Electrical Activity and Structure of Retinal Cells of the Aplysia Eye: II. Photoreceptors

1. Photoreceptors of the eye of Aplysia were studied by intracellular recording and Lucifer yellow injection. 2. Two basic photoreceptor types were observed, R and H. Two other types of cells were occasionally encountered: one was neurone-like, giving only a slight depolarization but large action potentials (APs) in response to light; the other was presumably glial. 3. Type R photoreceptors were found in the pigmented layer of the retina, had large distal (photoreceptor) processes extending toward the lens and an axon in the optic nerve. They are probably the large, microvillous receptor type with vesicle-filled cytoplasm observed previously in electron microscope studies. Action potentials were observed in the axon but not the cell body of the R receptor. The light response was an increasing conductance, 2 component depolarization followed by hyperpolarization. All 3 components were affected by light adaptation. Electrical coupling between R receptors and secondary neurones was apparent and the system produces the synchronous compound action potentials (CAPs) in the optic nerve. 4. Type H photoreceptors gave a slight depolarization to light with APs, followed by a hyperpolarization, followed by a late depolarization and more APs. They were in the pigmented layer of the retina and had smaller cell bodies and distal processes, but larger axons than R receptors. They may correspond to the photoreceptors with short microvilli and occasional cilia described previously in electron microscope studies. Electrical and dye coupling occurred between the receptors. The H receptors do not contribute to the CAP, but produce separate unitary potentials in the optic nerve.