Lecture 5: BLOOD SUPPLY TO THE CNS; VENTRICLES & CEREBRAL SPINAL FLUID

Question 1

The ____ Arteries and ____ Arteries Supply the Brain

Answer 1

The Internal Carotid Arteries and Vertebral Arteries Supply the Brain.

The arterial supply of the brain and much of the spinal cord is derived from 2 pairs of vessels, the internal carotid arteries and the vertebral arteries.

Question 2

internal carotid arteries

Answer 2

Large distributing arteries, originating from the bifurcation of the common carotid artery and running cranially in the neck to enter the base of the skull and eventually the cranial vault.

The internal carotid artery branches at the circle of Willis into anterior and middle cerebral arteries. The 2 internal carotids account for 85% of cerebral blood flow and thus supply most of the blood to the brain.

Question 3

vertebral artery

Answer 3

One of the two major arteries that supply each side of the CNS (see internal carotid artery).

The vertebral artery originates as the first branch of the subclavian, runs cranially through foramina in cervical vertebrae, enters the base of the skull through the foramen magnum, and ascends along the medulla. At the pontomedullary junction it unites with its contralateral counterpart to form the basilar artery.

The vertebral artery and its posterior inferior cerebellar branch (PICA) supply blood to the medulla and inferior part of the cerebellum, and it supplies the cervical spinal cord via the posterior and anterior spinal arteries.

The vertebral system provides 20% of the brain’s arterial supply, supplying the brainstem and cerebellum, parts of the diencephalon, spinal cord, and occipital & temporal lobes.

Before joining the basilar artery, each vertebral artery gives rise to 3 branches: the posterior spinal artery, anterior spinal artery, and posterior inferior cerebellar artery.

Question 4

posterior inferior cerebellar branch (PICA)

Answer 4

A long, circumferential branch of the vertebral artery, supplying much of the inferior surface of the cerebellum; en route it sends shorter branches to the choroid plexus of the fourth ventricle and to much of the lateral medulla.

Question 5

The Internal Carotid Arteries Supply Most of the Cerebrum

Answer 5

An internal carotid artery ascends through each side of the neck, traverses the petrous temporal bone, passes through the cavernous sinus, and finally reaches the subarachnoid space at the base of the brain.

Question 6

ophthalmic artery

Answer 6

As the internal carotid artery leaves the cavernous sinus, it gives rise to the ophthalmic artery, which travels along the optic nerve to the orbit, where it supplies the eye, other orbital contents, and some nearby structures.

Question 7

middle cerebral artery

Answer 7

The more posterior of the 2 terminal branches of the internal carotid. The middle cerebral artery (MCA) runs laterally beneath the basal forebrain to reach the insula, where many branches arise and exit from the lateral sulcus.

It supplies the insula, most of the lateral surface of the cerebral hemisphere, and the anterior tip of the temporal lobe.

Most of the precentral and postcentral gyri are within this area of supply, so occlusion of a MCA causes major motor and somatosensory deficits. In addition, if the left hemisphere is the one involved, language deficits are almost invariably found.

Question 8

anterior cerebral artery

Answer 8

The more anterior of the 2 terminal branches of the internal carotid artery (the other is the middle cerebral). Anterior cerebral branches (the pericallosal and callosomarginal arteries) curve around and above the corpus callosum to supply orbital cortex, the medial surface of the frontal and parietal lobes, and an adjoining narrow band of cortex along their superior surfaces.

occlusion of an anterior cerebral artery causes restricted contralateral motor and somatosensory deficits (affecting the leg more than other parts of the body, because of the somatoto.occlusion of an anterior cerebral artery causes restricted contralateral motor and somatosensory deficits (affecting the leg more than other parts of the body, because of the somatotopic arrangement.

Question 9

anterior choroidal artery

Answer 9

A long, thin, branch of the internal carotid artery that accompanies the optic tract and supplies many structures along the way: the optic tract, choroid plexus of the inferior horn of the lateral ventricle, part of the cerebral peduncle, and deep regions of the internal capsule, thalamus, and hippocampus.

Question 10

posterior communicating artery

Answer 10

A short vessel connecting the posterior cerebral artery to the internal carotid, thereby forming one link in the circle of Willis.

Normally pressures in the internal carotid and posterior cerebral arteries are balanced so that little or no blood flows around the circle, but if one vessel is occluded the posterior communicating artery may allow anastomotic flow and thus prevent neural damage.

Question 11

posterior cerebral artery

Answer 11

A prominent artery that arises from the bifurcation of the basilar artery at the level of the midbrain. The posterior cerebral artery forms the posterior part of the circle of Willis andsupplies the rostral midbrain, posterior thalamus, medial occipital lobe, and inferior and medial surfaces of the temporal lobe.

part of the vertebral artery system

Question 12

anterior communicating artery

Answer 12

A short vessel at the anterior end of the circle of Willis interconnecting the 2 anterior cerebral arteries just in front of the optic chiasm; occasionally it may be very small or, very rarely, absent.

Question 13

pericallosal artery

Answer 13

A branch of the anterior cerebral artery that travels just above the corpus callosum.

Question 14

callosomarginal artery

Answer 14

A branch of the anterior cerebral artery that follows the cingulate sulcus.

Question 15

lenticulostriate arteries

Answer 15

A collection of about a dozen small branches of the middle cerebral artery along its course toward the lateral sulcus. They penetrate the overlying brain near their origin and pass upward to supply deep structures (internal capsule, globus pallidus, putamen).

The lenticulostriate arteries exemplify a large collection of small perforating or ganglionic arteries that arise from all arteries around the base of the brain; these narrow, thin-walled vessels are involved frequently in strokes that deprive deep cerebral structures of blood and thus cause neurological deficits out of proportion to their size.

Other groups of perforating arteries include the thalamogeniculate arteries, arising more posteriorly from the posterior cerebral artery. The anterior choroidal artery is, in effect, a very large perforating artery.

Question 16

perforating arteries

Answer 16

Small arteries, also known as ganglionic arteries, that arise from larger arteries in and near the circle of Willis and supply deep cerebral structures such as the diencephalon and basal ganglia.

Question 17

posterior perforated substances

Answer 17

The ventral surface of the rostral midbrain, between the cerebral peduncles. So named because numerous small perforating branches of the posterior cerebral artery penetrate the brain here, on their way to deep structures such as the thalamus.

Question 18

anterior perforated substances

Answer 18

The inferior surface of the forebrain, roughly between the orbital gyri and the hypothalamus. So named because numerous lenticulostriate and other small perforating branches penetrate the brain there.

The narrow, thin-walled vessels of the anterior perforated substance are involved frequently in strokes. The deep cerebral structures they supply are such that damage to these small vessels can cause neurological deficits out of proportion to their size.

For example, the somatosensory projection from the thalamus to the postcentral gyrus must pass through the internal capsule; damage to a small part of the internal capsule from rupture or occlusion of a perforating artery can cause deficits similar to those resulting from damage to a large expanse of cortex.

Question 19

The Vertebral-Basilar System Supplies the ____ and Parts of the ____ and ____

Answer 19

The Vertebral-Basilar System Supplies the Brainstem and Parts of the Cerebrum and Spinal Cord

The 2 vertebral arteries run rostrally alongside the medulla and fuse at the junction between the medulla and pons to form the midline basilar artery, which proceeds rostrally along the anterior surface of the pons.

Question 20

posterior spinal artery

Answer 20

A small branch of each vertebral artery that travels near the line of attachment of dorsal roots, supplying the posterior third of the spinal cord. Like the anterior spinal artery, it receives additional blood from the thoracic/abdominal aorta through numerous anastomoses with radicular arteries below the upper cervical region.

Question 21

anterior spinal artery

Answer 21

A single midline vessel that originates rostrally as two arteries (one from each vertebral) which shortly join and then course within theanterior median fissure along the entire spinal cord. It receives additional blood from the thoracic/abdominal aorta through numerous anastomoses with radicular arteries below the upper cervical region, and gives rise to hundreds of central and circumferential branches that supply the anterior two-thirds of the cord.

Question 22

anterior inferior cerebellar artery (AICA)

Answer 22

A long, circumferential branch of the basilar artery arising just above the union of the 2 vertebrals. It supplies anterior regions of the inferior cerebellar surface, including the flocculus, and parts of the caudal pons.

Question 23

superior cerebellar artery

Answer 23

A branch of the basilar artery that arises just caudal to its bifurcation. Long circumferential branches supply the superior surface of the cerebellum, and shorter branches supply much of the rostral pons and caudal midbrain.

Question 24

pontine arteries

Answer 24

The many smaller branches of the basilar artery, collectively called pontine arteries, supply the remainder of the pons.

One of these, the internal auditory or labyrinthine artery (which is often a branch of the AICA), though hard to distinguish from the others by appearance, is functionally important because it also supplies the inner ear.

Question 25

internal auditory artery

Answer 25

the internal auditory or labyrinthine artery (which is often a branch of the AICA), though hard to distinguish from the others by appearance, is functionally important because it also supplies the inner ear. Its occlusion can lead to vertigo and ipsilateral deafness.

It is a pontine artery

Question 26

posterior choroidal arteries

Answer 26

The posterior cerebral artery gives rise to several posterior choroidal arteries, which supply the choroid plexus of the third ventricle and the body of the lateral ventricle.

The anterior & posterior choroidal arteries form anastomoses in the vicinity of the glomus. The primary visual cortex is located in the occipital lobe, so occlusion of a posterior cerebral artery at its origin leads to visual field losses in addition to other deficits referable to the midbrain and diencephalon.

Question 27

The ____ Interconnects the Internal Carotid and Vertebral-Basilar Systems

Answer 27

The Circle of Willis Interconnects the Internal Carotid and Vertebral-Basilar Systems

Question 28

Circle of Willis

Answer 28

The anastomotic polygon at the base of the brain, consisting of parts of the internal carotid, anterior cerebral, and posterior cerebral arteries, interconnected by the anterior and posterior communicating arteries.

Normally, little blood flows around this circle because the appropriate pressure differentials are not present: the arterial pressure in the internal carotid arteries is about the same as that in the posterior cerebral arteries, so little blood flows through the posterior communicating arteries.

If one major vessel becomes occluded, either within the circle of Willis or proximal to it, the communicating arteries may allow critically important anastomotic flow and prevent neurological damage. Thus it would be theoretically possible (though unlikely) for the entire brain to be perfused by just one of the four major arteries that normally supply it.

Question 29

Imaging Techniques Allow Arteries and Veins to Be Visualized

Answer 29

Cerebral angiography utilizes the intravenous injection of iodinated dyes to make blood much more opaque than brain to x-rays. A cerebral angiogram is typically produced by introducing a catheter into the femoral artery, threading it (under fluoroscopic guidance) into the aortic arch, then steering the catheter tip into the artery of interest. In this way, the contrast material can be introduced into a single vertebral or internal carotid artery. Once the dye has been introduced, a rapid series of x-ray pictures can follow it as it flows through the artery, into capillaries, and then into veins.

Photographic or digital techniques can be used to remove bone images and reveal blood vessels in relative isolation.

Angiography was the first technique developed for making images of normal and diseased vessels, and for decades it was also a major tool for inferring changes in the brain that caused distortion of the vasculature. It still produces the most detailed images of cerebral vasculature.

Computed tomography (CT) and magnetic resonance imaging (MRI) are less invasive and can simultaneously show the CNS itself. Hence they have become widely used for imaging studies of blood vessels.

Question 30

angiography

Answer 30

Visualization of blood vessels, traditionally by injecting an x-ray dense substance before an x-ray or CT study.

Vessels can now also be seen using MRI (magnetic resonance angiography, or MRA).

Question 31

The Meninges Intro

Answer 31

Living brain is soft and mushy, despite the network of cytoskeletal proteins contained in neurons and glial cells. Without support of some kind, the CNS would be unable to maintain its shape, particularly as we walk and run around and occasionally bump our heads.

The brain & spinal cord are protected from outside forces by their encasement in the skull and vertebral column. In addition, the CNS is suspended within a series of 3 meninges, that stabilize the shape and position of the CNS in 2 different ways during head & 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 with the head.

Second, there is a layer of cerebrospinal fluid (CSF) 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 1500g in air effectively weighs less than 50g in its normal CSF environment, where it is easily able to maintain its shape. An isolated fresh brain, unsupported by its usual surroundings, becomes seriously distorted and may even tear under the influence of gravity.

Question 32

meninges

Answer 32

from the Greek word meninx, meaning “membrane”

The 3 Meningeal Layers: Dura Mater, Arachnoid, and Pia Mater

the CNS is suspended within a series of 3 membranous coverings, the meninges, that stabilize the shape and position of the CNS in 2 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 with the head.

Second, there is a layer of CSF 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 1500g in air effectively weighs less than 50g in its normal CSF 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.

Question 33

dura mater

Answer 33

The outermost and most substantial of the 3 meningeal layers.

Intracranial dura mater is firmly attached to the inside of the skull and serves as its periosteum.

Spinal dura mater forms a sac, separate from the vertebral periosteum, within which the spinal cord is suspended.

Because the dura mater is by far the most substantial of the meninges, it is also called the pachymeninx (from the Greek word pachy, meaning “thick,” as in thick-skinned pachyderms).

See epidural space, subdural space, and venous sinus.

Question 34

arachnoid mater

Answer 34

The thin meningeal layer that lines and is attached to the dura mater, and is interconnected with the pia mater by arachnoid trabeculae.

Question 35

pia mater

Answer 35

The innermost and thinnest of the 3 meninges, attached to the surface of the CNS and connected to the arachnoid by arachnoid trabeculae.

The pia mater is attached to the brain, following all its contours, and the space between the arachnoid and pia mater is filled with CSF.

Question 36

arachnoid and pia mater

Answer 36

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 (from the Greek word lepto, meaning “thin” or “fine”).

Question 37

leptomeninges

Answer 37

The pia mater and arachnoid considered together.

Question 38

The Dura Mater Provides Mechanical Strength

Answer 38

The cranial dura is a thick, tough, collagenous membrane that adheres firmly to the inner surface of the skull (dura is the Latin word for “hard,” as in durable).

It is often described as consisting of 2 layers: an outer layer that serves as the periosteum of the inner surface of the skull and an inner layer, the meningeal dura. Because these two layers are tightly fused, with no sharp histological boundary between them, the entire complex is ordinarily referred to as dura mater.

With few exceptions, no space exists on either side of the dura under normal circumstances because one side is attached to the skull and the other side adheres to the arachnoid. However, two potential spaces, the epidural and subdural spaces, are associated with the dura.

Epidural (or extradural) space refers to the potential space between the cranium and the periosteal layer.

Subdural space is commonly described as the potential space between dura and arachnoid. The dura and arachnoid are normally attached to each other, and when they appear to separate, the splitting actually occurs within the innermost cellular layers of the dura. Parts of these potential spaces can become actual fluid-filled cavities in certain pathological conditions, most often as a result of hemorrhage.

Question 39

Epidural Space

Answer 39

Generally, Epidural (or extradural) space refers to the potential space between the cranium and the periosteal layer.

1. In the cranium, the potential space between the periosteal layer of the dura mater and the inner surface of the skull. This potential space can become a real space in certain pathological conditions, most commonly as a result of tearing a meningeal artery.
2. In the vertebral canal, the normally present space between the dural sac surrounding the spinal cord and the vertebral periosteum.

Question 40

Subdural Space

Answer 40

Nominally, the potential space between the dura mater and the arachnoid (although when it occurs it is actually a splitting of the innermost layers of the dura).

This potential space can become a real space in certain pathological conditions, most commonly as a result of tearing a cerebral vein at the point where it enters a dural venous sinus.

Question 41

Subarachnoid Space

Answer 41

The normally present, CSF-filled space between the arachnoid and pia mater. Subarachnoid space is enlarged in cisterns, and is the space through which cerebral and spinal arteries &veins travel over the surface of the CNS.

Question 42

Dural Septa Partially Separate Different Intracranial Compartments

Answer 42

There are several places where the inner dural layer folds in on itself as a sheetlike protrusion into the cranial cavity, each called a dural reflection or dural septum.

The principal dural reflections are the falx cerebri between the cerebral hemispheres and the tentorium cerebelli between the cerebral hemispheres and the cerebellum.

Question 43

Dural Septa

Answer 43

Inward, sheetlike extensions of dura mater that define intracranial compartments.

See also diaphragma sellae, falx cerebri, and tentorium cerebelli.

Question 44

falx cerebri

Answer 44

from the Latin word falx, meaning “sickle”

The sickle-shaped dural septum that separates the two cerebral hemispheres.

a long, arched, vertical dural sheet that occupies the longitudinal fissure. Anteriorly it is attached to the crista galli of the ethmoid bone. The falx curves posteriorly and fuses with the middle of the tentorium cerebelli, ending posteriorly at the internal occipital protuberance. The inferior, free edge of the falx generally parallels the corpus callosum, but the falx is somewhat broader posteriorly than it is anteriorly, so the free edge comes closer to the splenium of the corpus callosum than to the genu. The anterior portion of the falx frequently contains a number of perforations.

Question 45

tentorium cerebelli

Answer 45

The dural septum between the cerebellum and the inferior surfaces of the occipital and temporal lobes.

The midbrain passes through a midline notch in the tentorium; this notch provides an aperture through which parts of the medial temporal lobe can herniate in response to expanding masses.

The tentorium cerebelli separates the superior surface of the cerebellum from the occipital and temporal lobes, defining supratentorial and infratentorial compartments. The supratentorial compartment contains the cerebrum, and the infratentorial compartment (or posterior fossa) contains the brainstem and cerebellum. Because the cleft between the cerebrum and cerebellum is not horizontal or flat, neither is the tentorium. Rather, it is roughly the shape of a bird with its wings extended in front of it; the bird’s body corresponds to the midline region where the falx joins the tentorium, and its wings correspond to the rest of the tentorium, which extends anteriorly. Posteriorly the tentorium is attached mainly to the occipital bone. This line of attachment continues anteriorly and inferiorly along the petrous temporal bone. The free edge of the tentorium also 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). It is of great clinical significance.

Question 46

falx cerebelli

Answer 46

a small reflection that partially separates the two cerebellar hemispheres.

Question 47

diaphragma sellae

Answer 47

small reflection that covers the pituitary fossa, admitting the infundibulum through a small perforation.

Question 48

The Dura Mater Contains Venous Sinuses That Drain the Brain

Answer 48

The two layers of the cranial dura are tightly fused, and there are no pathological conditions in which an intradural space (i.e., a space between the two layers) develops. However, at some edges of dural reflections (most often attached edges), the two layers are normally separated to form venous 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 along the posterior line of attachment of the tentorium, and the straight sinus 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 (also called the torcular, or torcular Herophili—“winepress of Herophilus”) 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 most of the blood from the superior sagittal sinus flows into the right transverse sinus, whereas blood from the straight sinus flows into the left transverse sinus. Occasionally, the two transverse sinuses are not interconnected at all.
In addition to receiving cerebral veins, the major dural sinuses are connected with several smaller sinuses. The inferior sagittal sinus, in the free edge of the falx cerebri, empties into the straight sinus. The small occipital sinus, in the attached edge of the falx cerebelli, empties into the confluence of the sinuses. The superior petrosal sinus, in the edge of the tentorium attached to the petrous temporal bone, carries blood from the cavernous sinus to the transverse sinus at the point where the latter leaves the tentorium to become the sigmoid sinus. The inferior petrosal sinus follows a groove between the temporal and occipital bones, carrying blood from the cavernous sinus to the internal jugular vein.

Question 49

dural venous sinuses

Answer 49

at some edges of dural reflections, the two dura layers are normally separated to form venous 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.

Question 50

superior sagittal sinus

Answer 50

A prominent venous sinus in the attached edge of the falx cerebri. Venous blood traveling posteriorly in this sinus meets up with blood from the straight sinus at the confluence of the sinuses.

Question 51

transverse sinuses

Answer 51

The laterally directly sinus on each side in the attached edge of the tentorium cerebelli, conveying blood from the confluence of the sinuses to the sigmoid sinus.

Question 52

straight sinus

Answer 52

The final recipient of blood flowing through deep cerebral veins. The straight sinus travels in the line of attachment of the falx cerebri and tentorium cerebelli and empties into the confluence of the sinuses.

Question 53

confluence of the sinuses

Answer 53

also called the torcular, or torcular Herophili—“winepress of Herophilus”

The meeting point, near the internal occipital protuberance, where venous blood arrives through the straight and superior sagittal sinuses and leaves through the transverse sinuses.

Question 54

sigmoid sinus

Answer 54

The continuation of each transverse sinus after it leaves the attached edge of the tentorium cerebelli. Named for the sinuous course it takes on the way to the internal jugular vein.

Question 55

several smaller sinuses

Answer 55

In addition to receiving cerebral veins, the major dural sinuses are connected with several smaller sinuses.

The inferior sagittal sinus, in the free edge of the falx cerebri, empties into the straight sinus.

The small occipital sinus, in the attached edge of the falx cerebelli, empties into the confluence of the sinuses.

The superior petrosal sinus, in the edge of the tentorium attached to the petrous temporal bone, carries blood from the cavernous sinus to the transverse sinus at the point where the latter leaves the tentorium to become the sigmoid sinus.

The inferior petrosal sinus follows a groove between the temporal and occipital bones, carrying blood from the cavernous sinus to the internal jugular vein.

Question 56

The Dura Mater Has Its Own Blood Supply

Answer 56

The arterial supply of the dura comes from a collection 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.

Anteriorly the dura is supplied by branches of the ophthalmic artery, and posteriorly it is supplied by branches of the occipital and vertebral arteries. Meningeal veins, also located in the periosteal layer, generally parallel the arteries.

Question 57

The Dura Mater Is Pain Sensitive

Answer 57

The brain itself, as well as the arachnoid and pia mater, is not sensitive to pain (in the sense that physical stimulation of these structures is not painful). Consequently, some neurosurgical procedures can be carried out without general anesthesia. The principal pain-sensitive intracranial structures are the dura mater and proximal portions of blood vessels at the base of the brain.

Most of the cranial dura, except for that of the posterior fossa, receives sensory innervation from the trigeminal nerve. Dural nerves follow the meningeal arteries and end near either the arteries or the dural sinuses. Except in the floor of the anterior cranial fossa, areas of dura between branches of meningeal arteries are innervated sparsely, if at all. Deformation of these endings is painful and is the cause of certain types of headache. Interestingly, the way the pain is perceived depends on whether endings near meningeal arteries or endings near dural sinuses are stimulated. In the former case, the pain is fairly accurately localized to the area of stimulation. In the latter case, as in the case of stimulating the dura in the floor of the anterior cranial fossa, the pain is referred to portions of the peripheral distribution of the trigeminal nerve, such as the eye, temple, or forehead.

The dura of the posterior fossa is supplied primarily by fibers of the vagus nerve and the second and third cervical nerves. As in the case of supratentorial dural innervation, the pain-sensitive endings in the posterior fossa are mostly located near dural arteries and venous sinuses. Deformation in these areas causes pain referred to the area behind the ear or the back of the neck.

Question 58

The Dura Mater Has an Arachnoid Lining

Answer 58

The arachnoid is a thin, avascular membrane composed of a few layers of cells interspersed with bundles of collagen. It is semitransparent and resembles a substantial cobweb, for which it is named (the Greek word arachne means “spider’s web”).

The outer portion of the arachnoid consists of several layers of flattened cells adhering to the innermost cellular layer of the dura mater. This interface region of cell layers, partially dura and partially arachnoid, contains no collagen.

Small strands of collagenous connective tissue called arachnoid trabeculae, covered with fibroblast-like arachnoid cells, leave this interface layer and extend to the pia, with which they merge. Arachnoid trabeculae help keep the brain suspended within the meninges, much the way the Lilliputians stabilized Gulliver’s position. Dural septa extend this suspension system inward, preventing the cerebral hemispheres from bumping up against each other or the cerebellum.

Question 59

arachnoid trabeculae

Answer 59

Thin strands of collagenous connective tissue that interconnect the arachnoid and pia, contributing to the mechanical suspension of the CNS within subarachnoid space.

Question 60

The Arachnoid Bridges over CNS Surface Irregularities, Forming Cisterns

Answer 60

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. Therefore there is a subarachnoid space, filled with CSF, between the arachnoid and the pia mater, because the pia closely covers all the external surfaces of the CNS. This is the only substantial fluid-filled space normally found around the outside of the brain. The subarachnoid space is very narrow 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 is the space between the inferior surface of the cerebellum and the posterior surface of the medulla. Regions like this, which contain a considerable volume of CSF, are called subarachnoid cisterns.

An example is the cerebellomedullary cistern on anatomical grounds, and because it is the largest cranial cistern, it is also referred to as cisterna magna. Other prominent cisterns include (1) the pontine cistern, which is located around the anterior surface of the pons and medulla and is continuous posteriorly with the cerebellomedullary cistern; (2) the interpeduncular cistern, which is located between the cerebral peduncles and contains the posterior part of the arterial circle of Willis. The superior cistern (also referred to as the quadrigeminal cistern and the cistern of the great cerebral vein), a radiological landmark above the midbrain.

The superior cistern is continuous laterally with a thin, curved layer of subarachnoid space on each side that partially encircles the midbrain before opening into the interpeduncular cistern. The combination of the superior cistern and these sheetlike extensions is referred to as the ambient cistern. The transverse cerebral fissure, a fingerlike extension of subarachnoid space between the fornix and the roof of the third ventricle, continues anteriorly from the superior cistern; it becomes trapped in this location as the cerebral hemispheres grow backward over the diencephalon during development.

Arachnoid trabeculae are particularly prominent in subarachnoid cisterns, sometimes coalescing into delicate membranes that partially occlude the subarachnoid space.

Question 61

cerebellomedullary cistern

AKA, the cisterna magna

Answer 61

A large subarachnoid cistern between the medulla and the inferior vermis.

the largest cranial cistern

Question 62

CSF Enters the Venous Circulation through _____

Answer 62

CSF Enters the Venous Circulation through Arachnoid Villi

The CSF contained in the subarachnoid space is generally separated from the venous blood in dural sinuses by a layer of arachnoid, a 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, arachnoid villi, herniate through the wall of the sinus. At these sites the connective tissue of the dura is mostly missing, and only a loose layer of arachnoid cells and a layer of endothelium intervene between the subarachnoid space and venous blood.

Question 63

arachnoid villi

Answer 63

Small evaginations of the arachnoid that protrude through the dura mater and into the lumen of a dural sinus of the brain (esp. the superior sagittal sinus), so that only loosely arranged arachnoid cells and endothelium intervene between subarachnoid space and venous blood. Arachnoid granulations are the major sites of reabsorption of cerebrospinal fluid into the venous system.

The villi are esp. numerous in laterally directed dilations of the superior sagittal sinus, called venous lacunes or lateral lacunes, but some are found along all the sinuses and even along some dural veins.

Question 64

arachnoid granulations
and
pacchionian bodies

Answer 64

Large arachnoid villi are called arachnoid granulations, and those that become calcified with age are referred to as pacchionian bodies.

Question 65

The arachnoid villi are the major sites of reabsorption of CSF into the venous system.

Answer 65

Functionally, they behave like one-way valves, allowing flow from subarachnoid space into venous blood but not in the reverse direction. Because CSF pressure is ordinarily greater than venous pressure, the villi normally allow continuous movement of CSF, more or less as though by bulk flow, into the sinuses; however, even if the pressure gradient reverses, the flow does not. The exact mechanism of this flow is a subject of debate.

Question 66

The Arachnoid Has a Barrier Function

Answer 66

The CNS is isolated in some respects from the rest of the body and lives in a tightly controlled environment. This control is achieved partly by a system of diffusion barriers between the extracellular spaces in and around the nervous system and extracellular spaces elsewhere.

One barrier is between the CSF in the subarachnoid space and the extracellular fluids of the dura. Marker substances injected into the middle meningeal artery spread throughout the dura but do not enter the subarachnoid space. The barrier resides in those cellular layers of the arachnoid in the interface region with the dura, where the cells are connected to one another by a series of tight junctions that occlude extracellular space.

Question 67

Pia Mater Covers the Surface of the CNS

Answer 67

the Latin word pia means “tender”

The pia mater is a second delicate membrane that, unlike the arachnoid, closely invests all external surfaces of the CNS. Pia follows all the contours of the brainstem and all the folds of the cerebral & cerebellar cortices, abutting the layer of astrocyte end-feet at the surface of the CNS. 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. For this reason, some speak of the entire leptomeningeal complex as one entity—the pia-arachnoid.

Cerebral arteries and veins travel in subarachnoid space, held against the pia by sheets and strands of connective tissue, before penetrating the brain. As each small vessel enters or leaves the brain, it carries with it a sleeve of perivascular space (or Virchow-Robin space). This space extends inward, filled with connective tissue and extracellular fluid, to the level at which the vessel becomes a capillary. The actual nature and extent of this microscopic space and the question of whether it provides a functional pathway of communication between the extracellular space around neurons and the subarachnoid space are matters of controversy. The traditional view holds that the connective tissue elements of the perivascular space arise as an inwardly directed cuff of pia that accompanies each vessel, but there are indications that the pia may in fact be left behind on the surface of the CNS. Similarly, some claim that the perivascular space is small and restricted, although there are indications that it may provide an important route for the movement of extracellular fluid that may be continuous with the cervical lymphatics through the adventitia of larger vessels.

Question 68

Differences between Cranial and Spinal Meninges:

Dura mater

Answer 68

Cranial:
Double layered, attached to inner calvarial surface

Spinal:
Single layered, suspended in vertebral canal

Question 69

Differences between Cranial and Spinal Meninges:

Epidural space

Answer 69

Cranial:
Potential space between periosteum and calvaria

Spinal:
Real space between dura and vertebral periosteum

1. Cranial epidural space is a potential space in almost all parts of the skull, whereas spinal epidural space is an actual space.
2. Cranial epidural space, when present in pathological conditions, is located between periosteum and cranium. Spinal epidural space is located between periosteum and dura. This spinal epidural space is filled with fatty connective tissue and a vertebral venous plexus.

Question 70

Differences between Cranial and Spinal Meninges:

Arachnoid

Answer 70

Cranial:
Attached to inner surface of dura

Spinal:
Attached to inner surface of dura

Question 71

Differences between Cranial and Spinal Meninges:

Pia mater

Answer 71

Cranial:
Attached to CNS surface

Spinal:
Attached to CNS surface, expanded as denticulate ligaments

Question 72

spinal dura mater

Answer 72

The spinal dura mater is a single-layered membrane, lacking the periosteal component of the cranial dura.

Question 73

cranial dura

Answer 73

The inner layer of the cranial dura is continuous at the foramen magnum with the spinal dural sheath, which is separated from the vertebral periosteum by an epidural space.

Question 74

spinal arachnoid

Answer 74

The spinal arachnoid, like its cranial counterpart, is closely applied to the inner surface of the dura, leaving a CSF-filled subarachnoid space between itself and the spinal cord.

The spinal dural sheath and its arachnoid lining end at about the second sacral vertebra, whereas the spinal cord itself ends at about the level of the disk between the first and second lumbar vertebrae. There is a large subarachnoid cistern, the lumbar cistern, between these two points. This is the favored site for sampling CSF, because a needle can be inserted here with relatively little risk of damaging the CNS.

Question 75

lumbar cistern

Answer 75

The CSF-filled subarachnoid cistern extending from the conus medullaris, at about vertebral level L1-L2, to the end of the spinal dural sac, at about vertebral level S2. Nerve roots of the cauda equina travel through the lumbar cistern.

Question 76

denticulate ligament

Answer 76

A thickened, lateral, serrated sheet of pia mater on each side of the spinal cord, with periodic extensions that attach to the arachnoid and dura, stabilizing the position of the cord within the dural sac.

The pial covering of the spinal cord is relatively thick and gives rise to a toothed longitudinal projection on each side called a denticulate (or dentate) ligament. The denticulate ligament anchors the spinal cord to the arachnoid and through it to the dura.

Question 77

filum terminale

Answer 77

literally, the “terminal thread”

The pial prolongation extending through the cauda equina, from the caudal end of the spinal cord (the conus medullaris) to the end of the dural sac surrounding the spinal cord.

Question 78

Bleeding Can Open Up Potential Meningeal Spaces

Answer 78

The 3 meningeal coverings of the brain have various real or potential spaces associated with them. There is no space between the pia and the brain, but there is a subarachnoid space between the pia and the arachnoid, along with potential subdural and epidural spaces. Both of these potential spaces can become actual fluid-filled spaces under certain conditions.

Question 79

conus medullaris

Answer 79

The tapering caudal end of the spinal cord, at about vertebral level L1-L2.

Question 80

Source of Blood:

Meningeal artery

Answer 80

Nature of Bleeding or Hematoma:

Epidural hematoma

Question 81

Source of Blood:

Dural venous sinus

Answer 81

Nature of Bleeding or Hematoma:

Subdural or epidural hematoma

Question 82

Source of Blood:

Vein at attachment to sinus

Answer 82

Nature of Bleeding or Hematoma:

Subdural hematoma

Question 83

Source of Blood:

Cerebral artery or vein

Answer 83

Nature of Bleeding or Hematoma:
Subarachnoid hemorrhage
Intraparenchymal hemorrhage
Intraventricular hemorrhage

Question 84

Tearing of Meningeal Arteries Can Cause an Epidural Hematoma

Answer 84

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 epidural hematoma.

As the hematoma expands, it compresses and distorts the underlying brain and is likely to be fatal unless promptly treated. Less commonly, tearing of a dural venous sinus can cause an epidural hematoma.

Question 85

The meningeal arteries run in the _____ layer of the dura.

Answer 85

The meningeal arteries run in the periosteal layer of the dura.

Question 86

Tearing of Veins Where They Enter Venous Sinuses Can Cause a Subdural Hematoma

Answer 86

Bleeding can also occur into the potential subdural space, resulting in a subdural hematoma. The most common cause of subdural hematomas is tearing of a cerebral vein as it penetrates the arachnoid and enters a dural sinus. This can result from rapid accelerations or decelerations of the head: the venous sinuses are attached to the skull and move with it, but the brain can lag behind, so a vein extending from the brain to a sinus can tear loose right at the point where it penetrates the arachnoid.

Some subdural hematomas are acute and produce symptoms much like those of an epidural hematoma, whereas others may progress very slowly and become surprisingly large before producing symptoms.