Synapse ( Foster & Sherrington, 1897 ) : Generally, a natural junction achieving functional conduction between distinct neurons (Waldeyer, 1891), or a neuron and another cell (Hooke, 1665), and accomplished through contact or near contact without regard to mechanism; see Bullock & Horridge (1965, pp. 181, 196). They are usually divided into chemical synapses, electrical synapses, and mixed synapses. The most common in adult vertebrates, the chemical synapse, is a structure-function differentiation consisting of three parts: a neuronal presynaptic compartment (De Camilli et al., 2001) where neurotransmitter is released from the terminal (Cajal, 1899), a synaptic cleft of variable width, and a postsynaptic compartment (De Camilli et al., 2001) that may or may not be neuronal but does have plasma membrane receptors for neurotransmitters. At classical unidirectional chemical synapses neurotransmitters may have feedforward effects on the postsynaptic membranes and feedback effects on presynaptic membranes. It is thought, however, that these synapses are rectifiers, with neural network information flowing from presynaptic neuron to postsynaptic cell, modulated by presynaptic neurotransmitter feedback. In contrast, reciprocal or bidirectional chemical synapses release neurotransmitter on both sides of the synaptic cleft and can thus transmit neural network information in either direction depending on network activity. Bidirectional chemical synapses are characteristic of amacrine extensions rather than axons (K├Âlliker, 1896). Finally, the distance between presynaptic membrane and postsynaptic membrane can vary enormously, from about 20-30 nm at classical chemical synapses like the neuromuscular junction, to microns for the sympathetic innervation of blood vessels, to centimeters or meters for neurotransmitter released into the blood. Foster & Sherrington (1897, pp. 929, 969) supplied the term; Cajal (1894b, p. 447) had earlier called it an articulation. See Bullock & Horridge (1965, Tab. 2.1), Peters et al. (1991, Ch. 5), Cowan et al. (2001), Sandring (2008, p. 44-48).

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