Final answer:
A small diameter or unmyelinated axon has a slower speed of signal transmission due to continuous conduction and the increased resistance due to the smaller diameter.
Step-by-step explanation:
The speed of a small diameter or unmyelinated axon is generally slower than that of a myelinated axon. This reduced speed is due to a process called continuous conduction, where voltage-gated Na+ channels open sequentially along the axon, causing a slower propagation of the action potential. In contrast, saltatory conduction occurs in myelinated axons where the action potential rapidly leaps from one Node of Ranvier to the next, resulting in a much faster transmission. While myelin sheaths can greatly enhance the speed of conduction, they are not present in unmyelinated axons. The diameter of the axon also affects the speed of conduction, as larger diameters decrease the resistance and allow for faster propagation of the electrical signal. Therefore, a small diameter, unmyelinated axon would have a relatively low speed of signal propagation.