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Meningeal Coverings of the Brain and Spinal Cord
Living brain is soft and mushy, despite the network of cytoskeletal proteins contained in neurons and glial cells. Without support of some kind, the central nervous system (CNS) would be unable to maintain its shape, particularly as we walk and run around and occasionally bump our heads. 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 (from the Greek word meninx, meaning “membrane”), which stabilize the shape and position of the CNS 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 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 (e.g., gravity) to distort the brain. Thus a brain that weighs 1500 g in air effectively weighs less than 50 g 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 ( Fig. 4.1 ).
Effects of gravity and of partial flotation on brain. (A) An unfixed human brain in a vat of isotonic saline; normal shape is maintained. (B) The same brain in air, obviously distorted by its own weight.
(From Oldendorf W: The quest for an image of brain, New York, 1980, Raven Press.)
There Are Three Meningeal Layers: the Dura Mater, Arachnoid, and Pia Mater
The three meninges, from the outermost layer inward, are the dura mater, the arachnoid mater, and the pia mater ( Fig. 4.2 ). In common usage, the “mater” is often dropped and the three are referred to simply as the dura, arachnoid, and pia. The dura mater is by far the most substantial of the meninges, and for this reason is also called the pachymeninx (from the Greek word pachy meaning “thick,” as in thick-skinned pachyderms). The arachnoid and pia mater, in contrast, are thin and delicate. They are continuous with each other, often regarded as two parts of the single layer from which they are derived, and so are sometimes referred to together as the pia-arachnoid or the leptomeninges (from the Greek word lepto, meaning “thin” or “fine”). The dura mater is attached to the inner surface of the skull, and the arachnoid is adherent to the inner surface of the dura mater by dural border cells and pressure from the CSF. 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.
The same three meningeal layers continue around the spinal cord but have a slightly different arrangement there, so spinal meninges are described separately toward the end of this chapter.
The Dura Mater Provides Mechanical Strength
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 two 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 cranial dura under normal circumstances because one side is attached to the skull and the other side adheres to the arachnoid. a
a There are a few places near the inferior surface of the brain where a real intracranial epidural space between the inner layer of the dura and the intracranial periosteum exists. For example, the epidural venous plexus surrounding the dural sac of the spinal cord (described later in this chapter) continues through a similarly situated basilar venous plexus anterior to the brainstem and inferior to the diencephalon; this plexus in turn communicates with the cavernous sinus.
However, two potential spaces, the epidural and subdural spaces, are associated with the dura ( Table 4.1 ; see Fig. 4.14 ). Epidural (or extradural ) space refers to the potential space between the cranium and the periosteal layer. Subdural space is commonly described as potential space between dura and arachnoid and is sometimes said to contain a thin film of fluid. However, electron microscopic evidence indicates that 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 (see Fig. 4.14 ).
TABLE 4.1
Spaces in the Cranial Meninges
| Space | Location |
|---|---|
| Epidural | Potential space between dura and calvaria |
| Subdural | Potential space in the innermost dural layer, near the dura-arachnoid interface |
| Subarachnoid | Normally present, CSF-filled space; enlarged in cisterns |
Dural Folds Partially Separate Different Intracranial Compartments
There are several places where the inner dural layer separates from its external counterpart and protrudes into the cranial cavity. Such extensions of dura reflect back on themselves to form double-layered dural folds (dural reflections or dural septa). The principal dural folds are the falx cerebri b
b Named for its shape: falx is Latin for “sickle.”
between the two cerebral hemispheres, and the tentorium cerebelli between the cerebral hemispheres and the cerebellum ( Fig. 4.3 ). The falx cerebelli is a small reflection that partially separates the two cerebellar hemispheres. The diaphragma sellae, another small reflection, covers the pituitary fossa, admitting the infundibulum through a small perforation.
The falx cerebri is a long, arched, vertical dural fold (see Fig. 4.3 ; Fig. 4.4A ) that occupies the longitudinal fissure and separates the two cerebral hemispheres. 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 tentorium cerebelli separates the superior surface of the cerebellum from the occipital and temporal lobes, defining supratentorial and infratentorial compartments within the cranial vault. The supratentorial compartment contains the forebrain, and the infratentorial compartment (or posterior fossa ) contains the brainstem and cerebellum. Because the cleft between the cerebral hemispheres and the 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 would correspond to the midline region where the falx cerebri joins the tentorium, and its wings would correspond to the rest of the tentorium, which extends anteriorly (see Fig. 4.3 ). Posteriorly the tentorium is attached mainly to the occipital bone. This line of attachment continues anteriorly and inferiorly along the petrous part of the temporal bone. The free edge of the tentorium also curves anteriorly on each side, almost encircling the midbrain (see Figs. 4.3 and 4.4 ). This space in the tentorium, through which the brainstem passes, is called the tentorial notch (or tentorial incisure ). It is of great clinical significance, as discussed later in this chapter.
The Dura Mater Contains Venous Sinuses That Drain the Brain
As noted previously, 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 the attached edges of dural folds, 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 ( Fig. 4.5 ). The locations of the major sinuses can be inferred by considering the lines of attachment of the dural folds. The superior sagittal sinus is found along the attached edge of the falx cerebri, the left and right transverse sinuses along the posterior line of attachment of the tentorium cerebelli, and the straight sinus along the line of attachment of the cerebral falx and tentorium cerebelli to each other ( Figs. 4.6 and 4.7 ). All four of these sinuses meet in the confluence of the sinuses (also called the torcular, or torcular Herophili —“the 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 as the sigmoid sinus, which proceeds anteriorly and inferiorly through an S -shaped course and empties into the internal jugular vein (see Figs. 4.6 and 4.7 ; see also Fig. 6.32 ).
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 (see Figs. 4.6 and 4.7 ; see also Fig. 6.32 ). 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 (see Fig. 6.32 ). 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 (see Fig. 4.7 ). The superior petrosal sinus, in the edge of the tentorium attached to the petrous part of the 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 (see Fig. 4.6 ). The inferior petrosal sinus follows a groove between the temporal and occipital bones, carrying blood from the cavernous sinus, it passes through the jugular foramen to open into the internal jugular vein.
The Dura Mater Has Its Own Blood Supply
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. 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.
The Dura Mater Is Pain Sensitive
Remarkably, the brain as well as the arachnoid and pia mater are not sensitive to pain (in the sense that physical stimulation of these structures is not painful). As a consequence, 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. Nerves innervating the dura 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 dural endings is painful and is the cause of certain types of headache ( Fig. 4.8 ). Interestingly, the way the pain is perceived depends on whether nerve 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 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 cranial fossa is supplied primarily by fibers of the vagus nerve and the second and third cervical nerves. c
c The sensory component of the first cervical nerve typically is minor and does not contribute substantially to this innervation.
As in the case of supratentorial dural innervation, the pain-sensitive endings in the posterior cranial fossa are located primarily 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.
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