Ultrastructure of photoreceptors and circadian pacemaker neurons in the eye of a gastropod, Bulla.

Each of the paired cephalic eyes of the marine gastropod, Bulla, is about 0.5 mm in diameter and contains about 1000 large photoreceptors, small photoreceptors, numerous pigmented support cells and about 130 neurons. The photoreceptors are of three types: large (90 micron X 20-30 micron) dense ones (PRLD) with elaborate narrow microvilli and aggregates of 650 A clear vesicles in the cytoplasm; large clear ones (PRLC) with elaborate wide microvilli and aggregates of 650A clear vesicles; small slender receptors (PRS) with a tuft of microvilli and lacking vesicle aggregates. Neurons (15-25 micron) containing dense-core 1000 A vesicles are in the periphery of the retina or grouped in a collar around the neuropil below the photoreceptor layer. The 4-5 largest neurons are in the collar area. Correlation of neuron morphology with electrical activity was done with intracellular recording and Lucifer yellow injection of some of the larger neurons in the collar area whose action potentials contribute to the optic nerve impulses. Each one has an axon in the optic nerve and processes that go to the neuropil. They are the pacemaker neurons of the circadian rhythm in impulse frequency that is recorded from the optic nerve, since only their action potentials are correlated 1:1 with the optic nerve impulses. Gap junctions (with pentalaminar structure) are common between photoreceptors, glial cells, photoreceptors and glial cells, and neuronal processes in the neuropil, providing a basis for electrotonic coupling among retinal cells. There are about 2000 axons (diameter less than 3 micron) in the optic nerve, possibly one from each retinal photoreceptor and neuron plus efferent fibres from the brain. Accessory nerves, containing a few large axons, are seen in the optic nerve sheath.