Chemical synapse, in which the signal is relayed by means of a chemical messenger called a neurotransmitter. Arrival of an action potential triggers neurotransmitter release into the synaptic cleft, a narrow (20–50‑nm) gap between the pre- and post-synaptic membranes, which excites or inhibits the postsynaptic cell. An example of a chemical synapse is the neuromuscular junction (NMJ): the terminal bouton of an α-motor neuron forms a synapse with the motor end plate of a skeletal muscle cell. Action potential transmissions at chemical synapses are typically unidirectional: the signal can only be transmitted from pre- to post-synaptic cells. Transmission of an action potential across a chemical synapse is associated with a synaptic delay, as it takes time for each of the processes of neurotransmitter release, diffusion and combination with postsynaptic receptors to occur.