Midbrain control of the swim behaviour of the Xenopus laevis tadpole

The survival of all animals is crucially dependent on motor responses that are timely and well-coordinated. In vertebrates, the central pattern generators (CPG) in the spinal cord, which generate the rhythmic pattern of locomotion, are influenced by descending commands from brainstem structures, particularly the midbrain. However, it remains unclear how the midbrain neurons interact with the CPG networks to initiate and control locomotion. Using behavioural and electrophysiological techniques, this study assessed the effects the disconnection of the midbrain from the rest of the brainstem has on the following parameters: (1) the side of the initiation of swimming, (2) the latency to swim initiation and (3) frequency of swimming. My behavioural data showed that the lesions did not change the 'preference' for first contralateral response seen among the control tadpoles. Differently, the latency to swim initiation in the lesioned tadpoles was significantly shorter than that of the control group. Extracellular ventral root recordings, indicative of fictive swimming, indicated that the control tadpoles strongly 'favoured' initiation on one side over the other based on the strength of the electrical stimulus applied to the trunk skin. These strong side preferences were eliminated following midbrain-hindbrain border (MHB) lesions, suggesting a lack of sensory discrimination caused by the midbrain lesion. In addition, within the same set of experiments I observed that the lesioned tadpoles responded significantly quicker than the controls to distinct strengths of electrical stimulus (at threshold and suprathreshold for swim initiation). Finally, assessment of the fictive swim frequency at the beginning of each swim episode revealed that MHB-lesioned tadpoles swim at significantly higher frequency in response to threshold electrical stimulus only. The conclusion, therefore, is that the midbrain is highly instrumental in survival by ensuring the timely initiation of locomotion. The tadpole is no different to any other animal and even at this early developmental stage, the tadpole needs to make the right motor decisions.