Many animals make strong lashing movements when they are grasped. Amphibian tadpoles are no exception and the struggling movements of the Xenopus tadpole have been studied extensively. When the tadpole is held strong waves of bending start near the tail and propagate towards the head. This helps the tadpole to escape backwards.
Young Xenopus can also swim so they have two distinct rhythmic locomotor "gaits": swimming and struggling. This provides an opportunity to explore how vertebrates switch between motor patterns. (Soffe, 1991).
(Soffe, 1997) Both swimming and struggling can be evoked by stimulation of a single skin sensory pathway: brief Rohon-Beard sensory neuron discharge triggers long lasting swimming while their sustained discharge elicits struggling. This ability of the same sensory neurons to determine the pattern of motor output, just by their pattern of discharge, provides a simple form of behavioural selection according to stimulus.
(Soffe, 1993) Swimming and struggling are both generated by neural circuitry within the spinal cord. Although behaviorally distinct, their motor patterns employ the same kinds of synaptic drive. This seems to reflect the common nature of much of their premotor circuitry. Extra neurons are recruited to the spinal circuitry during struggling, but only from within neuron classes that also participate in swimming.
(Soffe, 1996) Expression of the struggling pattern, like swimming, is actually not critically dependent on sensory discharge. Both patterns can be elicited artificially by applied pharmacological excitants. Transient applications of high concentrations of glutamate or its agonists produce struggling; lower concentrations maintain swimming. Expression of the two very different motor patterns for swimming or struggling in this simple vertebrate system can apparently be controlled by the level of excitation within the spinal motor circuitry, and need not involve the activity of external neuromodulators.