Meningeal Coverings of the Brain and Spinal Cord

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4 Meningeal Coverings of the Brain and Spinal Cord

The meninges form a major part of the mechanical suspension system of the CNS, necessary to keep it from self-destructing as we move through the world. In addition, one layer of the meninges participates in the system of barriers that effectively isolates the extracellular spaces in the nervous system from the extracellular spaces in the rest of the body.

There Are Three Meningeal Layers: The Dura Mater, Arachnoid, and Pia Mater

The dura mater, or dura, is a thick connective tissue membrane that also serves as the periosteum of the inside of the skull (Fig. 4-1). The arachnoid and the pia mater (or pia) are much thinner collagenous membranes. The arachnoid is attached to the inside of the dura and the pia is attached to the outer surface of the CNS. Hence the only space normally present between or around the cranial meninges is subarachnoid space (not counting the venous sinuses found within the dura). The arrangement of spinal meninges is slightly different, as described later in this chapter.

Figure 4-1 Overview of the meninges. The dark bars interconnecting the innermost arachnoid cells indicate the bands of tight junctions that form the arachnoid barrier.

The Dura Mater Provides Mechanical Strength

The thickness and abundant collagen of the dura make it the mechanical link that connects the skull to the delicate strands of arachnoid (arachnoid trabeculae) that suspend the CNS in its bath of cerebrospinal fluid (CSF). This combination of partial flotation of the CNS in subarachnoid CSF, together with mechanical suspension by the skull-dura-arachnoid-arachnoid trabeculae-pia-CNS connections (see Fig. 4-3), stabilizes the fragile CNS during routine head movements.

Dural Septa Partially Separate Different Intracranial Compartments

The skull-dura-CNS suspension just described would not prevent different brain parts (e.g., the two cerebral hemispheres, or the cerebellum and the occipital lobes) from slapping against each other during head movements. This is addressed by sheetlike extensions of the dura that form dural reflections or dural septa, carrying the suspension system inward. The two most prominent of these are the falx cerebri, separating the two cerebral hemispheres, and the tentorium cerebelli, separating the cerebellum and brainstem (below it) from the forebrain (above it). These can’t be complete separations because the two cerebral hemispheres are interconnected beneath the falx by the corpus callosum, and the brainstem continues upward through the tentorial notch into the diencephalon. The free edges of these dural reflections are sites at which expanding masses can cause herniation of part of the brain from one compartment into another (THB6 Figure 4-19, p. 96).

The Dura Mater Contains Venous Sinuses That Drain the Brain

Along attached edges of dural reflections (and along some free edges) there is an endothelium-lined venous channel within the dura, called a dural venous sinus. Prominent sinuses are the superior sagittal sinus (along the attachment of the falx cerebri to the skull), left and right transverse sinuses (along the attachment of the tentorium cerebelli to the skull), and the straight sinus (along the attachment of the falx and tentorium to each other). All four meet at the back of the head in the confluence of the sinuses (also called the torcular, or torcular Herophili, Greek for “the winepress of Herophilus”). Venous drainage from the brain eventually reaches these sinuses (THB6 Figure 6-31, p.144).

The Dura Mater Has Its Own Blood Supply

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