neurotransmitter /-transmit″ər/ [Gk, neuron + L, transmittere, to transmit] , a chemical that modifies or results in the transmission of nerve impulses between synapses. Neurotransmitters are released from synaptic knobs into synaptic clefts and bridge the gap between presynaptic and postsynaptic neurons. Each vesicle within a synaptic knob stores as many as 10,000 neurotransmitter molecules. When a nerve impulse reaches a synaptic knob, thousands of neurotransmitter molecules squirt into the synaptic cleft and bind to specific receptors. This flow allows an associated diffusion of potassium and sodium ions that causes an action potential. Excitatory neurotransmitters decrease the negativity of postsynaptic membrane potentials; inhibitory neurotransmitters increase such potentials. Kinds include acetylcholine, gamma-aminobutyric acid, norepinephrine.
