely debilitating state�

It is most important to distinguish this state from that seen in a Parkinsonian patient who has a change of muscle tone called rigidity (discussed with Figure 5�14), with no change in reflex responsiveness and a normal plantar response�

This state should be contrasted with a lower motor neuron lesion of the anterior horn cell, with hypotonia and hyporeflexia, as well as weakness (e�g�, polio; discussed with Figure 5�7)�

These pathways are very important in providing a link between the vestibular influences (i�e�, gravity and balance) and the control of axial musculature, via the spinal cord� The main function is to provide corrective muscle activity when the body (and head) tilts or changes orientation in space (activation of the vestibular system, cranial nerve VIII; see Figure 3�6)� This motor activity has been classified as non-voluntary (discussed in Motor Systems-Introduction)�

The lateral vestibular nucleus, which is located in the lower pontine region (see Figure 3�6, Figure 6�8, and Appendix Figure A�7), is found at the lateral edge of the 4th ventricle and is characterized by extremely large neurons� (This nucleus is also called the Deiter nucleus in some texts, and the large neurons are often called by the same name�) It gives rise to a tract, the lateral vestibulospinal tract, that descends through the medulla and traverses the entire spinal cord in the ventral white matter (see Figure 6�10)� It does not decussate� The fibers terminate in the medial portion of the anterior horn, namely, on those motor cells that control the axial musculature (see Figure 5�7)�

The lateral vestibular nucleus receives its major inputs from the vestibular system and from the cerebellum; there is no cerebral cortical input�

Functionally, this pathway increases extensor muscle tone and activates extensor muscles� It is easier to think of these muscles as anti-gravity muscles in a four-legged animal; in humans, one must translate these muscles as functionally the extensors of the lower extremity and the flexors of the upper extremity�

The medial vestibulo-spinal tract originates from the medial vestibular nucleus (see Figure 3�4, Figure 6�8, and Appendix Figure A�7) and also from the inferior vestibular nucleus� The descending tract is situated medial to that from the lateral vestibulo-spinal tract (see Figure 6�10)� These nuclei are involved in other functions, particularly to do with connecting vestibular influences with head and eye movements and only projects to the upper cervical spinal cord (discussed with Figure 6�8 and Figure 6�9)�

The vestibular nuclei are found at the lower pontine level, and are seen through to the mid-medulla; the tracts descend throughout the spinal cord, as seen at C8 and L3� In the spinal cord, the lateral vestibulo-spinal tract is positioned somewhat anteriorly, just in front of the ventral horn, and it innervates the medial group of motor nuclei; the medial vestibulo-spinal tract is located in the ventral white matter, along with the other tracts�

A lesion of these pathways would occur with spinal cord injuries, and this would be classified with an “upper motor neuron” lesion, leading to spasticity and hyperreflexia�

Decorticate Rigidity (Flexor Posturing)

This term is to be used with caution, particularly because of the presumed location of the lesion and the wrong use of the term “rigidity” (see comment below)�

This state of abnormal posturing is sometimes seen in comatose patients with severe lesions at the midbrain level or above including severe lesion of the cerebral hemispheres�

In this postural state known as decorticate rigidity there is a state of flexion of the forearm (at the elbow) and extension of the legs�

Decerebrate Rigidity (Extensor Posturing)

Again this term is to be used with caution, particularly because of the presumed location of the lesion and the wrong use of the term “rigidity” (see comment below)�

This state of abnormal posturing is sometimes seen in comatose patients with severe lesions lower down in the brainstem�

In this condition they exhibit a postural state in which all four limbs are rigidly extended (including at the wrist)� Sometimes the back is arched, and this may be so severe as to cause a posture known as opisthotonus, in which the person is supported by the back of the neck and the heels�

Physiologically, these conditions are not related to Parkinsonian rigidity, but they are related to the abnormal state of spasticity (see discussion with Figure 5�12B)� The postulated mechanism involves the relative influence of the pontine and medullary reticular formation, along with the vestibulo-spinal pathway, with and without the input from the cerebral cortex�

The basal ganglia are involved with the initiation and cessation of movement, as well as the force or amplitude of the intended movement� The parts of the basal ganglia that are involved with the modulation of movement are the putamen, the globus pallidus, both internal and external divisions, and as part of this system the subthalamic nucleus and the substantia nigra�

This is a view of the “distal” basal ganglia, seen from the medial perspective, with the head of the caudate nucleus removed (see Figure 2�5B)� The illustration includes the two other parts of the functional basal ganglia “system”:

• The subthalamic nucleus (STh) is situated in a small region below the level of the diencephalon, the thalamus�

• The substantia nigra (SN) is a flattened nucleus located in the midbrain region� It has two parts (see Appendix Figure A�3):

• The pars compacta has pigment-containing cells (see Figure 1�5 and Figure 4�2C)� These neurons project their fibers to the caudate and putamen (the striatum or neostriatum)� This is called the nigro-striatal “pathway,” although the fibers do not form a compact bundle; the neurotransmitter involved is dopamine�

• The pars reticulata is situated more ventrally� It receives fibers from the striatum and is also an output nucleus from the basal ganglia to the thalamus, like the internal segment of the globus pallidus (see later)�

Information flows into the caudate (C) and putamen (P) from all areas of the cerebral cortex (in a topographic manner; see Figure  5�18), from the SN (dopaminergic from the pars compacta), and from the centromedian nucleus of the thalamus (see later)�

There are two circuits from the putamen for motor modulation:

1� The information is processed and passed through to the globus pallidus, internal segment (GPi) (and the pars reticulata of the SN); these are the output

nuclei of the basal ganglia for the facilitation of the intended movement-the “go” message�

2� In the other circuit, the information goes first to the external segment of the globus pallidus (GPe), which sends fibers to the subthalamic nucleus (S), which then connects with GPi� This output system is involved with  the restraint of the intended movement-the “stop” message�

The pathway from both outputs is relayed to the specific relay nuclei of the thalamus, the ventral anterior (VA) and ventral lateral (VL) nuclei (see Figure 4�3 and Figure 5�18)� These project to the premotor and supplementary motor cortical areas (see Figure 4�5, Figure 5�8, and Figure 6�13)�

Note to the Learner: Some of these connections are animated on the atlas Web site� The circuitry involving the basal ganglia, the thalamus, and the motor cortex areas is described in detail with Figure 5�18 (and animated on the atlas Web site)�

Another sub-loop of the basal ganglia involves the centromedian nucleus of the thalamus, a non-specific nucleus (see Figure 4�3)� The loop starts in the striatum (only the caudate nucleus is shown here), to both segments of the globus pallidus; then fibers from the GPi are sent to the centromedian nucleus, which then sends its fibers back to the striatum (see Figure 6�13)�

Parkinson’s disease: The degeneration of the dopaminecontaining neurons of the pars compacta of the SN, with the consequent loss of their dopamine input to the basal ganglia (the striatum), leads to this clinical entity (see also Figure  2�5A and Figure  2�5B)� Those afflicted with this disease have slowness of movement (bradykinesia), reduced facial expressiveness (mask-like facies), and a tremor at rest, typically a “pill-rolling” type of tremor� On examination, there is rigidity, manifested as an increased resistance to passive movement of both flexors and extensors that is not velocity dependent� (This is to be contrasted with spasticity; discussed with Figure 5�12B�) In addition, there is no change in reflexes�

The medical treatment of Parkinson’s disease has limitations, although various medications and combinations (as well as newer drugs) can be used for many years� The medications are used to replenish dopamine for the receptors involved in “go” aspect of movements� Too little or too much medication has clear effects on the modulation of motor control� After many years, the medical management becomes more difficult or ceases to work�

For some of these and other select patients, a surgical approach for the alleviation of the symptoms of Parkinson’s disease has been recommended�

MOTOR MODULATORY SYSTEM (ii) The cerebellum regulates the ongoing movement and also compares the actual movement with the intended (planned) motor act� The part of the cerebellum primarily involved is the neocerebellum (see Figure 3�7 and Figure 3�8)�

To understand the role of the cerebellum in the modulation of voluntary motor control, it is necessary to review the afferents to the cerebellum, the intra-cerebellar circuitry, and the efferents from the cerebellum�

FIGURE 5.15-CEREBELLUM A: AFFERENTS Information relevant to the role of the cerebellum in motor regulation comes from the cerebral cortex, the brainstem, and the muscle receptors in the periphery� The information is conveyed to the cerebellum mainly via the middle and inferior cerebellar peduncles�

The inferior cerebellar peduncle goes from the medulla to the cerebellum� It lies behind the inferior olivary nucleus and can sometimes be seen on the ventral view of the brainstem (as in Figure 1�8)� This peduncle conveys a number of fiber systems to the cerebellum� These are shown schematically in this diagram of the ventral view of the brainstem and cerebellum� They include the following:

• The posterior (dorsal) spino-cerebellar pathway is conveying proprioceptive information from most of the body� This is one of the major tracts of the inferior peduncle� These fibers, carrying information from the muscle spindles, relay in the dorsal nucleus of Clarke in the spinal cord (see Figure 5�1)� They ascend ipsilaterally in a tract that is found at the edge of the spinal cord (see Figure 6�10)� The dorsal spinocerebellar fibers terminate ipsilaterally; these fibers are distributed to the spino-cerebellar areas of the cerebellum�

• The olivo-cerebellar tract is also carried in this peduncle� The fibers originate from the inferior olivary nucleus (see Figure  1�8, Figure  3�1, Appendix Figure A�8, Appendix Figure A�9, and Appendix Figure A�10), cross

in the medulla, and are distributed to all parts of the cerebellum� These axons have been shown to be the climbing fibers to the main dendritic branches of the Purkinje neuron�

• Other cerebellar afferents from other nuclei of the brainstem, including the reticular formation, are conveyed to the cerebellum via this peduncle� Most important are those from the medial (and inferior) vestibular nuclei to the vestibulocerebellum� Afferents from the visual and auditory system are also known to be conveyed to the cerebellum�

The homologous spino-cerebellar tract for the upper limb is the cuneo-cerebellar tract� These fibers relay in the accessory (external) cuneate nucleus of the lower medulla (see Appendix Figure A�9 and Appendix Figure A�10)� This pathway is not shown in the diagram�

All parts of the cerebral cortex contribute to the massive cortico-pontine system of fibers (also described with Figure  3�8, Figure  5�10, and Figure  6�12)� These fibers descend via the anterior and posterior limbs of the internal capsule, then the inner and outer parts of the cerebral peduncle, and terminate in the pontine nuclei� The fibers synapse and cross and then project mainly to the neocerebellum via the middle cerebellar peduncle� This input provides the cerebellum with the cortical information relevant to motor commands and the planned (intended) motor activities�

Only one afferent tract enters via the superior cerebellar peduncle (see later)� This peduncle carries the major efferent pathway from the cerebellum (discussed with Figure 5�17 and Figure 5�18)�

One group of cerebellar afferents, those carried in the ventral (anterior) spino-cerebellar tract, enters the cerebellum via the superior cerebellar peduncle� These fibers cross in the spinal cord, ascend (see Figure 6�10), enter the cerebellum, and cross again, thus terminating on the same side from which they originated�

FIGURE 5.16-MOTOR MODULATORY SYSTEM (ii) CEREBELLUM B: INTRACEREBELLAR CIRCUITRY The cerebellum is presented from the dorsal perspective (as in Figure 1�9, Figure 6�11, and Figure 6�12)� To review, the 3rd ventricle is situated between the two diencephala; the pineal gland is seen attached to the posterior aspect of the thalamus� Below are the colliculi, superior and inferior� On the right side of the illustration, the cerebellar hemisphere has been cut away, revealing the “interior” on this side�

The cerebellum is organized with cortical tissue on the outside, the cerebellar cortex� The cortex consists of three layers, and all areas of the cerebellum are histologically alike� The most important cell of the cortex is the Purkinje neuron, which forms a layer of cells; their massive dendrites receive the input to the cerebellum, particularly along their extensive dendritic tree in the outer (granular) layer� Various interneurons are also located in the cortex� The axon of the Purkinje neuron is the only axonal system to leave the cerebellar cortex� It relays to the efferent nuclei of the cerebellum, the deep cerebellar nuclei�

Deep within the cerebellum are the intracerebellar nuclei or the deep cerebellar nuclei, now shown from the posterior view (see Figure 3�8)�

All (excitatory) afferents to the cerebellum go to both the deep cerebellar nuclei (via collaterals) and the cerebellar cortex� After processing in the cortex, the Purkinje neuron sends its axon on to the neurons of the deep cerebellar nuclei-all Purkinje neurons are inhibitory� Their influence modulates the activity of the deep cerebellar neurons, which are tonically active (described in more detail later)� The output of the deep cerebellar neurons, which is excitatory, influences neurons in the brainstem and cerebral cortex via the thalamus (discussed with Figure 5�17)�

The connections of the cortical areas with the intracerebellar nuclei follow the functional divisions of the cerebellum:

• The vestibulocerebellum is connected to the fastigial nucleus, as well as to the lateral vestibular nucleus�

• The spinocerebellum connects with the intermediate nuclei (the globose and emboliform)�

• The neocerebellum connects to the dentate nucleus�

Axons from the deep nuclei neurons project from the cerebellum to many areas of the central nervous system, including brainstem motor nuclei (e�g�, vestibular, reticular formation) and thalamus (to motor cortex)� In this way, the cerebellum exerts its influence on motor performance� This is discussed with Figure 5�17�

The cerebellum receives information from many parts of the nervous system, including the spinal cord, the vestibular system, the brainstem, and the cerebral cortex� Most of this input is related to motor function, but some is also sensory� These afferents are excitatory and influence the ongoing activity of the neurons in the intracerebellar nuclei, as well as projecting to the cerebellar cortex�

The incoming information to the cerebellar cortex is processed by various interneurons of the cerebellar cortex and eventually influences the Purkinje neuron� This leads to either increased or decreased firing of this neuron� Its axon is the only one to leave the cerebellar cortex, and these axons project, in an organized manner, to the deep cerebellar nuclei�

The Purkinje neurons are inhibitory, and their influence modulates the activity of the deep cerebellar nuclei� Increased firing of the Purkinje neuron increases the ongoing inhibition onto these deep cerebellar nuclei, whereas decreased Purkinje cell firing results in a decrease in the inhibitory effect on the deep cerebellar cells, that is, this results in the increased firing of the deep cerebellar neurons (called disinhibition)�

The cerebellar cortex projects fibers directly to the lateral vestibular nucleus� As would be anticipated, these fibers are inhibitory� The lateral vestibular nucleus could therefore, in some sense, be considered one of the intracerebellar nuclei� This nucleus also receives input from the vestibular system and then projects to the spinal cord (discussed with Figure 5�13)�

FIGURE 5.17-MOTOR MODULATORY SYSTEM (ii) CEREBELLUM C: EFFERENTS This is again a dorsal view of the diencephalon, brainstem, and cerebellum, with the deep cerebellar (intracerebellar) nuclei� The cerebellar tissue has been removed in the midline to reveal the 4th ventricle (as in Figure 1�9); the three cerebellar peduncles are also visualized from this posterior perspective (also in Figure 1�9)�

The output from the cerebellum is described, following the functional division of the cerebellum:

• Vestibulocerebellum: Efferents from the fastigial nuclei go to brainstem motor nuclei (e�g�, vestibular nuclei and reticular formation), thus influencing balance and gait� They exit in a bundle that is found adjacent to the inferior cerebellar peduncle (named the juxtarestiform body)�

• Spinocerebellum: The emboliform and globose, the intermediate nuclei, also project to brainstem nuclei, including the red nucleus of the midbrain� They also project to the appropriate limb areas of the motor cortex via the thalamus (see later); these are the fibers involved in the comparator function of this part of the cerebellum, whereby the cerebellum adjusts the actual movement with the intended movement�

• Neocerebellum: The dentate nucleus is the major outflow from the cerebellum via the superior cerebellar peduncle (see Figure 6�11)� This peduncle connects the cerebellar efferents, through the midbrain, to the thalamus on their way to the motor cortex� Some of the fibers terminate in the red nucleus of the midbrain, in addition to those from the intermediate nucleus� Most of the fibers, those from the dentate nucleus, terminate in the ventral lateral nucleus (VL) of the thalamus (see Figure  4�3 and Figure 5�18)� From here they are relayed to the motor cortex, predominantly area 4, and also the premotor cortex, area 6� The neocerebellum is involved in motor coordination and planning� (This is to be compared with the influence of the basal ganglia on motor activity; see Figure 5�18�)

The outflow fibers of the superior cerebellar peduncles originate mainly from the dentate nucleus, with some from the intermediate nucleus (as shown)� The axons start laterally and converge toward the midline (see Figure 5�3), passing in the roof of the upper half of the 4th ventricle (see Figure  1�9 and Figure  3�3)� The fibers continue to

“ascend” through the upper part of the pons (see Appendix Figure A�5 and Appendix Figure A�6)� In the lower midbrain, there is a complete decussation of the peduncles (see Appendix Figure A�4)�

The cerebral cortex is linked to the neocerebellum by a circuit that forms a loop� Fibers are relayed from the cerebral cortex via the pontine nuclei to the cerebellum� The ponto-cerebellar fibers cross and go to the neocerebellum of the opposite side� After cortical processing in the cerebellar cortex, the fibers project to the dentate nucleus� These efferents cross (decussate) in the lower midbrain and project to the thalamus� From the thalamus, fibers are relayed mainly to the motor areas of the cerebral cortex� Because of the two (double) crossings, the messages are returned to the same side of the cerebral cortex from which the circuit began�

Note to the Learner: This is also described and illustrated in The Integrated Nervous System�

Lesions of the neocerebellum (of one side) cause motor deficits to occur on the same side of the body, that is, ipsilaterally for the cerebellum� The explanation for this lies in the fact that the cortico-spinal tract is also a crossed pathway (see Figure 5�9)� For example, the errant messages from the left cerebellum which are delivered to the right cerebral cortex cause the symptoms to appear on the left sidecontralaterally for the cerebral cortex but ipsilaterally from the point of view of the cerebellum�

The cerebellar symptoms associated with lesions of the neocerebellum are collectively called dyssynergia, in which the range, direction, and amplitude of voluntary muscle activity are disturbed� The specific symptoms include the following:

• Distances are improperly gauged when pointing, called dysmetria, and this includes past-pointing�

• Rapid alternating movements are poorly performed, called dysdiadochokinesis�

• Complex movements are performed as a series of successive movements, which is called a decomposition of movement�

• There is a tremor seen during voluntary movement, an intention tremor (this is in contrast to the Parkinsonian tremor, which is present at rest)�

• Disturbances also occur in the normally smooth production of words, resulting in slurred and explosive speech�

In addition, cerebellar lesions in humans are often associated with hypotonia and sluggish deep tendon reflexes�

FIGURE 5.18-MOTOR MODULATORY SYSTEMS (III) THALAMUS: MOTOR CIRCUITS The specific relay nuclei of the thalamus that are linked with the motor systems, the basal ganglia, and the cerebellum are the ventral lateral (VL) and the ventral anterior (VA) nuclei (see Figure  4�3)� These nuclei project to the different cortical areas involved in motor control, the motor strip, the premotor area, and the supplementary motor area (as shown in the upper insets-see Figure 5�8)�

These thalamic nuclei also receive input from these cortical areas, in line with the reciprocal connections of the thalamus and cortex (discussed with Figure 6�13)� One of the intralaminar nuclei, the centromedian nucleus, is also linked with the circuitry of the basal ganglia (discussed with Figure 5�14)�

The neostriatum receives input from wide areas of the cerebral cortex, as well as from the dopaminergic neurons of the substantia nigra� The putamen, which is mainly involved with motor regulation, then connects with the globus pallidus� The major outflow from the basal ganglia, from the globus pallidus, follows two slightly different pathways to the thalamus, as pallidothalamic fibers� One group of fibers passes around and the other passes through the fibers of the internal capsule which is (represented on the diagram by large stippled arrows)� These merge and end in the VA and VL nuclei of the thalamus� (The VA nucleus is not seen on this section through the thalamus�) The other outflow from the basal ganglia via the pars reticulata of the substantia nigra generally follows the same projection to these thalami nuclei (not shown)�

The pathway from thalamus to cortex is excitatory� The basal ganglia influence is to modulate the level of excitation of the thalamic nuclei� Too much inhibition leads to a situation in which the motor cortex has insufficient activation, and the prototypical syndrome for this is Parkinson’s disease (discussed with Figure  2�5A and Figure  5�14)� Too little inhibition leads to a situation in which the motor cortex receives too much stimulation and the prototypical syndrome for this is Huntington’s chorea (discussed with Figure 2�5A)� The analogy that has been used to understand these diseases is to a motor vehicle, in which a balance is needed between the brake and the gas pedal for controlled forward motion in traffic�

The motor areas of the cerebral cortex that receive input from these two subsystems of the motor system are shown in the small insets at the top, both on the dorsolateral surface and on the medial surface of the hemispheres (see Figure 1�3, Figure 1�7, and Figure 4�5)� The cortical

projection from these thalamic nuclei is to the premotor and supplementary motor areas, as shown, cortical areas concerned with motor regulation and planning (see Figure 5�8 and Figure 6�13)�

The other part of the motor regulatory systems, the cerebellum, also projects (via the superior cerebellar peduncles) to the thalamus� The major projection is to the VL nucleus, but to a different portion of it than the part that receives the input from the basal ganglia� From here, the fibers project to the motor areas of the cerebral cortex, predominantly the precentral gyrus and the premotor area, areas 4 and 6, respectively (see Figure 4�3, Figure 5�8, and Figure 6�13)�

Note to the Learner: These connections are animated on the atlas Web site (www�atlasbrain�com)�

Many years ago, it was commonplace to refer to the basal ganglia as part of the extrapyramidal motor system (in contrast to the pyramidal motor system [discussed with Figure  5�9, the cortico-spinal tract])� This could lead to the  idea that there is a descending projection from the basal ganglia, analogous to the “pyramidal” (cortico-spinal) pathway� It is now known that the basal ganglia exert their influence via the appropriate parts of the thalamus to the cerebral cortex (see Figure 6�13), which then acts either directly (i�e�, using the cortico-spinal [pyramidal] tract) or indirectly via certain brainstem nuclei (cortico-bulbar pathways) to alter motor activity� The term extrapyramidal should probably be abandoned, but it is still frequently encountered in a clinical setting�

Tourette’s syndrome is a motor disorder manifested by tics, involuntary sudden movements; occasionally, these individuals have bursts of involuntary language that rarely contains vulgar expletives� This disorder starts in childhood and usually has other associated behavioral problems, including difficulty with attention� There is growing evidence that this disorder is centered in the basal ganglia� The condition may persist into adulthood�

Note to the Learner: A hypothetical clinical case with this syndrome is discussed in the Integrated Nervous System text�

The motor abnormality associated with an isolated lesion of the subthalamic nucleus is called hemiballismus� The person is seen to have sudden flinging movements of a limb, on the side of the body opposite to the lesion� The usual cause of hemiballismus is a vascular lesion�