ABSTRACT
To investigate the mechanism of smooth muscle contraction, the frequency response of the muscle stiffness of single beta-escin permeabilized smooth muscle cells in the relaxed state was studied. Also, the response was continuously monitored for 3 min from the beginning of the exchange of relaxing solution to activating solution, and then at 5-min intervals for up to 20 min. The frequency response (30 Hz bandwidth, 0.33 Hz (or 0.2 Hz) resolution) was calculated from the Fourier-transformed force and length sampled during a 3-s (or 5-s) constant-amplitude length perturbation of increasing-frequency (1-32 Hz) sine waves. In the relaxed state, a large negative phase angle was observed, which suggests the existence of attached energy generating cross-bridges. As the activation progressed, the muscle stiffness and phase angle steadily increased; these increases gradually extended to higher frequencies, and reached a steady state by 100 s after activation or approximately 40 s after stiffness began to increase. The results suggest that a fixed distribution of cross-bridge states was reached after 40 s of Ca2+ activation and the cross-bridge cycling rate did not change during the period of force maintenance.
Subject(s)
Calcium/pharmacology , Muscle Contraction/drug effects , Muscle Contraction/physiology , Muscle, Smooth, Vascular/drug effects , Muscle, Smooth, Vascular/physiology , Animals , Biomechanical Phenomena , Biophysical Phenomena , Biophysics , In Vitro Techniques , Muscle Relaxation/physiology , Muscle, Smooth, Vascular/chemistry , RabbitsABSTRACT
We developed muscle-tendon models incorporating Hill-type structure and length-dependent coupling between activation and velocity. The models were evaluated in electrically stimulated cat soleus muscles. Dynamic model parameters were estimated by a nonlinear parameter estimation algorithm from input-output data obtained during simultaneous random stimulation and length changes. Static parameters were estimated from the length-tension curve. A model with length history-dependent activation-velocity coupling predicted the behavior of the muscle under a wide variety of conditions, including during random perturbations and during isovelocity movements, where it captured short range stiffness and length history-dependent postyielding behavior. Furthermore, the model predicted twitch responses. The generality of this fixed parameter model makes it especially suitable for simulation and feedforward control, where muscle responses are not available for on-line parameter adaptation.
