What are nodes of Ranvier?

Nodes of Ranvier are specialized gaps in the myelin sheath that encase certain axons. These gaps are crucial because they contain a high concentration of ion channels, which are essential for the propagation of action potentials along the axon. The presence of these nodes allows for the rapid transmission of electrical signals through a process known as saltatory conduction. Instead of propagating the action potential continuously along the entire length of the axon, the impulse "jumps" from one node to the next. This significantly increases the speed of signal transmission compared to unmyelinated axons, where the action potential must travel continuously across the entire membrane surface.

In this context, the other options do not accurately define what nodes of Ranvier are. Neurotransmitter synthesis occurs in the neuronal soma and presynaptic terminals, not specifically at the nodes. While glial cells support neuronal function, the nodes of Ranvier are not defined by high glial cell activity, but rather by their role in facilitating rapid signal transmission.