Peripheral Nervous System

This can be answered by two ways: First consider the nerve structure. It is surrounded by myelin sheath(髓鞘) formed form fat and exposed in extra-cellular fluid. The nerve impulse or action potential, A.P.(動作電勢) is an electrical signal transmitting along the nerves via depolarization(去極化). Second the way for A.P. transmitted via either depolarization or saltatory conduction(跳躍式傳導).
A.P. can conduct signals more quickly if (1)reducing the electrical resistance,R, of the nerve by having a thicker nerve. (2)a thicker nerve also provides a larger surface area for ions exchange during depolarization. (3)temperature. The higher the temperature,the faster will the A.P. be transmitted. This is due to greater kinetic energy gained by ions during depolarization. (在足球比賽有球員受偒時會用一些冷凍的液體(神仙水)射它的腳也是這個原因) (4)If the A.P. is not transmitted via depolarization, but jumping from one node of ranvier to another, A.P. can also be conducted more quickly. This is called saltatory conduction.
R = ρ×L÷A where L is length and A is the cross section area and ρ is resistivity
I hope you will find them helpful.