Single‐unit recording from antidromically activated optic radiation neurones

Antidromic activation of cells in the central nervous system has proved to be a most useful method of studying the properties of neurones and the part these play in synaptic transmission (see e.g. Renshaw, 1941, 1946; Araki, Otani & Furukawa, 1953; Brock, Coombs & Eccles, 1953; Frank & Fuortes, 1955; Granit & Phillips, 1956; Phillips, 1956, 1959; Coombs, Curtis & Eccles, 1957a, b; Fatt, 1957; Fuortes, Frank & Becker, 1957; Martin & Branch, 1958; Bennett, Crain & Grundfest, 1959; Freygang & Frank, 1959; Machne, Fadiga & Brookhart, 1959). The method is also valuable in establishing the identity of various cells and in determining the extent of a tract, on the principle that an antidromic impulse will not cross a synapse (e.g. Woolsey & Chang, 1947). However, the existence of recurrent collaterals is a complicating factor here. We have stimulated the visual cortex in cats whilst recording extracellularly from units in the lateral geniculate nucleus (LGN) and optic radiation with both these objects in mind. The LGN projects to a large area of cortex and in these experiments no attempt was made to stimulate this entire area but only the most anterior part. Evidence will be produced to show that the majority of our records result from a true antidromic activation of the unit. This is a necessary step, because it has been suggested that the visual cortex sends an efferent supply to the LGN (e.g. Vastola, 1957; Wid6n & Marsan, 1960 a, who also give a list of references to earlier work). Our results do, however, lend some support to this idea, because a few units could be activated only after rather long latencies. However, the results are most useful for the study of the process of impulse initiation in the cell body and in this respect support the theories advanced by Araki & Otani (1955), Coombs et al. (1957a, b) and Fuortes et al. (1957).