Triggering of locust jump by multimodal inhibitory interneurons

The locust jump is triggered by a sudden inhibition of activity in hindleg flexor tibiae motoneurons following cocontraction of the hindleg flexor and extensor tibiae muscles. The main result of this investigation was the identification of two interneurons (one for each hindleg) that monosynaptically inhibit flexor tibiae motoneurons and whose properties are all consistent with them being the trigger interneurons for initiating a jump. These interneurons receive strong excitatory input from many sensory modalities (visual, auditory, tactile, and proprioceptive). Because of their multimodal response characteristics, we designated them M-neurons. A particularly strong excitatory input to each M-neuron is from both descending contralateral movement detector (DCMD) interneurons. The threshold for spike initiation in the M-neurons is high (~ 14 mV). As a consequence, input from any one sensory modality alone rarely initiates action potentials. Each M-neuron is depolarized by sensory input from leg proprioceptors. We propose that proprioceptive feedback during the cocontraction phase depolarizes the M-neurons to decrease their threshold, thus enabling extrinsic sensory stimuli to generate action potentials in both M-neurons and in so doing trigger a jump. The function of the proprioceptive gating of inhibitory transmission from the various sensory systems to the flexor motoneurons (via the M-neurons) is to ensure the development of a strong isometric contraction of the extensor tibiae muscle, and thus a powerful jump in response to external stimuli. Insofar as the initiation of the locust jump depends on sensory convergence onto large identified interneurons, this behavior is similar to ballistic movements in some other animals such as the crayfish tail flip and the startle response in fish. The unique feature of the locust jump is that the trigger interneurons initiate the jump only after a preceding phase (cocontraction) has been accomplished.