the nervous system is composed of billions of communicating cells called neurons neurons conduct electrical signals or action potentials along the cell's membrane along the axon to the terminus where synapses are formed the synapse consists of the membrane of the presynaptic cell a synaptic cleft and the postsynaptic cell membrane since an action potential can only propagate along the neurons membrane a chemical known as a neurotransmitter must bridge the gap between the adjacent cells the presynaptic cell releases the neurotransmitters into the synaptic cleft where they will move to bind the receptor proteins in the postsynaptic cell membrane when the action potential arrives at the synapse it opens voltage-gated calcium channels resulting in an inward flow of calcium the calcium binds to the vesicular membrane this binding activates the vesicles to fuse with the presynaptic membrane as fusion occurs neurotransmitter molecules are released into the synaptic cleft once in the cleft the neurotransmitter molecules diffuse throughout the interstitial fluid in the cleft some of the neurotransmitters will cross the synaptic cleft and bind with receptors in a postsynaptic membrane this binding produces an effect in the postsynaptic cell by opening the protein channels and allowing the flow of ions into and out of the cell if the effect of a neurotransmitter excites the postsynaptic membrane an excitatory postsynaptic membrane potential also known as epsp is generated this localized depolarizing signal generally causes long-distance signals action potentials in the dendrite of the postsynaptic cell if the effect of the neurotransmitter inhibits the postsynaptic membrane an inhibitory postsynaptic membrane potential also known as an eye PSP is generated this localized hyperpolarizing signal generally prevents the formation of long-distance signals in the postsynaptic cell neurotransmitters influence the postsynaptic membrane electrical state for only a short time and are removed from the synaptic cleft in one of three ways diffusion away from the receptors enzymatically degraded or recycled by the presynaptic cell some of the neurotransmitter molecules diffuse out of reach of the neurotransmitter receptors some neurotransmitter molecules are removed by enzymatic degradation some neurotransmitter molecules are transported back into the presynaptic cell by neurotransmitter transporters where they are recycled an example of a specific neurotransmitter that functions in these ways is serotonin some serotonin will cross the synaptic cleft and bind with the specific serotonin receptors within the membrane of the postsynaptic cell producing an effect in the target cell seratonin will eventually unbind from the receptors and either diffuse out of the synaptic cleft and be lost or be captured by the serotonin reuptake transporter in the presynaptic membrane this protein complex will transport the biologically active serotonin back into the presynaptic neuron for repackaging and re-release acetylcholine the first neurotransmitter discovered also diffuses across the synaptic cleft and temporarily binds with its specific membrane bound receptor on the postsynaptic cell affecting the cell's electrical State some acetylcholine in the cleft will interact with the enzyme acetylcholinesterase which will hydrolyze the acetylcholine molecule into acetate and choline acetate diffuses away but choline interacts with the presynaptic membrane bound transporter to be recycled in the presynaptic neuron into new acetylcholine unlike serotonin which is intact and biologically active following its uptake into the presynaptic cell new synthesis of acetylcholine is necessary with the recycled choline as the precursor