ISSN:
1432-2013
Keywords:
Fast and slow muscle fibres
;
Synaptic currents
;
Cholinesterase inhibitors
;
Potential dependency
;
Voltage clamp
;
ACh-induced noise
;
Frogs
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Abstract Miniature end-plate currents (mepcs) were recorded from fast and slow muscle fibres in two muscles, m. cruralis and m. ileofibularis, under voltage clamp or extracellularly. Both fibre types showed an identical sensitivity to α-bungarotoxin, and an exponential dependence of mepc decay time-constant upon membrane potential, but there were some important differences: i) the rise time and half-time of decay were approximately 3 times longer in slow junctions than in fast ones: ii) inhibition of AChE had a more prominent effect on mepcs recorded in fast fibres while preserving the significant difference in mepc time course between fast and slow fibres; iii) treatment by α-bungarotoxin had a greater effect on mepc time course in slow junctions even in the absence of anticholinesterase; iv) a more prominent variability of mepc time course was observed in slow junctions, especially after treatment by anticholinesterase drugs. Fluctuations of conductance of the postjunctional membrane induced by application of acetylcholine to different frog muscle fibres have been analysed. The autocorrelation function of these fluctuations was exponential and gave information about the mean life-time of single ionic channels and their conductance. There were two main classes of ionic channels: “fast” with mean life-time 2.9 ms and conductance 18.3 pS in single-innervated fast muscle fibres of phasic bundle of m. ileofibularis, m. sartorius and m. cutaneous pectoris; and “slow” −8.3 ms and 12.5 pS correspondingly (t=8–10°C, holding potential −80 mV) in multi-innervated fibres of tonic bundle of m. ileofibularis. An intermediate type of singly-innervated muscle fibres was found in tonic bundles of m. cruralis and m. ileofibularis. The time course of mepcs and life-time of ionic channels recorded in these fibres were intermediate between typical fast and slow mepcs. Simultaneous analysis of mepcs and ACh-induced current fluctuations suggests that the decay of mepcs in typical fast fibres is governed mainly by the closing of ionic channels while the decay of most mepcs in slow fibres is substantially slower than the rate of channel closing. Evidently some additional factors may prolong the effect of ACh-quanta in slow junctions. The relationship between the functional properties of different muscle fibres and rates of sequential steps of postjunctional activation is discussed.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00589092
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