EPAXIAL AND LIMB MUSCLE-ACTIVITY DURING SWIMMING AND TERRESTRIAL STEPPING IN THE ADULT NEWT, PLEURODELES WALTL

We have investigated the patterns of activation of epaxial musculature during both swimming and overground stepping in an adult newt (Pleuro deles waltl) with the use of electromyographic (EMG) recordings from d ifferent sites of the myomeric muscle dorsalis trunci along the body a xis. The locomotor patterns of some limb muscles have also been invest igated. During swimming, the epaxial myomeres are rhythmically active, with a strict alternation between opposite myomeres located at the sa me longitudinal site. The pattern of intersegmental coordination consi sts of three successively initiated waves of EMG activity passing post eriorly along the anterior trunk, the midtrunk, and the posterior trun k, respectively. Swimming is also characterized by a tonic activation of forelimb (dorsalis scapulae and extensor ulnae) and hindlimb (puboi schiotibialis and puboischiofemoralis internus) muscles and a rhythmic activation of muscles (latissimus dorsi and caudofemoralis) acting bo th on limb and body axis. The latter matched the activation pattern of epaxial myomeres at the similar vertebral level. During overground st epping, the midtrunk myomeres express single synchronous bursts wherea s the myomeres of the anterior trunk and those of the posterior trunk display a double bursting pattern in the form of two waves of EMG acti vity propagating in opposite directions. During overground stepping, t he limb muscles and muscles acting on both limb and body axis were fou nd to be rhythmically active and usually displayed a double bursting p attern. The main conclusion of this investigation is that the patterns of intersegmental coordination during both swimming and overground st epping in the adult newt are related to the presence of limbs and that they can be considered as hybrid lampreylike patterns. Thus it is hyp othesized that, in newt, a chain of coupled segmental oscillatory netw orks, similar to that which constitutes the central pattern generator (CPG) for swimming in the lamprey, can account for both trunk motor pa tterns if it is influenced by Limb CPGs in a way depending on the loco motor mode. During swimming, the segmental networks located close to t he girdles receive extra tonic excitation coming from the limb CPGs, w hereas during stepping, the axial CPGs are entrained to some extent by the limb oscillators.