Biomedical subjects
Publications and source records attributed to J Nolte.
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Two types of photoreceptors are found in the median ocellus of Limulus. One type is maximally sensitive to ultraviolet (UV) light, the other to green light; they are called UV and VIS cells, respectively. Biphasic receptor potentials, consisting of a small initial hyperpolarizing phase and a later slow depolarizing phase, can be recorded from both receptor types. These biphasic responses are elicited in UV cells in response to long-wavelength light, and in VIS cells in response to ultraviolet light. Another type of hyperpolarizing response can be recorded in UV cells: after a bright ultraviolet stimulus, the cell remains depolarized; long-wavelength light rapidly returns the membrane potential to its value preceding ultraviolet illumination (this long-wavelength-induced potential change is called a "repolarizing response"). Also, a long-wavelength stimulus superimposed during a UV stimulus elicits a sustained repolarizing response. A third cell type (arhabdomeric cell) found in the median ocellus generates large action potentials and is maximally sensitive to UV light. Biphasic responses and repolarizing responses also can be recorded from arhabdomeric cells. The retina is divided into groups of cells; both UV cells and VIS cells can occur in the same group. UV cells in the same group are electrically coupled to one another and to an arhabdomeric cell.
In the UV-sensitive photoreceptors of the median ocellus (UV cells), prolonged depolarizing afterpotentials are seen following a bright UV stimulus. These afterpotentials are abolished by long-wavelength light. During a bright UV stimulus, long-wavelength light elicits a sustained negative-going response. These responses to long-wavelength light are called repolarizing responses. The spectral sensitivity curve for the repolarizing responses peaks at 480 nm; it is the only spectral sensitivity curve for a median ocellus electrical response known to peak at 480 nm. The reversal potentials of the repolarizing response and the depolarizing receptor potential are the same, and change in the same way when the external sodium ion concentration is reduced. We propose that the generation of repolarizing responses involves a thermally stable intermediate of the UV-sensitive photopigment of UV cells.
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Spectral sensitivity curves can be distorted by screening pigments. We have determined whether this is true for Limulus polyphemus by determining, from receptor potentials recorded using intracellular microelectrodes, spectral sensitivity curves for normal animals and for white-eyed animals (which lack screening pigment). Our results show: (a) In median ocelli, the curve for UV-sensitive receptor cells peaks at 360 nm and does not depend on the presence of screening pigment, (b) The curve for ventral eye photoreceptors is identical to that for retinular cells from the lateral eyes of white-eyed animals and peaks at 520-525 nm. (c) In normal lateral eyes, when the stimulating light passes through screening pigment, the curve indicates relatively more sensitivity in the red region of the spectrum than does the curve for white-eyed animals. Therefore, the screening pigment is probably red-transmitting, (d) In median ocelli, the curve for visible-sensitive cells peaks at 525 nm and is approximately the same whether the ocelli are from normal or white-eyed animals. However, the curve is significantly broader than that for ventral eyes and for lateral eyes from white-eyed animals.
The spectral sensitivities of single Limulus median ocellus photoreceptors have been determined from records of receptor potentials obtained using intracellular microelectrodes. One class of receptors, called UV cells (ultraviolet cells), depolarizes to near-UV light and is maximally sensitive at 360 nm; a Dartnall template fits the spectral sensitivity curve. A second class of receptors, called visible cells, depolarizes to visible light; the spectral sensitivity curve is fit by a Dartnall template with lambda(max) at 530 nm. Dark-adapted UV cells are about 2 log units more sensitive than dark-adapted visible cells. UV cells respond with a small hyperpolarization to visible light and the spectral sensitivity curve for this hyperpolarization peaks at 525-550 nm. Visible cells respond with a small hyperpolarization to UV light, and the spectral sensitivity curve for this response peaks at 350-375 nm. Rarely, a double-peaked (360 and 530 nm) spectral sensitivity curve is obtained; two photopigments are involved, as revealed by chromatic adaptation experiments. Thus there may be a small third class of receptor cells containing two photopigments.
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There are two classes of photoreceptor cells in the median ocellus of Limulus. One class of cells respond to long wavelength (visible) stimuli with a depolarizing receptor potential and to near ultraviolet light with a biphasic, initially hyperpolarizing, receptor potential. The other class of receptors respond with a depolarization to near ultraviolet and with a biphasic response to visible light. In the latter type of cell, visible light can counteract the depolarization elicited by near ultraviolet light. The evidence suggests that there are two photopigments in each cell and that both are involved in the generation of receptor potential.