The Central Nervous System Is Protected by the Meninges and Cerebrospinal Fluid
The entire CNS is surrounded by three protective layers called meninges: the pia mater, arachnoid, and dura mater (see Figure 3-1). The innermost layer, lying next to the CNS, is the pia mater, which is a single layer of fibroblast cells joined to the outer surface of the brain and spinal cord.
The middle layer, the arachnoid, so named because of its spiderweb appearance, is a thin layer of fibroblast cells that traps cerebrospinal fluid between it and the pia mater (in the subarachnoid space). 'Γhe outermost meningeal layer, the dura mater, is a much thicker layer of fibroblast cells that protects the CNS. Within the brain cavity of the skull, the dura mater is often fused with the inner surface of the bone.Cerebrospinal fluid (CSF) is a clear, colorless fluid found within the subarachnoid space, the central canal of the spinal cord and the ventricular system of the brain (see Chapter 15). CSF is produced primarily in the ventricles of the brain, flows down a pressure gradient from the ventricles to the subarachnoid space, where it bathes the surface of the CNS, and from the subarachnoid space eventually passes into the venous system. It is a dynamic fluid, being replaced several times daily. Because CSF can exchange freely with the extracellular fluid of the CNS, it is an important determinant of the neuronal microenvironment, both carrying away metabolic waste and providing certain micronutrients. CSF also serves as a shock absorber for the CNS during abrupt body movement.