In Search of the Motor Pattern Generator for Snapping in Toads

Abstract

The hypothesis that a neuronal network interposed between the output elements of the optic tectum, and hypoglossal and other cranial motoneurons subserves ballistic snapping in anurans is critically examined. Morphological and electrophysiological data have not revealed a direct monosynaptic connection between the optic tectum and hypoglossal motoneurons. On the contrary, the available evidence supports the presence of interneurons mediating impulse traffic from the tectum to cranial motoneurons. It is argued that these interneurons are part of a motor pattern generator (MPG). The afferent connections of the bulbar reticular formation (RetF) and the hypoglossal nucleus were studied using the horseradish peroxidase method. The results substantiate the hypothesis that a vital component of the neural substrate for snapping generation is located in the medial RetF (MRF), has access to the motoneurons relevant for snapping, and receives inputs from the tectum and the torus semicircularis. Bilateral tectal and toral inputs to MRF suggest that the MPG consists of two unilateral subunits each having independent access to the relevant motoneurons. This is consistent with the observations that toads with unilateral tectal, or medullary lesions are still capable of snapping. Connections between the nucleus of the solitary tract and the hypoglossal nucleus may be important for elementary bulbar reflexes and manipulation of prey within the mouth. Interconnections between hypoglossal nucleus and other craniomotor nuclei might play a role in the coordination of tongue and jaw musculature necessary for precise timing of the ballistic snapping movement. From a review of the available evidence, a more detailed scheme of the MPG is proposed.