myelin sheath

Meaning: Myelin is layered plasma membrane, rich in lipid, wrapped around an axon. In the PNS one Schwann cell myelinates one segment of one axon. In the CNS one oligodendrocyte extends processes to several axons. The gaps between segments are nodes of Ranvier, where voltage-gated Na⁺ channels are concentrated. Myelin raises membrane resistance and lowers capacitance, so little current leaks out. Depolarizing current spreads passively and quickly inside the axon to the next node, where it brings the membrane to threshold and the action potential is regenerated. This is saltatory conduction. It also saves ATP, because fewer ions cross the membrane and the Na⁺/K⁺ pump has less to restore.

On the MCAT: A passage may describe demyelination, such as multiple sclerosis in the CNS, and ask you to predict slower or failed conduction because current leaks and may not reach threshold at the next node. You may compare myelinated and unmyelinated axons, or axons of different diameter. Conduction is faster in myelinated axons and in larger-diameter axons. Myelinated axon tracts look white, while cell bodies make up gray matter.

Common confusion: Myelin changes the speed of the action potential, not its size. Amplitude stays the same, because action potentials are all-or-none. Ion exchange does not happen across the membrane under the myelin, but at the nodes. Also keep the cells straight: Schwann cells are PNS, oligodendrocytes are CNS.