unmyelinated

Meaning: An unmyelinated axon has no myelin wrapped around it, so its membrane is exposed along its length and voltage-gated Na⁺ and K⁺ channels are spread along it. Each patch of membrane depolarizes the next patch, which is continuous conduction. In a myelinated axon, myelin (made by oligodendrocytes in the CNS and Schwann cells in the PNS) raises membrane resistance and lowers capacitance. Current then spreads passively to the next node of Ranvier, where channels are concentrated, and the signal appears to jump (saltatory conduction). A larger axon diameter also speeds conduction because it lowers internal resistance.

On the MCAT: Passages compare conduction velocity in myelinated and unmyelinated fibers, or describe demyelination (as in multiple sclerosis) and ask you to predict the result: slower or failed conduction as current leaks out through the membrane. Expect questions where you must compare two axons by both myelination and diameter. The squid giant axon is unmyelinated but conducts quickly because it is so wide. Unmyelinated conduction also involves more ion flux across membrane, so the Na⁺/K⁺ pump has more work to restore gradients afterward.

Common confusion: Unmyelinated does not mean the neuron cannot fire. It still produces full-size action potentials. Myelin changes how fast and how efficiently the signal travels, not whether the spike occurs or how large it is. Also, white matter looks white because of myelin, while gray matter is rich in cell bodies and dendrites.