A reflex behavior mediated by monosynaptic connections between hair afferents and motoneurons in the larval tobacco hornworm,Manduca sexta

Abstract

In the tobacco hornworm caterpillar, tactile stimulation of sensory hairs located on the tip of a proleg (the planta) evokes ipsilateral or bilateral retraction of the prolegs in that segment (Figs. 1,2). We have used electrophysiological and anatomical methods to investigate the excitatory neural pathways linking the planta hair afferents and the proleg retractor motoneurons (MNs). An important technical innovation was the development of an isolated proleg and desheathed ganglion preparation that permits rapid and reversible ionic manipulations and drug applications (Fig. 3). Action potentials (spikes) in individual planta hair afferents produce time-locked excitatory postsynaptic potentials (EPSPs) in ipsilateral proleg MNs (Fig. 3) which appear to be chemically-mediated and monosynaptic: the EPSPs have a short and constant latency, they follow afferent spikes without failure, they are reversibly abolished in elevated Mg⁺⁺ saline (Fig. 7), and they persist in saline with elevated Mg⁺⁺ and Ca⁺⁺ levels (Fig. 8). Planta hair afferents also excite ipsilateral MNs by polysynaptic pathways, and their excitation of contralateral proleg MNs is exclusively polysynaptic. Cobalt-staining of the proleg MNs and planta hair afferents (Fig. 6) shows that the afferents terminate in ventral neuropil, and the proleg MNs have an unusual ventral projection into this region. The ventral projection is on the ipsilateral side, which is consistent with the electrophysiological finding that time-locked EPSPs are found only from ipsilateral hairs. Two factors that contribute to the strong monosynaptic excitation of proleg MNs by ipsilateral planta hairs are the convergence of many hair afferents onto each MN (Fig. 5), and the facilitation shown at each afferent-MN synapse (Fig. 9). At least 6 afferents converge on each MN, and at short interspike intervals the afferent-evoked EPSPs are enhanced by as much as 400% by homosynaptic facilitation. The EPSP is abolished reversibly by the cholinergic antagonists curare and atropine (Fig. 10), suggesting that the neurotransmitter at the synapse is acetylcholine (ACh). This is of particular interest because the ACh receptors of tobacco-feedingManduca larvae are reported to be less nicotinesensitive than those of other insects.