Centrifugal actions on amacrine and ganglion cells in the retina of the turtle.

1. An electrophysiological investigation of efferent synapses in the retina of the turtle was conducted by recording intracellularly from amacrine cells. These cells have been selected because in birds they have been shown to have direct anatomical connexions with centrifugal fibre terminals. 2. Amacrine cells could be easily distinguished from most other retinal cells, except ganglion cells, by their different photo-responses. Because both amacrine and ganglion cells may generate action potentials they were distinguished by their responses to optic nerve stimulation. 3. The response of ganglion cells to single shock stimulation of the optic nerve consists of an antidromic action potential followed by a late synaptic potential. 4. Cells which did not show antidromic responses but were electrically excitable, by passing direct current through the recording electrode, were considered to be amacrine cells. 5. Amacrine cells generate an e.p.s.p. in response to optic nerve stimulation. An analysis of the e.p.s.p. suggests that it may be due to a single afferent fibre terminating in the proximity of the cell soma. By analogy to the bird, it is concluded that the amacrine cells e.p.s.p.s result from the activation of centrifugal fibres.