The Absolute Refractory Period

Repolarization of the membrane potential

Causing the membrane potential to reach E_Na+

Depolarization to threshold

The Relative Refractory Period

the nerve is thicker at these two best sites

these two best sites contain unmyelinated nodes of Ranvier 

the amount of myelin on the nerve is thinner at these two best sites

there is less subcutaneous fat at the two best sites

these sites have the lowest thresholds for AP generation because they contain the highest density of voltage-gated Na+ channels 

Using two pins placed some distance apart

Using two pins to record from an afferent neuron

Using a fine microelectrode inserted into an individual neuron

Using two pins to record from an individual neuron

Using a fine microelectrode inserted into the nerve trunk

Activate the Na⁺/K⁺ ATPase pump to  start pumping ions

Induce a flux of positively charged ions from outside to inside of the cell

Trigger the opening of voltage-gated K⁺ ion channels to initiate the Action Potential

Induce a flux of positively charged ions from inside to outside of the cell

Speed up the repolarization of the membrane potential

Summing APs to get a larger AP

Changing the size of the APs it produces

Reducing the Absolute refractory period

Changing the rate of APs

Reducing the Relative but not the absolute refractory period

about 10-20 km/h

about 10-20 m/s

about 50-60 km/h

likely to cause you to get demerit points for speeding

about 50-60 m/s