The Electrical Properties of A Crustacean Sensory Dendrite

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Abstract
The input properties and the response to stretch of a coxal receptor, the S fibre of the crab Scylla serrata, were studied using two and three intracellular microelectrodes. In the relaxed receptor the transmembrane potential ranged from about –60 to –70 mV, and the input resistance, RT, from 1 to 3 MΩ. The input IV relationship, studied by injecting slow-rising current ramps, was not linear either in the hyperpolarizing or in the depolarizing quadrants. Low values of RT and a linear IV relationship were associated with a large leakage of the microelectrodes. The response to step stretches was complex, consisting of an initial depolarizing transient, Va, and a steady-state depolarizing plateau, Vs. Both Va and Va propagated with decrement in the fibre which was about 9 mm long. The spatial decrement of Va and Vs was equal to that of the response to distally injected current pulses of comparable duration and amplitude. On the basis of the spatial decrement of both Va and Vs the dendrite can be considered equivalent, for current flowing from its distal to its proximal end, to a semi-infinite cable having a length constant of between 4 and 6 cm. The voltage transients recorded in response to long current pulses reached 84% of their final value in a time (t84%)ranging from 150 to 180 ms in fibres in which RT was 2 MΩ or larger. t84% was smaller in fibres having a lower RT. The time course of the transients recorded in response to injected current pulses deviated from the semi-infinite cable model in a manner suggesting the presence of a partial short circuit. For this reason the membrane time constant of the fibre is considered larger (by an undetermined amount) than t84%. The fibre presented less resistance to current flowing from its proximal to its distal end than to current flowing in the opposite direction. For this reason, and also because of the time course of the voltage transient, it is concluded that the distal sensory endings of the fibre have the properties of a leaky end termination, even in the non-stimulated receptors.