White Matter: Basic Principles of Axonal Organization and Function

Abstract

White matter occupies nearly half of the human brain and accommodates a variety of pathways interconnecting different areas of the CNS via predominantly myelinated axons. Being topographically segregated from gray matter, the white matter can be viewed, in a functional sense, as the “nerves within the brain”. However, unlike peripheral nerves, the white matter does not have protective layers of connective tissue around it or its constituent axonal bundles, which makes them potentially susceptible to diffusional influences from neighboring tissues. White matter receives less blood supply compared to gray matter, and its axons are well adapted to minimal energy supply while maintaining high fidelity delivery signals from one gray matter area to another. The CNS myelinated axons are well designed space-savers, having smaller diameters compared to PNS and more compact myelin sheaths. Myelin covers nearly 99 % of the length of myelinated axons, the rest being nodes of Ranvier that serve as “relay stations” for saltatory propagation of action potentials. The axonal conduction velocities in white matter are well tuned to specific physiological needs and provide timely delivery of signals to target neurons for summation with other synaptic inputs. A minor decrease in conduction velocity or in the number of conducting axons due to injury or stroke may have catastrophic consequences due to disturbed coordination of signal arrivals to target neurons and their summation with other synaptic inputs, potentially halting further transfer of signals to other neurons. The myelin-forming cells of the CNS, the oligodendrocytes, myelinate different numbers of axons depending on their calibers, ranging from one in case of large (10 μm) axons to 50–60 in case of smallest (<0.5 μm) axons. Myelinating multiple axons by single oligodendrocyte has its drawback, as an injury to one oligodendrocyte may have a “multiplication effect,” shutting down a number of axons at once. The chapter discusses in detail the organization of myelin sheaths and their relationships with axons and periaxonal glia.