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When ionotropic receptors are activated, certain ion species such as Na to enter the postsynaptic neuron, which depolarizes the postsynaptic membrane.If more of the same type of postsynaptic receptors are activated, then more Na will enter the postsynaptic membrane and depolarize cell.An action potential can be divided into several sequential phases: threshold, rising phase, falling phase, undershoot phase, and recovery.Following several local graded depolarizations of the membrane potential, the threshold of excitation is reached, voltage-gated sodium channels are activated, which leads to an influx of Na ions.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..After neurotransmitters are synthesized, they are packaged and stored in vesicles.These vesicles are pooled together in terminal boutons of the presynaptic neuron.

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.Once bounded with Ca2 , the vesicles dock and fuse with the presynaptic membrane, and release neurotransmitters into the synaptic cleft by a process known as exocytosis.The neurotransmitters then diffuse across the synaptic cleft and bind to postsynaptic receptors embedded on the postsynaptic membrane of another neuron.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.

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