Voltage-gated channels open and close in response to a change in the voltage across a plasma membrane. Chemically gated channels open or close in response to the presence or absence of a specific chemical that binds to the channel protein or to a separate receptor that in turn alters the channel protein. Figure 41.9 illustrates the course of an action potential. Figure 41.10 illustrates the spread of an action potential along a neuron. The refractory period is a time during which the neuron cannot be restimulated because it is hyperpolarized and the inactivation gates are closed.
Synapses can also occur between neurons.
Activated receptors open chemically gated sodium channels and depolarize the postsynaptic membrane. The spreading depolarization fires an action potential in the adjacent membrane just outside the motor end plate. The motor end plate is the portion of the postsynaptic membrane that is part of the synapse. Acetylcholine is broken down by acetylcholinesterase and the components of acetylcholine are taken back into the presynaptic cell. Figure 41.15 illustrates the arrival of an action potential at a synapse.