Final answer:
An increase in the diameter of the axon would cause the action potential to travel faster down the axon.
Step-by-step explanation:
An increase in the diameter of the axon (d) would cause the action potential to travel faster down the axon. The action potential is conducted down the axon as voltage-gated ion channels are opened by the spreading depolarization. In myelinated axons, propagation is described as saltatory conduction, where the electrical events seem to "jump" from one node to the next. A wider axon diameter reduces the resistance for ion diffusion within the cell, allowing for faster conduction of the action potential. The mechanism by which this happens involves voltage-gated ion channels that open in response to depolarization. In myelinated axons, this process is known as saltatory conduction, where the action potential effectively 'jumps' from one node of Ranvier to the next, leading to faster propagation than in unmyelinated axons. Consequently, an increase in axon diameter would cause the action potential to travel faster down the axon.