Dendritic spines, sometimes also called dendritic thorns, are tiny, specialized protoplasmic protuberances that cover the surface of many neurons. First described by Ramón y Cajal (1909; 1991) in light-microscopic studies of Golgi stained tissue, they are among the most striking subneuronal features of many neurons. Indeed, the presence of a high density of dendritic spines allows the unambiguous classification of neuronal types into spiny and aspiny, sparsely spiny, or smooth neurons. Over 90% of all excitatory synapses that occur in the cortex are located on dendritic spines. Spines can be found in all vertebrates as well as in invertebrates (e.g., the dendrites of Kenyon cells in the mushroom bodies in the olfactory system of the insect brain). The intimate association of spines with synaptic traffic suggests some crucial role in synaptic transmission and plasticity. Because of their submicrometer size (see below), physiological hypotheses as to the function of dendritic spines have only very recently become accessible to the experimentalist. For the previous two decades, spine properties have been investigated through analytical and computational studies based on morphological data, providing a very fertile ground for the crosspollination of theory and experiment. (For a very readable historical account of this see Segev et al., 1995.) The recent technical advances in the direct visualization of calcium dynamics in dendrites and spines are now permitting direct tests of some of these theoretical inferences (Guthrie, Segal, and Kater, 1991; Müller and Connor, 1991; Yuste and Denk, 1995; Denk, Sugimori, and Llinás, 1995; Svoboda, Tank, and Denk, 1996). As discussed in this chapter and, more extensively, in Chap. 19, the theoretical models that have endowed spines with active properties giving rise to all-or-none behavior (Perkel and Perkel, 1985; Shepherd et al., 1985; Segev and Rail, 1988; Baer and Rinzel, 1991) have, in general, been confirmed experimentally. Historically, the possibility of implementing synaptic memory by modulating the electroanatomy of spines was recognized early on (Chang, 1952) and was subsequently analyzed in depth by Rail (1970, 1974, 1978) and many others. Because small changes in the spine morphology can lead to large changes in the amplitude of the EPSP induced by the excitatory synapse on the spine, spines have been considered to contribute to the modulation of synaptic “weight” during long-term potentiation (see Chap. 13).
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