Functional morphology of the telson‐uropod stretch receptor in the sand crab Emerita analoga

Abstract

The telson-uropod stretch receptor in Emerita analoga belongs to a segmental array of axial-coxal receptors having sensory neurons with central somata. It consists of an elastic strand innervated by four giant, nonspiking, mechanoreceptive neurons. We examined the elastic (sensory) strand by light and electron microscopy to discover whether 1) functional differences between the four sensory neurons could be surmised from their peripheral terminations and 2) the process of mechano-electric transduction in this type of receptor might be revealed by the ultrastructure of the dendritic terminals. The elastic strand is divided into nonoverlapping domains by the mechanoreceptive neurons. We discerned no differences other than their serial order along the strand. Each neuron has three morphological zones in the periphery. In the zone of dendrite entry, the dendrite bifurcates into 40-60 microns primary branches that enter the strand at acute to right angles and turn rostrally and caudally along the strand's axis. The zone of branching consists of 4-35 microns diameter dendritic branches within the strand. Approximately 21,000 dendritic tips (diameters, 0.1-0.7 micron; lengths, 3-20 microns) per sensory neuron arise via short stubs from all levels of the zone of branching and constitute the zone of dendrite termination. The repeated bifurcations of dendrites within the zone of branching take place by radial intrusion of longitudinally oriented, partitions of extracellular matrix each of which extends the length of the pair of dendritic branches it divides. Tips project into these partitions. Since they change shape when the elastic strand is stretched, which compresses the extracellular matrix of the partitions, they are probably the site of mechanosensory transduction. Cross-sectional profiles of dendritic tips are significantly larger but less numerous in stretched than in relaxed receptors. We propose a model in which stretch compresses distal portions of tips until they are too small to be recognized. The large profiles are the proximal portions of tips that have been expanded by hydrostatic pressure. The very large number of dendritic tips/nonspiking mechanoreceptor may be what endows each neuron with 1) high sensitivity to stretch of the elastic strand and 2) ability to respond to a broader range of tensions than is usual for single sensory cells (the entire range experienced by the stretch receptor).