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1983; Halliday and To¨rk, 1986)). , 1987). Neuronal cell bodies in the SNc have various shapes (ovoid, polygonal, or fusiform), and sizes. Halliday and To¨rk (1986) reported that the perikaryal diameter of the SN neurons ranges from 6 to 33 mm in the rat, and SN neurons are relatively larger in primates (with diameters ranging from 11 to 43 mm in the SNc of the macaque monkey, and from 14 to 50 mm in the human SNc). , 1983). , 1983). , 1977; Phillipson, 1979b), neurons of the SNc were seen to emit long dendrites which branched infrequently (exhibiting features that overall matched the Cajal’s drawings shown in Fig.

Gap junctions provide in the nervous system the structural correlate of one class of electrical synapses, characterized by very close apposition between the presynaptic and postsynaptic membranes. It should be noted, in this respect, that different junctional specializations can mediate different forms of electrical transmission between neurons (Bennett, 1997). Electrical synapses transmit preferentially, but not exclusively, lowfrequency stimuli, that allow the rapid transfer of a presynaptic impulse into an electrical excitatory potential in the postjunctional cells.

Serotonin receptors are distributed throughout the SNr. , 2002). , 2004). These findings are in contrast with the wealth of evidence indicating that the noradrenergic system influences the activity of the nigrostriatal dopaminergic system (as also shown by the electrophysiological findings of Paladini and Williams, 2004). , 2004). Opioid receptors (and especially m receptor binding) are abundant in all the subregions of the ventral midbrain tegmentum, including the SNr (Fallon and Loughlin, 1995).