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After depolarization: is the slow repolarization phase which follows a rapid fall in spike potential and extends up to attainment of the RMP level. It is called phase of negative after potential and lasts for about 4 ms.

After reaching the resting level (–70 mV) the potential further falls and becomes more negative (–72 mV).This phase is called after hyperpolarization or phase of positive after potential.

What is the reason for naming them as negative and positive after potentials?

My attempt: I think we are naming them based on the direction of current, i.e. during after depolarisation membrane potential is going further negative , so the name negative after potential, and during after hyper polarisation, membrane potential is going from hyper polarised state to resting state, so its named as positive after potential.

Am I correct?

Source:Textbook of physiology

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Yes, you are correct. The negative current is hyperpolarising the axon and the positive current is returning it to baseline from a hyperpolarised state.

They are often referred to as inward/outward (negative/positive respectively) currents as well, due to the direction of the flow of ions. Which is further compounded when you start looking positive/negative ions.

Inward currents are inflow of +ve charge, or outflow of -ve charge. Outward currents are vice-versa.

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  • $\begingroup$ One doubt: we can see that during After hyperpolarization phase only last half of curve is having positive slope ( so positive current) then what about initial half in which curve is having negative slope( so negative current). $\endgroup$
    – JM97
    Jul 18, 2017 at 15:23
  • $\begingroup$ I'm not sure I follow, from your source, the trace drops to about -72mV immediately before point E. The bit from where it crosses -70 to E is the negative after potential. $\endgroup$
    – Oliver Houston
    Jul 18, 2017 at 15:32
  • $\begingroup$ Ah! got it, I was just confused by the Fig2.1-12 on pg52. Thanks. $\endgroup$
    – JM97
    Jul 18, 2017 at 15:40

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