<P> Neurons form elaborate networks through which nerve impulses (action potentials) travel . Each neuron has as many as 15,000 connections with other neurons . Neurons do not touch each other (except in the case of an electrical synapse through a gap junction); instead, neurons interact at close contact points called synapses . A neuron transports its information by way of an action potential . When the nerve impulse arrives at the synapse, it may cause the release of neurotransmitters, which influence another (postsynaptic) neuron . The postsynaptic neuron may receive inputs from many additional neurons, both excitatory and inhibitory . The excitatory and inhibitory influences are summed, and if the net effect is inhibitory, the neuron will be less likely to "fire" (i.e., generate an action potential), and if the net effect is excitatory, the neuron will be more likely to fire . How likely a neuron is to fire depends on how far its membrane potential is from the threshold potential, the voltage at which an action potential is triggered because enough voltage - dependent sodium channels are activated so that the net inward sodium current exceeds all outward currents . Excitatory inputs bring a neuron closer to threshold, while inhibitory inputs bring the neuron farther from threshold . An action potential is an "all - or - none" event; neurons whose membranes have not reached threshold will not fire, while those that do must fire . Once the action potential is initiated (traditionally at the axon hillock), it will propagate along the axon, leading to release of neurotransmitters at the synaptic bouton to pass along information to yet another adjacent neuron . </P> <Ol> <Li> Synthesis of the neurotransmitter . This can take place in the cell body, in the axon, or in the axon terminal . </Li> <Li> Storage of the neurotransmitter in storage granules or vesicles in the axon terminal . </Li> <Li> Calcium enters the axon terminal during an action potential, causing release of the neurotransmitter into the synaptic cleft . </Li> <Li> After its release, the transmitter binds to and activates a receptor in the postsynaptic membrane . </Li> <Li> Deactivation of the neurotransmitter . The neurotransmitter is either destroyed enzymatically, or taken back into the terminal from which it came, where it can be reused, or degraded and removed . </Li> </Ol> <Li> Synthesis of the neurotransmitter . This can take place in the cell body, in the axon, or in the axon terminal . </Li> <Li> Storage of the neurotransmitter in storage granules or vesicles in the axon terminal . </Li>

How can a neurotransmitter affect the activity of a postsynaptic neuron