Document Detail

Multiple mechanisms of spike-frequency adaptation in motoneurones.
MedLine Citation:
PMID:  10084714     Owner:  NLM     Status:  MEDLINE    
Spike-frequency adaptation is the continuous decline in discharge rate in response to a constant stimulus. We have described three distinct phases of adaptation in rat hypoglossal motoneurones: initial, early and late. The initial phase of adaptation is over in one or two intervals, and is primarily due to summation of the calcium-activated potassium conductance underlying the medium duration afterhyperpolarization (mAHP). The biophysical mechanisms underlying the later phases of adaptation are not well understood. Two of the previously-proposed mechanisms for adaptation are an increase in outward current flowing through calcium-activated potassium channels and increasing outward current produced by the electrogenic sodium-potassium pump. We found that neither of these mechanisms are necessary for the expression of the early and late phases of adaptation. The magnitude of the initial phase of adaptation was reduced when the calcium in the external solution was replaced with manganese, but the magnitudes of the early and late phases were consistently increased under these conditions. Partial blockade of the sodium-potassium pump with ouabain had no significant effect on any of the three phases of adaptation. Our current working hypothesis is that the magnitude of late adaptation depends upon the interplay between slow inactivation of sodium currents, that tends to decrease discharge rate, and the slow activation or facilitation of a calcium current that tends to increase discharge rate. Adaptation is often associated with a progressive decrease in the peak amplitude and rate of rise of action potentials, and a computer model that incorporated slow inactivation of sodium channels reproduced this phenomenon. However, the time course of adaptation does not always parallel changes in spike shape, indicating that the progressive activation of another inward current might oppose the decline in frequency caused by slow sodium inactivation.
R K Powers; A Sawczuk; J R Musick; M D Binder
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Publication Detail:
Type:  Journal Article; Research Support, U.S. Gov't, P.H.S.    
Journal Detail:
Title:  Journal of physiology, Paris     Volume:  93     ISSN:  0928-4257     ISO Abbreviation:  J. Physiol. Paris     Publication Date:    1999 Jan-Apr
Date Detail:
Created Date:  1999-05-25     Completed Date:  1999-05-25     Revised Date:  2007-11-15    
Medline Journal Info:
Nlm Unique ID:  9309351     Medline TA:  J Physiol Paris     Country:  FRANCE    
Other Details:
Languages:  eng     Pagination:  101-14     Citation Subset:  IM    
Department of Physiology and Biophysics, School of Medicine, University of Washington, Seattle 98195, USA.
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MeSH Terms
Action Potentials / physiology
Adaptation, Physiological*
Calcium / physiology
Computer Simulation
Electric Conductivity
Membrane Potentials / physiology
Motor Neurons / physiology*
Rats, Sprague-Dawley
Sodium-Potassium-Exchanging ATPase
Grant Support
Reg. No./Substance:
0/Buffers; 7440-70-2/Calcium; EC ATPase

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