The arrival of a nerve impulse at an axon terminal causes the sudden release of molecules of a transmitter substance from the terminal The transmitter molecules then diffuse across the fluid-filled gap between the tow cells and act on specific receptors on the surface of the adjacent neuron The electrical activity of the receptor neuron is altered by this action of the transmitter substance To date more than 30 different transmitter substances have been identified each has a characteristic excitatory or inhibitory effect on the postsynaptic cell There are three types of neurotransmitter substances (1)those affecting the ionic gate mechanisms (2)monoamine transmitters and (3)neuropeptides