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There are two families of receptors: ionotropic and metabotropic receptors.

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..

Some neurons such as photoreceptor cells, for example, do not have myelinated axons that conduct action potentials.

Other unipolar neurons found in invertebrates do not even have distinguishing processes such as dendrites.

After neurotransmitters are synthesized, they are packaged and stored in vesicles.

These vesicles are pooled together in terminal boutons of the presynaptic neuron.

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 .

When there is a change in voltage in the terminal bouton, voltage-gated calcium channels embedded in the membranes of these boutons become activated.

These allow Ca2 ions to diffuse through these channels and bind with synaptic vesicles within the terminal boutons.

As Na ions enter the cell, the membrane potential is further depolarized, and more voltage-gated sodium channels are activated.