Distributed motor pattern underlying whole-body shortening in the medicinal leech

Whole-body shortening was studied in the leech, Hirudo medicinalis, by a co mbination of videomicroscopy and multielectrode recordings. Video microscop y was used to monitor the animal behavior and muscle contraction. Eight suc tion pipettes were used to obtain simultaneous electrical recordings from f ine roots emerging from ganglia. This vital escape reaction was rather repr oducible. The coefficient of variation of the animal contraction during who le-body shortening was between 0.2 and 0.3. The great majority of all leech longitudinal motoneurons were activated during this escape reaction, in pa rticular motoneurons 3, 4, 5, 8, 107, 108, and L. The firing pattern of all these motoneurons was poorly reproducible from trial to trial, and the coe fficient of variation of their firing varied between 0.3 and 1.5 for differ ent motoneurons. The electrical activity of pairs of coactivated motoneuron s did not show any sign of correlation over a time window of 100 ms. Only t he left and right motoneurons L in the same ganglion had a correlated firin g pattern, resulting from their strong electrical coupling. As a consequenc e of the low correlation between coactivated motoneurons, the global electr ical activity during whole-body shortening became reproducible with a coeff icient of variation below 0.3 during maximal contraction. These results ind icate that whole-body shortening is mediated by the coactivation of a large fraction of all leech motoneurons, i.e., it is a distributed process, and that coactivated motoneurons exhibit a significant statistical independence . Probably due to this statistical independence this vital escape reaction is smooth and reproducible.