Effects of light- and dark-adaptation processes on the generator potential of the Limulus eye

Direct intracellular measurement of non-linear postreceptor transfer functions in dark and light adaptation in Limulus

Using one microelectrode inserted into a Limulus retinular cell, simultaneous recordings were made of the receptor potential evoked by light falling on that retinular cell and of the optic nerve action potentials generated in the eccentric cell coupled to that retinular cell. The postreceptor transfer function for sensory quantity was thereby examined directly between the retinular cell receptor potential and the eccentric cell action potentials over a more extended range than had been reported in previous studies. This transfer function exhibited a strong non-linearity; the function had 3 discrete linear segments. It is similar to the function relating extrinsic current to spike frequency in cat motoneuron. Light adaptation induced a shift of the transfer function. There were certain phenomenological similarities between the effect of light adaptation on the receptor and postreceptor transfer functions. But there were also enough dissimilarities to suggest that the mechanism of postreceptor adaptation is different than the receptor adaptation mechanism.

Reduction of mechanical sensitivity in an insect mechanoreceptor correlated with destruction of its tubular body

The modified cilium (dendrite) of epithelial mechanoreceptors of insects contains microtubules in different arrangements: (1) microtubules distributed over the entire receptor and not fixed in a special configuration, therefore called free microtubules, (2) densely packed, interconnected microtubules called the tubular body, and (3) 9 doublet microtubules. These groups of microtubules have been discussed in relation to mechanotransduction. In a preceding paper the free microtubules were proved to be not involved in mechanotransduction. In this paper the hypothesis is examined that the tubular body may be essential to mechanotransduction. For this purpose the effect of the microtubule-disassembling drug vinblastine on both the tubular body and the sensitivity is examined in a femoral mechanoreceptor of the cricket Acheta domesticus. After 6- to 26-h exposure to vinblastine the tubular body is partially or totally destroyed. Simultaneously, mechanical sensitivity decays to zero. In contrast, the pacemaker property for nerve impulses of the apical dendritic segment is only slightly altered. We conclude from these results that the tubular body is essential to mechanotransduction. Three experiments in which a (small) response persisted, despite a totally destroyed tubular body, suggest that receptor potentials can in principle be evoked without an intact tubular body. In addition to the irreversible reduction of receptor sensitivity, vinblastine causes a reversible reduction during repetitive stimulation. This adaptation is supposed to be the consequence of altered properties of the tubular body.

Quantitative model for the electric response of invertebrate and vertebrate photoreceptors

We propose that the same mechanism which leads to light-adaptation in invertebrate photoreceptors is responsible for the excitation of the receptor potential in vertebrates. Several qualitative and quantitative features of the vertebrate receptor response support this hypothesis.

Simple photoreceptors in Limulus polyphemus

The "olfactory nerve," the endoparietal eye, and the rudimentary lateral eyes of Limulus (polyphemus) contain simple photoreceptor cells that duplicate many of the electrical responses of the retinular cells of the lateral eye; the responses are a receptor potential consisting of aninitial transient phase and a subsequent steady phase,low-amplitude fluctuations, and a small locally regenerative response to pulses of both light and current. Photic stimulation does not induce conducted action potentials, but does increase the membrane conductance. The receptor potentialrequires the presence of sodium ions in the external medium. Measurements of action and absorption spectra indicate a photopigment whose maximum absorption is of light with wavelength of 535 nanometers. The functional significance of these cells has not been ascertained.