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The Journal of Neurophysiology Vol. 82 No. 3 September 1999, pp. 1339-1351
Copyright ©1999 by the American Physiological Society
1Department of Physiology and Biophysics and
2The Fishberg Center for Research in
Neurobiology,
Evans, Colin G.,
Ferdinand S. Vilim,
Orna Harish,
Irving Kupfermann,
Klaudiusz R. Weiss, and
Elizabeth C. Cropper.
Modulation of Radula Opener Muscles in Aplysia. J. Neurophysiol. 82: 1339-1351, 1999. We observed fibers immunoreactive (IR) to serotonin (5-HT), the
myomodulins (MMs), and FMRFamide on the I7-I10 complex in the marine
mollusk Aplysia californica. The I7-I10 muscle complex, which produces radula opening, is innervated primarily by one motor
neuron, B48. B48 is MM-IR and synthesizes authentic MMA. When B48 is stimulated in a physiological manner, cAMP levels are
increased in opener muscles. cAMP increases also are seen when the MMs
are applied to opener muscles but are not seen with application of the
B48 primary neurotransmitter acetylcholine (ACh). Possible
physiological sources of 5-HT and FMRFamide are discussed. When
modulators are applied to resting opener muscles, changes in membrane
potential are observed. Specifically, 5-HT, MMB, and low
concentrations of MMA all depolarize muscle fibers. This
depolarization is generally not sufficient to elicit myogenic activity
in the absence of neural activity under "rest" conditions. However,
if opener muscles are stretched beyond rest length, stretch- and
modulator-induced depolarizations can summate and elicit contractions. This only occurs, however, if "depolarizing" modulators are applied alone. Thus other modulators (i.e., FMRFamide and high concentrations of MMA) hyperpolarize opener muscle fibers and can prevent
depolarizing modulators from eliciting myogenic activity. All
modulators tested affected parameters of motor neuron-elicited
contractions of opener muscles. MMB and 5-HT increased
contraction size over the range of concentrations tested, whereas
MMA potentiated contractions when it was applied at lower
concentrations but decreased contraction size at higher concentrations.
FMRFamide decreased contraction size at all concentrations and did not
affect relaxation rate. Additionally, the MMs and 5-HT increased muscle
relaxation rate, decreased contraction latency, and decreased the rate
at which tension was developed during motor neuron-elicited muscle
contractions. Thus these modulators dramatically affect the ability of
opener muscles to follow activity in the opener motor neuron B48. The possible physiological significance of these findings is discussed.
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