EXTERNAL FEATURES

Structures to indentify: intact brain

MAJOR SUBDIVISIONS

  • cerebral hemispheres
  • diencephalon
  • cerebellum
  • brainstem
    • midbrain
    • pons
    • medulla


CEREBRAL HEMISPHERES AND LOBES OF THE BRAIN

  • cortex
  • gyrus
  • sulcus (fissure)
  • longitudinal fissure (interhemispheric fissure)
  • corpus callosum
  • frontal lobe
  • parietal lobe
  • temporal lobe
  • occipital lobe
  • central sulcus (Rolandic fissure)
  • lateral sulcus (Sylvian fissure)
  • insular lobe
  • parietooccipital sulcus (midsagittal brain)
  • preoccipital notch

FRONTAL LOBE

  • cingulate sulcus (midsagittal brain)
  • orbital portion of frontal lobe
  • precentral sulcus
  • precentral gyrus
  • primary motor area
  • superior and inferior frontal sulci
  • superior frontal gyri
  • middle frontal gyri
  • inferior frontal gyri
    • pars orbitalis
    • pars triangularis
    • pars opercularis
  • Broca's area
  • olfactory bulb
  • olfactory tract
  • olfactory trigone
  • lateral olfactory stria (tract)
  • media olfactory stria (tract)
  • olfactory sulcus
  • gyrus rectus
  • orbital gyri
  • optic tract
  • anterior perforated substance
PARIETAL LOBE
  • postcentral gyrus
  • postcentral sulcus
  • primary somatosensory cortex
  • interparietal sulcus (may not be visible)
  • superior and inferior parietal lobules
  • supramarginal gyrus
  • angular gyrus
  • superior temporal sulcus
  • Wernicke's area

TEMPORAL LOBE

  • lateral sulcus
  • superior and inferior temporal sulci
  • superior, middle and inferior temporal gyri
  • transverse temporal gyri
  • primary auditory cortex
  • occipitotemporal sulcus
  • collateral sulcus
  • occipitotemporal gyri (fusiform gyri)
  • parahippocampal gyrus
  • uncus

OCCIPITAL LOBE

  • parietooccipital sulcus (midsagittal brain)
  • calcarine sulcus (midsagittal brain)
  • occipital pole
  • occipital gyri

LIMBIC LOBE

  • cingulate gyrus (midsagittal brain)
  • parahippocampal gyrus (midsagittal brain)
  • isthmus (midsagittal brain)


BRAINSTEM

MEDULLA
  • ventral median fissure
  • pyramids
  • pyramidal decussation
  • olive
  • preolivary sulcus
  • hypoglossal nerve (CN XII)
  • postolivary sulcus (retroolivary sulcus)
  • glossopharyngeal nerve (CN IX)
  • vagus nerve (CN X)
  • accessory nerve (CN XI)
  • fourth ventricle
  • obex
  • dorsomedial sulcus
  • tuberculum gracilis and fasciculus gracilis
  • dorsal intermediate sulcus
  • tuberculum cuneatum and fasciculus cuneatus
  • dorsolateral sulcus
  • tuberculum cinerum
  • median sulcus
  • median eminence
  • hypoglossal trigone
  • vagal trigone
  • vestibular area
  • striae medullares (of IV ventricle)
  • inferior cerebellar peduncle

PONS

  • basilar pons
  • basilar sulcus
  • middle cerebellar peduncle
  • abducens nerve (CN VI)
  • facial nerve (CN VII)
  • vestibulocochlear (auditory, CN VIII)
  • trigeminal nerve (CN V)
  • median eminence
  • facial colliculus
  • superior cerebellar peduncle
  • anterior (superior) medullary velum
  • cerebral aqueduct

MIDBRAIN

  • mammillary bodies
  • cerebral peduncles
  • interpeduncular fossa
  • posterior perforated substance
  • oculomotor nerves (CN III)
  • superior colliculus
  • inferior colliculus
  • tectum (corpora quadrigemina)
  • brachium of the inferior colliculus
  • medial geniculate body
  • trochlear nerve (CN IV)

Additional structures to identify that are not listed in text

  • optic nerve (CN II)
  • optic chiasm
  • optic tract


STRUCTURES TO IDENTIFY: MIDSAGITTAL SECTION

  • frontal lobe
  • parietal lobe
  • temporal lobe
  • occipital lobe
  • corpus callosum
    • rostrum
    • genu
    • body
    • splenium
  • pyramids
  • superior (anterior) medullary velum
  • paraterminal gyrus
  • lamina terminalis
  • optic chiasm
  • anterior commissure
  • callosal sulcus
  • cingulate gyrus
  • cingulate sulcus
  • cingulate sulcus, marginal branch (pars marginalis or marginal sulcus)
  • superior frontal gyrus, medial surface
  • paracentral lobule
  • paracentral sulcus
  • parietooccipital sulcus
  • precuneus
  • calcarine sulcus
  • cuneus
  • lingual gyrus
  • septum pellucidum
  • fornix
  • thalamus
  • anterior commissure
  • interventricular foramen (of Monro)
  • optic chiasm
  • lamina terminalis
  • posterior commissure
  • mammillary bodies
  • pineal body
  • habenula
  • stria medullaris thalami
  • hypothalamic sulcus
  • cerebral aqueduct
  • massa intermedia (may be absent)
  • hypothalamus
  • infundibulum (intact brain)
  • tectum
  • superior colliculus
  • inferior colliculus
  • superior cistern
  • tegmentum
  • basilar pons
  • fourth ventricle
  • medulla 


CEREBELLUM

  • vermis
  • hemispheres
  • vallecula (vallecula cerebelli)
  • paravermal zone
  • folia
  • sulci
  • primary fissure
  • anterior lobe
  • posterior lobe
  • posterolateral fissure
  • flocculonodular lobe
  • nodulus
  • flocculi
  • tonsils
  • inferior cerebellar peduncle (restiform body)
  • middle cerebellar peduncle
  • superior cerebellar peduncle (brachium conjunctivum)
  • anterior (superior) medullary velum

 

INTRODUCTION (see Martin Fig. 1.3, p. 16 of this lab manual chapter)

The CNS consists of the brain and spinal cord. The brain consists of four basic subdivisions: the paired cerebral hemispheres, the diencephalon, the cerebellum and the brainstem. The brainstem can be further subdivided into three parts: midbrain (mesencephalon), pons and medulla.

This laboratory will concentrate on the surface and midline internal features of the brain. You will examine an intact brain, a midsagittally sectioned brain and a cerebellum and attempt to identify all of the structures listed above. To assist you, each of these structures is underlined in the chapter.

Goals: The goals of this laboratory are to familiarize you with the more prominent external and midline internal features of the brain. Many areas will receive a more thorough examination in later laboratory exercises.

Supplies required: You will need a whole intact brain, a brain lacking a cerebellum, and/or a brainstem, a brain sectioned in the midsagittal plane, and a detached cerebellum. Use of available brain models is also encouraged. Refer to your Haines atlas, especially chapter 2, and the DeArmond et al. and Martin figures included at the end of this chapter.


CEREBRAL HEMISPHERES

Histologically, the cerebral hemispheres consist of three layers. From superficial to deep they are: the cortex (gray matter that is seen on the surface), white matter and basal ganglia, a collection of deeply located neuronal masses.

The surface of the human cerebrum is highly convoluted. This highly convoluted type of cortex is called gyrencephalic cortex. The crest of a single convolution is called a gyrus (pl. gyri), whereas the depression or groove that separates adjacent gyri is called a sulcus (pl. sulci) or fissure. Many lower mammals, for example rodents, lack the large numbers of gyri and sulci of the human brain. The cerebrum in these animals is smooth and is called lissencephalic cortex.

LOBES OF THE BRAIN (Haines Figs. 2-5, 2-6, 2-9, 2-12, 2-15, 2-40, 3-1, 3-7; DeArmond et al. Figs. 1, 2, 3 and 4, pp. 17-20 of this lab manual chapter)

The cerebral hemispheres are incompletely separated from each other by the longitudinal fissure (interhemispheric fissure). At the bottom of this fissure is a broad band of fibers called the corpus callosum that forms the principal connection between the two hemispheres. On the basis of the more consistent sulci and fissures, each cerebrum is divided into four true lobes: Frontal, parietal, temporal, occipital. Two "synthetic" lobes, the insular and limbic lobes, are not visible on the surface of the brain. In some instances the demarcation between lobes is not clear cut and the boundaries between them are somewhat arbitrary.

The two sulci important for topographical orientation of the cerebral hemispheres are the central sulcus and lateral sulcus. On the lateral surface of the brain identify the lateral sulcus (Sylvian fissure). Anteriorly, the lateral sulcus separates the frontal and temporal lobes, while further posterior it separates portions of the parietal lobe and temporal lobe. Separation of the lateral sulcus will reveal the insular lobe and the supereomedial surface of the temporal lobe. The portions of the frontal, parietal and temporal lobes that overlie the insular region (i.e. lips of the lateral sulcus) constitute the opercular portions of these lobes.

Locate the central sulcus (Rolandic fissure) on the lateral surface of the hemisphere. The sulcus starts near the midpoint of the superior margin of the hemisphere and courses downward toward the lateral sulcus. The frontal lobe lies rostral to the central sulcus and the parietal lobe lies caudal to it. On the lateral surface of the hemispheres, the boundaries between the occipital, parietal and temporal lobes are rather indistinct and arbitrary. The rostral border of the occipital lobe (hence, the caudal borders of the parietal and temporal lobes) is generally considered to be an imaginary line between the parietooccipital sulcus (see midsagittal brain; this sulcus only extends for a short distance on the superior lateral surface of the brain) to the preoccipital notch (temporo-occipital incisure), a small indentation on the lateral inferior margin of the cortex.

Note: some authors describe the boundary of the temporal lobe and occipital lobe to be a small sulcus called the anterior occipital sulcus. This sulcus is rarely labeled in most atlases, but it generally runs superiorly from the region of the preoccipital notch.

Frontal lobe

Boundaries: The frontal lobe is the largest of the hemispheric lobes, comprising about 1/3 of the hemisphere. Laterally, the surface of the lobe extends from the frontal pole of the hemisphere to the central sulcus. Its inferior boundary is the lateral sulcus. The medial surface of the frontal lobe lies superior and rostral to the cingulate sulcus (identified in midsagittal dissection). The orbital portion of the frontal lobe consists of a large cortical area on the ventral surface, anterior to the temporal lobe.

On the lateral surface of the frontal lobe are 3 prominent sulci: precentral sulcus, superior frontal sulcus and the inferior frontal sulcus. The precentral sulcus is anterior and parallel to the central sulcus. Between the central and precentral sulci is the precentral gyrus. The precentral gyrus and anterior bank of the central sulcus comprise the primary motor area (MI) that is responsible for controlling voluntary muscle movement on the contralateral side of the body. More anteriorly, the frontal lobe is divided by two horizontal sulci, the superior and inferior frontal sulci into 3 horizontal gyri, the superior, middle and inferior frontal gyri. The posterior area of the superior frontal gyrus includes much of the premotor cortex, an area important for motor programming and execution. The posterior aspect of the middle frontal gyrus includes the frontal eye field, an area of cortex concerned with eye movements. The inferior frontal gyrus is further subdivided into three parts by the anterior ascending and anterior horizontal rami of the lateral sulcus. From rostral to caudal these regions are the pars orbitalis, pars triangularis and pars opercularis. Broca's speech area, a region involved with the production of speech, is composed of the pars triangularis and pars opercularis in the dominant cerebral hemisphere (usually the left).

Located in the ventral surface (orbital surface) of the frontal lobe are the olfactory bulb, olfactory tract, olfactory trigone, and the lateral and medial olfactory stria. The olfactory bulb receives inputs from the olfactory nerve (CN I), but these fine axons were cut when the brain was removed from the skull and are not visible. The olfactory bulb and olfactory tract (or more properly olfactory peduncle which includes the lateral olfactory tract, anterior olfactory nucleus and centrifugal fibers headed for the olfactory bulb) reside in the olfactory sulcus. Medial to the olfactory tract is the gyrus rectus, while lateral to the sulcus are the orbital gyri. Just posterior to the lateral and medial olfactory stria and anterior to the optic tract is a region studded with numerous perforations made by incoming blood vessels. This region is called the anterior perforated substance.

Parietal lobe

The parietal lobe is divided into three major sections: postcentral gyrus, superior parietal lobule and inferior parietal lobule. The postcentral gyrus lies posterior and parallel to the central sulcus and anterior to the postcentral sulcus. The postcentral gyrus is the primary somatosensory cortex (SI) upon which cortical perception of general body and facial sensation is laid out topographically. The cortex of the caudal parietal lobe is located caudal to the postcentral sulcus. This region of the parietal lobe is further subdivided by the intraparietal sulcus (interparietal sulcus in DeArmond figs.) into superior and inferior parietal lobules. The intraparietal sulcus is located on the superolateral surface the parietal lobe and courses in a anteroposterior direction starting at the upper half of the postcentral sulcus (note: the intraparietal sulcus is very difficult to identify in many brains since it is often discontinuous or even absent, see DeArmond et al. Figs. 1 and 2 and see if you can identify this structure in your specimen). The superior parietal lobule is somatosensory association cortex. The inferior parietal lobule consists of the supramarginal and angular gyri. The supramarginal gyrus surrounds the ascending, terminal ramus of the lateral sulcus, whereas the angular gyrus is located more posteriorly and caps the ascending, terminal ramus of the superior temporal sulcus. These two gyri are important cortical association areas that are located between the primary cortical areas for vision, audition and somatosensation. In the left hemisphere, this region of the cortex is important for the understanding of speech and symbolic language and it is part of a larger cortical area known as Wernicke's area. In the right hemisphere, this area is specialized for the perception of spatial orientation.

Temporal lobe

The temporal lobe lies ventral to the lateral sulcus. The nomenclature used to describe the sulci and gyri varies among authors and can be confusing. Most authors divided the lateral surface of the temporal into three gyri, the superior, middle and inferior temporal gyri separated by the superior and inferior temporal sulci. The superior temporal sulcus is usually easily identified and separates the superior temporal gyrus from the middle temporal gyrus. If the margins of the lateral sulcus are separated, it is possible to see on the posterior wall of the sulcus (superior aspect of the temporal lobe) the transverse temporal gyri (Heschl's gyri). These short convolutions represent the primary auditory cortex (AI). The inferior temporal sulcus separates the middle temporal gyrus from the inferior temporal gyrus and in contrast to the superior temporal sulcus is more difficult to identify because it is often discontinuous. Continuing onto the inferior and surfaces of the temporal lobe locate the occipitotemporal sulcus (temporo-occipital sulcus). This sulcus forms the mesial boundary of the inferior temporal gyrus and, like the inferior temporal sulcus, is often discontinuous. On the inferior surface of the temporal lobe is the rhinal fissure (a shallow groove near the temporal pole that continues onto the dorsal surface, more easily seen in midsagittal section) and its deeper posterior extension, the collateral sulcus. Medial to occipitotemporal sulcus and lateral to the collateral sulcus are the occipitotemporal gyri (fusiform gyri). Medial to the collateral sulcus is the parahippocampal gyrus. There is a medial extension of the parahippocampal gyrus that extends over the olfactory tract known as the uncus. Portions of both the uncus and parahippocampal gyrus are associated with olfaction.

Note: Some authors, including DeArmond, divide the lateral surface of the temporal lobe into superior, middle and inferior temporal gyri, but the sulci dividing these gyri are referred to as the superior, middle (= inferior of most authors) and inferior (= occipitotemporal of most authors) sulci. The remainder of the nomenclature is the same as that noted previously. I personally find DeArmond's method easier to remember, but for the lab exam I will expect you to learn the most widely used nomenclature.

Occipital lobe

The occipital lobe is the smallest of the surface lobes and is concerned primarily with vision. The anterior boundary of the occipital lobe is the parietooccipital sulcus on the medial aspect of the cerebral hemisphere (best seen in midsagittal section). This sulcus extends for only a small distance onto the superior surface of the cerebrum. The calcarine sulcus, also seen best in the midsagittal brain, courses superiorly and anteriorly from the occipital pole on the medial aspect of the hemisphere. The primary visual cortex is found on both banks of the calcarine sulcus. The surrounding areas of the occipital lobe constitute the visual association cortex. The occipital gyri are located on the inferior, and lateral and superior regions of the occipital cortex.

Insula (see Haines Fig. 2-40, 3-1)

The insula (Island of Reil) is located in the depths of the lateral sulcus and is difficult to visualize in the intact specimen. It is concealed by the frontal, parietal and temporal opercula. The insula is outlined by a circular sulcus and is divided into two regions by a central sulcus (not to be confused with the central sulcus noted above). Several short gyri lie anterior to the sulcus and one or two long gyri are located posterior to it. The specific functions of the insula are not completely understood, but it may be involved in visceral sensation, and have some general sensory and motor functions.

Limbic Lobe (see Haines Fig. 2-5; best seen in midsagittal brain)

The structures of the limbic lobe are best seen in a hemisected brain, but they will be discussed here. The limbic lobe consists of a dorsally situated cingulate gyrus, the ventrally located parahippocampal gyrus and a narrow cortical area that connects them, the isthmus. The cingulate gyrus begins inferior to the genu of the corpus callosum and parallels the corpus callosum as far as the splenium (enlarged posterior area of the corpus callosum, see below) where it connects to the isthmus. The parahippocampal gyrus is located between the hippocampal sulcus and the collateral sulcus. This gyrus extends almost as far anteriorly as the temporal pole where it hooks backward toward the uncus. The hippocampus and dentate gyrus are also parts of the limbic lobe, but cannot be seen unless the temporal lobe is further dissected (see Haines 3-6 and 3-7). The limbic lobe is involved in memory, behavior, emotion, olfaction and the control of some visceral functions.

 

BRAINSTEM (use brain minus cerebellum, or brainstem specimens) (Haines Figs. 2-15, 2-18, 2-20, 2-21, 2-32, 2-33, 2-34, 2-35, 2-36, 3-10; Figs. 5-6 of DeArmond et al., Figs. 5-6 pp. 21-22 of this lab manual chapter)

The brainstem is subdivided into three parts: midbrain (mesencephalon), pons, and medulla. Examine your brainstem and identify each of the structures noted in the list at the beginning of the chapter.

Medulla

The medulla or medulla oblongata is the caudal part of the brainstem and it is shaped roughly like a cone. Its narrow lower end is continuous with the spinal cord while the anterior end is broader and merges with the pons. The medulla is about 3 cm in length.

Several structures should be identified on the ventral surface of the medulla. The pyramids are elongated pyramid-shaped columns located on each side of the ventral median fissure. Each pyramid consists of motor fibers descending from the cortex to spinal cord (corticospinal fibers). Most of these corticospinal fibers cross the midline just rostral to the junction of the medulla and spinal cord in the pyramidal decussation (may or may not be present on your specimen). On the ventrolateral surface identify the olive, a smooth, oval swelling produced by the underlying inferior olivary nucleus. The pyramid is separated from the olive by the preolivary sulcus (fissure) (ventrolateral or anterolateral sulcus) from which fibers of the hypoglossal nerve (CN XII) exit the brainstem. These hypoglossal fibers are responsible for motor control of the tongue. The shallow groove dorsolateral to the olive is the postolivary sulcus (retroolivary sulcus). Emerging from the postolivary sulcus are in anterior to posterior order the glossopharyngeal nerve (CN IX), vagus nerve (CN X) and the accessory nerve (CN XI). In your specimens, the identity of individual bundles can only be approximated by their position.

On the dorsal surface, the medulla can be divided into a rostral open part and a caudal closed part. The closed part contains the rostral continuation of the spinal cord central canal (seen best in microscopic sections). Further rostrally, the central canal opens into the fourth ventricle. This open part of the medulla forms the floor and lateral walls of the fourth ventricle. The apex of the V-shaped boundary of the caudal portion of the fourth ventricle is called the obex. At this region a small transverse fold (may be missing) overhangs the opening of the central canal into the fourth ventricle.

The midline furrow on the dorsal surface of the medulla is called the dorsomedial sulcus (posteromedial sulcus or posterior medial sulcus). On either side of the dorsomedial sulcus is the fasciculus gracilis, an uncrossed fiber tract that extends the entire length of the spinal cord to terminate in the nucleus gracilis. The mass of the nucleus gracilis produces a prominent bulge on the dorsal surface of the medulla called the tuberculum gracilis (gracile tubercle). Lateral to the fasciculus gracilis and separated by the dorsal (posterior) intermediate sulcus is the fasciculus cuneatus. The fasciculus cuneatus consists of uncrossed myelinated fibers that terminate in the nucleus cuneatus. The bulge formed on the surface of the caudal medulla by the underlying nucleus cuneatus is termed the tuberculum cuneatum (cuneate tubercle). Between the tuberculum cuneatum and the olive can sometimes be identified a third, subtler swelling called the tuberculum cinerum that is formed by the underlying spinal trigeminal tract and nucleus (see Haines 2-32 and 2-34). The tuberculum cinerum is separated from the tuberculum cuneatum by the dorsolateral (posterolateral or posterior latera)l sulcus. The dorsolateral sulcus is a shallow groove on the dorsolateral side of the spinal cord that extends into the medulla. In the spinal cord the dorsolateral sulcus marks the line of entry of the dorsal root fibers.

Note: Haines 2-32 and 2-35 should be especially helpful for the following section.

The floor of the fourth ventricle is divided into halves by the median sulcus. On either side of the median sulcus there is an elevation called the median eminence that extends through both the medulla and pons. It is most prominent at rostral levels of the floor of the fourth ventricle. This structure contains the medial longitudinal fasciculus. Within the medullary portion of the median eminence is the hypoglossal trigone formed by the underlying hypoglossal nucleus. Lateral to the hypoglossal trigone is a smaller, more difficult to identify structure called the vagal trigone produced by the underlying dorsal motor nucleus of the vagus. The lateral floor of the fourth ventricle is called the vestibular area because it overlies the vestibular nuclei.

The dorsal border between the medulla and pons is demarcated by the striae medullares (of fourth ventricle). These thin fascicles emerge from the area of the median sulcus and run transversely across the vestibular area into the lateral recess of the fourth ventricle to enter the inferior cerebellar peduncle. The inferior cerebellar peduncle runs along the lateral border of the fourth ventricle and turns dorsally into the cerebellum.

Pons

The pons lies rostral to the medulla and caudal to the midbrain. On the dorsal surface of the brain, its caudal border is demarcated by the striae medullares and its rostral border by the caudal inferior colliculi. The ventral surface of the pons (basilar pons) is large and is formed by transverse running pontocerebellar fibers. These fibers decussate at the midline and head laterally to form the middle cerebellar peduncle. A ventral median depression called the basilar sulcus indicates the position occupied by the basilar artery.

Four cranial nerves emerge from the ventral or ventrolateral surface of the pons. The abducens nerve (CN VI) emerges from the ventral surface of the brainstem at the junction of the medulla and pons in line with the exit of the hypoglossal nerve. The facial (CN VII) and vestibulocochlear (CN VIII) nerves emerge from the ventrolateral surface of the pons in the cerebellopontine angle formed by the junction between the pons, medulla and cerebellum. The facial nerve lies anterior and medial to vestibulocochlear nerve, and both emerge from the brainstem more or less in line with cranial nerves IX, X and XI. The trigeminal nerve (CN V) enters the ventrolateral surface of the pons at the junction of the basilar pons and middle cerebellar peduncle. The trigeminal nerve has two roots: a larger sensory root and a smaller motor root. The motor root emanates from the brain just rostral and medial to the sensory root.

The dorsal surface of the pons is the floor of the fourth ventricle. On either side of the midline is the continuation of the median eminence. In the median eminence at the level of the caudal pons is a slight swelling termed the facial colliculus. The facial colliculus is formed by fibers of the facial nerve (motor fibers) looping over the abducens nucleus, the cells of origin for the abducens nerve.

The superior cerebellar peduncles (brachium conjunctivum) emerge from the cerebellum forming the dorsolateral wall of the fourth ventricle. As these fibers course rostrally, they gradually converge to enter the midbrain. The superior cerebellar peduncles consist mainly of cerebellar efferent fibers from the dentate nucleus which pass rostrally to decussate in the midbrain en route to the thalamus. The space between the superior cerebellar peduncles is bridged by a thin sheet of white matter, the anterior (superior) medullary velum, which may or may not be present in your specimen. At the anterior end of the pons the fourth ventricle narrows and merges with the cerebral aqueduct.

Midbrain

On the ventral surface of the brainstem the midbrain extends from the rostral border of the basilar pons to the posterior aspect of the mammillary bodies. On the dorsal surface the midbrain extends from the caudal borders of the inferior colliculi rostrally to the posterior commissure (this is best seen in a midsagittal section of the brain).

On the ventral surface of the midbrain are located the cerebral peduncles (crus cerebri), separated by the interpeduncular fossa (see Haines fig. 2-21). The cerebral peduncles may not be visible in your specimen if the temporal lobes are in place since in the intact brain the temporal lobe may make visualization of these structures difficult. In the depths of the interpeduncular fossa can be seen numerous small holes. These holes represent the entry point of the posterior cerebral artery. Because of its appearance this region is frequently referred to as the posterior perforated substance. Exiting from the interpeduncular fossa near the junction of the pons and midbrain are the oculomotor nerves (CN III). This cranial nerve supplies all but two of the extraocular muscles.

On the dorsal surface of the midbrain are four prominent rounded elevations, the inferior and superior colliculi. Collectively the four colliculi are called the tectum or corpora quadrigemina. The inferior colliculi function with the auditory pathways and the superior colliculi are involved in vision. Coursing anterior and lateral to the inferior colliculus, and lateral to the superior colliculus, is a fiber bundle called the brachium of the inferior colliculus. These fibers run from the inferior colliculus to the medial geniculate body, an oval eminence on the ventral surface of the caudal diencephalon (Haines 2-32, DeArmond et al. Fig. 5, p. 22 below ). The trochlear nerves (CN IV) arise from the dorsal midbrain immediately caudal to the inferior colliculi and pass inferiorly around the lateral side of the midbrain. The trochlear nerve is the only cranial nerve to exit from the dorsal surface of the brainstem.

 

MIDSAGITTAL SECTION OF THE BRAIN (Haines Figs. 2-24, 2-27, 2-28; and DeArmond et al. Fig. 4, p. 23 of this lab manual chapter)

The most prominent structure in the midsagittally-sectioned brain is the corpus callosum, a massive interhemispheric commissure. The corpus callosum is a thick plate of myelinated fibers which constitutes the main commissure between the two hemispheres. The corpus callosum is usually divided into the following main structures: the rostrum (head), genu (bend), body, and splenium (tail). The rostrum and genu interconnect the rostral parts of the frontal lobes, the body interconnects caudal parts of the frontal lobes and the parietal lobes, and the splenium interconnects areas of the temporal and occipital lobes. Throughout much of its length, the corpus callosum forms the floor of the interhemispheric fissure and the roof of the lateral ventricles. Note the paraterminal gyrus (part of the frontal lobe) located immediately anterior to the lamina terminalis. The lamina terminalis is a thin membrane that extends from the optic chiasm to the anterior commissure forming part of the anterior wall of the diencephalon/third ventricle (see below).

Cerebral Hemispheres

Identify the frontal, parietal, temporal and occipital lobes on your midsagittal brain. The callosal sulcus (corpus callosum, sulcus) separates the corpus callosum from the overlying cingulate gyrus. Posteriorly, the callosal sulcus curves around the splenium and continues as the hippocampal sulcus, which is not visible in your midsagittal brain. The cingulate sulcus lies superior to the cingulate gyrus. Near the splenium, the cingulate sulcus turns superiorly as the cingulate sulcus, marginal branch or (marginal sulcus or pars marginalis). The cortex superior to the anterior part of the cingulate sulcus is the medial surface of the superior frontal gyrus. Posteriorly, the paracentral lobule (paracentral gyrus) is formed by those portions of the precentral and postcentral gyri that extend onto the medial surface of the hemisphere. The central sulcus forms a notch on the superior surface of this lobule. The cortex in the anterior half of the paracentral lobule has a motor function and in the posterior half it has a sensory function. The anterior limit of the paracentral lobule is demarcated by the paracentral sulcus (a medial, often interrupted continuation of the precentral sulcus) and the caudal limits by the marginal sulcus. The portion of the parietal lobe located caudal to the marginal sulcus and rostral to the parietooccipital sulcus is the precuneus (precuneate gyrus). The occipital lobe is divided into a superior cuneus (cuneate gyrus) and an inferior lingual gyrus by the calcarine sulcus The calcarine sulcus travels anteriorly from the vicinity of the occipital pole to join the parietooccipital sulcus and then continues on anteriorly. Remember that primary visual cortex resides along the banks of the calcarine sulcus. Note: the calcarine sulcus is extremely variable in its course and configuration and it can be extraordinarily different in the right and left hemisphere in the same individual.

The septum pellucidum (sometimes shortened to septum) is a membranous structure attached to the ventral surface of the corpus callosum (may be absent in your brain). It separates the posterior half of the anterior horns of lateral ventricles. The fornix is located inferior to the septum pellucidum. The fornices are C-shaped fiber bundles that arise in the hippocampus (located inside the temporal lobe), fuse at the midline (now termed the body of the fornix), sweep anteriorly over the thalamus by coursing within the inferior edge of the septum pellucidum. The fused bundles then separate (now termed the columns of the fornix) and descend just caudal and ventral to the anterior commissure, a small bundle of nerve fibers that crosses the midline. Posterior to the columns of the fornix is a small opening called the interventricular foramen (of Monro) which connects the lateral and third ventricles.

Diencephalon

The diencephalon (thalamus, epithalamus, subthalamus and hypothalamus) is located between the cerebral hemispheres and the midbrain. Much of the diencephalon is buried in the ventral and medial portion of each hemisphere. Although it is only a small portion of the brain, it has many important functions. The relatively indistinct rostral border of the diencephalon is near the interventricular foramen and the lamina terminalis, which extends from the optic chiasm to the anterior commissure forming the anterior wall of the third ventricle. The caudal border is an imaginary line drawn from the posterior commissure (located just superior to the rostral entrance of the cerebral aqueduct) to the posterior border of the mammillary bodies.

The epithalamus occupies the most dorsal part of the diencephalon and consists of: 1) a pine cone-shaped structure called the pineal body, 2) the habenula, and the 3) the stria medullaris thalami (do not confuse with the striae medullares on the floor of the fourth ventricle). The pineal body is an endocrine gland whose rich vascularization gives it a reddish-brown appearance in the gross specimen. The pineal tends to calcify with age making it visible on x-ray films of the skull in about 70% of adults and thus a useful landmark for x-ray diagnosis.

The hypothalamic sulcus is a shallow groove in the lateral wall of the third ventricle posterior to the interventricular foramen and just ventral to the massa intermedia (if it is present; see below). If followed posteriorly, the hypothalamic sulcus can be traced to the cerebral aqueduct. The hypothalamic sulcus separates the thalamus from the hypothalamus. The thalamus is a paired egg-shaped structure located dorsal to the sulcus. The posterolateral region of the thalamus is greatly expanded and it overlies part of the midbrain. Portions of the thalami may be fused together in the midline as the massa intermedia (absent in about 20% of human brains). The stria medullaris thalami, identified above, overlies the dorsomedial border of the thalamus.

The hypothalamus is located ventral to the hypothalamic sulcus and is important for visceral and endocrine regulation. It forms the inferior and lateral walls of the third ventricle and is the only part of the diencephalon that is visible on the ventral surface of the intact brain. The rostral border of the hypothalamus is demarcated by the lamina terminalis and its caudal border is the caudal extent of the mammillary body. Three regions of the hypothalamus can be seen in the midsagittal brain. The supraoptic area above the optic chiasm (where approximately 50% of retinal efferents cross the midline), the tuberal region located dorsal to the infundibulum (best seen on the ventral surface of the intact brain, this region is where the pituitary gland is attached by the pituitary stalk) and the posterior area which includes the mammillary bodies and the area dorsal to them.

The subthalamus lies lateral and caudal to the hypothalamus and is not visible in your midsagittal brain.

Brainstem

There are several structures previously identified in the intact brain that can be seen in the midsagittally sectioned brain. The tectum (L. for roof) is composed of the inferior and superior colliculi and forms the roof of the midbrain over the cerebral aqueduct. Regions of the subarachnoid space that contain substantial amounts of cerebrospinal fluid are called subarachnoid cisterns. Note the cistern dorsal to the midbrain between the cerebellum and the pineal gland. This is the superior cistern. The tegmentum (L. for cover) is the dorsal portion of the pons lying between the basilar pons and fourth ventricle/cerebral aqueduct and the part of the midbrain located above the crus cerebri and below the tectum. Superior to the fourth ventricle and inferior to the cerebellum is the anterior medullary velum (superior medullary velum), a structure that forms part of the roof of the fourth ventricle. Locate the medulla caudal to the basilar pons. At the base of the medulla can be seen the prominent pyramids.

 

GROSS ANATOMY OF THE CEREBELLUM

Supplies: midsagittal, whole brains and a separated cerebellum; Haines 2-29--2-31; Nolte Figs. 14-1--14-5 pp. 23-24 of this chapter.

The cerebellum is housed in the posterior cranial fossa. Its superior surface is separated from the cerebrum by the tentorium cerebelli and its convex inferior surface rests upon the occipital bone. The cerebellum overlies the pons and medulla, forming the roof of the fourth ventricle.

Regions of the cerebellum

Certain terms are used to identify regions of the cerebellum. The vermis is the region in and near the midline while the reminder of the cerebellum is referred to as the hemispheres. The superior vermis is not well demarcated from the hemispheres, but the inferior vermis is well delineated, lying in a deep depression called the vallecula (vallecula cerebelli). The term paravermal zone is used for the medial parts of the hemispheres for 1-2 cm on either side of the vermis.

The cerebellar surface is folded to form long, transverse convolutions called folia, separated by parallel sulci. The surface is divided into lobes or lobules by a number of fissures and sulci, most of which are transversely oriented. You should identify the following two fissures and three major lobes on the midsagittal brain. Identify the primary fissure which separates the anterior and posterior lobes of the cerebellum. The anterior lobe is everything rostral to the primary fissure while the posterior lobe is everything between the primary fissure and the posterolateral fissure. The posterolateral fissure separates the posterior and flocculonodular lobe. The flocculonodular lobe is a small component that lies at the rostral edge of the inferior surface. The nodulus is the rostral portion of the inferior vermis, and the flocculi are irregularly shaped masses on each side. In fact, each lobe consists of a vermal and hemispheric component.

Cerebellar lobules

In the midsagittal view of the cerebellum, the vermis is divided into 10 lobules (I-X). Lobules I-V are part of the anterior lobe, VI-IX are part of the posterior lobe and X is part of the flocculonodular lobe. These lobule designations are primarily useful for research purposes, but you should be familiar with their approximate locations and the designations of each. However, you will not be responsible for identifying these lobules in the laboratory exam

I -- lingual

II, III -- central lobule

IV, V -- culmen

VI -- declive

VII -- folium and tuber vermis

VIII -- pyramis

IX -- vermis

X -- nodulus

 

Cerebellar hemispheres

As noted previously, the surfaces of the hemispheres contain several transverse fissures. Do not bother tracing these fissures and learning the lobes on the surface of the cerebellum. Their identification on the surface is difficult and beyond the scope of this laboratory. Furthermore, the names given surface lobes have little functional significance and there is no uniform system of accepted nomenclature. The exception is the tonsils. The position of the tonsils is clinically significant because these parts of the cerebellar hemispheres are close to the medulla and can, in some situations, compress this part of the brain stem (tonsillar herniation).

Cerebellar peduncles

Fibers connecting the cerebellum with the rest of the brain enter/exit via three stout fiber bundles, the cerebellar peduncles. The cerebellar peduncles also form the attachment of the cerebellum to the rest of the brain. The inferior cerebellar peduncle (restiform body) is difficult to locate on gross specimens (note: technically, the inferior cerebellar peduncle is composed of the restiform body and the juxtarestiform body, but in general usage the terms inferior cerebellar peduncle and restiform body are often used interchangeably). It ascends along the dorsolateral surface of the medulla and curves sharply at the pontocerebellar angle to enter the cerebellum on the medial surface of the middle cerebellar peduncle. This latter structure is a massive fiber bundle which heads dorsolaterally from the pons to enter the cerebellum. Both the inferior and middle cerebellar peduncles contain mostly afferent fibers traveling to the cerebellum. The superior cerebellar peduncle (brachium conjectivum) is the smallest and most medial of the peduncles. It emerges beneath the anterior lobe of the cerebellum and extends rostrally into the mesencephalon at the level of the inferior colliculus. A thin sheet of white matter, the anterior(superior medullary velum, extends between the two superior peduncles to form the anterior roof of the fourth ventricle. The superior cerebellar peduncle is primarily an efferent fiber bundle. Note that the three cerebellar peduncles are fused to one another on the inferior surface of the cerebellum.