demyelinating disease

Meaning: Myelin is a lipid-rich insulating sheath made by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. It raises membrane resistance, so depolarizing current spreads along the axon to the next node of Ranvier, where voltage-gated Na⁺ channels are clustered (saltatory conduction). In a demyelinating disease the sheath is damaged, current leaks out through the exposed membrane, and conduction slows or fails. Multiple sclerosis, an immune attack on CNS myelin, is the standard example.

On the MCAT: A passage may describe a patient or animal model with slowed nerve conduction and ask you to predict the result: lower conduction velocity, weaker or blocked signals, and effects in white matter tracts, which are the myelinated axons. Questions may also compare myelinated and unmyelinated axons, or ask which glial cell is involved in the CNS versus the PNS.

Common confusion: Demyelination is not the same as death of the neuron. The axon and cell body are often intact at first, although lasting damage can follow. Myelin also does not change the size of an action potential; it changes how fast and how reliably the signal propagates.