Barry W. Connors
After meticulous study of spinal reflexes, Charles Sherrington N10-2 deduced that neurons somehow communicate information, one to the next, by a mechanism that is fundamentally different from the way that they conduct signals along their axons. Sherrington had merged his physiological conclusions with the anatomical observations (Fig. 13-1) of his contemporary, the preeminent neuroanatomist Santiago Ramón y Cajal. N10-1 Ramón y Cajal had proposed that neurons are distinct entities, fundamental units of the nervous system that are discontinuous with each other. Discontinuous neurons must nevertheless communicate, and Sherrington in 1897 proposed that the synapse, a specialized apposition between cells, mediates the signals. The word synapse implies “contiguity, not continuity” between neurons, as Ramón y Cajal himself explained it. When the fine structure of synapses was finally revealed with the electron microscope in the 1950s, the vision of Ramón y Cajal and Sherrington was amply sustained. Neurons come very close together at chemical synapses (see p. 206), but their membranes and cytoplasm remain distinct. At electrical synapses (see p. 205), which are less common than chemical synapses, the membranes remain distinct, but ions and other small solutes can diffuse through the gap junctions, a form of continuity.
FIGURE 13-1 Synapses of the mammalian brain. Drawings by Ramón y Cajal made from Golgi-stained cortex of the cerebellum. A, Basket cell (B) from the mouse, making synaptic contacts onto the somata of numerous postsynaptic Purkinje cells (A). The axon (c) of the basket cell branches several times to make baskets (a and b) that synapse onto the somata of Purkinje cells (axosomatic synapse). Ramón y Cajal used the osmic method to stain the Purkinje cell and the Golgi method to stain the basket cell. B, A single climbing fiber (a) from a human, making numerous synaptic contacts onto the dendrites of a single Purkinje cell (b). This is an example of axodendritic synapses. Ramón y Cajal used the Golgi method to stain the climbing fiber. (From Ramón y Cajal S: Histology of the Nervous System of Man and Vertebrates. New York, Oxford University Press, 1995.)