Keywords
meningeal layers, arachnoid villi, cisterns, blood brain barriers, CNS herniations, epidural space, CNS bleeds
Chapter Outline
There Are Three Meningeal Layers: The Dura Mater, Arachnoid, and Pia Mater, 22
The Dura Mater Provides Mechanical Strength, 22
Dural Septa Partially Separate Different Intracranial Compartments, 22
The Dura Mater Contains Venous Sinuses That Drain the Brain, 22
The Dura Mater Has Its Own Blood Supply, 23
The Dura Mater Is Pain-Sensitive, 23
The Dura Mater Has an Arachnoid Lining, 23
The Arachnoid Bridges Over CNS Surface Irregularities, Forming Cisterns, 23
CSF Enters the Venous Circulation Through Arachnoid Villi, 23
The Arachnoid Has a Barrier Function, 23
Pia Mater Covers the Surface of the CNS, 24
The Vertebral Canal Contains Spinal Epidural Space, 24
Bleeding Can Open Up Potential Meningeal Spaces, 24
Parts of the CNS Can Herniate From One Intracranial Compartment Into Another, 24
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.
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. 4.1 ), 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, since the two cerebral hemispheres are interconnected just 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.
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, as well as CSF from the brain eventually reaches these sinuses.
The Dura Mater Has Its Own Blood Supply
In addition to the venous sinuses that drain the brain, the dura contains a mostly separate set of arteries and veins that help with its periosteal role for the skull (see Fig. 4.1 ). Meningeal arteries can be important clinically because tearing one can cause bleeding between the skull and dura (epidural bleeding, described later).
The Dura Mater Is Pain-Sensitive
The dura and some subarachnoid blood vessels are the only pain-sensitive intracranial structures. So inflammation (e.g., meningitis) or traction (e.g., by an expanding mass) causes headache. The brain itself has no pain-sensitive endings.
The Dura Mater Has an Arachnoid Lining
The arachnoid is attached to the skull by virtue of its attachment to the dura. The pia is attached to the CNS. Hence, the arachnoid trabeculae that interconnect the pia and arachnoid form a relatively weak mechanical suspension system for the CNS. The CNS is completely immersed in the CSF that fills subarachnoid space. The partial flotation effect of this CSF reduces the effective weight of the CNS to the point that the meningeal suspension system can support the brain and spinal cord (see Fig. 4.1 ).
The Arachnoid Bridges Over CNS Surface Irregularities, Forming Cisterns
Subarachnoid space is filled with CSF, contains the arteries and veins that supply and drain the CNS, and is traversed by arachnoid trabeculae. Large pockets of subarachnoid space, corresponding to major irregularities in the surface of the CNS, are called subarachnoid cisterns . Prominent intracranial cisterns include cisterna magna (between the medulla and the inferior surface of the cerebellum) and the superior cistern (above the midbrain).
CSF Enters the Venous Circulation Through Arachnoid Villi
Arachnoid outpouchings poke through holes in the walls of venous sinuses as arachnoid villi ( Fig. 4.2 ). Here only an arachnoid layer and an endothelial layer separate CSF and venous blood, the arachnoid barrier (see next section) is missing, and CSF can move directly from subarachnoid space into venous blood. Arachnoid villi act like mechanical flap valves, so when CSF pressure is higher than venous pressure (the usual situation), CSF moves into the venous system. If the reverse situation occurs, the villi snap shut and venous blood doesn’t enter subarachnoid space.
