What factors determine the speed of action potentials?

The speed of action potentials is primarily influenced by the axonal diameter and the presence of myelination. Larger axonal diameters reduce the internal resistance to the flow of ions, thereby allowing the action potential to propagate more quickly along the axon. Myelination, on the other hand, enhances this process through a mechanism known as saltatory conduction. In myelinated axons, the action potential jumps from one node of Ranvier (gaps in the myelin sheath) to the next, which significantly increases the conduction velocity as compared to unmyelinated axons.

While factors like the type of neurotransmitter released can influence synaptic transmission, they do not directly affect the speed of action potentials along the axon. Similarly, the length of the axon can influence the overall signal transmission time but does not dictate the speed of the action potentials themselves, as longer axons can still have varying diameters and myelination characteristics. The frequency of action potentials pertains to how often action potentials occur but does not affect the velocity at which individual action potentials travel along the axon. Thus, the axonal diameter and myelination are indeed the primary contributors to the speed of action potentials.