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As the rising phase reaches its peak, voltage-gated Na channels are inactivated whereas voltage-gated K channels are activated, resulting in a net outward movement of K ions, which repolarizes the membrane potential towards the resting membrane potential.
Repolarization of the membrane potential continues, resulting in an undershoot phase or absolute refractory period.
If more of the same type of postsynaptic receptors are activated, then more Na will enter the postsynaptic membrane and depolarize cell.
The Hodgkin–Huxley model of an action potential in the squid giant axon has been the basis for much of the current understanding of the ionic bases of action potentials.
Briefly, the model states that the generation of an action potential is determined by two ions: Na and K .
Metabotropic receptors on the other hand activate second messenger cascade systems that result in the opening of ion channel located some place else on the same postsynaptic membrane.
Although slower than ionotropic receptors that function as on-and-off switches, metabotropic receptors have the advantage of changing the cell's responsiveness to ions and other metabolites, examples being gamma amino-butyric acid (inhibitory transmitter), glutamic acid (excitatory transmitter), dopamine, norepinephrine, epinephrine, melanin, serotonin, melatonin, and substance P.