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As Na ions enter the cell, the membrane potential is further depolarized, and more voltage-gated sodium channels are activated.
Repolarization of the membrane potential continues, resulting in an undershoot phase or absolute refractory period.
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.
Moreover, the distinctions based on function between neurons and other cells such as cardiac and muscle cells are not helpful.
Thus, the fundamental difference between a neuron and a nonneuronal cell is a matter of degree.