Somatic preganglionic axons are typically _______ and ________. Therefore have a ______ conduction velocity.

• A. myelinated and larger; faster
• B. unmyelinated and smaller; slower
• C. myelinated and smaller; slower
• D. unmyelinated and larger; faster

Answer: (A)

Conduction velocity is faster when axons are myelinated and have a larger diameter because myelin enables saltatory conduction and a bigger diameter lowers internal resistance to current flow. Preganglionic autonomic fibers are typically both myelinated and relatively large compared with postganglionic fibers, so they conduct signals more rapidly. If the fibers were unmyelinated or smaller, the signal would propagate more slowly. Therefore, being myelinated and larger leads to faster conduction velocity.