It is a narrow communicating junction between two neurons. The axon terminal of one neuron forms a presynaptic knob. The dendrite tip of the next neuron is expanded but depressed to form postsynaptic depression. The narrow space between the two is called the synaptic cleft. The presynaptic knob possesses a number of neurotransmitters containing synaptic vesicles. The membrane of the postsynaptic depression possesses a number of chemoreceptor sites.
As the impulse reaches the presynaptic knob, it activates the synaptic vesicles. They reach the presynaptic membrane and burst open. Neurotransmitter (e.g., acetylcholine) is released. Neurotransmitter molecules come in contact with chemoreceptor sites of the postsynaptic membrane. The latter becomes depolarized to generate an electric current or impulse. The impulse travels across the synapse only in one direction from the axon terminal to the dendrite tip.