MORPHOLOGY AND PHYSIOLOGY OF THE THORACIC AND ABDOMINAL STRETCH RECEPTORS OF THE ISOPOD CRUSTACEAN LIGIA EXOTICA

摘要

In the terrestrial isopod Ligia exotica, paired stretch receptors, each comprising a separate rapidly and slowly adapting receptor cell, were found in the third to eighth thoracic segments and first five abdominal segments, The dendritic endings of the two sensory cells in each receptor terminate on a common receptor muscle; the cross-striation of this fiber is homogeneous throughout the segments. But the dendritic endings of the receptor cells differ: the rapidly adapting cell has a club-shaped ending restricted to the middle of the receptor muscle, whereas the slowly adapting receptor cell has a bifurcating ending that extends along the entire length of the muscle. Stretch applied to the receptor muscle evokes characteristically different responses in the two sensory cells, The slowly adapting receptor cell has a lower firing threshold and fires continuously for the duration of the stretch, while the rapidly adapting receptor cell has a higher threshold and fires a brief burst at the beginning of the stimulus. However, application of an intense stimulus will evoke continuous firing of the rapidly adapting receptor, which then changes to intermittent bursts. The adaptive significance of such a response is not known, nor is it likely to occur in nature. However, this unusual response is intrinsic to the rapidly adapting cell, as it can be evoked by current injection. In the second thoracic segment, instead of rapidly and slowly adapting cells, we found a single slowly adapting cell with a long robust dendrite attached to the extensor muscle.