Mechanosensory integration in the crayfish abdominal nervous system: Structural and physiological differences between interneurons with single and multiple spike initiating sites

Summary Cobalt backfills were used to demonstrate a population of approximately 50 paired interneurons in the 6th abdominal ganglion of the crayfish,Procambarus clarkii. Intracellular recordings from somata were used to study the response properties of individual interneurons, which were subsequently injected with Lucifer yellow. This report deals with 22 identified mechanosensory interneurons, which were each studied 2 to 20 times. (The total number of cells studied was 177). All but two of the interneurons could be assigned to one of two homogeneous classes, based on their receptive field sizes and four other consistent features: amplitude of soma spikes, duration of afterdischarge, presence of postsynaptic inhibition, and structure of the neuropilar processes. Unisegmental interneurons (Type I) (n=9) had excitatory receptive fields restricted to one segment, small soma spikes, little afterdischarge, and received extensive postsynaptic inhibition from contralateral and occasionally anterior sensory fields. All of these interneurons had a large diameter neuropilar segment (integrating segment) that was separated from the main axon by a constricted region. Multisegmental interneurons (Type II) (n=11) had excitatory receptive fields of at least six hemisegments (one half of the abdomen), large (sometimes overshooting) soma spikes, prolonged afterdischarge, and little evidence of postsynaptic inhibition. These interneurons lacked any expanded region of the dendritic tree that could be called an integrating segment. Anomalous interneurons (n=2) had multisegmental receptive fields, but in all other respects they resembled unisegmental interneurons, although their soma spikes were somewhat larger in amplitude.We hypothesize that the fundamental difference between the two main kinds of interneurons is that Type II interneurons have multiple spike initiating sites distributed throughout their dendritic trees, with any site being capable of initiating a spike that propagates to the main axon, while Type I interneurons have a single spike initiating site. The properties of anomalous interneurons are consistent with them having a single spike initiating site in each of several ganglia.