System Components

The spinal cord was introduced in the orientation section of the atlas (Section 1; see Figure 1�10, Figure 1�11, and Figure 3�9)� The organization of the nervous tissue in the spinal cord has the gray matter inside, in a typical “butterfly” or “H”-shaped configuration, with the white matter surrounding it�

The dorsal horn of the gray matter is associated with the incoming sensory information� Its nuclei are described with the sensory systems (see Figure 5�1)�

The ventral horn of the gray matter is associated with the outgoing motor fibers� Its nuclei are described with the motor systems (see Figure 5�7)�

The white matter surrounding the gray matter is divided into three funiculi with the ascending and descending tracts (see also Figure 3�9)�

The two spinal cord levels that are used to delineate the pathways are lumbar (the lower illustration and radiology) and cervical (the upper illustration and radiology)�

This cross-sectional level of the spinal cord is used in the various illustrations of the pathways in Section 2� This cross-section is similar in appearance to the cervical section because both are innervating the limbs (see Figure 3�9)�

There is, however, proportionately less white matter at the lumbar level than at the cervical level� The descending tracts are smaller because many of the fibers have terminated at higher levels� The ascending tracts are smaller because they are conveying information only from the lower regions of the body�

This T2-weighted scan is taken through the lower vertebral thoracic region, which is at the level of spinal lumbar region (see Figure 1�10)�

Note the orientation of the radiograph, with ventral above and dorsal below, which is the opposite of the usual way in which the spinal cord is depicted diagramatically�

The gray/white matter of the cord can be distinguished� The cord is surrounded by fibers descending to exit at a lower level; these fibers are seen as little black “dots” in the CSF surrounding the cord�

This is a cross-section of the spinal cord through the cervical enlargement� This level is used in the illustrations of the various pathways� The incoming dorsal (afferent) root is associated with a group of nuclei in the dorsal horn (see Figure  5�3)� The dorsal horn is large because of the amount of afferents coming from the skin, particularly of the fingers and hand� Given that the cervical enlargement contributes to the formation of the brachial plexus to the upper limb, the gray matter of the ventral horn is also very large because of the number of neurons involved in the innervation of the upper limb, particularly the muscles of the hand�

The white matter is comparatively larger at this level because all the ascending tracts are present and are carrying information from the lower parts of the body as well as the upper limb� In addition, all the descending tracts are fully represented as many of the fibers terminate in the cervical region of the spinal cord� In fact, some of them do not descend to lower levels�

This T2-weighted scan is taken through the vertebral cervical region, which is at the level of spinal cervical region (see Figure 1�10)�

Note the orientation of the radiograph, as with the lumbar region�

The gray and white matter can be distinguished (see also Figure 3�9)� The roots are exiting at about the same level and are seen as black lines, particularly on the right side of the illustration�

The description of the cross-sections (axial) through the brainstem follows a uniform “floor plan” (see Introduction to the Appendix and Appendix Figure A�2)� This part of the brainstem has a distinct appearance because of the presence of two distinct structures: the pyramids and the inferior olivary nucleus (see Figure 1�8 and Figure 3�1)�

The pyramids, located ventrally, are an elevated pair of structures located on either side of the midline� They contain the cortico-spinal fibers that have descended from the motor areas of the cortex (see Figure  5�9 and Figure  5�10)� The olive (inferior olivary nucleus) is a prominent structure that has a distinct scalloped profile when seen in cross-section (see Appendix Figure A�8, Appendix Figure A�9, and Appendix Figure A�10)� It is so large that it forms a prominent bulge on the lateral surface of the medulla� Its fibers relay to the cerebellum (see Figure 5�15)�

The tegmentum is the area of the medulla that contains the cranial nerve nuclei, the nuclei of the reticular formation, the ascending tracts, and the inferior olivary nucleus�

Cranial nerves (CN) IX, X, and XII are attached to the medulla and have their nuclei here, as well as some nuclei of CN VIII (see Figure 3�4 and Figure 3�5)� The reticular formation occupies the central region of the tegmentum�

Included in the tegmentum are the two ascending tracts, the large medial lemniscus and the small anterolateral system, both conveying the sensory modalities from the opposite side of the body� The spinal trigeminal tract and nucleus, conveying the modalities of pain and temperature from the ipsilateral face and oral structures, is also found throughout the medulla� The solitary nucleus and tract, which subserves both taste and visceral afferents, are similarly found in the medulla�

The nuclei gracilis and cuneatus, the relay nuclei for the dorsal column tracts, are found in the lower part of the medulla, on its dorsal aspect (see Figure 3�3 and Appendix Figure A�10)� The 4th ventricle lies behind the tegmentum and separates the medulla from the cerebellum (see Figure 3�2)� The roof of this (lower) part of the ventricle has choroid plexus (see Figure  7�8)� Cerebrospinal fluid (CSF) escapes from the 4th ventricle via the various foramina located here and then flows into the subarachnoid space, the cisterna magna (see Figure 3�2 and Figure 7�8)�

This is a photographic image, enlarged, at the level of the middle of the medulla, with the cerebellum attached (see

Figure 1�7 and Figure 1�8)� The upper left image shows the level of the section through the medulla; the corresponding level is shown on a medial view of the brain� Many of the structures visible on this “gross” specimen are seen in more detail on the histological sections in the Appendix�

This specimen shows the principal identifying features of the medulla as described, the pyramids located ventrally and the more laterally placed inferior olivary nucleus, with its scalloped borders� Between the olivary nuclei, are two dense structures, the medial lemniscus, one on each side of the midline, which is the major sensory (ascending) pathway (see Figure 5�2)� The medial longitudinal fasciculus (MLF) is still a distinct tract in its usual location (see Figure 6�8 and Figure 6�9)� The other dense tract that is recognizable in this specimen is the inferior cerebellar peduncle located at the outer posterior edge of the medulla (see Figure 5�15)�

The space behind is the 4th ventricle, narrowing in its lower portion (see Figure 3�1 and Figure 3�3)� There is no “roof” to the ventricle in this section, and it is most likely that the plane of the section has passed through the median aperture, the foramen of Magendie (see Figure 7�8)�

The cerebellum remains attached to the medulla, with the prominent vermis and the large cerebellar hemispheres� The cerebellar lobe adjacent to the medulla is the tonsil (see Figure  1�9)� The extensive white matter of the cerebellum is seen, as well as the thin outer layer of cerebellar cortex�

The medulla is represented by three sections in the Appendix:

• Upper medulla: The upper level typically includes CN VIII (both parts) and its nuclei (see Appendix Figure A�8)�

• Mid-medulla: This section through the midmedulla includes the nuclei of CN IX, X, and XII (see Appendix Figure A�9)�

• Lower medulla: The lowermost section is at the level of the dorsal column nuclei, the nuclei gracilis, and cuneatus (see Appendix Figure A�10)�

The T2-weighted magnetic resonance image of the medulla shown in the lower part of the illustration is a mirror image of the specimen, so that the ventral (anterior) aspect is at the top and the cerebellum (with its narrow folia) is below; the CSF (white) of the 4th ventricle is in between� The medulla is surrounded by CSF (in the subarachnoid space)�

The small size of the medulla is notable� Other than the size and relationships, the medullary outline is recognizable by the “bump” of the pyramids and by the “bump” of the inferior olivary nucleus immediately behind�

The pons is characterized by its protruding anterior (ventral) portion, the pons proper (see Figure  1�8 and Figure 3�1)� The upper left image shows the level of the section through the mid-to upper pons; the corresponding level is shown on a medial view of the brain� Many of the structures visible on this “gross” specimen are seen in more detail on the histological sections in the Appendix�

The basilar portion of the pons (or pons proper) contains the pontine nuclei, the site of relay of the corticopontine fibers (see Figure  5�10 and Figure  5�15); the ponto-cerebellar fibers then cross and enter the cerebellum via the large middle cerebellar peduncle� Intermingled with the pontine nuclei are the dispersed fibers that belong to the cortico-spinal system (see Figure 5�9)�

Behind the pons proper is the tegmentum, which seems quite compressed� This region of the brainstem contains the cranial nerve nuclei, most of the ascending and descending tracts, and the nuclei of the reticular formation� The cranial nerves attached to the pons include the trigeminal (cranial nerve [CN] V) at the mid-pontine level and the abducens (CN VI), the facial (CN VII), and part of CN VIII (the vestibulocochlear) at the lowermost pons; the fibers of CN VII form an internal loop over the abducens nucleus in the pons (see Figure 6�12 and Appendix Figure A�7)� The fibers of CN VII and CN VIII are located adjacent to each other at the cerebello-pontine angle (see Figure 1�8 and Figure 3�4)�

The ascending tracts present in the tegmentum are those conveying sensory information from the body and face� These include the medial lemniscus and the anterolateral fibers (see Figure 5�2 and Figure 5�3)�

One of the distinctive nuclei of the upper pons is the locus ceruleus, a pigment-containing nucleus (discussed with Figure 3�6B and Appendix Figure A�5)� The nuclei of the reticular formation of the pons have their typical location in the tegmentum (see Figure 3�6A and Figure 3�6B)�

The 4th ventricle in the pontine region begins as a widening of the aqueduct and then continues to enlarge so that it is widest at about the level of the junction between the pons and medulla (see Figure 3�1 and Figure 3�3)� This ventricle separates the pons and medulla anteriorly from the cerebellum posteriorly (see Figure 3�2)�

There is no pontine nucleus dorsal to the 4 ventricle; the cerebellum is located above (posterior to) the roof of the ventricle�

The thin folia of the cerebellum are easily recognized, with an inner strip of white matter bounded on either side by the thin gray matter of the cerebellar cortex�

This is a photographic image, enlarged, of the pontine region, with the cerebellum attached� The section is done at the level of the mid-upper pons, as indicated in the upper images of the ventral view of the brainstem and in the mid-sagittal view�

The unique nucleus present at this level is the locus ceruleus, a small nucleus whose cells have pigment (see Appendix Figure A�5), much like those of the substantia nigra, pars compacta� As with that nucleus, the pigment is lost during histological processing�

The ventral region has the distinctive appearance of the pontine nuclei, with the cortico-spinal and corticopontine fibers dispersed among them� The pontine tegmentum seems quite compressed� The space in the middle of the tissue section is the 4th ventricle, as it begins to widen� Behind the ventricle is a small area of white matter called the superior medullary velum and the superior cerebellar peduncles (see Figure 3�2 and Figure 3�3)� The thin folia of the cerebellum are easily recognized, with an inner strip of white matter bounded on either side by the thin gray matter of the cerebellar cortex�

The pons is represented by three sections in the Appendix:

• Upper pons: The uppermost pons is at the level of the locus ceruleus (see Appendix Figure A�5)�

• Mid-pons: The mid-pons (middle pons) is at the level of the attachment of the trigeminal nerve� It includes the massive middle cerebellar peduncles (see Appendix Figure A�6)�

• Lower pons: The lowermost pons is just above the junction with the medulla (see Appendix Figure A�7)� This lowermost level has the nuclei of CN VI and VII and parts of both divisions of CN VIII�

The magnetic resonance imaging (T2-weighted) scan of the pons shown in the lower part of the figure is a mirror image of the specimen, so that the ventral (anterior) aspect is at the top and the cerebellum (with its narrow folia) below; the cerebrospinal fluid (CSF) (white) of the 4th ventricle is in between� The distinctive feature of the ventral pons, the pons proper, can be recognized in the radiograph� The large trigeminal nerve (CN V) is seen� (The locus ceruleus is not seen�) The pons is surrounded by CSF (in the subarachnoid space)� The black dot in the midline ventrally (at the top, within the CSF) is the basilar artery (see Figure  8�1)� The temporal lobe and internal carotid arteries are also visualized�

The midbrain is the smallest of the three parts of the brainstem� The temporal lobes of the hemispheres tend to obscure its presence on an anterior and inferior view of the whole brain (see Figure 1�1 and Figure 2�8)� The midbrain area is easily recognizable from the anterior view in a dissected specimen of the isolated brainstem (see Figure 1�8 and Figure 3�1)�

The ventral portion of the midbrain contains the massive cerebral peduncles located anteriorly, with a fossa in between� These peduncles contain axons that are a direct continuation of the fiber systems of the internal capsule (see Figure 2�4 and Figure 4�4)� Within them are found the pathways descending from the cerebral cortex to the brainstem (cortico-bulbar), cortico-pontine to the cerebellum via the pons (see Figure 5�10), and to the spinal cord (cortico-spinal; see Figure 5�9)�

The tegmentum contains the nucleus of cranial nerve (CN) III to the eye muscles and the associated EdingerWestphal (EW, parasympathetic) nucleus and the nucleus of CN IV (to one eye muscle), as well as two special nuclei in the midbrain region-the substantia nigra and the red nucleus, both involved in motor control�

The substantia nigra is found throughout the midbrain and is located behind the cerebral peduncles� It derives its name from the dark melanin-like pigment found (not in all species) within its neurons in a freshly dissected specimen, as seen in the present illustration (see also Figure 1�5)� (The nucleus has thus been color-coded in black�) The pigment is not retained when the tissue is processed for sectioning (discussed with Appendix Figure A�3)� The function of the substantia nigra is related to the basal ganglia (see Figure 5�14 and Figure 5�18)�

The red nucleus derives its name from the observation that this nucleus has a reddish color in a freshly dissected specimen, presumably because of its marked vascularity� (This nucleus has therefore been color-coded in red�) The red nucleus is found at the superior collicular level� Its function is discussed with the motor systems (see Figure 5�11)�

The aqueduct of the midbrain helps to identify this crosssection as the midbrain area (see Figure 3�2 and Figure 7�8)� The periaqueductal gray, surrounding the aqueduct, has been included as part of the reticular formation (see Figure 3�6B) and is thought to be important in the maintenance of consciousness as part of the ARAS (discussed with Figure 3�6A); this area participates as part of the descending control system for pain modulation (see Figure 5�6)� The reticular formation is found in the core area of the tegmentum�

Posterior to the aqueduct are the two pair of colliculi, which can also be seen on the dorsal view of the isolated brainstem (see Figure 1�9 and Figure 3�3)� The four nuclei together form the tectal plate, or tectum, also called the quadrigeminal plate�

The pretectal region, located in front of and somewhat above the superior colliculus, is the nuclear area for the pupillary light reflex (see Figure 6�7)�

This is a photographic image, enlarged, of the sectioned midbrain� As shown in the upper left image, the brainstem was sectioned at the level of the cerebral peduncles; the corresponding level is shown on a medial view of the brain, thus indicating that the section is through the superior colliculus� Many of the structures visible on this “gross” specimen are seen in more detail on the histological sections in the Appendix�

The distinctive features identifying this section as midbrain are anteriorly, the outline of the cerebral peduncles with the fossa in between� Immediately behind is a dark band, the substantia nigra, pars compacta, with pigment present in the cell bodies� A faint outline of the red nucleus can be seen in the tegmentum, which identifies this section as the superior collicular level� In the middle toward the back of the specimen is a narrow channel, which is the aqueduct of the midbrain, surrounded by the periaqueductal gray� The gray matter behind the “ventricle” is the superior colliculus at this level�

There are two levels presented for a study of the midbrain in the Appendix:

• Upper midbrain: This includes the CN III nucleus and the superior colliculus (see Appendix Figure A�3)�

• Lower midbrain: This is at the level of the CN IV nucleus and the inferior colliculus and the decussation of the superior cerebellar peduncles (see Appendix Figure A�4)�

The magnetic resonance imaging (T2-weighted) scan of the midbrain shown at the bottom of the figure is a mirror image of the specimen, so that the ventral (anterior) aspect is at the top; the cerebrospinal fluid (CSF) (white) of the cerebral aqueduct is not really visible�

The shape of the midbrain is quite distinct, with the cerebral peduncles prominent� The space between the peduncles, the interpeduncular fossa, is where the oculomotor nerve (CN III) emerges (see Figure 1�8 and Figure 3�5)�

The midbrain is surrounded by CSF (in the subarachnoid space); the area behind it (below in the image) is an enlargement of the CSF space, a cistern, called the quadrigeminal cistern� It is of some importance as a radiological landmark�

The level of the image includes the temporal lobe, with the amygdala and inferior horn of the lateral ventricle� The optic chiasm and mammillary bodies are also seen (see Figure 1�5 and Figure 1�6)� The middle cerebral artery is also indicated (see Section 3)�

To lay the groundwork for understanding the functional organization of the sensory and motor pathways (see Chapter 5), it is necessary to have a familiarity with the nuclei of the thalamus, their organization, and their names�

There are two ways of dividing up the nuclei of the thalamus, namely, topographically and functionally�

The thalamus is subdivided by bands of white matter into a number of component parts� The main white matter band that runs within the thalamus is called the internal medullary lamina, and it is shaped like the letter “Y�” It divides the thalamus into a lateral group with its lower ventral set of nuclei, a medial group, and an anterior group of nuclei�

The thalamus has three different types of nuclei:

• Specific relay nuclei: These nuclei relay sensory and motor information to specific sensory and motor areas of the cerebral cortex� Included with these are the medial and lateral geniculate bodies, relay nuclei for the auditory and visual systems� In addition, motor regulatory information from the basal ganglia and cerebellum is also relayed in the thalamus as part of this set of nuclei� These nuclei are located in the ventral nuclear group�

• Association nuclei: These nuclei are connected to broad areas of the cerebral cortex known as the association areas� One of the most important nuclei of this group is the dorsomedial nucleus, located in the medial group, which projects to the frontal lobe�

• Non-specific nuclei: These scattered nuclei have other or multiple connections including the cerebral cortex, basal ganglia, other thalamic nuclei, the brainstem and the spinal cord� Some of these nuclei are located within the internal medullary lamina and are often referred to as the intralaminar nuclei� Some of these nuclei form part of the ascending reticular activating system, which is involved in the regulation of our state of consciousness and arousal (discussed with Figure  3�6A)� The reticular nucleus, which lies on the outside of the thalamus, is also part of this functional system�

The following detailed classification system is given at this point but will be understood only as the functional systems of the central nervous system are described (see the Note to the Learner at the end of this section)�

Specific Relay Nuclei (and Function)

The cortical connections (reciprocal) of these nuclei are given at this point (to be used with the description of the pathways-see note below)�

VA-ventral anterior (motor) ↔ premotor area and supplementary motor area�

VL-ventral lateral (motor) ↔ precentral gyrus and premotor area�

VPL-ventral posterolateral (somatosensory) ↔ postcentral gyrus�

VPM-ventral posteromedial (trigeminal) ↔ postcentral gyrus�

MGB-medial geniculate (body) nucleus (auditory) ↔ temporal cortex�

LGB-lateral geniculate (body) nucleus (vision) ↔ occipital cortex�

Association Nuclei (and Association Cortex)

These nuclei are reciprocally connected to association areas of the cerebral cortex�

DM-dorsomedial nucleus ↔ prefrontal cortex� AN-anterior nucleus ↔ limbic lobe� P-pulvinar ↔ visual cortex (and other areas)� LP-lateral posterior ↔ parietal lobe� LD-lateral dorsal ↔ parietal lobe�

Non-Specific Nuclei (diffuse projections)

IL-intralaminar� CM-centromedian� Mid-midline� R-reticular�

For schematic purposes, this presentation of the thalamic nuclei, which is similar to that shown in a number of textbooks, is quite usable� Histological sections through the thalamus are challenging and beyond the scope of an Introductory course�

Note to the Learner: The thalamus is introduced at this point because it is involved throughout the study of the brain� The learner should become familiar with the names and understand the general organization of the various nuclei at this point� It is advised to consult this diagram as the cerebral cortex is described in the following illustrations� Each of the specific relay nuclei involved in one of the pathways will be introduced again with the functional systems, and at that point the student should return to this illustration� A summary diagram showing the thalamus and the cortex with the detailed connections is presented in Chapter 6 (see Figure 6�13)� Various nuclei are also involved with the limbic system (see Section 4)�

The white matter bundles that course between parts of the basal ganglia and the thalamus are collectively grouped together and called the internal capsule� These are projection fibers, axons going to and coming from the cerebral cortex (see Figure 2�4)� The internal capsule is defined as a group of fibers located at a specific plane within the cerebral hemispheres in a region that is situated among the head of the caudate, the lentiform nucleus (see Figure 2�10A), and the thalamus (see Figure  2�10A and Figure 2�10B; also the dissection, Figure 9�4)�

The internal capsule has three parts:

• Anterior limb: A group of fibers separates the two parts of the neostriatum from each otherthe head of the caudate from the putamen� This fiber system carries axons that are coming down from the cortex, mostly to the pontine region, which are then relayed to the cerebellum� Other fibers in the anterior limb relay from the thalamus to the cingulate gyrus (see Figure 10�1A) and to the prefrontal cortex (see Figure 10�1B)�

• Posterior limb: The fiber system that runs between the thalamus (medially) and the lentiform nucleus (laterally) is the posterior limb of the internal capsule� The posterior limb carries three extremely important sets of fibers�

• Sensory information from thalamus to cortex, as well as the reciprocal connections from cortex to thalamus�

• Most of the descending fibers to the brainstem (cortico-bulbar) and spinal cord (cortico-spinal)� In addition, there are fibers from other parts of the cortex that are destined for the cerebellum, after synapsing in the pontine nuclei�

• Genu: In a horizontal section, the internal capsule (of each side) is seen to be “V”-shaped (see Figure 2�10A)� Both the anterior limb and the posterior limb have been described-the bend of the “V” is called the genu and it points medially (also seen with neuroradiological imaging, both computed tomography [CT, see Figure 2�1A] and magnetic resonance imaging [MRI, see Figure 2�10B])�

The internal capsule fibers are shown from the medial perspective in a dissection in which the thalamus has been partially removed (see Figure 2�7 and Figure 9�1B)� The fibers of the internal capsule are also seen in a dissection of the brain from the lateral perspective, just medial to the lentiform nucleus (see Figure 9�4)�

Below the level of the internal capsule is the midbrain� The descending fibers of the internal capsule continue into the midbrain (see Figure 5�15 and Figure 9�5A, also seen in coronal MRIs in Figure 2�9B and 9�5B) and are next located in the structure called the cerebral peduncle of the midbrain (see Figure 1�8 and Figure 4�2C; also seen in cross-sections of the brainstem in Appendix Figure A�3)�

In summary, at the level of the internal capsule, there are both the ascending fibers from thalamus to cortex and descending fibers from widespread areas of the cerebral cortex to the thalamus, the brainstem and cerebellum, and the spinal cord� These ascending and descending fibers, called projection fibers, are sometimes likened to a funnel, with the top of the funnel the cerebral cortex and the stem the cerebral peduncle� The wider upper portion of the funnel is also called the corona radiata, as part of the projection fibers (see Figure  2�2B and Figure  2�4)� The base of the funnel, where the funnel narrows, would be the internal capsule, and the bottom stem of the funnel the cerebral peduncle� The main point is that the various fiber systems, both ascending and descending, are condensed together in the region of the internal capsule�

Note to the Learner: Many students have difficulty understanding the concept of the internal capsule and where it is located� One way of thinking about it is to look at the projection fibers as a busy two-lane highway� The internal capsule represents one section of this pathway, analogous to highways where many lanes of traffic are reduced to a narrowed roadway�

The posterior limb of the internal capsule is a region that is apparently particularly vulnerable to small vascular events (discussed with Figure 8�6)� These small occlusions lead to the destruction of the fibers supplied� Because of the high packing density of the axons in this region, a small lesion can cause extensive disruption of descending motor and/or ascending sensory pathways� This is one of the most frequent types of cerebrovascular accidents (often shortened to CVA), commonly called a “lacunar stroke�” (The details of the vascular supply to this region are discussed with Figure 8�6�)

This is a photographic image of the same brain as shown previously (see Figure  1�3)� The central fissure (often called the fissure of Rolando) divides the frontal lobe anteriorly from the parietal lobe posteriorly� The deep lateral fissure is clearly visible (see later)�

Some cortical areas are functionally directly connected with either a sensory or motor system; these are known as the primary areas. The gyrus in front of the central fissure is called the precentral gyrus, also called area 4, and it is the primary motor area, specialized for the control of voluntary movements (see Figure 5�9 and Figure 6�13)� The area in front of this gyrus is called the (lateral) premotor area, also called area 6, which is similarly involved with voluntary motor actions (see Figure  5�8)� An area in the frontal lobe (colored) has a motor function in regard to eye movements; this is called the frontal eye field (area 8; see Figure 6�9)� The gyrus behind the central fissure is the postcentral gyrus, including areas 3, 1, and 2 (in that order; see Figure  5�2 and Figure  6�13), and it has a somatosensory function for information from the skin (and joints)� (Other sensory primary areas will be identified at the appropriate time�)

The representation of the body on the cortical surface is not proportional to the size of the body part but is more representative of the “usage”; for example, the hand has an extensive cortical representation, both sensory and motor, and particularly the thumb� This “figurine” is classically known as a homunculus� (This is shown particularly well in The Integrated Nervous System�)

The remaining cortical areas that are not directly linked to either a sensory or motor function are called association cortex� The most anterior parts of the frontal lobe are the newest in evolution and are known as the prefrontal cortex (in front of the frontal eye fields, as previously mentioned)� This broad cortical area seems to be the chief “executive” part of the brain� The parietal areas are connected to sensory inputs and have a major role in integrating sensory information from the various modalities� In the parietal lobe are two special gyri, the supramarginal and angular gyri; these areas, particularly on the non-dominant side, seem to be involved in visuo-spatial activities�

Some cortical functions are not equally divided between the two hemispheres� One hemisphere is therefore said to be dominant for that function� This is the case for language ability, which, in most people, is located in

the left hemisphere� This photograph of the left hemisphere shows the two language areas: Broca’s area for the motor aspects of speech and Wernicke’s area for the comprehension of written and spoken language (near the auditory area)�

The lateral fissure (also known as the fissure of Sylvius) divides the temporal lobe below from the frontal and parietal lobes above� Extending the line of the lateral fissure posteriorly continues the demarcation between the temporal and parietal lobes� The temporal lobe seen on this view is a large area of association cortex whose function is still being defined, other than the portions involved with the auditory system (see Figure 6�2 and Figure 6�3) and language (on the dominant side)� Other portions of the temporal lobe include the inferior parts (see Figure 1�5) and the medial portion, which is part of the limbic system (see Section 4)�

The location of the parieto-occipital fissure is indicated on this photograph� This fissure, which separates the parietal lobe from the occipital lobe, is best seen when the medial aspect of the brain is visualized after dividing the hemispheres (see Figure 1�7 and the next paragraph)�

The medial aspect of the hemisphere is also shown (as in Figure 1�7)� Both the primary somatosensory and motor areas of the cortex extend onto this part of the hemisphere, located in the interhemispheric fissure (see Figure 2�2A)— with representation of the lower extremity� In front of the motor area is the supplementary motor area, involved in motor planning (see Figure 5�8)�

The prefrontal cortex extends onto the medial aspect of the brain, as well as onto its inferior (orbital) aspect (see Figure 1�5 and Figure 1�6; also discussed with the limbic system in Section 4)� Likewise, the temporal lobe has an extensive inferior aspect�

The occipital lobe is concerned with the processing of visual information� The primary sensory area for vision is located along the calcarine fissure (see Figure 6�5)�

The cerebellum lies below the occipital lobe, with the large dural sheath, the tentorium cerebelli (not labeled; see Figure 7�4, Figure 7�5, and Figure 7�6) separating these parts of the brain�

It is most important to delineate anatomically the functional areas of the cortex� This forms the basis for understanding the clinical implications of damage (called lesions) to the various parts of the brain, including loss of the blood supply (clinically, an ischemic stroke)� Clinicians are now being assisted in their tasks by modern imaging techniques, including CT and MRI�

Ar m

Fo ot

The illustrations of the sensory and motor pathways in this section of the atlas are all done in a standard manner�

On the left side: The central nervous system (CNS) is depicted, including spinal cord, brainstem, thalamus, and a coronal section through the hemispheres, with small diagrams of the hemisphere at the top showing the area of the cerebral cortex involved�

The diagram of the hemispheres is a coronal section, similar to the one already described in Chapter 3, at the plane of the lentiform nucleus (see Figure  2�8 and Figure  2�9A)� Note the basal ganglia, the thalamus, the internal capsule, and the ventricles; these labels are not repeated in the following diagrams� This diagram is used to convey the overall course of the tract, and particularly at what level the fibers cross (i�e�, decussate)�

Note to the Learner: The symbol indicates a synaptic relay in the pathway� The symbol indicates a decussation (crossing) of the pathway (from one side to the other)�

On the right side: Cross-sections of the brainstem and spinal cord are shown, at standardized levels; the exact levels are indicated on the left� In all, there are five cross-sections-three through the brainstem and two through the spinal cord�

The cross-sections of the brainstem and the spinal cord include:

• The midbrain-upper�

• The pons-mid-upper�

• The medulla-mid�

• Cervical spinal cord�

• Lumbar spinal cord�

The exact position of the tract under consideration is indicated in these cross-sections�

These brainstem and spinal cord cross-sections are the same as those shown in the Appendix� In the Appendix, details of the histological anatomy of the brainstem and spinal cord are given� We have titled the Appendix “Neurological Neuroanatomy” because it allows the precise location of the tracts that is necessary for the localization of an injury or disease� The learner may wish to consult these detailed diagrams at this stage�

This section is a foundation for the student in correlating the anatomy of the pathways with clinical symptoms following lesions caused by disease, injury or vascular problems�

Studying pathways in the CNS necessitates visualizing the pathways, a challenging task for many students� The pathways that are under study extend longitudinally through the CNS, going from spinal cord and brainstem to thalamus and cortex for sensory (ascending) pathways, and from cortex to brainstem and spinal cord for motor (descending) pathways� As is done in other texts and atlases, diagrams are used to facilitate this visualization exercise for the student; color adds to the ability to visualize these pathways, as does the illustration-with animation-on the Web site (www�atlasbrain�com)�

Note to the Learner: In this overview of the pathways, the student is advised to return to the description of the cortex (see Figure  1�3 and Figure  4�5)� This will inform the student which areas of the cerebral cortex are involved in the various sensory modalities and the relevant motor areas� This will assist in integrating the anatomical information presented in the previous section� Similarly, the student is advised to review the thalamus and its connections with each of the pathways (see Figure 4�3 and Figure 6�13)� It is also suggested that the learner refer to Figure 6�10 which presents a summary of all the pathways in the spinal cord�