Which of the following factor regulate rate of firing of axons during nerve conduction?

Chemical, mechanical, or electrical excitation can activate a neuron leading to the generation of a nerve impulse. When the depolarization exceeds the threshold level of -55 mV, activated sodium channels cause a flood of Na+ ions into the cell which drives fuher depolarization. This causes a relative excess of cations in the cell, which eventually leads to a membrane potential of +35 mV. If the initial stimulus is less than -55 mV, few Na+ channels are opened and the threshold level is never reached. As a result, nerve conduction is an all-or-none response. Ref: Beck-Yoo K. (2012). Chapter 19. The Nervous System. In L.W. Janson, M.E. Tischler (Eds), The Big Picture: Medical Biochemistry.