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Thus, not all neurons correspond to the stereotypical motor neuron with dendrites and myelinated axons that conduct action potentials.Some neurons such as photoreceptor cells, for example, do not have myelinated axons that conduct action potentials.Briefly, the model states that the generation of an action potential is determined by two ions: Na and K .An action potential can be divided into several sequential phases: threshold, rising phase, falling phase, undershoot phase, and recovery.These allow Ca2 ions to diffuse through these channels and bind with synaptic vesicles within the terminal boutons.

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.Such a process is also known as a positive feedback loop.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.In the human brain alone, there are over eighty billion neurons.Neurons are diverse with respect to morphology and function.

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