Chapter 3:

The illustration of the isolated brainstem and diencephalon is based on the dissection shown in Figure 1�8� It is used repeatedly for portraying aspects of the brainstem� At this point, it is presented with the focus now on its internal structures, starting with the ventricular system�

The ventricular system is shown “inside” the brainstem, where it is situated posteriorly�

The ventricular system continues from the diencephalon, where the slit-like 3rd ventricle lies in the midline and separates the thalamus of each hemisphere (see Figure  1�9, Figure  2�8, Figure  2�9A, and Figure  2�10A)� The system now enters into the brainstem� In the midbrain region, the system narrows considerably and is known as the cerebral aqueduct, otherwise called the aqueduct of the midbrain, also known as the aqueduct of Sylvius� (An appreciation of the narrow aqueduct can be obtained in Figure 3�2, a mid-sagittal view of the brainstem; see also Appendix Figure A�3 and Appendix Figure A�4�)

In the area between the pons and the cerebellum, the ventricular space expands considerably into a lozenge-shaped space-the 4th ventricle (see Figure 3�3)� At the bottom of this ventricle, the system narrows again and becomes the central canal of the spinal cord (discussed with Figure 7�8)�

From the 4th ventricle, cerebrospinal fluid (CSF) leaves the interior of the brain and flows into the subarachnoid space-which is outside the brain-via three foramina, as discussed with Figure 3�2 and with Figure 7�8�

Other structures of the brainstem can now be explained (see also Figure 1�8)�

The descending fibers present in the internal capsule (see Figure 4�4) continue in the brainstem (see the radiograph B below and also see Figure 2�9B, Figure 9�5A, and Figure 9�5B) as the cerebral peduncle (see Figure 4�2C; also Appendix Figure A�3 and Appendix Figure A�4)� This includes the cortico-spinal tract (see Figure 5�9) and the

cortico-pontine and cortico-bulbar fibers (see Figure 5�10 and Figure 5�15)�

The protruding or bulging portion of the pontine region (see Figure 3�2) is created by a massive set of nuclei, the pontine nuclei (see Figure 4�2B and Appendix Figure A�6)� These are relay nuclei in the pathway from the cerebral cortex to the cerebellum (see Figure 5�10 and Figure 5�15)�

The structure labeled the pyramid has this (approximate) shape when seen in an axial section (see Figure 4�2A)� It is noteworthy because the fibers of the cortico-spinal tract in fact create the pyramid (see Appendix Figure A�8, Appendix Figure A�9, and Appendix Figure A�10)� These fibers cross (decussate) at the lower level of the medulla and become the lateral cortico-spinal tract (see Figure 5�9 and Figure 6�12)�

This nucleus of the medulla, better known as the inferior olivary nucleus, is large enough to create this “bump” (see Figure 4�2A, Appendix Figure A�9, and Appendix Figure A�10)� It is one of the nuclei that sends afferents to the cerebellum (see Figure 5�15)�

The four magnetic resonance imaging scans are T1weighted images of the hemispheres and the brainstem in the coronal plane, including the cerebellum� The shape of the brainstem can be seen in the “locator” image (and in Figure 3�2)� The CSF in this instance is dark (as seen in the lateral ventricles of the hemispheres)�

The CSF spaces of the brainstem can be seen in the images C and D, with the enlargement of the 4th ventricle�

Subsequent illustrations detail the various nuclei of the brainstem� In Section 2, all the pathways ascending and descending through the midbrain are described (see Figure 6�11 and Figure 6�12), as well as the cerebellar connections (see Figure 5�15 and Figure 5�17)� Further details of the brainstem are found in the Appendix�

This mid-sagittal view of the brain is a higher magnification of the view presented in Figure 1�7� The section of the brain goes through the interhemispheric fissure, the corpus callosum (see Figure 2�2A) and the midline 3rd ventricle, as well as sectioning the brainstem in the midline�

On this view, the thalamic portion of the diencephalon is separated from the hypothalamic part by a groove, the hypothalamic sulcus� This sulcus starts at the foramen of Monro (the interventricular foramen, discussed with the ventricles; see Figure 2�9B and Figure 7�8) and ends at the aqueduct of the midbrain� The optic chiasm is found at the anterior aspect of the hypothalamus, and behind it is the mammillary body (see Figure  1�5 and Figure 1�6)�

The three parts of the brainstem can be distinguished on this view-the midbrain, the pons with its bulge anteriorly, and the medulla (refer to the ventral view shown in Figure 3�1 and in Figure 1�8)� Through the midbrain is a narrow channel for cerebrospinal fluid (CSF), the aqueduct of the midbrain� The area of the midbrain behind the aqueduct includes the superior and inferior colliculi (see Figure 1�9), referred to as the tectum or tectal plate (see Figure 3�3)�

The aqueduct connects the 3rd ventricle with the 4th ventricle, a space with CSF that separates the pons and medulla from the cerebellum (see later)� CSF escapes from the ventricular system at the bottom of the 4th ventricle through the foramen of Magendie (see Figure 7�8), and the ventricular system continues as the narrow central canal of the spinal cord (see also Figure 7�8)�

The cerebellum lies behind (or above) the 4th ventricle� It has been sectioned through its midline portion, the vermis (see Figure 1�9)� Although it is not necessary to name all its various parts, it is useful to know two of them-the lingula and the nodulus� (The reason for this will become evident when describing the cerebellum; see Figure 5�17�)

The tonsil of the cerebellum can also be seen in this view (see Figure 1�7; also Figure 1�9 and Figure 5�16)�

The cut edge of the tentorium cerebelli, one of the major main folds of dura, is seen separating the cerebellum from the occipital lobe (discussed with the meninges in Section 3; see Figure 7�4, Figure 7�5, and Figure 7�6)� This view clarifies the separation of the supratentorial space, namely, the cerebral hemispheres, from the infratentorial space, the brainstem, and the cerebellum in the posterior cranial fossa�

The lower image is a T1-weighted magnetic resonance imaging scan image of the brainstem in the mid-sagittal plane� This is an extremely important image, one that is most frequently seen in the clinical setting�

The corpus callosum is clearly seen, as well as the fornix, with the septum pellucidum attached between the two, thus separating one lateral ventricle from the other� The diencephalon is situated below the fornix outlining the “space” occupied by the 3rd ventricle� CSF (dark) can be seen in the aqueduct of the midbrain and filling the 4th ventricle�

The space outside the brainstem also contains CSF (explained with Figure 7�8), with enlargements of that space called cisterns� The large cistern below the cerebellum and behind the medulla is the important cistern magna�

Should there be an increase in the mass of tissue occupying the posterior cranial fossa (e�g�, tumor, hemorrhage), the cerebellum would be pushed downward� This would force the cerebellar tonsils into the foramen magnum, thereby compressing the medulla� The compression, if severe, could lead to a compromising of function of the vital centers located in the medulla (discussed with Figure 3�6A)�

The complete syndrome is known as tonsillar herniation, or coning� This is a life-threatening situation that may cause cardiac arrest or respiratory arrest, or both� (This will be further discussed in the context of Increased Intracranial Pressure, ICP, in the introduction to Section 3 and with Figure 7�1�)

This diagram shows the brainstem and 4th ventricle from the dorsal perspective, with the cerebellum removed� A similar view of the brainstem is used for some of the later diagrams (see Figure 6�11 and Figure 6�12)� This dorsal perspective is useful for presenting the combined visualization of many of the cranial nerve nuclei and the various pathways of the brainstem�

The posterior aspect of the midbrain has the superior and inferior colliculi, as previously seen (see Figure  1�9), as well as the emerging fibers of cranial nerve (CN) IV, the  trochlear nerve� The posterior aspect of the cerebral peduncle is also seen�

Now that the cerebellum has been removed, the dorsal aspect of the pons is seen� The space separating the pons from the cerebellum is the 4th ventricle-the ventricle has been “unroofed” by removal of the cerebellum� The upper portion of the 4th ventricle is still covered by a sheet of nervous tissue that bears the name superior medullary velum; more relevant, it contains an important connection of the cerebellum, the superior cerebellar peduncles (discussed with Figure 5�17 and Figure 6�11)� The choroid plexus, which is found in the lower half of the roof of the 4th ventricle (see Figure 7�8), has also been removed�

As seen from this perspective, the 4th ventricle has a “floor�” Noteworthy are two large bumps, one on each side of the midline called the facial colliculi, where facial nerve CN VII makes an internal loop (discussed with Figure 6�12 and also with the pons in Appendix Figure A�7)�

Because the cerebellum has been removed, the cut edges of the middle and inferior cerebellar peduncles are seen� The cerebellar peduncles are the connections between the brainstem and the cerebellum, and there are three pairs of them (see Figure 5�15)� The inferior cerebellar peduncle connects the medulla and the cerebellum, and the prominent middle cerebellar peduncle brings fibers from the pons to the cerebellum� Both can be seen in the ventral view of the brainstem (see Figure  1�8)� Details of the information carried in these pathways are

outlined when the functional aspects of the cerebellum are studied with the motor systems (see Figure  5�15)� The superior cerebellar peduncles convey fibers from the cerebellum to the thalamus that pass through the roof of the 4th ventricle and the midbrain to synapse in the thalamus (see Figure  5�17)� This peduncle can be visualized only from this perspective�

CN V emerges through the middle cerebellar peduncle (see also Figure 1�8 and Figure 3�4)�

The lower part of the 4th ventricle separates the medulla from the cerebellum (see Figure 3�2)� The special structures below the 4th ventricle are two large protuberances on either side of the midline-the gracilis and cuneatus nuclei, relay nuclei that belong to the ascending somatosensory pathway (discussed with Figure 1�9, Figure 5�2, and Figure 6�11; see also Appendix Figure A�10)�

The cranial nerves seen from this view include the entering nerve CN VIII (vestibulocochlear nerve)� More anteriorly, from this oblique view, are the fibers of the glossopharyngeal (CN IX) and vagus (CN X) nerves as these emerge from the lateral aspect of the medulla, behind the inferior olive�

A representative cross-section of the spinal cord is also shown from this dorsal perspective�

There are fibers (not labeled) shown crossing the floor of the 4th ventricle, continuing (on the left side of the illustration) from CN VIII and an expansion (which is one of cochlear nuclei, see Figure 6�1)� The fibers are part of the auditory projection, called the dorsal acoustic stria (described with Figure 6�1)� These fibers of CN VIII, the auditory portion, take an alternative route to relay in the lower pons before they ascend to the inferior colliculi of the midbrain�

Two additional structures are shown in the midbrainthe red nucleus (described with Figure  4�2C and Figure 5�11; see also Appendix Figure A�3) and the brachium of the inferior colliculus, which is a connecting pathway between the inferior colliculus and the medial geniculate body, part of the auditory system (fully described with Figure 6�1 and Figure 6�2)�

The medial and lateral geniculate bodies (nuclei) belong with the thalamus (see Figure  4�3)� The lateral geniculate body (nucleus) is part of the visual system (see Figure 6�4)�

The cranial nerve nuclei with sensory functions are discussed in this diagram (see also Appendix Figure A�1)� The olfactory nerve (cranial nerve [CN] I) and the optic nerve (CN II) are not attached to the brainstem and are not considered at this stage�

Sensory information from the region of the head and neck includes the following:

• Somatic afferents: general sensations, consisting of touch (both discriminative and crude touch), pain, and temperature� These afferents come from the skin of the scalp and face and the mucous membranes of the head region via branches of the trigeminal nerve, CN V�

• Visceral afferents: sensory input from the pharynx and other homeostatic receptors of the neck (e�g�, for blood pressure) and from the organs of the thorax and abdomen� This afferent input is carried mainly by the vagus, CN X, but also by the glossopharyngeal nerve, CN IX�

• Special senses: auditory (hearing) and vestibular (balance) afferents with the vestibulocochlear nerve, CN VIII, as well as the special sense of taste with CN VII and CN IX�

This diagram shows the location of the sensory nuclei of the cranial nerves superimposed on the ventral view of the brainstem (on one side only)� These nuclei are also shown in Figure 6�11, in which the brainstem is presented from a dorsal perspective� The details of the location of the cranial nerve nuclei within the brainstem are described in the Appendix�

The major sensory nerve of the head region is the trigeminal nerve, CN V, through its three divisions peripherally (ophthalmic, maxillary, and mandibular)� The sensory ganglion for this nerve, the trigeminal ganglion, is located inside the skull� The nerve supplies the skin of the scalp and face, the conjunctiva of the eye and the eyeball, the teeth, and the mucous membranes inside the head, including the surface of the tongue (but not taste-see later)�

The sensory components of the trigeminal nerve are found at several levels of the brainstem (see trigeminal pathways in Figure 5�4, Figure 5�5, and Figure 6�11):

• The principal nucleus, which is responsible for the discriminative aspects of touch, is located at the mid-pontine level, adjacent to the motor nucleus of CN V�

• A long column of cells that relays pain and temperature information, known as the spinal nucleus of V or the descending trigeminal nucleus, descends through the medulla and reaches the upper cervical levels of the spinal cord�

• Another group of cells extends into the midbrain region, the mesencephalic nucleus of V� These cells appear to be similar to neurons of the dorsal root ganglia and are thought to be the sensory proprioceptive neurons for the muscles of mastication�

Note to the Learner: The location of the sensory nucleus of the CN V inside the brainstem does not correspond exactly to the level of attachment of the nerve to the brainstem as seen externally�

• Cochlear nuclei: The auditory fibers from the spiral ganglion in the cochlea are carried to the central nervous system in CN VIII, and these fibers form their first synapses in the cochlear nuclei as it enters the brainstem at the uppermost level of the medulla (see Figure 6�1)� The auditory pathway is presented in Section 2 (see also Figure 6�2 and Figure 6�3)�

• Vestibular nuclei: Vestibular afferents enter the central nervous system as part of CN VIII� There are four nuclei-the medial and inferior located in the medulla, the lateral located at the ponto-medullary junction, and the small superior nucleus located in the lower pontine region� The vestibular afferents terminate in these nuclei� The vestibular nuclei are further discussed in Section 2 with the motor systems (see Figure 5�13) and with the special senses (see Figure 6�8)�

The special sense of taste from the surface of the tongue is carried in CN VII and CN IX, and these terminate in the solitary nucleus in the medulla (see Appendix Figure A�8)�

Trigeminal neuralgia is discussed with the trigeminal pathways (see Figure 5�4)�

The visceral afferents with CN IX and CN X from the pharynx, larynx, and internal organs are also received in the solitary nucleus (see Appendix Figure A�8)�

Remembering that each cranial nerve is unique and may have one or more functional components, the motor functions of the cranial nerves are now reviewed�

There are two kinds of motor functions:

1� Most neuroanatomy texts distinguish between the motor supply to the muscles derived from somites (including cranial nerve [CN] III, IV, VI, and XII) and the motor supply to the muscles derived from the branchial arches (called branchiomotor, including CN V, VII, IX, and X)� No distinction is made among these muscle types in this atlas�

2� The parasympathetic supply to smooth muscles and glands of the head is included with CN III, VII, and IX, and the innervation of the viscera in the thorax and abdomen with CN X�

This diagram (see also Appendix Figure A�1) shows the location of the motor nuclei of the cranial nerves, superimposed on the ventral view of the brainstem (again on one side only)� These nuclei are also shown in Figure 6�12, in which the brainstem is presented from a dorsal perspective� The details of the location of the cranial nerve nuclei within the brainstem are described at various levels in the Appendix�

• CN III, the oculomotor nerve, has both motor and autonomic fibers� The motor nucleus that supplies most of the muscles of the eye is found at the upper midbrain level (see Appendix Figure A�3)� The parasympathetic nucleus, known as the Edinger-Westphal (E-W) nucleus, supplies the pupillary constrictor muscle (the reflex response of the pupil to light) and the muscle that controls the curvature of the lens; both are part of the accommodation reflex (discussed with Figure 6�7)�

• CN IV, the trochlear nerve, is a motor nerve to one eye muscle, the superior oblique muscle� The trochlear nucleus is found at the lower midbrain level (see Appendix Figure A�4)�

• CN V, the trigeminal nerve, has a motor component to the muscles of mastication (chewing)� The nucleus is located at the mid-pontine level; the small motor nerve is attached to the

brainstem at this level, along the route of the middle cerebellar peduncle, adjacent to the much larger sensory root (not shown here-see Figure 6�12)�

• CN VI, the abducens nerve, is a motor nerve that supplies one extraocular muscle, the lateral rectus muscle� The nucleus is located in the lower pontine region�

• CN VII, the facial nerve, is a mixed cranial nerve� The motor nucleus that supplies the muscles of facial expression is found at the lower pontine level� The parasympathetic fibers, to salivary and lacrimal glands, are part of CN VII (see the additional details section later)�

• CN IX, the glossopharyngeal nerve, and CN X, the vagus nerve, are also mixed cranial nerves� These supply the muscles of the pharynx (CN IX) and larynx (CN X) and originate from the nucleus ambiguus� In addition, the parasympathetic component of CN X, coming from the dorsal motor nucleus of the vagus, supplies the organs of the thorax and abdomen� Both nuclei are found throughout the middle and lower portions of the medulla (see Appendix Figure A�9 and Appendix Figure A�10)�

• CN XI, the spinal accessory nerve, originates from a cell group in the upper four to five segments of the cervical spinal cord� This nerve supplies the large muscles of the neck (the sternomastoid and trapezius)� As mentioned previously, CN XI enters the skull and exits again, as if it were a true cranial nerve�

• CN XII, the hypoglossal nerve, innervates all the muscles of the tongue� It has an extended nucleus in the medulla situated alongside the midline�

Note to the Learner: In this diagram, it appears that the nucleus ambiguus is the origin for CN XII� This is not the case but is a visualization problem� A clearer view can be found in Figure 6�11 and in the cross-sectional views (see Appendix Figure A�9)�

Two small parasympathetic nuclei are also shown but are rarely identified in brain sections-the superior and inferior salivatory nuclei� The superior nucleus supplies secretomotor fibers for CN VII (to the submandibular and sublingual salivary glands, as well as nasal and lacrimal glands)� The inferior nucleus supplies the same fibers for CN IX (to the parotid salivary gland)�

The reticular formation (RF) is the name of a group of neurons found throughout the brainstem� Using the ventral view of the brainstem, the RF occupies the central portion or core area of the brainstem, the tegmentum (see Appendix Figure A�2), from midbrain to medulla (see also brainstem cross-sections in the Appendix)�

This collection of neurons is a phylogenetically old set of neurons that functions as a network or reticulum, from which it derives its name� The RF receives afferents from most of the sensory systems (see Figure 3�6B), and it projects to virtually all parts of the nervous system�

Functionally, it is possible to localize different subgroups within the RF:

• Cardiac and respiratory “centers”: Subsets of neurons within the medullary RF and also in the pontine region are responsible for the control of the vital functions of heart rate and respiration� The importance of this knowledge was discussed in reference to the clinical emergency, tonsillar herniation (discussed with Figure 3�2 and Figure 7�1; also discussed in the context of increased intracranial pressure (ICP) in the Introduction to Section 3)�

• Motor areas: Both the pontine and medullary nuclei of the reticular formation contribute to motor control via the cortico-reticulo-spinal system (discussed in Motor SystemsIntroduction in Chapter 5 and also with Figure  5�12A and Figure  5�12B)� In addition, these nuclei exert a very significant influence on muscle tone, which is very important clinically (discussed with Figure 5�12B)�

• Ascending projection system: Fibers from the RF ascend to the thalamus and project to various non-specific thalamic nuclei (e�g�, intralaminar; see Figure 4�3)� From these nuclei, there is a diffuse distribution of connections to all parts of the cerebral cortex� This whole system is concerned with consciousness and is known as the ascending reticular activating system (ARAS)�

• Pre-cerebellar nuclei: There are numerous nuclei in the brainstem that are located within the boundaries of the RF that project to the cerebellum� These are not always included in discussions of the RF�

It is also possible to describe the RF topographically� The neurons appear to be arranged in three longitudinal sets; these are shown on the left side of this illustration:

• The lateral group consists of small neurons� These are the neurons that receive the various inputs to the RF, including those from the anterolateral system (pain and temperature; see Figure  5�3), the trigeminal pathway (see Figure 5�4), and auditory and visual input�

• The next group is the medial group� These neurons are larger and project their axons upward and downward� The ascending projection from the midbrain area is particularly involved with the consciousness system, the ARAS� Nuclei within this group, notably the nucleus gigantocellularis of the medulla and the pontine reticular nuclei, caudal (lower) and oral (upper) portions, give origin to the two reticulo-spinal tracts (discussed with Figure  3�6B; see also Figure  5�12A and Figure 5�12B)�

• Another set of neurons occupies the midline region of the brainstem, the raphe nuclei, which use the monoamine serotonin for neurotransmission� The best-known nucleus of this group is the nucleus raphe magnus, which plays an important role in the descending pain modulation system (discussed with Figure 5�6)�

In addition, both the periaqueductal gray located in the midbrain (see Figure 3�6B, Appendix Figure A�3, and Appendix Figure A�4) and the locus ceruleus in the upper pons (see Figure 3�6B and Appendix Figure A�5) are considered part of the RF (discussed with Figure 3�6B)�

In summary, the RF is connected with almost all parts of the central nervous system (CNS)� Although it has a generalized influence within the CNS, it also contains subsystems that are directly involved in specific functions� The most clinically significant aspects are:

• The cardiac and respiratory centers in the medulla�

• The descending systems in the pons and medulla that participate in motor control and influence muscle tone�

• The ascending pathways in the upper pons and midbrain that contribute to the consciousness system�

• The pain modulation pathway�

In this diagram, the reticular formation (RF) is viewed from the dorsal (posterior) perspective (as in Figure 3�3, Figure 6�11, and Figure 6�12)� Various nuclei of the RF that have a significant (known) functional role are depicted, as well as the descending tracts emanating from some of these nuclei�

Functionally, there are afferent and efferent nuclei in the RF and groups of neurons that are distinct because of the catecholamine neurotransmitter used, either serotonin or norepinephrine� The afferent and efferent nuclei of the RF include the following:

• The neurons that receive the various inputs to the RF are found in the lateral group (as discussed with Figure  3�6A)� In this diagram, these neurons are shown receiving collaterals (or terminal branches) from the ascending anterolateral system, carrying pain and temperature (see Figure 5�3 and also Figure 5�4)�

• The neurons of the medial group are larger, and these are the output neurons of the RF, at various levels� These cells project their axons upward and downward� The nucleus gigantocellularis of the medulla and the pontine reticular nuclei, caudal and oral portions give rise to the descending tracts that emanate from these nuclei-the medial and lateral reticulospinal pathways, part of the indirect voluntary and non-voluntary motor system (see Figure 5�12A and Figure 5�12B)�

• The raphe nuclei use the monoamine neurotransmitter serotonin and project to all parts of the central nervous system (CNS)� Studies indicate that serotonin plays a significant role in emotional equilibrium, as well as in the regulation of sleep� One special nucleus of this group, the nucleus raphe magnus located in the upper part of the medulla, plays a special role in the descending pain modulation pathway (described with Figure 5�6)�

Other nuclei in the brainstem appear functionally to belong to the RF yet are not located within the core region� These include the periaqueductal gray and the locus ceruleus�

• The periaqueductal gray of the midbrain (for its location, see Appendix Figure A�3 and

Appendix Figure A�4) includes neurons that are found around the aqueduct of the midbrain� This area also receives input (illustrated but not labeled in this diagram) from the ascending sensory systems conveying pain and temperature, the anterolateral pathway; the same occurs with the trigeminal system� This area is part of a descending pathway to the spinal cord that is concerned with pain modulation (see Figure 5�6)�

• The locus ceruleus is a small nucleus in the upper pontine region (see Appendix Figure A�5)� In some species (including humans), the neurons of this nucleus accumulate a pigment that can be seen when the brain is sectioned (before histological processing; see the photograph of the pons in Figure 4�2B)� Output from this small nucleus is distributed widely throughout the brain to virtually every part of the CNS, including all cortical areas, subcortical structures, the brainstem and cerebellum, and the spinal cord� The neurotransmitter involved is norepinephrine (dopamine and norepinephrine are catecholamines)� Although the functional and electrophysiological role of this nucleus is still not clear, the locus ceruleus has been thought to act like an alarm system in the brain� It has also been implicated in a wide variety of CNS activities, such as mood, reaction to stress, and various autonomic activities�

One additional area-the paramedian pontine RF (PPRF)—comprises a group of neurons involved in controlling horizontal saccadic eye movements; the role of these neurons is discussed with eye movements (see Figure 6�8 and Figure 6�9)�

The cerebral cortex sends fibers to the RF nuclei, including the periaqueductal gray, thus forming part of the cortico-bulbar system of fibers (see Figure 5�10)� The nuclei that receive this input and then give off the pathways to the spinal cord form part of an indirect voluntary motor system-the cortico-reticulo-spinal pathways (discussed in Motor Systems-Introduction in Chapter 5; see Figure 5�12A and Figure 5�12B)� In addition, this system is known to play an extremely important role in the control of muscle tone (discussed with Figure 5�12B)�

Lesions of the cortical input to the RF in particular have a very significant impact on muscle tone� In humans, the end result is a state of increased muscle tone called spasticity, accompanied by hyper-reflexia, which is an increase in the responsiveness of the deep tendon reflexes (discussed with Figure 5�12B)�

The cerebellum has been subdivided anatomically according to some constant features and fissures (see Figure 1�9)� In the midline, the worm-like portion is the vermis; the lateral portions are the cerebellar hemispheres� The horizontal fissure lies approximately at the division between the superior and the inferior surfaces� The deep primary fissure is found on the superior surface, and the area in front of it is the anterior lobe of the cerebellum� The only other parts to be noted are the nodulus and lingula of the vermis, as well as the tonsil�

To understand the functional anatomy of the cerebellum and its contribution to the regulation of motor control, it is necessary to subdivide the cerebellum into operational units� The three functional lobes of the cerebellum are:

• The vestibulocerebellum�

• The spinocerebellum�

• The neocerebellum or cerebrocerebellum�

These lobes of the cerebellum are defined by the areas of the cerebellar cortex involved, the related deep cerebellar nucleus, and the connections (afferents and efferents) with the rest of the brain�

There is a convention of portraying the functional cerebellum as if it is found in a single plane, using the lingula and the nodulus of the vermis as fixed points (see also Figure 1�7)�

Note to the Learner: The best way to visualize this is to use the analogy of a book, with the binding toward yourepresenting the horizontal fissure� Place the fingers of your left hand on the edge of the front cover (the superior surface of the cerebellum) and the fingers of your right hand on the edges of the back cover (the inferior surface of the cerebellum), then (gently) open up the book to expose both the front and the back covers� Both are now laid out in a single plane; now, the lingula is at the “top” of the cerebellum and the nodulus is at the bottom of the diagram� This same “flattening” can be done with an isolated brainstem and attached cerebellum in the laboratory� (The Video on the web site titled Diencephalon, Brainstem and Cerebellum demonstrates how the exercise described is done with an actual specimen�)

Having done this, as is shown in the upper part of this figure, it is now possible to discuss the three functional lobes of the cerebellum�

• The vestibulocerebellum is the functional part of the cerebellum responsible for balance and gait� It is composed of two cortical components, the flocculus and the nodulus; hence it is also called the flocculonodular lobe� The flocculus is a small lobule of the cerebellum located on its inferior surface and oriented in a transverse direction below the middle cerebellar peduncle (see Figure  1�8 and Figure  3�1); the nodulus is part of the vermis�

The vestibulocerebellum sends its fibers to the fastigial nucleus, one of the deep cerebellar nuclei (discussed with Figure 3�8 and Figure 5�17)�

• The spinocerebellum is concerned with coordinating the activities of the limb musculature� It receives information from muscles of the body via spino-cerebellar pathways (see Figure 5�15 and Figure  6�10)� It is made up of three areas:

• The anterior lobe of the cerebellum, the cerebellar area found on the superior surface, in front of the primary fissure (see Figure 1�9)�

• Most of the vermis (other than the parts mentioned earlier, see Figure 1�9)�

• A strip of tissue on either side of the vermis called the paravermal or intermediate zone-there is no anatomical fissure demarcating this functional area�

The output deep cerebellar nuclei for this functional part of the cerebellum are mostly the intermediate nuclei, the globose and emboliform nuclei (see Figure 3�8)� Part of their role is to act as a comparator between the intended and the actual movements�

• The neocerebellum includes the remainder of the cerebellum, the areas behind the primary fissure and the inferior surface of the cerebellum (see Figure 1�9), with the exception of the vermis itself and the adjacent strip, the paravermal zone� This is the largest part of the cerebellum and the newest from an evolutionary point of view� It is also known as the cerebrocerebellum because most of its connections are with the cerebral cortex (see also Figure 5�15)�

The output nucleus of this part of the cerebellum is the dentate nucleus (see Figure 3�8 and Figure 5�17)� The neocerebellum is involved in the overall coordination of voluntary motor activities and also in motor planning�

The brainstem is presented from the anterior perspective, with the cerebellum attached (as in Figure 1�8, Figure 3�1, Figure  3�4, and Figure  3�5)� This diagram shows the intracerebellar nuclei-also called the deep cerebellar nuclei-within the cerebellum�

There are four pair of deep cerebellar nuclei-the fastigial nucleus, the intermediate group (named the globose and emboliform), and the lateral or dentate nucleus� Each belongs to a different functional part of the cerebellum� These nuclei are the output nuclei of the cerebellum to other parts of the central nervous system (discussed with Figure 5�17)�

• The fastigial (medial) nucleus is located next to the midline�

• The globose and emboliform nuclei are slightly more lateral; these are often grouped together and called the intermediate (or interposed) nucleus�

• The dentate nucleus, with its irregular margin, is the most lateral� This nucleus is sometimes called the lateral nucleus and is by far the largest (see also Figure 4�2A)�

The nuclei are located within the cerebellum at the level of the junction of the medulla and the pons� Therefore, the cross-sections shown at this level (see Appendix Figure A�8) may include these deep cerebellar nuclei� Usually, only the dentate nucleus can be identified in sections of the gross brainstem and cerebellum done at this level�

Two of the afferent fiber systems are shown-representing cortico-ponto-cerebellar fibers and spino-cerebellar fibers (further described with Figure 5�15)� It is important to

note that the cortico-pontine/ponto-cerebellar afferents cross� All afferent fibers send collaterals to the deep cerebellar nuclei en route to the cerebellar cortex, and these are excitatory� Therefore, these neurons are maintained in a chronic state of activity�

Note to the Learner: The lateral vestibular nucleus functions as an additional deep cerebellar nucleus because its main input is from the vestibulocerebellum (shown in Figure 5�16 and Figure 5�17); its output is to the spinal cord non-voluntary motor system (see Figure 5�13)�

The accompanying magnetic resonance imaging scan (T2-weighted) in the axial plane shows the thin, leaf-like folia of the cerebellum hemispheres and the middle cerebellar peduncles, with the 4th ventricle (cerebrospinal fluid is white) separating the cerebellum from the pons anteriorly� The outline of the dentate nucleus can be discerned�

This “cut” shows the incoming vestibulocochlear nerve (both divisions are seen on the left side of the radiograph), as well as the horizontal semi-circular (fluidfilled) canal�

Although it is rather difficult to explain in words what the cerebellum does in motor control, damage to the cerebellum leads to quite dramatic alterations in ordinary movements (discussed with Figure 5�17)� Lesions of the cerebellum result in the decomposition of the activity, or fractionation of movement, so that the action is no longer smooth and coordinated� Certain cerebellar lesions also produce a tremor that is seen when performing voluntary acts, better known as an intention tremor�

A human spinal cord was sectioned at four levels-cervical, thoracic, lumbar, and sacral-from the top of the page to the bottom, as shown on the left side� The gray matter of the spinal cord, which consists of the nuclei, is found on the inner aspect, surrounded by the white matter, which consists of tracts, with the pathways coursing up and down the spinal cord connecting the spinal cord with the higher centers of the brain, the brainstem and cerebral cortex�

The gray matter is said to be arranged in the shape of a butterfly or somewhat like the letter “H�” The gray matter of the spinal cord contains a variety of cell groups (i�e�, nuclei) that serve different functions (see Appendix Figure A�11)� The division of the gray matter is shown in the upper drawing on the right side� Although rather difficult to visualize, these groups are continuous longitudinally throughout the length of the spinal cord�

The dorsal region of the gray matter, called the dorsal or posterior horn, is associated with the incoming (afferent) dorsal root and is thus related to sensory functions� The cell body of these sensory fibers is located in the dorsal root ganglion (see Figure 1�12, Figure  4�1, and Figure  5�1)� The dorsal horn is quite prominent in the region of the cervical and lumbar plexuses because of the very large sensory input to these segments of the cord from the limbs�

The ventral gray matter, called the ventral or anterior horn, is the motor portion of the gray matter� The ventral horn has the large motor neurons, the anterior horn cells, that are efferent to the muscles (see Figure 1�12 and Figure  5�7) via the ventral nerve roots� These neurons, because of their location in the spinal cord, which is “below” the brain, are also known as lower motor neurons� (The neurons in the cerebral cortex, at the “higher” level, are called upper motor neurons-discussed with Figure 5�8, Figure 5�9, and Figure 5�10�) The ventral horn is again prominent at the level of the limb plexuses because of the large number of motor neurons supplying the limb musculature�

The area in between is usually called the intermediate gray and has a variety of cell groups with some association-type functions�

The autonomic innervation to the organs of the chest, abdomen, and pelvis is controlled by neurons located in the spinal cord�

• The preganglionic sympathetic neurons form a distinctive protrusion of the gray matter called the lateral horn, which extends throughout the thoracic region, from spinal cord level T1 to L2� The post-ganglionic nerves supply the organs of the thorax, abdomen, and pelvis�

• The parasympathetic preganglionic neurons are located in the sacral area and do not form a separate horn� This region of the spinal cord in the area of the conus medullaris (the lowest illustration) controls bowel and bladder function, subject to commands from higher centers, including the cerebral cortex�

The parasympathetic control of the organs of the thorax and abdomen comes from the vagus nerve, which is cranial nerve X (see Figure 1�8 and Figure 3�5)�

The central canal of the spinal cord (not well visualized in these specimens) is located in the center of the commissural gray matter� This represents the remnant of the neural tube and is filled with cerebrospinal fluid (see Figure  7�8)� In the adult, the central canal of the spinal cord is probably not patent throughout the whole spinal cord�

A histological view of these levels of the spinal cord (except sacral) is shown in the Appendix (see Appendix Figure A�11)� The blood supply of the spinal cord is discussed in Section 3 (with Figure 8�7 and Figure 8�8)�

On the right side of the illustration are drawings of the cross-sections of the spinal cord, based on the actual specimens, that highlight certain features (e�g�, the lateral horn in the thoracic region)�

The white matter, which contains the ascending sensory and descending motor pathways, surrounds the gray matter and is usually divided into regions called funiculi (singular: funiculus) which are indicated in the drawing of the lumbar region: these have some functional significance� The posterior (dorsal) funiculus is located between the dorsal horns, the lateral funiculus between the dorsal and ventral horns, and the anterior funiculus between the anterior horn and the anterior median fissure� The distinct pathways will be described with the functional systems in Section 2; a summary diagram with all the tracts is shown after all the spinal cord pathways have been discussed in Section 2 (see Figure 6�10)�

Note to the Learner: The cervical and lumbar levels illustrated on the left side are used, without the attached nerve roots, in the illustrations of all the pathways in Section 2�

This section explains how the nervous system is organized to assess sensory input and execute motor actions� Detailed knowledge of the various central nervous system (CNS) systems allows a clinician to deduce whether there is a problem involving the CNS� Usually (often), this type of problem is referred to a neurologist�

The functioning nervous system has a hierarchical organization to carry out its activities� Incoming sensory fibers, called afferents, have their input into the spinal cord (via the peripheral nerves) as well as the brainstem (via the cranial nerves), except for the special senses (which are discussed separately)� This sensory input is processed by relay nuclei, including the thalamus, before the information is analyzed by the cortex� In the cortex, there are primary areas that receive the information, and other cortical association areas that elaborate the sensory information, and still other areas that integrate the various sensory inputs�

On the motor side, the outgoing motor fibers, called efferents, originate from motor neurons in the brainstem and the spinal cord� These motor nuclei are under the control of motor centers in the brainstem and cerebral cortex� In turn, these motor areas are influenced by other cortical areas and by the basal ganglia, as well as by the cerebellum�

Simpler motor patterns and reflexes are built into the spinal cord� Most notable is the reaction of muscles in response to stretch-called the stretch reflex, also known as the myotatic reflex (discussed with Figure 5�7); this reflex has only one synapse (monosynaptic)� This is of very key significance, both physiologically and clinically, because it is this reflex that is tested clinically, called the deep tendon reflex� Beyond that, simpler motor patterns, which are often reflexive, such as the withdrawal of a limb from a painful stimulus or stimulation of the sole of the foot (plantar reflex), involve processing in the CNS, requiring interneurons in the spinal cord, brainstem, thalamus, or cortex�

The processing of both sensory and motor activities, beyond simple reflexes, involves a series of neuronal connections, creating functional systems� These include nuclei of the CNS at the level of the spinal cord, brainstem, and thalamus� In almost all functional systems in humans, the cerebral cortex is also involved� The axonal connections

among the nuclei in a functional system usually run together to form a distinct bundle of fibers, called a tract or pathway� These tracts are named according to the direction of the pathway (e�g�, spino-thalamic means that the pathway is going from the spinal cord to the thalamus; cortico-spinal means that the pathway is going from the cortex to the spinal cord)� Along their way, these axons may distribute information to several other parts of the CNS by means of axon collaterals�

ORGANIZATION OF THIS SECTION Chapter 4 uses the information developed in Section 1 of the atlas (Chapters 1 to 3) to assemble the parts of the nervous system called system components, which are used to illustrate the pathways (tracts)� These include the spinal cord, brainstem, thalamus, internal capsule and cerebral cortex� A standardized diagram is used to show the pathways (see Figure 4�6)�

In Chapter 5 we are concerned with the sensory tracts (also called pathways) and their connections in the CNS, and the motor pathways and brain regions concerned with movements, including the reticular formation and vestibular system� Included in this section is the issue of motor modulation by the basal ganglia and cerebellum�

Chapter 6 includes the presentation of the special senses (audition, vision, and vestibular), as well as a summary of all the pathways and their cortical connections�

Note on the Web site (www.atlasbrain.com): All the pathways in Chapter 5 are presented on the Web site supplemented by the use of animation demonstrating activation of the pathway� After studying the details of a pathway with the text and illustration, the learner should then view the same figure on the Web site for a better understanding of the course of the tract, the synaptic relays, and the decussation of the fibers� The decussation is shown on the left diagram, and the location of the pathway in the spinal cord and brainstem is shown in the axial cuts on the right side�

Destruction of the nuclei and pathways of the CNS as a result of disease or injury leads to a neurological loss of function� How does the physician (neurologist) diagnose what is wrong?