Central and peripheral controls of swimming in anuran larvae.

The isolated nervous system of the bullfrog tadpole (Rana catesbeiana or clamitans) spontaneously exhibits patterned motoneuronal discharges which are the basis for swimming. In vitro recordings from the ventral roots showed that motoneurons on one side of the cord burst in alternation with those on the other side. On each side of the cord, motoneuron bursts occurred in synchrony the entire length of the cord. Patterned activity was found only in the medial motoneurons which innervate the muscles used for swimming. Lateral motoneurons innervating the limbs exhibited unpatterned bursting. The pattern generating mechanism is distributed along the length of the spinal cord. Proprioceptive information entering via the dorsal roots is not necessary for generation of the basic pattern of alternating activity, but interacts with the centrally generated pattern to produce a rostrocaudal lag of activity. These results are compared to those found using the dogfish and possible ontogenetic complications discussed.