Glial cells are support cells for neurons. With copious smooth ER, Schwann cells (in peripheral nerves) and oligodendrocytes (in the brain and spinal chord) are glial cells whose purpose is to produce myelin, a glycolipid insulator. The sections of axonal membrane between the mylenated sections, called the nodes of Ranvier, possess abundant Na+ channels. Saltatory propagation refers to the process of a nerve impulse jumping from node to node, which greatly speeds the wave of depolarization down the axon.

Saltatory conduction seems to be a favorite topic for the MCAT writers to touch on in discussions of nerve impulse transmission.