Lab

I. SURFACE FEATURES OF THE BRAIN

A. Lateral Surface

The lateral surface of the cerebral hemisphere may be divided into four lobes: frontal, parietal, occipital and temporal. The frontal lobe is demarcated from the parietal lobe by the central sulcus of Rolando, and from the temporal lobe by the lateral fissure of Sylvius. The parietal and temporal lobes are demarcated from the occipital lobe by a line drawn from the parietal-occipital fissure to the preoccipital notch. Each lobe may be divided into a number of gyri convolutions) separated by sulci (grooves). The precentral gyrus is known to subserve motor function. The postcentral gyrus subserves somatosensory function (touch, kinesthesis or position sense, vibration sense). The transverse temporal gyri subserve auditory function.

B. Medial Surface

In medial sagittal section of the brain, one sees a massive band of commissural fibers, the corpus callosum, which connects the two cerebral hemispheres. The caudal end of the corpus callosum is known as the splenium, the middle portion as the body, the rostral bend as the genu, and the rostral termination as the rostrum. Above the corpus callosum lies the cingulate gyrus. The region of cerebral cortex around the banks of the calcarine fissure consists of striate cortex which subserves visual sensation.

The diencephalon is divided into two major regions, the thalamus and the hypothalamus, by the hypothalamic sulcus. The left and right thalami are joined through the third ventricle by a bridge of tissue known as the massa intermedia.

The midbrain (as seen in median sagittal section) is divided into a ventral portion, the tegmentum, and a dorsal portion, the tectum (roof), by the cerebral aqueduct. The tectum consists of four protuberances corpora quadrigemina the rostral pair, known as the superior colliculi, are related to the visual (and other) systems; the caudal pair, known as the inferior colliculi, are related, to the auditory system. Rostral to the former is a diencephalic derivative, the pineal body (epiphysis). Caudal to the inferior colliculi of the midbrain, the superior medullary velum forms the rostral part of the roof of the fourth ventricle in the hind brain (rhombencephalon).

C. Basal Surface

Immediately lateral to the interhemispheric fissure is the gyrus rectus (straight gyrus), lateral to which is the olfactory bulb, which sends fibers into the olfactory tract. Medial to the rhinal sulcus are seen the uncus, involved in olfactory sensation, and the parahippocampal gyrus. Descending fibers course through the massive cerebral peduncles (crura cerebri), the most ventral parts of the midbrain (mesencephalon). In the interpeduncular fossa (the space between the peduncles), the oculomotor (third nerve) emerges. Note also the optic chiasm, mammillary bodies, and the structures of the hindbrain, including the cranial nerves.

II. MENINGES

The brain and spinal cord are protected from outside forces by their encasement in the skull and vertebral column, respectively. In addition, the CNS is suspended within a series of three membranous coverings, the meninges (Greek, meninx = membrane), that stabilize the shape and position of nerve tissue in two different ways during head and body movements. First, the brain is mechanically suspended within the meninges, which in turn are anchored to the skull so that the brain is constrained to move in parallel with the head. Second, there is a layer of cerebrospinal fluid within the meninges; the buoyant effect of this fluid environment greatly decreases the tendency of various forces (such as gravity) to distort the brain. Thus a brain weighing 1500 grams in air effectively weighs less than 50 grams in its normal cerebrospinal fluid environment, where it is easily able to maintain its shape. In contrast, an isolated fresh brain, unsupported by its usual surroundings, becomes seriously distorted and may even tear under the influence of gravity.

The three meninges, from the outermost layer inward, are the dura mater, the arachnoid, and the pia mater. The dura mater is by far the most substantial of the three meninges. The arachnoid and pia mater, in contrast, are thin and delicate. They are similar to and continuous with each other and so are sometimes referred to together as the pia-arachnoid or the leptomeninges (Greek, lepto = thin, fine). The dura mater is attached to the inner surface of the skull, and the arachnoid adheres to the inner surface of the dura mater. The pia mater is attached to the brain, following all its contours, and the space between the arachnoid and pia mater is filled with cerebrospinal fluid.

A. Dura Mater

The cranial dura is a thick, tough membrane that adheres firmly to the inner surface of the skull. It is often described as consisting of two layers: an outer layer that serves as the periosteum of the inner surface of the skull and an inner layer, the true dura.

No space exists on either side of the dura under normal circumstances, since one side is attached to the skull and the other side is adjacent to the arachnoid. However, two potential spaces, the epidural and subdural spaces, are associated with the dura. Epidural space refers the potential space between the cranium and the periosteal layer. Subdural space is commonly described as the potential space between dura and arachnoid and is said to contain a thin film of fluid.

1. Dural reflections

There are several places where the inner dural layer is reflected as sheet-like protrusions, called dural reflections or septa, into the cranial cavity. The principal dural reflections are the falx cerebri, which intervenes between the two cerebral hemispheres, and the tentorium cerebelli, which intervenes between the cerebral hemispheres and the cerebellum. The falx cerebelli is a small reflection that partially separates the two cerebellar hemispheres.

The falx cerebri (Latin, falx = sickle) is a long, arched vertical dural sheet that occupies the longitudinal fissure and separates the two cerebral hemispheres. The falx curves posteriorly and fuses with the middle of the tentorium cerebelli at the internal occipital protuberance. The inferior, free edge of the falx generally parallels the corpus callosum.

The tentorium cerebelli separates the superior surface of the cerebellum from the occipital lobes. The free edge of the tentorium curves anteriorly on each side, almost encircling the midbrain. This space in the tentorium through which the brainstem passes is called the tentorial notch (or tentorial incisure).

2. Dural sinuses

The two layers of the cranial dura are tightly fused. However, at some edges of dural reflections (most often attached edges), the two layers are normally separated to form channels, called dural venous sinuses, into which the cerebral veins empty. These sinuses are roughly triangular in cross section and are lined with endothelium. The locations of the major sinuses can be inferred by considering the lines of attachment of the falx and the tentorium. The superior sagittal sinus is found along the attached edge of the falx, the left and right transverse sinuses are found along the posterior line of attachment of the tentorium, and the straight sinus is found along the line of attachment of the falx and tentorium to each other. All four of these sinuses meet in the confluence of the sinuses near the internal occipital protuberance. Venous blood flows posteriorly in the superior sagittal and straight sinuses into the confluence, and from there through the transverse sinuses. Each transverse sinus continues, from the point where it leaves the tentorium, as the sigmoid sinus, which proceeds anteriorly and inferiorly through an S-shaped course and empties into the internal jugular vein.

The confluence of the sinuses is generally not a symmetrical structure. Usually blood from the superior sagittal sinus flows into the right transverse sinus, while blood from the straight sinus flows into the left transverse sinus. In an extreme case, the two transverse sinuses may not be interconnected at all.

3. Dural vasculature and innervation

The arterial supply of the dura comes from a large number of meningeal arteries. These are somewhat misnamed because they travel in the periosteal layer of the dura and function mainly in supplying the bones of the skull; however, many small arterial branches penetrate the dura itself. The largest of the meningeal arteries is the middle meningeal artery, a branch of the maxillary artery which ramifies over most of the lateral surface of the cerebral dura.

Most of the cranial dura, except for that of the posterior fossa, receives sensory innervation from the trigeminal nerve. The dura of the posterior fossa is supplied by fibers of the second and third cranial nerves.

B. Arachnoid

The arachnoid is a thin avascular membrane composed of a few layers of cells interspersed with bundles of collagen. It is semi-transparent and resembles a substantial cobweb, for which it is named (Greek, arachne = spider's web). Mesothelial cells cover both the inner and outer surfaces of the arachnoid. Small strands of mesothelium-covered arachnoid tissue, called arachnoid trabeculae, leave the inner surface and extend to the pia, with which they merge. Arachnoid trabeculae presumably serve to help keep the brain suspended within the meninges, much the way the Lilliputians stabilized Gulliver's position.

1. Subarachnoid cisterns

Because the arachnoid is attached to the inner surface of the dura mater, it, like the dura, conforms to the general shape of the brain but does not dip into sulci or follow the more intricate contours of the surface of the brain. There is, therefore, a subarachnoid space, filled with cerebrospinal fluid, between the arachnoid and the pia mater, since the pia closely covers all the external surfaces of the central nervous system. This is the only substantial fluid-filled space normally found around the brain. The subarachnoid space is nonexistent over the surfaces of gyri, relatively small where the arachnoid bridges over small sulci, and much larger in certain locations where it bridges over large surface irregularities. An example of such a location is the space between the inferior surface of the cerebellum and the dorsal surface of the medulla. Regions such as this, which contain a considerable volume of cerebrospinal fluid, are called subarachnoid cisterns. This particular example is called the cerebellomedullary cistern on anatomical grounds and, since it is the largest cranial cistern, it is also referred to as cisterna magna. Other prominent cisterns are indicated and include (1) the pontine cistern, around the anterior surface of the pons and medulla, which is continuous posteriorly with the cerebellomedullary cistern; (2) the interpeduncular cistern, between the cerebral peduncles, which contains the arterial circle of Willis; and (3) the superior cistern, a radiological landmark above the midbrain. The superior cistern is also referred to as the cistern of the great cerebral vein, the quadrigeminal plate cistern, and cisterna ambiens.

2. Arachnoid villi

The cerebrospinal fluid contained in the subarachnoid space is generally separated from the venous blood in dural sinuses by a layer of arachnoid, a thick layer of dura, and the endothelial lining of the sinus. However, at many locations along dural sinuses, particularly along the superior sagittal sinus, small evaginations of the arachnoid, called arachnoid villi, protrude into the sinus. At these sites the connective tissue of the dura is lacking, and only a loose layer of arachnoid cells and a layer of endothelium intervene between subarachnoid space and venous blood. Large arachnoid villi are called arachnoid granulations, and those that become calcified with age are referred to as pachionian bodies.

The arachnoid villi are the major sites of reabsorption of cerebrospinal fluid into the venous system. Functionally, they behave like one-way valves, allowing flow from subarachnoid space into venous blood but not in the reverse direction. Since cerebrospinal fluid pressure is ordinarily greater than venous pressure,,the villi normally allow continuous movement of cerebrospinal fluid, more or less as though by bulk flow, into the sinuses.

C. Pia Mater

The pia mater is a second delicate membrane (Latin, pia = tender) that, unlike the arachnoid, closely invests all surfaces, following all the contours of the brainstem and all the folds of the cerebral and cerebellar cortices.

Arachnoid trabeculae span the subarachnoid space and merge with the pia mater so subtly that it is difficult to decide where the arachnoid ends and the pia begins. The area of the pia immediately adjacent to nervous tissue is a very thin layer of collagen and scattered cells and is considered separately as the intima is by some authors. The intima pia merges with a more superficial region, which consists of loose connective tissue closely resembling the arachnoid. Those who recognize the intima pia as a separate entity designate this more superficial region the epipial layer. Others, however, consider the intima pia to represent the true pia mater an the epipia to be part of the arachnoid. Still, others speak of the entire leptomeningeal complex as one entity, the pia-arachnoid.

The pia mater is often referred to as a vascular membrane, but in fact the cranial epipial layer is rather sparse, and the cerebral arteries and veins travel in the subarachnoid space before penetrating the brain. The vessels essentially rest on the intima pia, held there by small strands of connective tissue. A cuff of pia surrounds each small vessel as it enters the brain, enclosing a shell of subarachnoid space around the vessel. This is the perivascular space (or space of Virchow-Robin), which ends abruptly at the point at which the vessel becomes a capillary.

As discussed previously, the three meningeal coverings of the brain have various real or potential spaces associated with them. There is no space between pia and brain, but there is a subarachnoid space between pia and arachnoid, along with potential subdural and epidural spaces. Both of these potential spaces can become actual fluid-filled spaces under certain conditions.

The meningeal arteries run in the periosteal layer of the dura. If one of these arteries is torn (typically as a result of traumatic skull injury), bleeding occurs between the periosteum and the skull, opening up the potential epidural space and causing an a idural hematoma. As the hematoma expands, it compresses and distorts the underlying rain and is almost always fatal unless promptly treated surgically.

Bleeding can also occur into the potential subdural. space, resulting in a subdural hematoma. The most common cause is the tearing of a cerebral vein as it enters a dural sinus. Some subdural hematomas are acute and produce symptoms much like those of an epidural hematoma, while others may progress very slowly and become surprisingly large before producing symptoms.

Dural reflections such as the falx cerebri and the tentorium cerebelli are firmly attached to the cranium. These reflections are stretched rather taut, which allows them to perform their mechanical support function, but this very tautness can result in additional problems in cases of increasing intracranial pressure (for example, subdural hematoma or an expanding tumor). The midbrain may be pushed against the edge of the tentorium as it passes through the tentorial notch, causing damage to a cerebral peduncle and one or more cranial nerves. Also, depending on where the expanding mass causing the increased pressure is located, certain portions of the brain may herniate from one side of a dural reflection to another. For example, increased pressure on the lateral surface of one cerebral hemisphere can cause the hemisphere to be displaced inferiorly and medially, causing the uncus and adjacent portions of the temporal lobe to herniate through the tentorial notch and compress the midbrain. Such pressure could also cause one cingulate gyrus to herniate under the falx. Similarly, downward pressure can cause portions of the cerebellum to herniate into the foramen magnum and compress the medulla. Herniations that compress the brainstem are likely to have very grave consequences.

III. ARTERIAL SUPPLY AND VENOUS DRAINAGE

The sources of the blood supply to the brain are the two internal carotid arteries and the two vertebral arteries; the latter merge to form the basilar artery lying along the base of the brainstem. These arteries provide the inflow to the anastomotic arterial ring at the base of the brain, known as the circle of Willis. The middle cerebral artery takes its origin from the circle of Willis and runs between the banks of the lateral fissure to reach the lateral surface of the hemisphere. Also originating from the circle of Willis are the anterior cerebral artery, which serves mainly the anterior part of the medial surface of the hemisphere, and the osterior cerebral artery, which serves

mainly the posterior part of the medial surface of the hemisphere. A single anterior communicating and paired posterior communicating arteries complete the circle of Willis. The cerebellum receives arterial blood from three arteries: the superior cerebellar artery, and the anterior inferior cerebellar artery arise from the basilar artery while the posterior inferior cerebellar artery arises from the vertebral.

A. Venous Drainage

The principal route of venous drainage of the brain is through a system of cerebral veins that empty into the ducal venous sinuses and ultimately into the internal jugular vein. There is also a collection of emissary veins connecting extracranial veins with ducal sinuses and a basilar venous plexus around the base of the brain that communicates with the epidura venous plexus of the spinal cord. These play a relatively minor role in the normal circulatory pattern of the brain, but emissary veins cay be very important clinically as a path for the spread of infection into the cranial cavity.

Cerebral veins are conventionally divided into superficial and deep groups. In general, the superficial veins lie on the surface of the cerebral hemispheres and empty into the superior sagittal sinus, while the deep veins drain internal structures and eventually empty into the straight sinus.

Cerebral veins are valueless and, in contrast to cerebral arteries, are interconnected by numerous functional anastomoses, both within a group and between superficial and deep groups.

IV. VENTRICULAR SYSTEM AND CEREBROSPINAL FLUID

The hollow core of the embryonic neural tube develops into a continuous fluid‑filled system of ventricles, lined with ependymal cells. In the adult each division of the CNS contains a portion of this ventricular system. Within each cerebral hemisphere is a relatively large lateral ventricle. The paired lateral ventricles communicate with the third ventricle of the diencephalons through the interventricular foramina (or foramina of Monro). The third ventricle in turn communicates with the fourth ventricle of the pons and medulla through the narrow cerebral aqueduct (or aqueduct of Sy vius) of the midbrain. The fourth ventricle continues caudally as the tiny central canal of the spinal cord and caudal medulla; this canal is usually not patent over much of its extent.

Cerebrospinal fluid is formed within the ventricles, fills them, and emerges from apertures in the fourth ventricle to fill the subarachnoid space.

A. Ventricles

1. Lateral ventricle

The lateral ventricle follows a long C‑shaped course through all the lobes of the cerebral hemisphere. It is customarily divided into five parts: (1) an anterior (or frontal) horn, in the frontal lobe anterior to the interventricular foramen; (2) a body in the frontal and parietal lobes, extending posteriorly to the region of the splenium of the corpus callosum; (3) a posterior (or occipital) horn, projecting backward into the occipital lobe; (4) an inferior (or temporal horn, curving down and forward into the temporal lobe; and (5) a collateral trigone (or atrium), the region near the splenium where the body and the posterior and inferior horns meet.

2. Third ventricle

The narrow, slit‑shaped third ventricle occupies most of the midline region of the diencephalon, and so its entire outline can be seen in a hemisected brain. It often looks like a misshapen doughnut in casts of the ventricular system. The hole in the doughnut corresponds to the interthalamic adhesion, which crosses the ventricle in most human brains.

Much of the medial surface of the thalamus and hypothalamus forms the wall of the third ventricle, and part of the hypothalamus forms its floor. It has a thin membranous roof. containing‑choroid plexus. At the posterior end of the mammillary bodies, the third ventricle narrows fairly abruptly to become the cerebral aqueduct, which traverses the midbrain.

3. Fourth ventricle

The fourth ventricle is sandwiched between the cerebellum posteriorly and the pons and rostral medulla anteriorly. It is shaped like a tent with a peaked roof, the peak protruding into the cerebellum. The floor is relatively flat, and since it narrows rostrally into the aqueduct and caudally into the central canal, it is somewhat diamond‑shaped. For this reason, the floor is sometimes referred to as the rhomboid fossa. At the location where the lateral point of the diamond would be expected, the entire ventricle becomes a narrow tube that proceeds anteriorly and curves around the brainstem, ending adjacent to the flocculus of the cerebellum. This tubular prolongation is the lateral recess of the fourth ventricle. The portion of the roof of the ventricle rostral to the peak is the su erior medullar velum, and the portion caudal to the peak is the inferior medullary velum. a superior medullary velum is a thin layer of white matter related to the cerebellum, while the inferior medullary velum is a membrane containing choroid plexus and is similar to the roof of the third ventricle.

The lateral and third ventricles are closed cavities, communicating only with other parts of the ventricular system. In contrast, there are three apertures in the fourth ventricle through which the ventricular system communicates freely with subarachnoid space. These are the unpaired median aperture (or foramen of Magendie) and the two lateral apertures (or foramina of Luschka) of the fourth ventricle. The median aperture is simply a hole in the inferior medullary velum; it is as though the caudal end of the membrane, where it should have closed off the ventricle at its junction with the central canal, had instead been lifted up and attached to the inferior surface of the cerebellar vermis. The result is a funnel‑shaped opening from the subarachnoid space (the cerebellomedullary cistern) into the ventricle. The inferior medullary velum also covers the lateral recess, and at the end of each recess is another opening in the velum, the lateral aperture.

3. Ventricle size

The ventricles are both smaller and more variable in size than one might expect. Although there is an average total of approximately 130 ml of cerebrospinal fluid within and around the brain and spinal cord, only about 20 ml of this fluid is contained within the ventricles. The rest occupies subarachnoid space. The third and fourth ventricles together have a volume of only about 2 ml, so the lateral ventricles contain nearly all the ventricular cerebrospinal fluid. The total volume of 20 ml is only an average figure, and the ventricles of some apparently normal brains have been found to have total volumes of less than 10 ml or more than 50 ml (however, volumes greater than 30 ml are usually considered suspicious).

B. Choroid plexus

All four ventricles contain strands of highly convoluted. and vascular membranous material called choroid plexus that secretes most of the cerebrospinal fluid. There is a long continuous band of choroid plexus in each lateral ventricle, extending from near the tip of the inferior horn, around in a C‑shaped course through the body of the ventricle to the interventricular foramen. The choroid plexus of each lateral ventricle grows through the interventricular foramen and becomes one of the two narrow strands of choroid plexus in the roof of the third ventricle. It does not continue through the aqueduct, which is completely surrounded by neural tissue. The choroid plexus of the fourth .ventricle is formed from a similar invagination of the inferior medullary velum into the caudal half of the ventricle.

1. Cerebrospinal fluid

Cerebrospinal fluid is a colorless liquid, low in cells and proteins, but generally similar to plasma in its ionic composition. Cerebrospinal fluid seems to be secreted primarily by choroid plexus.

The rate of formation of new cerebrospinal fluid (an average of about 350 ul/minute in man) is relatively constant and little affected by blood pressure or intraventricular pressure. This means that the total volume of cerebrospinal fluid is renewed more than three times per day. Little is known about mechanisms available to modify the secretion rate, but some very recent experiments indicate that stimulation of the sympathetic supply to the choroid plexus can decrease this rate markedly.

a. Circulation

If the cerebrospinal fluid is turned over several times per day, it must circulate from its site of formation to a site of removal. We have already discussed all the elements of the system involved: cerebrospinal fluid formed in the lateral ventricles passes through the interventricular foramina into the third ventricle, from there through the cerebral aqueduct into the fourth ventricle, and thence through the median and lateral apertures into cisterna magna and the pontine cistern. From the pontine cistern, the fluid slowly moves up over the cerebral hemispheres, through the arachnoid villi, and into the superior sagittal sinus. The flow should not be thought of as slow and steady, since arterial pulsations cause a constant ebb and flow, with a small net movement toward the superior sagittal sinus with each heartbeat. As would be expected from the rate of cerebrospinal fluid formation, it takes several hours for new cerebrospinal fluid to complete the journey.

In addition to this basic pattern of circulation, some cerebrospinal fluid moves from the cisterns around the fourth ventricle into the subarachnoid space around the spinal cord. Most of this fluid is returned to the venous system through small arachnoid villi that are found in the ducal sleeves accompanying spinal nerve roots.

Since the rate of production of cerebrospinal fluid is relatively independent of blood pressure and intraventricular pressure, the fluid will continue to be produced even if the path of its circulation is blocked or is otherwise abnormal. When this happens, cerebrospinal fluid pressure rises and ultimately the ventricles expand at the expense of surrounding brain, creating a condition known as hydrocephalus. .In principle, hydrocephalus can result from excess production of cerebrospinal fluid, from blockage of cerebrospinal fluid circulation, or from a deficiency in cerebrospinal fluid reabsorption. All three types occur, but that caused by a blockage of circulation is by far the most common.

Tumors of the choroid plexus, called papillomas, are sometimes associated with hydrocephalus.' In some of these cases,.a much greater than normal production of cerebrospinal fluid has been directly shown and is believed to be the cause of the hydrocephalus