SYNAPTIC-INTERACTIONS BETWEEN A MUSCLE-ASSOCIATED PROPRIOCEPTOR AND BODY-WALL MUSCLE MOTOR-NEURONS IN LARVAL AND ADULT MANDUCA-SEXTA

1. Synaptic remodeling of a proprioceptive circuit during metamorphosi s of the insect, Manduca sexta, is described. The stretch receptor org an is a muscle-associated proprioceptor that is innervated by a single sensory neuron. It inserts dorsolaterally in the abdomen in parallel with the intersegmental muscles of each abdominal segment. The synapti c input from the stretch receptor sensory neuron to select abdominal i nternal (intersegmental) and external muscle motor neurons was charact erized in both the larva and adult. 2. In the larva, the sensory neuro n provides excitatory synaptic input to motor neurons that innervate m uscles ipsilateral to the stretch receptor organ in the body wall; the strongest excitatory synaptic input is to motor neurons that innervat e targets in close proximity to the stretch receptor organ. The sensor y neuron also provides excitatory synaptic input to motor neurons that innervate contralateral, dorsal targets. However, it inhibits, appare ntly through a polysynaptic pathway, motor neurons innervating contral ateral, lateral, and ventral targets. 3. The synaptic input to interse gmental muscle motor neurons from the stretch receptor sensory neuron changes during metamorphosis. In contrast to the larva, all motor neur ons recorded in the adult (both ipsilateral and contralateral) were ex cited by the sensory neuron. As in the larva, the adult sensory neuron provides the strongest excitatory synaptic input to motor neurons inn ervating targets in close proximity to the stretch receptor organ. 4. The proprioceptive input to the body wall muscle motor neurons was eva luated to determine whether the pathway is monosynaptic, as has been d escribed in other systems. Spike-triggered signal averaging and synapt ic latency measurements suggested that the strongest excitatory synapt ic input to motor neurons involves a monosynaptic pathway.