The endplale potential is terminated by hydrolysis of acetylcholine by the enzyme acetylcholinesterase. Choline liberaled in the junctional cleft is actively transported back into the nerve terminal. The release of acetylcholine is quantal. A quantum corresponds to the amount of acetylcholine in a single prejunctional vesicle. Direct electrical transmission between adjacent cells is mediated by gap junctions.
Spinal motoneurons have hundreds of synaptic inputs.
An action potential in an excitatory input causes an excitatory postsynaptic potential that depolarizes the motoneuron and brings it closer to threshold. An action potential in an inhibitory input causes an inhibitory postsynaptic potential that hyperpolarizes the motorneuron. The efficacy of synaptic transmission depends on the timing and frequency of action potentials in the presynaptic neuron. Facilitation, augmentation, posttetanic potentiation, and long-term potentiation are examples of increased efficacy of synaptic transmission in response to multiple stimulations of the presynaptic neuron . A large number of neuropeptides function as neurotransmitters or as neuromodulators in the central nervous system.