Neural mechanisms underlying behavior in the locust Schistocerca gregaria I. Physiology of identified motorneurons in the metathoracic ganglion

Abstract

SUMMARYA preparation of the desert locust, Schistocera gregaria, has been developed, in which it was possible to work with identified neurons while still allowing some behavior. A total of 26 motorneurons to the hind leg were studied singly, and in various pairs, both by direct stimulation, and by recording during spontaneous activity and various reflex actions. Motorneurons were identified by passing current into their somata and correlating the evoked somata spikes with extracellularly or intracellularly recorded events in the muscles. Tension of the muscle was also recorded and motor axons were stimulated to evoke antidromic spikes in the somata. Both epsp's and ipsp's can be seen clearly in recordings from the somata; spikes appear as electrotonically conducted remnants only. Somata exhibited little or no electrogenesis. It is inferred that impulses are initiated in a zone tentatively identified with the region of emergence of the motor axon from the neuropil. Integration occurs in the neuropilar segment, with the soma serving as a parallel RC element. Data was obtained on the central mechanisms of coordination of synergistic and antagonistic motorneurons and on the modes of excitation of slow and fast neurons to the same muscles.

Thoracic Interneurons, Motorneurons and Sensory Neurons of Locusta Migratoria (Insecta: Orthoptera) in Primary Cell Culture

“…The membrane potentials of cells in vitro are similar to those measured in situ, although they tend to be less negative. For the hindleg motoneurons resting potentials of -45 to -60mV have been reported [36]. We observed similar potentials in cell culture.…”

“…The percentage was higher in motorneurons (26%) than in interneurons (8%). Since the cell somata of insect neurons are in situ usually inexcitable [36] the low percentage is not surprising. Accordingly, no action potentials have been found in thoracic neurons of cockroaches, except in DUM-neurons, which are known to be excitable in situ [12].…”

Local Innervation Patterns of the Metathoracic Flexor and Extensor Tibiae Motor Neurons in the Cricket Gryllus bimaculatus

“…Back-fills from the PF (n=4) also stained similar numbers of neurons (data not shown), indicating that an identical set of neurons bifurcates to innervate this pair of muscle bundles. The position of somata varied among individuals, making it difficult to subdivide the pool of flexor exciters into anterior-, lateral-and posterior groups as in locusts (Hoyle and Burrows, 1973). Also it was difficult to trace single axonal projections into each particular nerve branch.…”

“…It has only two excitatory motor neurons, one inhibitory motor neuron and one modulatory neuron. This number and combination are well conserved in the three pairs of legs (Wilson, 1979;Theophilidis, 1983) and in several families of orthopteroid insects: locusts (Hoyle and Burrows, 1973), katydids (Theophilidis, 1983) and stick insects (Godden, 1972).…”

Intracellular recordings from interneurons and motoneurons in intact flying locusts

“…It is important to note that the flexors received this initial depolarization independently of activity in FETi, as indicated by the absence of extensor spikes in the myogram at the time the flexors became active. Consequently the central excitatory connection that exists between FETi and the flexor motoneurones (Hoyle & Burrows, 1973) could not have been responsible for initiating flexor activity. This connection probably does contribute some excitation to the flexors during co-contraction, however, and may account for the oscillations in membrane potential sometimes seen superimposed on the plateau depolarization in our records of flexors.…”