Local interneurons in the terminal abdominal ganglion of the crayfish

Abstract

1. On the basis of comparisons between total cell counts and cell counts of cobalt backfills of all roots and connectives leaving the ganglion, we estimate that 38–50% of the approximately 650 neurons in the last abdominal ganglion of the crayfish are local interneurons. 2. The soma size distributions for all of the neurons and for projecting neurons were compared. The comparison indicates that most local neurons are small. All neurons larger than 40 μm (n = 121) have axons that leave the ganglion, while 77 % of neurons smaller than 21 μm (n = 140) are local interneurons (Figs. 1,2). 3. We have recorded and filled with Lucifer yellow 63 local interneurons from which 15 unambiguously different types could be recognized. Five criteria were used in our initial classification. In addition to structure, we noted if each local interneuron was spiking or nonspiking, and, in our most complete experiments, if it was capable of affecting motor activity or projecting interneuron activity when depolarized. We also tried to determine its optimal sensory modality and receptive field. 4. Of the 15 interneuron types we encountered, 5 types (represented by 11 fills) were unilateral, nonspiking, premotor interneurons in the uropod motor circuits (Figs. 3–6). 5. Two types of local premotor interneurons were bilateral and spiking. One excited exopodite motor neurons on one side only. Our evidence suggests that the cells had a nonspiking input region on one side and a spiking output region on the other (Figs. 7–9). 6. Six other types of interneuron were encountered for which we have no evidence of motor effects. All of these neurons had bilateral but asymmetric processes, and were spiking on the putative output side. Some of these neurons appear to be part of sensory processing circuits on the basis of clear modality, distinct receptive field, and short-latency response to sensory input (Figs. 10–12). 7. Two nonspiking, bilateral interneurons were found (Fig. 13). These cells had very large processes which permitted them to be penetrated repeatedly and studied in detail. Although nonspiking, these neurons had separate input and output sides. They are involved in lateral inhibition within the sensory circuits (Reichert et al., submitted). 8. Our results indicate considerable heterogeneity in the properties and functions of local interneurons. Three basic modes of operation were found: (i) five types of interneurons functioned without spikes and may have diffuse input and output; (ii) eight types of bilateral interneurons had properties suggestive of nonspiking input and spiking output sides; and (iii) two bilateral neurons were nonspiking but had distinct input and output sides.