myelin

Meaning: Myelin is a lipid-rich insulating sheath wrapped in segments around the axons of many neurons. Glial cells make it: oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. The sheath raises membrane resistance and lowers capacitance, so current spreads farther along the axon before it leaks out. Voltage-gated Na⁺ channels are concentrated at the gaps between segments, the nodes of Ranvier, so the action potential is regenerated there and appears to jump from node to node. This is saltatory conduction. It is faster and uses less energy than continuous conduction along a bare axon.

On the MCAT: Expect a passage on demyelination, such as multiple sclerosis, with data showing slower conduction velocity or failed transmission. You may compare myelinated and unmyelinated axons, or recall that a larger axon diameter also speeds conduction. Myelinated fibers form white matter, while cell bodies form gray matter.

Common confusion: Myelin changes the speed of the signal, not its size. Action potentials are all-or-none, so amplitude stays the same. The ion channels and the regeneration of the signal are at the nodes, not under the myelin.