Neurotransmitters are chemical substances that transmit information between nerve cells, neurons, and between neurons and effectors, muscles or glands. The transmission occurs across a narrow gap or cleft at the synapse, microscopic distance between a transmitting presynaptic cell and a receiving postsynaptic cell. Neurotransmitters are synthesized by neurons and stored in structures called synaptic vesicles that hold thousands of molecules of neurotransmitter. When a nerve impulse passes along a presynaptic axon, it reaches the axon bulb at the axon terminal and depolarizes the presynaptic membrane. This causes calcium ions to rush into the neuron through voltage gated channels. Stimulated by calcium ions, the synaptic vesicles then fuse with the presynaptic membrane and spill their neurotransmitter into the synaptic cleft. The neurotransmitter diffuses to the postsynaptic membrane on the other side of the synapse. Specialized receptor proteins that project from the outside of the postsynaptic membrane receive the neurotransmitter...