 Dear students, in this module we shall discuss the mechanism of release of neurotransmitters. The neurotransmitters are leased into the synaptic cleft in both the fast as well as slow chemical transmission, but the mechanism involving their release are common for both these transmissions. There are two basic patterns of release. First is the release due to action potential and second release of neurotransmitter without action potentials that is non-spiking release. Dear students, most neurons release neurotransmitters when an action potential reaches the exon terminals. The mechanism involves the activation of voltage-gated calcium channels which allow the entry of calcium ions into the exon terminal. When calcium ions enter the cell they bind to a protein molecule on the inner surface of the membrane at special sites which are called release sites. It means that when calcium ions enter the terminal, they come here and bind to a special protein on the membrane. This protein is present on the release site, so the place where this protein is present with which the calcium ions bind is called the release site. The binding of calcium ions causes the release sites to open through the membrane. Release sites open and as a result the vesicles which contain the neurotransmitter in them are released their neurotransmitter into the synaptic cleft. Dear students, for neurotransmitter release there are two important factors. First, an action potential reaches and second, calcium ions are inflow in the exon terminal. These two factors are essential for the neurotransmitter release. When an action potential ends and the influx of calcium ions ends then the neurotransmitter release also closes. Dear students, neurotransmitters are generally released as packets which contain about 2,000 to 10,000 molecules of neurotransmitter in them. This is known as the quantal release because we normally use quantum covered for packets but when a quantum is released then we call it the quantal release. Dear students, as we have just mentioned, the neurotransmitter release is related to depolarization i.e. the action potential. So the probability of quantal release increases if the presynaptic membrane is depolarized due to action potential. Now the amount of released neurotransmitter is directly proportional or directly related to the depolarization. More the depolarization of the membrane of the terminal, more is the release of neurotransmitter molecule and if depolarization will decrease or there will be weaker action potential then as a result the neurotransmitter is also less produced. Dear students, there are some neurons that do not generate action potentials but there are also neurotransmitters released on one of their exon terminals. Such releases are called non-spiking releases. In these neurons the information transfer occurs by electrotonically conducted graded potentials. In this case too, the amount of transmitter released depends on the membrane potential and depolarization.