Stochastic relations between cellular and molecular events in muscle mechanics

概要

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If a myosin head in a sarcomere could walk, it would step towards the M-line or towards the Z-disc. The probable result of innumerable units would be expressed by the wave form of the contraction. The probability density function of a Poisson process fits a twitch wave of skeletal muscle very well. Results of experiment reveal that (1) Poisson's parameter λ is load-sensitive and specifies the velocity of sliding; (2) when a nerve impulse triggers the SR, Ca concentration increases throughout the muscle. The uniformization of independent events takes place and the condition for a Poisson process is fulfilled. Though it looks dramatic, twitch is an "unusual" event. A 1-dimensional Brownian process with random configuration would be the "usual" process. Brownian motion of innumerable contractile proteins never stops in life. The internal energy produced by ATP is not only for the contraction but also for stabilizing structures. Each cycle of shortening and elongation is a phenomenon that consumes energy. The result of experiment related to the energy distribution is summarized in the diagram (Fig. 2. C). The energy of a single myosin-actin unit is proportional to βk_BT and is found to be 9.52"10^<-21> J, at 20℃.
2004-03-25