Interaction between the soma and the axon terminal of retinal horizontal cells in Cyprinus carpio.

Intracellular recordings were made from the monophasic horizontal cells of the carp retina which are known to respond with a sustained hyperpolarization to all visible monochromatic light. The receptive field of each subcellular structure, the soma and the axon terminal, was determined using a long narrow slit of light. Somata and axon terminals showed receptive fields that encompassed almost the entire retina. This observation suggests that each aggregate of the subcellular parts forms a synctial structure. However, with increasing distance from the slit, the response peak decayed more steeply in somata than in axon terminals. The spatial decline of the peak consisted of two exponential functions in somata, while a single exponential function in axon terminals. The length constant of the axon terminal was similar to the larger length constant revealed in the soma. This finding suggests an electrical communication at work between the soma and the axon terminal. A quantitative account was made in light of a discrete resistive network model which consists of a pair of syncytia coupled through connecting axons; one represents the contiguous layer of somata and the other the contiguous layer of axon terminals. Relevant response properties computed from the model analysis were in satisfactory agreement with experimental data. It was concluded that the soma and the axon terminal of the horizontal cell are electrically connected in the cyprinid retina.