Development of the gin trap reflex inManduca sexta: a comparison of larval and pupal motor responses

Abstract

1. Responses of motor neurons in larvae and pupae of Manduca sexta to stimulation of tactile sensory neurons were measured in both semi-intact, and isolated nerve cord preparations. These motor neurons innervate abdominal intersegmental muscles which are involved in the production of a general flexion reflex in the larva, and the closure reflex of the pupal gin traps. 2. Larval motor neurons respond to stimulation of sensory neurons innervating abdominal mechanosensory hairs with prolonged, tonic excitation ipsilaterally, and either weak excitation or inhibition contralaterally (Figs. 4A, 6). 3. Pupae respond to tactile stimulation of mechanosensory hairs within the gin traps with a rapid closure reflex. Motor neurons which innervate muscles ipsilateral to the stimulus exhibit a large depolarization, high frequency firing, and abrupt termination (Figs. 2, 4B). Generally, contralateral motor neurons fire antiphasically to the ipsilateral motor neurons, producing a characteristic triphasic firing pattern (Figs. 7, 8) which is not seen in the larva. 4. Pupal motor neurons can also respond to sensory stimulation with other types of patterns, including rotational responses (Fig. 3A), gin trap opening reflexes (Fig. 3B), and 'flip-flop' responses (Fig. 9). 5. Pupal motor neurons, like larval motor neurons, do not show oscillatory responses to tonic current injection, nor do motor neurons of either stage appear to interact synaptically with one another. Most pupal motor neurons also exhibit i-V properties similar to those of larval motor neurons (Table 1; Fig. 10). Some pupal motor neurons, however, show a marked non-linear response to depolarizing current injection (Fig. 11).

Postembryonic development of the dorsal longitudinal flight muscle and its innervation inManduca sexta

“…Others are maintained throughout metamorphosis and generate movements of the pupa (Finlayson, 1956;Levine and Truman, 1985;Waldrop and Levine, 1989), whereas yet others undergo partial degeneration followed by differentiation and growth of the adult muscle (Thorn and Truman, 1989;Fernandes et al, 1991;Hegstrom and Truman, 1996). In Drosophila, remnants of specific partially degenerated larval body wall muscles contribute to the formation of the adult DLM anlage (Fernandes et al, 1991;Fernandes and Keshishian, 1996).…”

Operation and Defensive Role of "Gin Traps" in a Coccinellid Pupa (Cycloneda Sanguinea)

“…Gin traps have been the subject of little physiological work, except in Sphingidae, where much has been learned from studies of the neuronal control and development of control of the pupal gin trap reflex (see, for instance, Levine et al 1986, Levine 1989, and Waldrop and Levine 1989. Coccinelid gin traps could doubtless also lend themselves to further study.…”

Intersegmental interneurons serving larval and pupal mechanosensory reflexes in the moth Manduca sexta

“…The reflex circuitry has been studied in the isolated abdominal nerve cord, by electrically stimulating sensory axons and recording responses from somata and axons of intersegmental muscle motor neurons (Waldrop and Levine 1989b). Larval motor neurons respond to stimulation of the axons of the future gin trap sensory neurons with a slow, prolonged excitation ipsilateral to the stimulus, and a weak excitation or inhibition contralaterally.…”

“…Motor neurons on the contralateral side exhibit a triphasic response pattern: a brief period of excitation is followed by inhibition and then a prolonged period of excitation, with the inhibitory phase concurrent with the excitation of ipsilateral motor neurons. This pattern corresponds to a rapid closure and re-opening of the gin trap, as has been observed in semi-intact preparations (Waldrop and Levine 1989b), and in electromyographic recordings from intact animals (T. Hodges-Stacy and B. Waldrop, unpubl, obs.…”

“…Although the larval and pupal reflex circuits use the same sensory and motor elements (Waldrop and Levine 1989b), modifications of both types of neurons may contribute to the altered responses. The retained sensory neurons expand their terminal arborizations during the larval-pupal transition, perhaps allowing them to form new, or stronger, connections within the CNS .…”