ABSTRACT
Schwann cells are the glial cells of the peripheral nervous system (PNS). The Schwann cell received its name from the seminal work by Professor Theodor Schwann1 on the cellular construction of animal tissues. Using teased nerve fibers from fetal pig sciatic nerve, he recognized that each fiber contained rows of closely aligned nuclei and correctly concluded that the mature nerve was produced by the growth and lengthening of these cells during development.2 We now know that during this process, the Schwann cells ensheathe all axons in the PNS, either by wrapping them with the greatly extended and highly compacted multilamellar extension of their plasma membrane, the myelin sheath,3 or by sequestering “nonmyelinated” axons within invaginations of the Schwann cell plasma membrane. Most of what we know about these cells and their relationship to nervous system function is with regards to myelin and its role in the rapid efficient conduction of impulses down axons. There are periodic interruptions between adjacent segments of myelin, termed nodes of Ranvier, where axonal membranes are exposed. The high resistance of myelin serves as an electrical insulator, and this property, coupled with the high concentration of voltage-gated sodium channels localized at the nodes, allows impulse conduction to jump from node to node; this is termed saltatory conduction (L. saltare, to leap). In contrast, conduction in unmyelinated axons involves a continuous wave of membrane depolarization that moves down the axon. The presence of the myelin sheath
thus greatly increases the efficiency of nervous system operation, facilitating conduction while conserving metabolic energy and space.4
