Journal of Neuroscience, Vol 7, 2476-2487, Copyright © 1987 by Society for Neuroscience
Projections of the wing stretch receptors to central flight neurons in the locust
DN Reye and KG Pearson
To investigate the cellular mechanisms by which proprioceptors modify the
activity of a central neuronal oscillator, we identified flight motoneurons
and interneurons in the thoracic ganglia of the locust Locusta migratoria,
which receive constant short-latency PSPs from the wing stretch receptors
(SRs). We also examined the changes in membrane potential oscillations of
individual neurons when the SRs were stimulated on every cycle so as to
mimic their discharge in intact animals. The SRs evoked constant
short-latency (4 msec or less) EPSPs of 2 mV or less in depressor
motoneurons. In general, the hindwing SRs project to both forewing and
hindwing depressor motoneurons, whereas the forewing SRs project only to
forewing depressor motoneurons. The SRs did not evoke IPSPs in elevator
motoneurons. Twenty-one thoracic interneurons were identified that receive
short-latency (4 msec or less) EPSPs from the forewing and/or hindwing SRs.
Amplitudes varied from 1 to 6 mV, depending on the particular neuron. Only
one neuron was found that responded with an IPSP to each SR afferent spike,
but the longer latency (5-8 msec) and the different polarity suggest at
least a disynaptic pathway. Fourteen interneurons received input from more
than one SR. Of these, 12 neurons received EPSPs from the forewing and
hindwing SRs ipsilateral to each neuron's soma, while 3 received bilateral
forewing SR input. Convergence of more than one SR with the remaining 7
interneurons cannot be ruled out. Most EPSPs were evoked in interneurons
depolarized in the depressor phase of the central oscillator cycle, which
corresponds to the natural phase of SR activity in intact animals. However,
elevator, nonrhythmic, and tonically active cells also received
short-latency EPSPs from the SRs. Stimulating the SRs with trains of
stimuli on every cycle produced pronounced changes in the membrane
potential oscillations of several neurons, including phase-shifts,
increased burst frequency, prolonged bursts, and abolition of activity.
These observations suggest that the concepts of proprioceptors as error
detectors and of the central oscillator as the primary generator of the
movement are too simplistic to be applied to the locust flight system.