Vision begins with the capture of light by specialized, photoreceptive neurons in the retina. Two types of visual receptors, rods and cones, divide the intensity range of vision. Rods are extraordinarily sensitive; they “count” single photons. Cones are not so sensitive, but they are able to signal changes over an enormous range of intensities. In addition, most vertebrates have more than one spectral type of cone so that they can see in color.
Rods and cones extend a unique cilium, called the outer segment (OS), that converts light into an electrical signal. The outer segment houses a stack of hundreds of membranous disks, each one jam packed with the visual pigment, rhodopsin. A photoisomerization transforms rhodopsin into an activated form that ignites a biochemical cascade culminating in the closure of ion channels in the plasma membrane. The change in membrane potential moves passively through the inner segment (IS) to the synapse, where it regulates the release of neurotransmitter onto second order neurons.
Researchers in the department seek to understand the molecular mechanisms underlying visual transduction and adaptation by using electron microscopy to visualize the photoreceptor ultrastructure,
Disks of wild type mouse (B) shrink slightly and the incisure (arrow) lengthens in hemizygous rhodopsin knockout mouse rods (A) that express half as much rhodopsin. On the other hand, transgenic over-expression of rhodopsin by the rod causes disk enlargement and disappearance of the incisure (C).
single cell microspectrophotometry to measure what wavelengths are absorbed by rods and cones,
Absorption spectra of visual receptors from salamander, mouse, alligator and zebrafish. A. Salamanders have 2 spectral types of rods (black and green traces) and 3 types of cones (red, blue and violet traces). B. Mice have 1 type of rod (black trace) and 2 types of cones (not shown). C. Alligators have rods (black trace) and at least 1 type of cone (green trace). D. Zebrafish have rods (black trace) and 5 types of cones (red, green, blue, violet and magenta traces). Spectra were measured side-on from single cells except in mouse, for which the measurement was made through a layer of many rods to compensate for their very small outer segment diameter.
and single-cell electrophysiological recording as well as electroretinogram recording to “listen in” on how rods and cones respond to light.
Bicarbonate is a neuromodulator of phototransduction. A. Flash responses of a salamander rod in Ringer’s solution (black traces) accelerate in the presence of bicarbonate (red traces). The maximal response also grows larger. The effect of bicarbonate reverses upon washing with Ringer’s solution (gray traces). B. Bicarbonate shifts the rod responses to higher flash strengths (black to red), i.e. decreases relative sensitivity, an effect that disappears after washing (gray). Bicarbonate, ubiquitously present in the body, thus improves temporal resolution and sets the operating range of rods.
There is even a setup that combines microspectrophotometry and electrical recording!
Flash responses (left) and absorption spectra (right) from the same mouse rod in the dark adapted state (black), after bleaching (red) and following regeneration of the rhodopsin with exogenously applied 11-cis retinal (blue).
For further details about specific laboratories and their research, please click on the following links:
