Heterogeneity of group Ia synapses on homonymous α-motoneurons as revealed by high-frequency stimulation of Ia afferent fibers

W. F. Collins, Marcia Honig, L. M. Mendell

Research output: Contribution to journalArticle

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Abstract

1. Excitatory postsynaptic potentials (EPSPs) were recorded in medial gastrocnemius (MG) motoneurons following intraaxonal electrical stimulation of single spindle afferent fibers in anesthetized cats. 2. High-frequency bursts of 32 shocks were delivered to the afferent axon and the EPSPs were averaged in the motoneuron. EPSP amplitude generally changed during the burst, in some cases increasing and in other cases decreasing, depending on the connection. 3. Interpretation of these changes was complicated by potentiation of the initial EPSPs in the burst that occurred with the repeated bursts. The extent of the potentiation varied from connection to connection. 4. The magnitude of facilitation or depression during a burst of standard frequency (167 Hz) was determined by comparison of EPSPs at the end of burst with the mean EPSP obtained during low-frequency stimulation (18 Hz). Large amplitudes EPSPs tended to depress, whereas the small amplitude EPSPs facilitated. 5. Facilitation was more prevalent in motoneurons with large rheobases and depression was more often observed in small rehebase motoneurons. The use of partial correlations, which was necessary because of the inverse correlation between EPSP amplitude and motoneuron rheobase, revealed that facilitation-depression behavior during repetitive stimulation is correlated primarily with EPSP amplitude rather than with motoneuron rheobase. 6. Acute transection of the spinal cord resulted in no change in motoneuron rheobase but considerable enlargement of mean EPSP amplitude at low frequencies of stimulation. A significant increase in the amount of depression during repetitive stimulation was noted under these conditions. 7. These results indicate considerable heterogeneity in the response of individual connections to repetitive stimulation. We sugest that this heterogeneity results from differences in transmitter release at different connections. 8. This heterogeneity must also have functional consequences related to susceptibility for firing of different motoneurons under various physiological conditions that can include afferent discharge frequencies equivalent to those used in this study.

Original languageEnglish (US)
Pages (from-to)980-993
Number of pages14
JournalJournal of Neurophysiology
Volume52
Issue number5
DOIs
StatePublished - Jan 1 1984
Externally publishedYes

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Excitatory Postsynaptic Potentials
Motor Neurons
Synapses
Depression
Spinal Cord Injuries
Electric Stimulation
Axons
Shock
Cats

All Science Journal Classification (ASJC) codes

  • Neuroscience(all)
  • Physiology

Cite this

Heterogeneity of group Ia synapses on homonymous α-motoneurons as revealed by high-frequency stimulation of Ia afferent fibers. / Collins, W. F.; Honig, Marcia; Mendell, L. M.

In: Journal of Neurophysiology, Vol. 52, No. 5, 01.01.1984, p. 980-993.

Research output: Contribution to journalArticle

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