Sensory initiation of a co-ordinated motor response: Synaptic excitation underlying simple decision-making

Abstract

Animals take time to make co-ordinated motor responses to a stimulus. How can sensory input initiate organized movements, activating all necessary elements at the same time as avoiding inappropriate co-excitation of antagonistic muscles? In vertebrates, this process usually results in the activation of reticulospinal pathways. Young Xenopus tadpoles can respond to head-skin touch by swimming, which may start on either side. We investigate how motor networks in the brain are organized, and whether asymmetries in touch sensory pathways avoid co-activation of antagonists at the same time as producing co-ordinated movements. We record from key reticulospinal neurons in the network controlling swimming. When the head skin is stimulated unilaterally, excitation builds up slowly and asymmetrically in these neurons such that those on both sides do not fire synchronously. This build-up of excitation to threshold is the key decision-making step and determines whether swimming will start, as well as on which side. In response to stronger stimuli, the stimulated side tends to 'win' because excitation from a shorter, trigeminal nucleus pathway becomes reliable and can initiate swimming earlier on the stimulated side. When this pathway fails or is lesioned, swimming starts later and on the unstimulated side. Stochasticity in the trigeminal nucleus pathway allows unpredictable turning behaviour to weaker stimuli, conferring potential survival benefits. We locate the longer, commissural sensory pathway carrying excitation to the unstimulated side and record from its neurons. These neurons fire to head-skin stimuli but excite reticulospinal neurons indirectly. We propose that asymmetries in the sensory pathways exciting brainstem reticulospinal neurons ensure alternating and co-ordinated swimming activity from the start.