CNS Compartments: The Anatomy and Physiology of the Cerebrospinal Fluid

Abstract

This chapter revises the anatomy and physiology of the central nervous system (CNS) compartments. The CNS compartments consist of the brain and spinal cord tissue, which are divided into the intracellular space containing intracellular fluids and the extracellular space containing the interstitial fluid, as well as the cerebrospinal fluid (CSF) space, which is located within the ventricular system, in the cranial and spinal subarachnoid space, and in the perivascular spaces. The meninges are three protective membranes surrounding the brain and spinal cord, providing physical support, shock absorption, and protection from infection. There are three layers of meninges, from outermost to innermost: the dura mater, the arachnoid mater, and the pia mater. Recently, a novel layer in the meninges with immunological functions has been described and named the subarachnoid lymphatic-like membrane. The tissues and fluids of the CNS are safeguarded from exogenous and endogenous compounds present in the systemic circulation by the blood–brain barrier and blood–CSF barrier. These barriers are critical in maintaining the proper function of the CNS. Finally, we will focus on the glymphatic system, the CSF flow and clearance, and its driving mechanisms. Glymphatic flow occurs through the paravascular space when CSF flows through astroglial AQP4 channels and mixes with ISF to flow toward the venous system, where the CSF-ISF mixture enters the paravascular space and is cleared from the brain parenchyma. This flow depends on arterial pulsations to create a pressure gradient for the mixture to flow down. Lastly, PBM plays an important role in regulating arterial motion and, consequently, CSF-ISF flow. Under normal conditions, CSF is secreted via a two-step process, involving plasma filtration and ion pumping. CSF is mainly absorbed in the cranial arachnoid granulations, but the absorptive process is influenced by changes in the subarachnoid space pressure. The complex CSF flow involves the cardiovascular system, nerve roots, and arachnoid trabeculae. We will learn that CSF flow might be voluntarily modified by controlling breathing, neural activity, and exercise.