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Photoreceptors and Image Processing: Part 1A Lecture NotesKey Words and Termseye, vision, retina, rhodopsin, 11-cis retinal isomerization, threshold sensitivity of human vision, single photon response, electrophysiology, phototransduction, signal amplification, enzyme cascade Lecture NotesSimilarities between the vertebrate eye and a camera. The retina is analogous to the film in a camera: both can adjust to different levels of illumination, and both can focus on objects at different distances. Vertebrate and insect eyes are very different anatomically, but they use related transcriptional control genes, suggesting a common evolutionary origin. The vertebrate retina has three layers of neurons, with photoreceptors occupying the outermost layer. Vertebrate photoreceptors (rods and cones) are highly specialized cells, with an outer segment that is filled with membranes. Visual pigments are integral membrane G-protein coupled receptors. Cis to trans photoisomerization of the 11-cis retinal chromophore is the fundamental event in phototransduction.
A recently discovered intrinsically photo-sensitive retinal ganglion cell controls non-image forming vision (circadian rhythms and pupil construction).
Selig Hecht and his colleagues determined the absolute threshold for human vision and found it to be remarkably low: 5-7 absorbed photons are sufficient to register a response.
The light response of single photoreceptors can be measured with a suction electrode. Under dim light conditions, single photon responses can be recorded, and in the absence of light there are rare single-photon like responses (due to thermal isomerization of 11-cis retinal).
The retinal ganglion cell solves the problem of reliably detecting a weak signal in the presence of thermal noise by acting as a coincidence detector: under conditions of complete dark adaptation, 5-7 single photon responses must arrive within a few milliseconds to be considered a real light flash and not just background thermal noise.
Phototransduction uses an enzymatic amplification cascade to convert a small stimulus (one absorbed photon) into a large membrane voltage signal. |
