BIOG 1105-1106 | Cornell Introductory Biology, Individualized Instruction

One class of glial cells wrap around and around the axons of many neurons in the CNS to form a heavily lipid myelin sheath (see Figure). The sheath is actually layer upon layer of the plasma membrane of the glial cells. In the peripheral nervous system, Schwann cells often perform much the same function, wrapping and enveloping the axon or dendrite to form a myelin sheath.

By insulating the axon, myelin sheaths reduce the area of neuron membrane that must be kept “charged,” thereby greatly reducing the associated metabolic cost, and substantially speeding up conduction of impulses. The myelin sheath is interrupted at intervals by nodes of Ranvier, points where one glial or Schwann cell ends and another begins.

Development of the myelin sheath. (A) Initially the unmyelinated axon lies in a pocket of the glial cell. (B) The glial-cell membrane then begins to coil around the axon. (C) The membrane winds tightly around the axon, forming a myelin sheath.

Myelin sheath. The myelin sheath is composed of hundreds of layers of adjacent membranes formed by the Schwann cell that wraps round and round the axon of the neuron. Where the two layers of Schwann cell membrane meet they form the mesaxon; as the Schwann cell grows, the mesaxon wraps round the axon of the neuron (a). Such a repeating pattern is very suitable for X-ray diffraction analysis, which confirms that myelin consists of a biomolecular leaflet of phospholipids and cholesterol coated on each outer surface by a layer of protein. (b) is a transmission electron micrograph of an early stage in the formation of the myelin sheath. The mesaxon (ma) has formed a few turns round the axon (a). The outer membrane of the Schwann cells (sm) separates the extracellular space (e) from the cytoplasm of the Schwann cell (c). (c) is a transmission electron micrograph showing a vertical section of a myelinated neuron at a later stage than (b). There are many layers of myelin (my).

Node of Ranvier. Vertical section through a node of Ranvier. The myelin sheath is interrupted at regular intervals of 2-3 mm by the nodes of Ranvier (nr). These are the only places where the axolemma (am) (the plasma membrane surrounding the axoplasm) is exposed to the extracellular fluid. Elsewhere the axoplasm is shielded by the myelin sheath (my). Only at the nodes can ion exchange take place, and as a result myelinated nerve conducts impulses much more rapidly than unmyelinated nerve.