Mandibular motor neurons of the caterpillar of the hawk moth Manduca sexta.

As part of a planned study of the central neural basis of feeding behaviour in larval Manduca sexta, the morphology and physiology of the mandibular motor system is here described. The gross neuroanatomy of the postoral head segments has been investigated, especially the course and structure of the mandibular nerves. The electrophysiology of the mandibular opener and closer muscles has been investigated by extra- and intracellular recording during feeding behaviour and during electrical stimulation of the motor nerve. All the muscle fibres examined are of the "fast," twitch type. Contraction is associated exclusively with locally or completely propagated overshooting action potentials, never with local junctional potentials. Control of the muscles is by recruitment of more motor units and/or an increase of frequency of action potentials. No inhibitory synaptic potentials could be found. The motor neurons of the mandibular muscles have been identified by cobalt backfills of the mandibular nerve, and characterized by intracellular recording and dye injection. There are 12 closer and 8 opener motor neurons. All motor neurons recorded so far evoke 1:1 twitches in the muscle, and none appear to be inhibitory. No GABA-immunoreactive axons could be found in the mandibular nerve.