Neural Photoreception in a Lamellibranch Mollusc

Open Access
Publisher Website
Google Scholar
Abstract
The pallial nerves of Spisula solidissima each contain a single afferent nerve fiber which responds directly to illumination of the nerve, and apparently mediates the "shadow" response of siphon retraction. These units show constant-frequency spontaneous activity in the dark; illumination abruptly inhibits this discharge, and cessation of the light stimulus then evokes a prolonged burst of impulses at high frequency (the off-response). Impulses are initiated at a point near the visceral ganglion, and propagated unidirectionally toward it. Stimulation with monochromatic light has revealed that more than one photoreceptor pigment is involved, since the discharge patterns evoked are wavelength-specific. Inhibition is relatively prominent at short wavelengths, excitation at long wavelengths. Following selective adaptation with blue light, "on" responses can be produced with red stimuli, demonstrating the unmasking of an excitatory event which takes place during illumination. The two photoreceptor pigments may be segregated in two or more cells presynaptic to the recorded unit, or,[long dash]more likely[long dash]may both be contained in the same cells. The spectral sensitivity function for inhibition shows a single maximum at 540 m[mu], and is probably dependent upon a carotenoid pigment. No photoreceptor function has been demonstrated for a hemoprotein, apparently identical with cytochrome h, which occurs in high concentration in Spisula nerve.