Synaptic cleft definition

The definition of a synapse requires at least the presence of a presynaptic element containing a concentration of vesicles and an apposed postsynaptic element, from which it is separated by the synaptic cleft (Peters et al., 1991). The vesicles of the dopamine synapses are concentrated around the points of synaptic contact rather than around the mitochondria that are part of the varicosities, so the release is probably punctate from the synaptic active zones. Notwithstanding the existence of synaptic specializations and... 

When an action potential traveling down the axon of a motoneuron reaches the myoneural endplate, a process occurs that releases acetylcholine into the synaptic cleft and consequently depolarizes the postsynaptic membrane. A similar process probably occurs at cholinergic synapses in the central nervous system. In 1950 Fatt and Katz discovered a spontaneous subthreshold activity (MEPP) of motor nerve endings and were thereby led to the concept that acetylcholine is released in definite units (quanta) of 10 to 10 molecules. Electron microscopy subsequently revealed characteristic vesicles about 40 nm in diameter, clustered near presynaptic membranes. Subcellular fractionation procedures were devised by Whittaker and de Robertis for the isolation of these vesicles from brain homogenates in sucrose density gradients, and it was soon demonstrated that they were indeed concentrated reservoirs of acetylcholine. The hypothesis that the vesicles discharge the quanta of transmitter became irresistible. 

The principal factors that influence the frequency of discharge of synaptic vesicles are the temperature, the state of polarization of the presynaptic membrane, and the osmotic pressure. The strong dependence of the frequency V of MEPP on these factors is explained by the hypothesis that they all contribute to the free energy of activation in the expression V = Voexp (—AG / kT). Another factor that is important in the discharge of synaptic vesicles is the concentration of calcium ions, [Ca +], in the synaptic cleft. This factor has a definite influence on the v of MEPP, but a more important effect on the amplitude of the end plate potential. Although MEPP will persist even in the absence of Ca + ions in the cleft, the EPP cannot be excited without Ca + ions. 

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