The nerve impulse is carried across the synapse by certain chemicals called neurotransmitters . These chemicals are enclosed in synaptic vesicles that fuse with the presynaptic membrane and are released in response to a nerve impulse that reaches the axon terminal. Neurotransmitters bind to the postsynaptic membrane of the target cell, which can be a muscle, a gland, or the terminals of another neuron that takes over to propagate the nerve impulse. After the impulse is propagated, enzymes in the synaptic space break down the remaining neurotransmitter chemicals. Surrounding the nerve cell body are short, branched extensions called dendrites , that bind neurotransmitters released by other neurons or by sensory receptors , cells associated with the five senses - touch, vision, hearing, taste, and smell.
Some sensory receptors are actually specialized neurons.
Axons of some nerve cells are surrounded by a protective sheath of membranous material called myelin , produced by accessory cells called Schwann cells . Myelin gives bundled nerves their white appearance. Myelin is not continuous along the axon, but the sheath is interrupted at regular intervals called nodes of Ranvier , where the nerve is unmyelinated.