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The Journal of Neuroscience, March 1, 2000, 20(5):1990-2002
A Proprioceptive Role for an Exteroceptive Mechanoafferent Neuron
in Aplysia
Dmitry
Borovikov1,
Colin G.
Evans1,
Jian
Jing1,
Steven C.
Rosen3, and
Elizabeth C.
Cropper1, 2
1 Department of Physiology and Biophysics and
2 the Fishberg Center for Research in Neurobiology, The
Mount Sinai Medical Center, New York, New York 10029, and
3 Center for Neurobiology and Behavior, New York State
Psychiatric Institute and College of Physicians and Surgeons of
Columbia University, New York, New York 10032
Afferent regulation of centrally generated activity is likely to be
more complex than has been established. We show that a neuron that is
an exteroceptor can also function as a proprioceptor. We study the
Aplysia neuron B21. Previous data suggest that B21 functions as an exteroceptor during the radula closing/retraction phase
of ingestive feeding. We show that the tissue innervated by B21, the
subradula tissue (SRT), is innervated by a motor neuron (B66) and that
B66-induced SRT contractions trigger centripetal spikes in B21. Thus,
B21 is also a proprioceptor. To determine whether exteroceptive and
proprioceptive activities occur during the same phase of ingestive
feeding, we further characterize B66. We show that B66 stimulation does
not close or retract the radula. Instead it opens it. Moreover, B66 is
electrically coupled to other opening/protraction neurons. Finally, we
elicit motor programs in semi-intact preparations and show that during
radula opening/protraction we observe B66 activity, SRT contractions,
and spikes in B21 that can be eliminated if B66 is indirectly
hyperpolarized. B21 is, therefore, likely to act as an exteroceptor
during one phase of ingestive feeding and as a proprioceptor during the
antagonistic phase. Previous experiments have shown that centripetal
spikes in B21 are only transmitted to one follower if they are "gated in" by depolarization. During ingestive programs B21 is centrally depolarized during closing/retraction, but it is not depolarized during
opening/protraction. We sought to determine whether there are other
followers that receive B21 input when it is not centrally depolarized.
We found one such cell. Moreover, we found that stimulation of B21
during radula opening/protraction significantly decreases the duration
of this phase of behavior. Thus, proprioceptive activity in B21 is
likely to have an impact on motor programs.
Key words:
sensorimotor integration; sensory gating; central pattern
generator; multisensory; multimodal sensory neuron; motor program
Copyright © 2000 Society for Neuroscience 0270-6474/00/2051990-13$05.00/0
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