MEDICAL NEUROBIOLOGY D525

Gross Anatomy of the Brain

It is particularly important to secure a clear orientation in this initial laboratory assignment. References are made to figures in Neuroanatomy. An Atlas of Structures, Sections and Systems by Duane E. Haines, 6th edition, Lippincott Williams & Wilkins 2004. Study the pictures of the gross specimens as well as the MRI provided.

Membranes of the brain

Two groups of membranes cover the brain (Pages 46-47)

The outer tough fibrous sheath of dura mater is representative of the system of pachymeninges and remains adherent to the bones of the skull.

Within the dura mater, the more delicate arachnoid and pia mater form secondary investments (leptomeninges).

The arachnoid membrane can be seen here as fine webs and filaments covering the brain surface and bridging the sulci (depressions) of the brain.

Enclosed within the arachnoid membrane is the subarachnoid space filled with cerebrospinal fluid which acts as a cushion to the cerebrum. The large blood vessels of the brain must cross this space.

By contrast, the pia mater is closely adherent to the brain surface and is a highly vascular covering. Unlike the arachnoid, it passes into all the sulci.

 

Brain

The human brain displays three basic subdivisions: Cerebrum, cerebellum and brainstem.  Identify all 3 on your specimen.

Next turn the brain upside down and differentiate the3 main parts of the brainstem:  the medulla oblongata (most caudal and attached to the spinal cord); the pons and the midbrain.  With the brain still in this position, study the:

ARTERIAL SUPPLY (Fig. 2-18 p. 23; Fig.2-21 p. 25; Fig. 2-24 p. 27)

The 2 vertebral arteries can be found along the ventral aspect of the medulla.  Identify the major branches arising from the vertebral arteries:

The posterior spinal artery

The anterior spinal artery

The posterior inferior cerebellar artery (PICA)

At the level of the junction between the medulla oblongata and the pons, the vertebral arteries unite to form the basilar artery, which runs rostrally along the midline of the ventral aspect of the pons.  Close to its beginning, it gives out bilaterally anterior inferior cerebellar arteries.  Its subsequent branches are named pontine arteries, which are small penetrating arteries.  Finally, it gives out bilateral superior cerebellar arteries.  The basilar artery terminates by dividing into the posterior cerebral arteries at the level of the junction between the pons and midbrain.

Next identify more rostrally the internal carotid arteries.  The remaining stub of these arteries may be seen lateral to the optic nerves and the optic chiasma.

Branching laterally from the internal carotid arteries are the middle cerebral arteries. The middle cerebral arteries course between the temporal lobes and the frontal lobes.  In order to see these vessels, you may have to gently pull the temporal lobes away from the frontal lobes.  Take care not to damage the brain tissue.

The anterior cerebral arteries arise medially from the internal carotid arteries and course rostrally between the 2 frontal lobes.  They are joined in the midline by the anterior communicating artery.

The posterior communicating arteries branch posteriorly from the internal carotid arteries and join with the posterior cerebral arteries arising from the basilar artery.

Realize that the combination of the anterior communicating artery, anterior cerebral arteries, internal carotid arteries, posterior communicating arteries, posterior cerebral arteries and basilar artery form the circle of Willis, an important anastomosis in the brain.

ATTACHMENT OF THE CRANIAL NERVES (p. 22, 23, 24, 25, 27)

Proceed with thesequential examination of the nerve attachments from anterior to posterior.  Identify each cranial nerve.

Olfactory bulb and tract (I). The bulb lies between the gyrus rectus medially and the medial orbital gyrus of the frontal lobe laterally.  Its tract extends posteriorly and bifurcates (Fig. 2-14 p. 20).

Optic nerves(II). These nerves are partially united in the optic chiasm, which straddles the diencephalon immediately in front of the stalk of the pituitary gland. Notice the mammillary bodies lying immediately caudal to the pituitary stalk (Fig. 2-14 p. 20).

Oculomotor nerve(III). This emerges at the medial side of the cerebral peduncle in the interpeduncular fossa, and then lies between the posterior cerebral and superior cerebellar arteries (Fig. 2-18 p.23).

Trochlear nerve(IV). This is the only nerve to emerge on the dorsal aspect of the brainstem. It appears in the groove posterior to the inferior colliculi (the lower 2 bumps of the quadrigeminal plate and curves around the sides of the midbrain (Fig. 2-34 p. 34).  In order to see the trochlear nerve, turn the brain right side up and gently depress the cerebellum away from the cerebrum.   Gently dissect the arachnoid and pial membranes away from the dorsal surface of the midbrain and look for the trochlear nerve in the revealed space.

Trigeminal nerve(V) is attached to the side of the brainstem at the junction of the pons and the middle cerebellar peduncle (Fig.2-18 p. 23). It has a small motor root on its medial aspect.

Abducent nerve(VI) emerges at the caudal border of the pons near the midline.

Facial nerve (VII) and Auditory nerve (VIII):

·         are attached further laterally at the same level as the abducent nerve in the cerebellopontine angle between the lower border of the pons and the lateral margin of the medulla.

Glossopharyngeal nerve (IX), Vagus nerve (X). and Accessory nerve (XI)

·         are attached by a common series of rootlets linearly arranged posterolateral to the inferior olive (oval bodies on the lateral aspect of the medulla). The accessory nerve has an additional spinal root which may be found on the lateral aspect of the medulla.  This spinal root has an oblique attachment to the spinal cord, inclining forward as it descends into the spinal canal.

Hypoglossal nerve (XII) is attached by a series of rootlets in the groove between the pyramid and the inferior olive.  The pyramids are located on either side of the midline on the ventral aspect of the medulla oblongata.

BRAINSTEM AND CEREBELLUM

The brainstem is organized into a midbrain, pons and medulla oblongata (p. 24-27; 34-37).  Identify all 3 parts.

Observe the pyramids, two bands of fibers on the ventral aspect of the medulla oblongata, appearing above at the lower border of the pons and disappearing below into the interior of the spinal cord at the spinomedullary junction (p. 24-25).

Find the inferior olive lying immediately lateral to the pyramid at the lower border of the pons (Fig.2-20 p. 24).

Examine the attachments of the cerebellum to the brainstem (p.32-33). Of the three peduncles (superior, middle and inferior) which attach it to the brainstem, the middle is the largest. Its fibers form a yoke around the sides of the brainstem and are responsible for the swelling of the pons on its ventral aspect (Fig. 2-20 p. 24). The broad bands of the pontine fibers separate the midbrain above from the medulla oblongata below.

Next, holding the brain with the frontal lobes pointing away from you, carefully depress the cerebellum away from the under surface of the cerebral hemispheres.  Observe in the revealed space:

The divergent cerebral peduncles (crus cerebri) will be seen on the ventral aspect of the midbrain at this level (p. 24), disappearing into the interior of the hemispheres.

Turn the brain right side up and observe that the cerebellum is organized into 2 cerebellar hemispheres laterally and the vermis in the midline (Fig. 2-31 p. 32).

CEREBRUM (p. 16-23)

The cerebrum is formed of 2 parts:

·         The very large cerebral hemispheres (or telencephalon) are budded embryologically as paired cerebral vesicles from the anterior end of the neural tube.

·         The median (unpaired) anterior end of the neural tube forms the diencephalon, which will be studied more extensively with the interior of the cerebral hemisphere.

Holding the brain right side up, carry out the following observations:

 The surface of each cerebral hemisphere is highly convoluted, with numerous grooves, or sulci, separating finger-like processes of neural tissue, the gyri.  Deeper grooves are called fissures.

The 2 cerebral hemispheres are separated by a deep median fissure, the longitudinal cerebral fissure (p. 16).

Note the gray color of the surface tissue due to the external aggregation of nerve cells, forming the cerebral cortex.  By contrast the subcortical nerve fibers form the white matter, and can be displayed through a small incision in the cortex.

Identify the three poles of the hemisphere (Fig. 2-14, p. 20):  frontal, temporal and occipital.

Each cerebral hemisphere has three surfaces:

·         superolateral,

·         medial,

·         and inferior.

Now examine successively certain markings on each surface.

The superolateral surface is deeply fissured by the lateral cerebral fissure (Sylvian fissure; Fig. 2-11 p. 18).

Gently open the lips of this sulcus and observe the buried cortex of the insula (Fig. 2-46 p. 45). The posterior part of this buried cortex is concerned with hearing.  The circuminsular fissure (or circular sulcus) surrounds the insula, separating it from the adjacent frontal, parietal, and temporal lobes.

Next, identify superior to the lateral cerebral fissure: the central sulcus (fissure of Rolando), the precentral sulcus and the postcentral sulcus (Fig. 2-11 p. 18).

The precentral gyrus, located between the central and precentral sulci, is concerned in motor activity of the body.

The postcentral gyrus, located between the central and poscentral sulci, contains a sensory representation of the whole body.

Then, find the parieto-occipital sulcus (Fig. 2-26 p. 28) on the medial surface of the posterior portion of the cerebral hemisphere and the preoccipitalnotch (Fig. 2-11 p. 18).

From these landmarks, the hemisphere may be divided into four lobes (Fig. 2-5, p. 13):

The frontal lobe extends from the frontal pole to the central sulcus posteriorly and is limited below by the lateral fissure.  The precentral gyrus and sulcus are located in the frontal lobe.  The superior and inferior frontal sulci extend anteriorly and inferiorly and divide the lateral surface of the frontal lobe into superior, middle and inferior frontal gyri (p.16,18).   Theinferior frontal gyrus is further subdivided from anterior to posterior into orbital, triangular and opercular parts (Do you see the letter M? p.18).

The parietal lobe stretches from the central sulcus as far back as an imaginary line joining the parieto-occipital sulcus to the preoccipital notch (p.13).  The postcentral gyrus and sulcus are located in the parietal lobe.  On the superolateral aspect, the intraparietal sulcus (Fig. 2-11, p.18) separates the superior parietal lobule from the inferior parietal lobule.  The supramarginal gyrus arches above the ascending end of the posterior ramus of the lateral cerebral fissure (p.18). The angular gyrus arches above the end of the superior temporal sulcus and is continuous with the middle temporal gyrus.  On the medial aspect, the precuneus (Fig.2-26, p. 28) is located between the parieto-occipital sulcus and the ascending end of the cingulate sulcus.

The occipital lobe extends forward from the occipital pole to the parieto-occipital sulcus, the posterior limit of the parietal lobe (Fig.2-26, p. 28).

The temporal lobe extends back from the temporal pole below the lateral fissure, as far as its junction with the occipital lobe, but much of it lies on the inferior aspect of the hemisphere (Fig. 2-14, p. 20).  The lateral surface of the temporal lobe is divided into the superior, middle and inferior temporal gyri separated by superior and inferior temporal sulci.

Turn the brain upside down and, with a sharp scalpel, sever  the brainstem from the brain at the level of the midbrain:  insure that your blade will cut through the cerebral peduncles ventrally and above the quadrigeminal plate dorsally.  Examine the inferior aspect of each cerebral hemisphere, formed jointly by the inferior aspect of the temporal lobe and the orbital surface of the frontal lobe.

The inferior aspect of the temporal lobe is bounded medially by a broad cortical strip, the (para)hippocampal gyrus (Fig. 2-14 p. 20) separated from the more lateral cortex by the collateral sulcus.  The posterior part of this sulcus lies below and parallel to the calcarine sulcus.  With a probe or the handle of a scalpel,gently evert the medial margin of the hippocampal gyrus and follow itforward.  This line is referred to as the hippocampal fissure and it ends anteriorly in a hook-shaped process of cortex (the uncus; p20, 22), which is marked off from the temporal pole by the short rhinal sulcus. The cortex of the uncus plays a role in olfaction.

Typically, the orbital surface of the frontal lobe is formed of three parallel gyri (lateral, intermediate and medial orbital gyri), and the olfactory bulb and tract rest upon the olfactory sulcus, between the medial orbital gyrus and the gyrus rectus (Fig. 2-17 p. 22).

With a large brain knife, separate the 2 hemispheres and study the medial aspect of each hemisphere.

Identify the main bands of fibers of commissures uniting the hemispheres.

·         The corpus callosum (Fig. 2-29 p. 30) is the largest of these bands. Its middle portion or body is narrower than either the posterior extremity (splenium) or the anterior end (genu). The genu of the corpus callosum continues inferiorly by a narrow connecting band (the rostrum) into the lamina terminalis (Fig. 2-30 p. 31), which embryologically formed the closing plate at the front end of the neural tube.

·         In the lamina terminalis a smaller anterior commissure can be seen (Fig. 2-30 p. 31).

·         Next, define the diverticulum of the pineal gland (Fig. 2-30 p. 31) below the splenium of the corpus callosum. Two small commissures, the habenular and the posterior commissures, are to be found above and below the root of the pineal gland, respectively.

·         Immediately below the body of the corpus callosum identify the arching fibers of the fornix system (p. 30-31), which are here united with those of the opposite side in the fornix commissure.

The medial aspect of the hemisphere bears two major sulci:

·         The cingulate gyrus (Fig. 2-29 p. 30) runs a course parallel to the corpus callosum, lying between the corpus callosum and cingulate sulcus (see below,  re: limbic lobe). From its origin anteriorly on the medial aspect of the frontal lobe, it arches above the genu and body of the corpus callosum.  Posteriorly, it divides into two diverging branches directed upward towards the medial margin of the hemisphere. These branches enclose the paracentral lobule, which is the continuation of the precentral and postcentral gyri.

·         Another Y-shaped sulcus is seen on the medial aspect of the occipital lobe. Its stem begins below the splenium of the corpus callosum (precalcarine sulcus) and bifurcates behind into an upper parieto-occipital sulcus or fissure and lower calcarine fissure (or postcalcarine sulcus; Fig. 2-26 p. 28).  The cortex surrounding the lips of the calcarine fissure is concerned with vision.   The calcarine fissure divides the occipital lobe into cuneus superiorly and the lingual (or lateral occipitotemporal) gyrus inferiorly.

Attention should bedirected to a girdle of cortex at the medial margin of the hemisphere (p. 13, 28), formed jointly of the subcallosal, cingulate, and parahippocampal gyri as well as the hippocampal formation and dentate gyrus.  This is referred to as the limbic lobe, originally defined by the French anatomist Paul Broca in 1878.

Observe the distribution of the branches of the anterior cerebral artery and posterior cerebral artery on the medial aspect of the hemisphere (Fig. 2-27 p. 29)

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