Nodes of Ranvier function to which of the following?

• A. Produce myelin
• B. Regulate ion balance in cerebrospinal fluid
• C. Speed up transmission by saltatory conduction
• D. Generate action potentials

Answer: (C)

Nodes of Ranvier are gaps in the myelin sheath along a myelinated axon where the membrane is exposed. The myelin insulation makes the membrane resistive and low in capacitance, so the electrical signal travels quickly through the insulated segments. At the nodes, many voltage-gated sodium channels re-depolarize the membrane, allowing the action potential to be boosted as it moves. This arrangement enables saltatory conduction, so the nerve impulse leaps from node to node and travels much faster than along unmyelinated fibers.

These nodes themselves don’t produce myelin (myelin is produced by glial cells—oligodendrocytes in the CNS and Schwann cells in the PNS). They also aren’t responsible for regulating ion balance in cerebrospinal fluid (that’s handled by other brain structures and cells like the choroid plexus and ependymal cells). And while action potentials are regenerated at the nodes, the primary purpose of the nodes is to speed up conduction, not to initiate the impulse.