Interneurons coactivating hindleg flexor and extensor motoneurons in the locust

1. 1.

   Using intracellular staining and recording techniques in the ventral nerve cord of locusts, we have identified a pair of large interneurons which monosynaptically excite hindleg flexor and extensor tibiae motoneurons. The somata and input processes of these interneurons are in the mesothoracic ganglion and each makes its connections to hindleg motoneurons via a large axon running in the meso-metathoracic connective contralateral to its soma.

2. 2.

   We refer to these interneurons as C-neurons because single action potentials in one interneuron often caused the coactivation of the fast extensor tibiae motoneuron and a variable number of flexor tibiae motoneurons in one hindleg. The probability of activity in a C-neuron activating hindleg motoneurons depended on the position of the tibia, being highest when the tibia was flexed.

3. 3.

   Coactivation of the fast extensor tibiae motoneuron and a number of flexor motoneurons could also be elicited by a variety of stimuli in intact animals when the tibiae were close to full flexion. This resulted in the tibiae being locked into full flexion and, very often, to the initiation of the co-contraction phase of the jump. We refer to this synchronous activation of hindleg flexor and extensor motoneurons in intact animals as the cocking response.

4. 4.

   We propose that the C-neurons function to produce the cocking response. Consistent with this proposal is that the C-neurons receive input from all those sensory sources which evoke the cocking response.

5. 5.

   A general feature of the interneuronal organization in insects derived from this and other investigations is the existence of individual interneurons for the production of single aspects of a behavior. By controlling transmission of sensory information to motoneurons via these unique interneurons, the same sensory stimulus can evoke a variety of motor response. This concept is discussed in relation to the jumping system of the locust.

DCMD :

descending contralateral movement detector

EMG :

electromyogram

FET :

Fast extensor tibiae

Authors and Affiliations

1. Department of Physiology, University of Alberta, T6G 2H7, Edmonton, Alberta, Canada

   K. G. Pearson & R. M. Robertson

We thank Dr. F. Delcomyn for his helpful comments on this manuscript and T. Abrams for valuable discussions in the initial phase of this investigation. The research was supported by a grant from the Canadian Medical Research Council.

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