The response patterns of single receptors in the crustacean statocyst

Abstract

The behaviour of single receptors in the statocyst of the lobster Homarus americanus was studied by moving single receptor hairs or the statolith with a micro-needle while recording the electrical activity from single sensory neurons innervating these hairs. After removing the statolith, the sensory hairs formerly in contact with it assume an angle of about 40° from the cyst floor and in this resting position the associated sensory neuron discharges spontaneously. Movement of a statolith hair toward the vertical causes an increase in frequency during the first 15°, however further displacement in this dirction causes the frequency to drop until it is blocked entirely at a vertical deflexion of 40° from the resting position. The block is reversible and the discharge returns as the hair is moved back toward its resting position. The hair has a peak position where discharge frequency reaches a maximum. Deflexion away from the peak position in either direction decreases the rate of discharge. The hairs may be set below their peak position thus resulting in the bell-shaped response curve observed with stimulation of this receptor type. These receptors function as static position receptors by firing at different non-adapting rates corresponding to specific degrees of hair deflexion. The threshold displacement of the statolith hair nerve terminal is approximately 0.5 μ . Different statolith hair receptors are set at various points within their response range by their relation to the sand mass. This provides a means for resolving the ambiguity inherent in the bell-shaped response curve of the single receptor and points to the importance of the multiplication of sensory channels within a single modality. The thread hairs, which are responsible for acceleration sensitivity, increase in frequency with either posterior or ventral deflexion and decrease during either anterior or dorsal deflexion. This differential response to movement accounts for the burst-depression type of coding obtained from acceleration receptors in the intact organ. It is argued that duplication of sensory channels is necessary to retain sensitivity and reliability in a sensory system where the response is superimposed upon an aperiodic back­ground discharge.