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.
Synapses can be excitatory or inhibitory.
Synapses can form on presynaptic membranes and thereby influence release of neurotransmitter by the presynaptic cell. Whether a synapse is excitatory or inhibitory depends on the types of ion channels and postsynaptic receptors on the postsynaptic membrane. The same neurotransmitter can be excitatory at some synapses and inhibitory at other synapses. The axon hillock is the critical part of the neuron for making the decision whether an AP is generated based on the combined excitatory and inhibitory inputs. If the combined potential leads to depolarization of the membrane to the threshold value, an action potential is generated.