Physiological and morphological identification of photosensitive neurons in the opisthosomal ganglia of Limulus polyphemus.

The motor outputs of the isolated opisthosomal ventral nerve cord in Limulus polyphemus are modulated by light. We have identified the photosensitive neurons and examined their physiological and morphological properties using intracellular recording and staining techniques. We found that photosensitive neurons are present in each ganglion of the opisthosomal ventral nerve cord. These neurons often discharged action potentials spontaneously in the dark, and they increased the frequency of this discharge in the light. The mean latency (+/-SD) of the light-induced action potential was 2.2 +/- 1.1 s. Cells responded in a graded fashion over a 2-log unit of light intensity. The peak spectral sensitivity was 425 nm or lower. The Lucifer-yellow-labeled photosensitive neurons had oval somata with mean (+/-SD) diameters of 102 +/- 3 microm (long axis) and 75 +/- 5 microm (short axis), and extended their axons to the contralateral region of the ventral nerve cord. The soma had no dendrites, and the axon had thin branches.