ESTIMATION OF THE SIZE AND DIRECTIONAL OUTPUT OF FUNCTIONAL-GROUPS OFINTERNEURONS UNDERLYING ABDOMINAL POSITIONING BEHAVIORS IN CRAYFISH

Quantitative studies were made of a large population of interneurons t hat controls postural flexion and extension of the crayfish abdomen. T he number of interneurons needed to produce a motor program was estima ted by stimulating a single abdominal positioning interneuron and reco rding interneuronal activity that was evoked from rostral and caudal c onnectives in an isolated abdominal nerve cord. We also examined the r ole that these functional groups have in producing a stronger motor ou tput in either a rostral or caudal direction and thus specifying vario us abdominal geometrics. The average number of interneurons responding to stimulation of a single abdominal positioning interneuron was 32 ( range: 3-50; n = 27). The average number of interneurons that decrease d activity was 10 (range: 2-32). Of 653 activated interneurons from 20 preparations, approximately 43% fired between 2 and 5 Hz, 33% fired b etween 6 and 15 Hz, and 25% fired >15 Hz. The size of a recruited grou p was usually but not always correlated with the strength of its motor response or with the direction of motor bias. Therefore, the contribu tion of a group may depend upon the number of active elements as well as synaptic efficacy. (C) 1997 Wiley-Liss, Inc.