The autonomic nervous system controls involuntary functions and consists of afferent, central, and efferent nerves. It uses neurohumoral transmission where nerves release chemical messengers called neurotransmitters at synapses. This involves impulse conduction along the nerve, calcium-dependent transmitter release from synaptic vesicles, binding of the transmitter to post-synaptic receptors, and termination of the transmitter signal via reuptake or diffusion. Transmitters can elicit excitatory or inhibitory post-synaptic potentials depending on their effects on ion channels.