Crayfish photoreceptors exhibit a voltage-dependent potassium conductance, G(K), that is generally similar to the delayed rectifier channel described in neurons and other arthropod retinular cells. G(K) activation (i.e. the apparent threshold, Vth) occurs near the resting potential and G(K) is substantially reduced by 25 mM extracellular tetraethylammonium (TEA) and by intracellular Cs+ injections. Light exposure, sufficient to reduce visual sensitivity 100-fold, increases Vth (shifts it in the depolarizing direction) by about 20 mV. The light-dependent change in Vth does not depend upon the corresponding increase (depolarization) of the steady-state membrane potential nor does it depend upon inward calcium currents. Vth is slightly influenced by fluctuations in Ko associated with the light-elicited currents. During light exposure Ko (measured with K+-sensitive electrodes) increases by 2.1 mM (equivalent to an 8 mV increase in E(K)). This increase in E(K) makes only a modest contribution to the light-dependent change in Vth as determined by perfusion with high potassium salines. Intracellular calcium injections increase Vth by 10 to 20 mV and reduce visual sensitivity by 5- to 10-fold. The results imply that during exposure to high levels of illumination, K+ currents at the steady-state membrane potential are diminished by a calcium-dependent change in G(K) gating and, to a smaller degree, by a reduced K+ concentration gradient. It is notable that Ca2+ appears to inhibit both G(K) and the light-elicited conductance from both inside and outside the plasma membrane. As a consequence of the light-dependent change in Vth, G(K) makes only modest contributions to the changes in sensitivity and speed normally associated with light adaptation. These functions are regulated by the transduction pathway and are revealed at the resting potential in the time course and magnitude of the light-elicited currents.

Adaptation, Ocular↗

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Visual responses of crayfish ocular motoneurons: an information theoretical analysis.

Motoneuron responses were elicited by global visual motion and stepwise displacements of an illuminated stripe. Stimulus protocols were identical to those used in previous behavioral studies of compensatory eyestalk reflexes. The firing rates and directional selectivity of the motoneuron responses were measured with respect to four stimulus dimensions (spatial frequency, contrast, angular displacement and velocity). The directional selectivity of the motoneuron response was correlated to the previously measured gain of the reflex for each stimulus dimension. The information theoretical analysis is based upon Kullback-Leibler (K-L) distances which measure the dissimilarity of responses to different stimuli. K-L distances for single neurons are strongly influenced by the mean rate difference of the responses to any pair of stimuli. Because of redundancy, the joint K-L distances of pairs of neurons were less than the sum of the K-L distances of the individual neurons. Furthermore, the joint K-L distances were only weakly influenced by correlations among coactivated neurons. For most of the stimulus dimensions, the K-L distances of single motoneurons were not sufficient to account for the stimulus discriminations exhibited by the eyestalk reflex which typically required the summed output of 2 to 5 motoneurons. Thus the behaviorally relevant information is encoded in the motoneuron ensemble. The minimum time required to discriminate the direction of motion (the encoding window) for a single motoneuron is about 380 to 480 ms (including a 175 ms response latency) for stepwise displacements and up to 1.0 s for global motion. During this period a motoneuron fires 2 to 3 impulses.

Action Potentials↗

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[Application of dry lyophilized vicasal plasma for the prevention and therapy of hemorrhages in otolaryngological practice].

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Freeze Drying↗