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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.After neurotransmitters are synthesized, they are packaged and stored in vesicles.These vesicles are pooled together in terminal boutons of the presynaptic neuron.

Thus, the fundamental difference between a neuron and a nonneuronal cell is a matter of degree.The undershoot phase occurs because unlike voltage-gated sodium channels, voltage-gated potassium channels inactivate much more slowly.Nevertheless, as more voltage-gated K channels become inactivated, the membrane potential recovers to its normal resting steady state..Neurons are diverse with respect to morphology and function.Thus, not all neurons correspond to the stereotypical motor neuron with dendrites and myelinated axons that conduct action potentials.

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