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Neurons are cells that are specialized to receive, propagate, and transmit electrochemical impulses.In the human brain alone, there are over eighty billion neurons.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.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 .
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.
Plastic change often results from the alteration of the number of neurotransmitter receptors located on a synapse.
There are several underlying mechanisms that cooperate to achieve synaptic plasticity, including changes in the quantity of neurotransmitters released into a synapse and changes in how effectively cells respond to those neurotransmitters.