Response properties of chemosensory peg sensilla on the pectines of scorpions.

By behavioral and anatomical criteria, the pectinal sensory appendages of scorpions appear to be chemoreceptive organs specialized for detection of substances on substrates. These comb-like, midventral appendages contain tens of thousands of minute (< 5 microns), truncated setae, called pegs, arranged in dense, two-dimensional arrays on the ventral surface. In this study we used extracellular recording techniques to examine spontaneous and stimulated activity of sensory neurons within individual pegs. Chronic recordings lasting several days showed long-term fluctuations in spontaneous activity of sensory units in single peg sensilla, with peak activity coinciding with the animal's normal period of foraging. Several units were identified by the stereotypical waveforms of action potentials they elicit. Near-range olfactory stimulation of peg sensilla by volatile alcohols, aldehydes, ketones, esters, and carboxylic acids produced dose-dependent patterns of neural response. Contact stimulation with these chemicals, or water, or mechanical deflection of the peg tip also evoked activity in identifiable units. The peg sensilla appear to be broadly sensitive to odorants and tastants, suggesting they function similarly to the antennae of mandibulate arthropods.

Electrophysiological evidence of synaptic interactions within chemosensory sensilla of scorpion pectines.

The pectines of scorpions are ventral bilateral appendages supporting 10(4)-10(5) chemosensory sensilla called pegs. Each peg contains 10-18 sensory neurons, some of which show ultrastructural evidence of axo-axonic synapses with other sensory neurons in the same sensillum. In extracellular recordings from single-peg sensilla, individual sensory units can be distinguished by impulse waveform and firing frequency. Cross-correlation analysis of impulse activity showed that at least two of these units, types 'A1' and 'A2', are inhibited during the 100-ms period immediately following activity of a third unit, type 'B'. This interaction between sensory units in a single sensillum also occurs in surgically isolated pectines, indicating that it does not involve efferent feedback from the central nervous system. Other sensillar neurons appear to have excitatory interactions. Thus, in scorpion pectine, chemosensory information undergoes some form of processing within individual sensilla prior to its relay to the CNS, making this an unusually accessible preparation for study of first-order chemosensory processing events.