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The mystery of the visual system K pathway

Vivien A. Casagrande

Gasser and Erlanger shared the 1944 Nobel Prize in Physiology and Medicine for their elegant demonstration of a direct relationship between axon diameter and conduction velocity. Their work established that different sensations within a single cutaneous nerve (e.g. pain and temperature versus touch) are transmitted via axons of different calibre. Inspired by their findings, Bishop (1933) demonstrated that axons in the optic nerve can be divided into three main populations according to their conduction velocities and diameters. By analogy to the work of Gasser & Erlanger (1929), Bishop proposed that the three optic nerve axon classes process different sensory qualities. Although Bishop ultimately attributed differences in the diameter of optic nerve axons to phylogenetic age, his original observations continue to be relevant to modern views of parallel processing within the mammalian visual system. Later work on the cat visual system (reviewed by Stone, 1983) established that retinal ganglion cells and their target cells in the lateral geniculate nucleus (LGN) form three distinct classes generally referred to as W, X and Y. These cell classes differ in morphology, axon diameter, response properties and conduction velocity. Information within each class is kept largely separate from retina to cortex. Thus the large retinal ganglion cells innervate large LGN cells, medium retinal ganglion cells innervate medium LGN cells, and small retinal ganglion cells innervate small LGN cells. The projection from each LGN cell class terminates in a distinct pattern in the cortex.