Morphology and responses to light of the somata, axons, and terminal regions of individual photoreceptors of the giant barnacle

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Abstract
The median eye of the giant barnacle Balanus nubilus, comprises 4 large photoreceptor neurons which are visible under the dissecting microscope for almost their entire length. Their structure and responses to light were recorded in the somata, axons and terminal regions. The photoreceptor somata, each 40-70 .mu.m in diameter, extend numerous light-sensitive dendritic processes whose membranes form rhabdomeric microvilli. Recordings from the soma show that dim light evokes a steady, noisy depolarization; brighter light elicits a transient depolarization which decays to a maintained plateau, followed by a hyperpolarization when the light is turned off. Light-induced voltage changes spread decrementally along the photoreceptor axons, which average 10 mm in length and 25 .mu.m in diameter. In distal parts of the axon, near the presynaptic terminals, depolarizations and hyperpolarizations can be as large as 50% or more of their values in the soma. There is no demonstrable electrical coupling between photoreceptor neurons as shown by simultaneous recordings from 2 receptor somata or axons. Each photoreceptor axon enters the mid line commissure of the supraesophageal ganglion, bifurcates and arborizes in a restricted zone of neuropil in each hemiganglion. The lare size of the terminal processes of these neurons and their characteristic cytoplasmic inclusions enable them to be traced with the EM as they branch in the neuropil. The terminal processes subdivide and end in 1-3 .mu.m diameter branches which are the sites of apparently chemical synapses. Vesicle-containing, presynaptic loci on these processes of the receptor cell are invariably apposed to 2 post-synaptic processes from cells as yet unidentified.