Impulse propagation

1. How do Na ions enter the neuron during depolarization?
2. What is the absolute refractory period and how does it differ from the relative refractory period?
3. Why does temperature change the AP? And what effect does it have on latency, amplitude, duration, and conduction velocity?
4. What are the areas between the myelin sheath covering called and what is the process of propagating a current from one node to another?
5. How are the depolarized membranes brought back to resting state?

Answers:
1. Na enters the cell through Na voltage gated channels with activation and inactivation gates. When the cell is activated, the activation gate is open. When the cell is inactivated, the inactivation gate is closed.
2. The absolute refractory period is the time after closure of the inactivation gates, when no action potential can be formed, no matter how strong the stimulus. The relative refractory period occurs after the absolute refractory period and a more intense stimulation is required to form an AP.
3. Decreased temperature affects the protein configuration of the Na channels and causes a delay in opening and closing of the gates. Decreased temperature results in prolonged latency, increased duration, and decreased C.V. Amplitude may be increased, but may also decrease due to increased temporal dispersion phase cancellation.
4. Nodes of Ranvier are the areas between the myelin. Saltatory conduction is propagating a current from one node to another.
5. K voltage-gated channels open from depolarization after a slight delay. K moves out of the cell to establish a charge equilibrium.