CHAPTER 2. TOPOGRAPHY OF THE CENTRAL NERVOUS SYSTEM

A. SPINAL CORD LEVELS

OBJECTIVE:
    BE ABLE TO DISTINGUISH SACRAL, LUMBAR, THORACIC, AND CERVICAL SECTIONS OF THE SPINAL CORD.

The spinal cord has essentially the same arrangement throughout its extent.  On its surface are several longitudinal grooves.  The most prominent of these is the anterior median fissure, occupied by the anterior spinal artery (Plate 19).  On the opposite side is a far less conspicuous groove, the posterior median sulcus (Plate 18).   The ventral and dorsal rootlets of the spinal nerves attach somewhat lateral to these median grooves, at the anterolateral and posterolateral sulci respectively.

The external and internal features of the spinal cord can be observed in transverse sections (slides 1, 2, 3, 4, 5, 6, 7).

These sections, as well as the sections of the brain, have been treated with the Weigert stain in which the myelin is colored black or dark blue.  Therefore, the white matter, made up primarily of myelinated fibers, is stained black or dark blue, and the gray matter, mostly devoid of myelin, is unstained and appears yellow or tan.

Although individual tracts and nuclei will be studied with the functional systems, some structures in slides 1, 2, 3, 4, 5, 6, 7 should be identified now so that the various levels of the spinal cord can be distinguished from one another.

  1. Identify the posterior, lateral, and anterior funiculi and account for their increase in size from caudal to rostral.
  2. Identify the posterior horn.  Compare its size and shape at lumbar and sacral levels with cervical and thoracic levels and account for the differences in size.
  3. Identify the anterior horn and compare its size and shape at the various levels.  Note that it is smaller in the thoracic level and larger in the cervical and lumbosacral enlargements.

B. THE BRAINSTEM AND ITS LEVELS

OBJECTIVES:
  1. BE ABLE TO IDENTIFY EACH PART OF THE BRAINSTEM AND ITS MAIN SURFACE FEATURES,
  2. BE ABLE TO IDENTIFY IN THE BRAINSTEM SLIDES THE LEVELS DESCRIBED AND THE MAIN FEATURES OF EACH.

In this topography chapter, the structures described in both gross specimens and slides are for the purpose of identifying: 1) the various subdivisions of the brain, and 2) the precise levels of the brainstem slides which will be used throughout the guide to localize the functional paths.  It is not necessary to learn the individual structures at this time because the responsibility of knowing a structure does not occur until its function and clinical significance as a part of a functional path has been covered.

Gross Anatomy of the Brainstem

The brainstem is the unpaired central part of the brain and is comprised of the medulla oblongata, pons, and midbrain. The "brainstem" specimens used in the neuroanatomy laboratory also include the diencephalon and basal forebrain structures on the ventral surface of the specimen.

Take extreme care in handling the gross specimens of the brain because they scar easily. For this reason, instruments should not be placed against the surface nor should the tissue be stabbed with probes or pointers. In identifying structures point at them, not into them.

Likewise utmost care must be taken to prevent the tissue from drying. When gross specimens are going to be studied, run tap water over them to remove excess formalin. While studying the tissues moisten them repeatedly with a wet paper towel to prevent drying which will cause discoloration and hardening that will make subsequent studies and dissection difficult.

Ventral Surface (Pls. 8, 10)

On the ventral surface of the medulla are the pyramids, which are located on either side of the anterior median fissure. Lateral to each pyramid is a prominent elevation, the olive. The sulcus between the olive and pyramid is the preolivary sulcus and is where the hypoglossal (XII) nerve rootlets emerge. The sulcus behind the olive is the postolivary sulcus and is the area of attachment of the glossopharyngeal (IX), vagus (X), and cranial accessory (XI) nerves, from superior to inferior.

The ventral part of the pons is called the basilar part. Its surface consists of transverse bands formed by bundles of fibers. The shallow basilar sulcus near the midline is normally occupied by the basilar artery.

The abducens (VI) nerve emerges at the pontomedullary junction near the lateral border of the pyramid. Attaching more laterally at the pontomedullary junction are the facial (VII) and vestibulocochlear (VIII) nerves with the small intermediate part of VII between them. The angle formed at the junction of the pons, medulla, and cerebellum is called the cerebellopontine or cerebellar angle.

Slightly above midway between the medulla and midbrain the trigeminal (V) nerve attaches to the ventrolateral surface of the pons. It consists of a larger inferolateral sensory root (portio major) and a smaller superomedial motor root (portio minor).

The ventral surface of the midbrain is formed by the cerebral peduncles and is much shorter than the dorsal surface. It consists of the converging cerebral crura, the most ventral parts of the cerebral peduncles, which are separated from each other by the interpeduncular fossa. The oculomotor (III) nerves emerge from the walls of the interpeduncular (Pl. 13).

Dorsal Surface (Pl. 9)

The dorsal surface of the caudal or inferior half of the medulla, presents the gracile tubercles on either side of the posterior median sulcus. Lateral to the gracile tubercle and extending slightly more rostrally is the cuneate tubercle. The dorsal surface of the rostral or superior half of the medulla and the dorsal surface of the pons form the floor of the fourth ventricle.

The floor of the fourth ventricle can be divided into medullary and pontine parts by an imaginary line passing horizontally through its widest part. In most brains the superior part of the medullary floor contains a variable number of white strands called the striae medullares which extend laterally from the median sulcus toward the lateral recess. The caudal tip of the fourth ventricle lies between the gracile tubercles and is called the obex.

The median sulcus divides the floor of the fourth ventricle into symmetrical halves. Each half is further subdivided into medial and lateral parts by the superior and inferior foveae, small depressions at pontine and medullary levels respectively. Lateral to the two foveae is the vestibular area. Also, in the medullary floor at the lateral recess is a small eminence, the acoustic tubercle. Between the inferior fovea and the median sulcus are two poorly distinguishable triangular areas, the hypoglossal trigone medially, and the vagal trigone laterally. Between the superior fovea and the median sulcus is the medial eminence. Its caudal part forms the facial colliculus.

The dorsal surface of the midbrain is the tectum which consists of two pairs of mounds, the inferior colliculi and the superior colliculi. The trochlear nerves emerge just caudal to the inferior colliculi. The midbrain area just rostral to the superior colliculi is the pretectum.

Sections of the Brainstem

After the surface features of the brainstem are familiar, these same structures should be identified in transverse sections taken from functionally important levels in these parts of the brainstem. By locating on the brainstem specimen the landmarks in a section, one is able to determine precisely from where the section was taken. Since the brainstem sections will be used repeatedly as the functional systems are studied, knowing precisely where they are located in the brain will enhance the development of a 3-dimensional image of the functional paths. This is important because the clinician must project knowledge of the nervous system, no matter what the source, onto the gross brain and ultimately to the living brain in situ.

X-section at Dorsal Column Nuclei (Sl. 10)

Ventrally are the pyramidal tracts, which form the pyramids, and dorsally are the gracile and cuneate nuclei, which form the gracile and cuneate tubercles respectively. This is the more rostral part of the closed medulla.

X-section at X and XII Nuclei (Sl. 14)

Ventrally is the pyramidal tract forming the pyramid, the inferior olivary nucleus forming the olive, and rootlets of the hypoglossal nerve between them. The floor of the fourth ventricle presents medially, the hypoglossal and dorsal vagal nuclei underlying their respective trigones, and laterally, the vestibular nuclei underlying the vestibular area.

X-section at Lateral Aperture (Sl. 17)

This is the more rostral part of the medulla and the widest part of the fourth ventricle. Ventrally, the surface of the medulla presents the pyramid formed by the pyramidal tract, and the olive formed by the inferior olivary nucleus. Dorsally, the ventricular floor is relatively smooth except in the lateral recess where the dorsal cochlear nucleus forms an eminence, the acoustic tubercle. Lateral to this tubercle is the lateral aperture. Most of the ventricular floor consists of the vestibular area, overlying the vestibular nuclei. The myelinated fibers in the floor form the striae medullares of the fourth ventricle.

X-section at VI and VII Nuclei (Sl. 21)

This is the most caudal part of the pons. The ventral or basilar part of the pons consists of the pontine nuclei, bundles of descending fibers (the corticospinal and corticobulbar tracts) and transverse pontine fibers. The most conspicuous structures in the dorsal or tegmental part of the pons at this level are the abducens and facial intramedullary nerve rootlets

X-section at Motor Trigeminal Nucleus (Sl. 23)

This section is at the midpontine level where the trigeminal nerve attaches. Although its size and shape may vary, the basilar part appears similar at all pontine levels. The most conspicuous structure in the pontine tegmentum at this level is the motor trigeminal nucleus located in its lateral part.

X-section at Isthmus (Sl. 27)

The most rostral part of the pons is referred to as the isthmus rhombencephali. Its basilar part contains larger numbers of fibers than more caudal levels but is still readily recognizable. The cavity of the isthmus is the narrow cerebral aqueduct and dorsal to it is the decussation of the trochlear nerve the only cranial nerve to cross and to emerge from the dorsal surface of the brainstem.

X-section at Inferior Colliculus (Sl. 29)

Due to the overlap by the basilar part of the pons in the plane of this section, the ventral third is through the basilar part of the pons and the dorsal two-thirds is through the midbrain. The tectum in the caudal midbrain consists of conspicuous paired structures, the inferior colliculi. Between the tectum and the basilar part of the pons is the midbrain tegmentum.

X-section at Superior Colliculus (Sl. 31)

This section is not symmetrical because the left side is slightly more rostral than the right and contains several thalamic nuclei, whereas the right side contains only one, the medial geniculate nucleus. Note that in viewing X-sections of the spinal cord and brainstem, the right and left side are reversed.

Dorsally, the tectum of the rostral midbrain is formed by the superior colliculi. The remainder of the midbrain is formed by the cerebral peduncle which consists of three parts: tegmentum, substantia nigra, and cerebral crus from dorsal to ventral. The oculomotor nerve is emerging in the wall of the interpeduncular fossa.

C. THE FOREBRAIN AND ITS LEVELS

OBJECTIVE:
    BE ABLE TO IDENTIFY THE SUBDIVISIONS OF THE DIENCEPHALON AND TELENCEPHALON (CEREBRAL HEMISPHERES) AND THE MAIN ANATOMICAL FEATURES OF EACH.

Gross Anatomy of the Diencephalon

This part of the forebrain includes the thalamus, hypothalamus, subthalamus, and epithalamus.

3rd Ventricle

The cavity of the diencephalon is the 3rd ventricle. Its features can be observed in the bisected brain (Pls. 12, 13). It communicates anteriorly with the lateral ventricles at the interventricular foramina (of Monro) and posteriorly with the cerebral aqueduct. Its lateral wall is formed by the thalamus and hypothalamus, and its floor by the hypothalamus.

In the median plane, the diencephalon is bounded posteriorly by an imaginary line passing from the front of the posterior commissure to the back of the mamillary body, and anteriorly by a line passing along the lamina terminalis and extending from the front of the optic chiasm to the interventricular foramen.

Hypothalamus

The only part of the diencephalon on the ventral surface of the brain is the hypothalamus (Pls. 10, 11). It is located at the median part of the middle cranial fossa above the diaphragma sellae. Most posteriorly it includes the mamillary bodies, paired spherical masses about the size of small peas located in the interpeduncular fossa. Most anteriorly is the optic chiasm. Between the mamillary bodies and the optic chiasm is a smooth area, the tuber cinereum to which the infundibular stem of the hypophysis is attached. The tuberal region surrounding the infundibulum protrudes slightly and forms the median eminence. The tuberal region is the widest part of the hypothalamus.

Epithalamus

The dorsal surface of the posterior part of the diencephalon is formed by the epithalamus (Pls. 9, 12).  This consists chiefly of the pineal gland.

Thalamus

The thalami are two egg-shaped masses in the dorsal part of the diencephalon. The hypothalamic sulcus traverses the lateral wall of the 3rd ventricle from the cerebral aqueduct to the interventricular foramen and demarcates the thalamus, above, from the hypothalamus, below (Pl. 12).  In most brains the right and left thalami are partially fused across the 3rd ventricle by the interthalamic adhesion (massa intermedia). At the interventricular foramen is a swelling, the anterior tubercle and on the dorsomedial surface of the thalamus is a bundle of fibers, the medullary stria. Posteriorly and dorsolaterally, the pulvinar overhangs the midbrain like a pillow (Pl. 9).  On the ventral surface of the pulvinar close to the midbrain is the medial geniculate body. Lateral to this is the lateral geniculate body which receives the optic tract after it arches around the cerebral peduncle (Pl. 10).

Gross Anatomy of the Cerebral Hemispheres

Each cerebral hemisphere has three surfaces: lateral, medial, and inferior. The lateral surface is convex and conforms to the concavity of the cranial vault. The medial surface of the hemisphere is flat and vertical and is related to the falx cerebri which is located in the longitudinal fissure between the two hemispheres. The inferior surface is divided into three parts each of which has a shape conforming to the structure upon which it rests. The anterior or orbital part is slightly concave and rests on the roofs of the orbit and nose. The middle or temporal part is convex and is adapted to the middle cranial fossa. The posterior or tentorial part is slightly concave and rests on the tentorium cerebelli which separates it from the cerebellum.

Lateral Surface (Pls. 2, 4)

The most prominent and uniform cleft on the lateral surface is the lateral sulcus or fissure (of Sylvius) which begins on the base of the brain, extends to the lateral surface of the hemisphere, and proceeds posteriorly and slightly superiorly. It separates the frontal and parietal lobes from the temporal lobe. The next most prominent and uniform cleft is the central sulcus or fissure of Rolando, which is between the frontal and parietal lobes. It is oriented in the dorso-ventral direction and can usually be identified as the sulcus behind the most anterior gyrus which rises uninterruptedly from the lateral sulcus to the superior margin of the hemisphere.

The frontal lobe extends anteriorly from the central sulcus to the anterior tip of the hemisphere which is called the frontal pole. The parietal lobe is superior to the lateral sulcus and behind the central sulcus. The temporal lobe is inferior to the lateral sulcus. Its most anterior part is called the temporal pole. Posteriorly, the parietal and temporal lobes become continuous with the occipital lobe. The occipital lobe is demarcated from the parietal and temporal lobes by an imaginary line between the parieto-occipital sulcus and the preoccipital notch. This anatomical boundary has no functional significance.  

Medial Surface (Pls. 3, 5)

The most prominent structure in the median plane in a bisected brain is the corpus callosum which connects the right and left cerebral hemispheres.

The most conspicuous clefts on the medial surface of the hemisphere are two horizontally-oriented sulci, the callosal sulcus, dorsal to the corpus callosum (Pl. 12), and the cingulate sulcus, dorsal to the cingulate gyrus (Pl. 5), and a vertically-oriented sulcus, the parieto-occipital (a short distance posterior to the corpus callosum). The latter separates the parietal and occipital lobes. The cingulate sulcus separates the frontal and parietal parts of the medial hemisphere from the cingulate gyrus, part of the limbic lobe. A vertical line from where the central sulcus reaches the medial surface down to the cingulate sulcus separates the parietal and frontal lobes.

As will be seen later, the frontal lobe, which is the largest lobe, is concerned primarily with higher mental functions and voluntary movements. The precentral gyrus, immediately anterior to the central sulcus, contains the primary motor area which commands skilled movements. The parietal lobe is concerned with the awareness of one’s body and of the objects surrounding it. The postcentral gyrus, immediately posterior to the central sulcus, is the primary somatosensory area in which sensations such as touch and proprioception are represented. The temporal lobe contains the primary auditory area and areas for various types of memory, while the occipital lobe contains the primary visual area. The limbic lobe is associated with emotions, behavior and some forms of memory.

Sections of the Diencephalon and Adjacent Structures

X-section at Mamillary Body (Sl. 37)

This section includes the diencephalon and the adjacent parts of the cerebral hemisphere. In the midline are, from ventral to dorsal, the mamillary region of the hypothalamus, the 3rd ventricle and the corpus callosum. The walls of the 3rd ventricle are formed by the hypothalamus ventrally and the thalamus dorsally. The thalamus extends laterally to the internal capsule. The area bounded by the hypothalamus medially, the thalamus dorsally, the internal capsule laterally, and the cerebral crus ventrally, is the subthalamus.

The lateral ventricle can be seen extending laterally beneath the corpus callosum. The caudate nucleus is in the lateral wall of the lateral ventricle. More ventrally, lateral to the internal capsule is the lentiform nucleus which is comprised of three segments. The medial and more darkly stained two segments form the globus pallidus. The most lateral segment forms the putamen, only a small part of which can be seen in this section. Its staining is similar to that of the caudate nucleus.

X-sections at Tuber Cinereum (Sl. 39), Infundibulum (Sl. 41), and Optic Chiasm (Sl. 43)

Identify in these sections of the intermediate and anterior parts of the diencephalon the 3rd ventricle and the parts of the diencephalon forming its walls. Also, identify the lateral ventricle, corpus callosum, caudate nucleus, internal capsule, medial and lateral segments of the globus pallidus, and the putamen.