Novel dual innervation of a larval proleg muscle by two similar motoneurons in the tobacco hornworm Manduca sexta.

In Manduca sexta, the accessory planta retractor muscle (APRM), which retracts the larval proleg, is innervated by two excitatory motoneurons, the accessory planta retractor motoneurons (APRs). These muscles and motoneurons have been the focus of a number of developmental and behavioral studies. The present study investigated properties of the pair of APRs that innervate each APRM and determined their pattern of innervation of APRM fibers. Members of APR pairs could not be distinguished by their anatomical or electrical properties (resting membrane potential, input resistance and spike threshold). Spontaneous synaptic inputs to members of APR pairs were highly correlated, whereas spontaneous synaptic inputs to APRs and functionally dissimilar motoneurons were not well correlated. Synaptic inputs from identified mechanosensory neurons and interneurons to the two APRs were qualitatively similar, but the magnitude of the response to sensory stimulation sometimes differed within a pair. Both APRs produced large, rapidly rising excitatory junction potentials in APRM fibers. Within the APRM, some fibers were singly innervated by one or the other APR while the remaining fibers were dually innervated by both APRs. In dually innervated fibers, the motor terminals of the two APRs were spatially segregated. This innervation pattern appears to be unique among insects and shares some properties with the innervation of vertebrate muscle.