The brain and spinal cord are covered by three layers (meninges) of mesodermal origin: the tough dura mater is outermost, followed by the arachnoid and, lastly, the pia mater. The pia matter lies directly on the surface of the brain and spinal cord. Between the dura mater and the arachnoid is the (normally only virtual) subdural space; between the arachnoid and the pia mater is the subarachnoid space. The subarachnoid space contains the CSF.
It flows through the ventricular system (internal CSF space) and then enters the subarachnoid space surrounding the brain and spinal cord (external CSF space). It is resorbed in the arachnoid granulations of the superior sagittal sinus and in the perineural sheaths of the spinal cord. An increased volume of CSF (because of either diminished resorption or—less commonly—increased production) manifests itself in increased CSF pressure and enlargement of the ventricles (hydrocephalus).
The three meninges (dura mater, arachnoid, pia mater) are depicted in ▶Fig. 10.1 and ▶Fig. 10.2. The dura mater is also called the pachymeninx (“tough membrane”), while the arachnoid and pia mater are collectively called the leptomeninges (“delicate membranes”).
Outer and inner layers. The outer layer of the cranial dura mater is the periosteum of the inside of the skull. The inner layer is the actual meningeal layer; it forms the outer limit of the very narrow subdural space. The two dural layers separate from each other at the sites of the dural sinuses. Between the superior and inferior sagittal sinuses, a double fold of the inner dural layer forms the falx cerebri, which lies in the midsagittal plane between the two cerebral hemispheres; the falx cerebri is continuous with the tentorium, which separates the cerebellum from the cerebrum. Other structures formed by a double fold of inner dura mater are the falx cerebelli separating the two cerebellar hemispheres, the diaphragma sellae and the wall of Meckel’s cave, which contains the gasserian (trigeminal) ganglion.
Blood supply of the dura mater. The dural arteries are relatively large in caliber because they supply the bony skull as well as the dura mater. The largest is the middle meningeal artery, whose branches are distributed over the entire lateral convexity of the skull. This artery is a branch of the maxillary artery, which is, in turn, derived from the external carotid artery; it enters the skull through the foramen spinosum. The anterior meningeal artery is relatively small and supplies the midportion of the frontal dura mater and the anterior portion of the falx cerebri. It enters the skull through the anterior portion of the cribriform plate. It is a branch of the anterior ethmoidal artery, which is, in turn, a branch of the ophthalmic artery; it therefore carries blood from the ICA. The posterior meningeal artery enters the skull through the jugular foramen to supply the dura mater of the posterior cranial fossa.
The middle meningeal artery makes an anastomotic connection in the orbit to the lacrimal artery, a branch of the ophthalmic artery. The ophthalmic artery branches off the ICA near the internal aperture of the optic canal. Thus, in some cases, the central retinal artery can obtain blood by way of the middle meningeal artery, even if the ophthalmic artery is proximally occluded.
Spinal dura mater. The two layers of the dura mater adhere tightly to each other within the cranial cavity but separate from each other at the outer rim of the foramen magnum. The outer dural layer continues as the periosteum of the spinal canal, while the inner layer forms the dural sac enclosing the spinal cord. The space between the two layers is called the epidural or extradural space, even though it is, strictly speaking, inside the dura mater. It contains loose connective tissue, fat, and the internal venous plexus (▶Fig. 10.2, ▶Fig. 11.20). The two layers of the spinal dura mater join where the spinal nerve roots exit from the spinal canal through the intervertebral foramina. The lower end of the dural sac encloses the cauda equina and terminates at the S2 level (▶Fig. 3.22). Its continuation below this level is the filum of the dura mater, which is anchored to the sacral periosteum by the fibrous coccygeal ligament.
Orbital dura mater. A similar division of the two layers of the dura mater is found in the orbit, which the dura mater reaches from the cranial cavity by extension along the optic canal. The outer dural layer is the periosteal lining of the bony orbit. The inner dural layer surrounds the optic nerve, together with its pia mater and arachnoid, as well as the perioptic subarachnoid space in between. This space communicates with the subarachnoid space of the cranial cavity. The inner dural layer is continuous with the sclera as the optic nerve enters the globe.
Papilledema. The dural sheath of the optic nerve can be stretched if elevated intracranial pressure is transmitted to the perioptic subarachnoid space. Retrobulbar stretching of the dural sheath is a major factor in the development of papilledema. Another cause of papilledema is acute intracranial SAH (due to a ruptured aneurysm or vascular malformation) with blood extending into the perioptic subarachnoid space.
Innervation. The dura mater above the tentorium is innervated by branches of the trigeminal nerve, and its infratentorial portion by branches of the upper cervical segmental nerves and the vagus nerve. Some of the dural nerves are myelinated, while others are unmyelinated. Their endings evidently respond to stretch, because mechanical stimulation of the dura can be consciously felt, and is often painful. The afferent fibers accompanying the meningeal arteries are particularly sensitive to pain.
The arachnoid of both the brain and the spinal cord is a thin, delicate, avascular membrane closely applied to the inner surface of the dura matter. The space between the arachnoid and the pia mater (the subarachnoid space) contains the CSF. The arachnoid and the pia mater are connected to each other across this space by delicate strands of connective tissue. The pia mater adheres to the surface of the brain along all of its foldings; thus, the subarachnoid space is narrower in some places, and wider in others. Enlargements of the subarachnoid space are called cisterns. The cranial and spinal subarachnoid spaces communicate directly with each other across the foramen magnum. Most of the arterial trunks supplying the brain, and most of the cranial nerves, run in the subarachnoid space.
Cisterns. The subarachnoid cisterns of the head have individual names, e.g., the cerebellomedullary cistern, also called the cisterna magna. The more important named cisterns are depicted in ▶Fig. 10.4.
The pia mater consists of thin layers of mesodermal cells resembling endothelium. Unlike the arachnoid, it covers not just the entire externally visible surface of the brain and spinal cord but also all of the hidden surfaces in the depths of the sulci (▶Fig. 10.1 and ▶Fig. 10.2). It is fixed to the central nervous tissue beneath it by an ectodermal membrane consisting of marginal astrocytes (pial-glial membrane). Blood vessels that enter or leave the brain and spinal cord by way of the subarachnoid space are surrounded by a funnel-like sheath of pia mater. The space between a blood vessel and the pia mater around it is called the Virchow–Robin space.
The sensory nerves of the pia mater, unlike those of the dura mater, do not respond to mechanical or thermal stimuli, but they are thought to respond to vascular stretch and changes in vascular wall tone.
The ventricular system (▶Fig. 10.3) consists of the two lateral ventricles (each of which has a frontal horn, central portion = cella media, posterior horn, and inferior horn); the narrow third ventricle, which lies between the two halves of the diencephalon; and the fourth ventricle, which extends from pontine to medullary levels. The lateral ventricles communicate with the third ventricle through the interventricular foramina (of Monro); the third ventricle, in turn, communicates with the fourth ventricle through the cerebral aqueduct. The fourth ventricle empties into the subarachnoid space through three openings: the single median aperture (foramen of Magendie) and the paired lateral apertures (foramina of Luschka).
Properties of the cerebrospinal fluid. The normal CSF is clear and colorless, containing only a few cells (up to 4/µL if obtained by lumbar puncture) and relatively little protein (ratio of CSF albumin to serum albumin = 6.5 ± 1.9 × 10–3). Its composition differs from that of blood in other respects as well. The CSF is not an ultrafiltrate of blood; rather, it is actively secreted by the choroid plexus, mainly within the lateral ventricles. The blood within the capillaries of the choroid plexus is separated from the subarachnoid space by the so-called blood–CSF barrier, which consists of vascular endothelium, basal membrane, and plexus epithelium. This barrier is permeable to water, oxygen, and carbon dioxide, but relatively impermeable to electrolytes and completely impermeable to cells. Some pathological processes in the brain (hypoxia, inflammation, infection, neurodegenerative diseases) can lead to the appearance in the CSF of proteins that are otherwise not found in it at all, or only in trace amounts.
The circulating CSF volume is generally between 130 and 150 mL. Every 24 hours, 400 to 500 mL of CSF are produced; thus, the entire CSF volume is exchanged three or four times daily. The CSF pressure (note that the CSF pressure is not the same as the intracranial pressure) in the supine position is normally 70 to 120 mm H2O.
Infectious or neoplastic processes affecting the CNS alter the composition of the CSF in characteristic ways, as summarized in ▶Table 10.1.