A study of the mechanical response of striated frog’s muscle in relation to the theory of impulse transmission by acetylcholine

Beznák, Aladár and Gáspár-Rády, Zsuzsanna (1948) A study of the mechanical response of striated frog’s muscle in relation to the theory of impulse transmission by acetylcholine. A Magyar Tudományos Akadémia Tihanyi Biológiai Kutatóintézetének évkönyve, 18. pp. 280-326. ISSN 0365-3005

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Abstract

1. The duration of the tetanus (contracture) of the frog’s rectus abdominis in an ACh bath of 0.01-20.000 µg/ml concentration is proportional to the concentration of ACh. 2. A rectus in fatigue relaxation due to a certain concentration of ACh will give a contracture to a higher concentration of ACh or to a stronger tetanizing current. This second contracture will be as much smaller as the height of the first contracture preceding it. 3. The fatigue relaxation curve in a concentration causing a longlasting slow relaxation is three-phasic: there is a first fast period’ followed by a long slow, and finished by a last, fast period. 4. The sartorius is easily fatigued, consequently a concentrationaction curve is very rarely seen. There is, however, a tendency towards increase in the height and duration of the contraction with the increase in ACh concentration. A sartorius in fatigue relaxation will respond to a higher concentration of ACh, but owing to its fatigability the quantitative relation seen in the rectus is absent. 5. Indirect electric stimulation by condenser discharges of 10-100 /sec. frequency and 0.5-50 µF intensity cause tetani, the shape and duration of which is very similar to that seen on ACh stimulation. In the case of the rectus the intensity/ contraction curve is not regularly a sigmoid skewed curve as it is in the case of ACh. The great fatigability of the sartorius prevents our seeing a quantitative relationship between intensity and length of contraction. If fatigue relaxation developed in either rectus or sartorius, the muscle would contract only to electric stimuli so strong as to have caused larger contraction previous to the ACh fatigue than the ACh did. If either of the two muscles relaxed during stimulation from ACh solutions or electric stimulation causing 100% contraction, no mechanical response could be elicited either with stronger ACh or with ES. The Jendrassik-Gelei and reversed Jendrassik-Gelei effects are due to not all the muscle fibres being fatigued and the remaining ones giving response to stronger electric or ACh stimuli. 6. The load necessary to stretch a rectus in 100% contraction to its 0 line is loosely correlated to the length of isotonic contraction. Averages of the length of contraction and stretching force dot not differ significantly in 100% electric or ACh contraction. The standard deviation of the former is considerably greater in both modes of stimulation. 7. The isometric tensions caused by ES are always much greater than those caused by ACh. Partly on model experiments with bundles of rubber bands, partly in experiments on the rectus, it has been shown that the length of the contraction is a function of the total length of the muscle fibres in the state of excitement. The tension developed is proportional to the crosssectional number of the fibres in excitatory state. The contraction of a given fibre within a bundle depends, amongst other things, on its position in the bundle. 8. These results can best be interpreted by the hypothesis that the muscle is composed of a population of receptors (or reacting units) of varying fatigability and sensitivity to ACh. Their distribution is abnormal. The concentration/action curve of the rectus is more truly to be regarded as a skew sigmoid curve than a hyperbola. All the differences seen in the mechanical response given to ACh or to electric stimulation find easy explanation in the circumstance that in the former mode of stimulation the ACh reaches the receptors by diffusion, in the latter by the nervous impulse. The Dale-Brown-Feldberg hypothesis of impulse transmission in the neuromuscular junction is capable of harmoniously explaining all the properties of the isotonic and isometric mechanical responses to ES or ACh described in this paper.

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