A multipurpose muscle ergometer.

A fast (0.1 mm steps within 2 ms), strong (40 N continuously) and accurate (resolution 0.002 N and 1.0 micron) muscle ergometer was developed to test dynamic and static properties of mammalian muscle. Both for twitches and for tetani isometric, isokinetic and isotonic contractions can be measured accurately. Force-velocity data and time to peak-force data of four EDL muscles, as well as force-extension data of their serial tendinous structures are shown to demonstrate the machine.

Functional morphology of the M. gastrocnemius medialis of the rat during growth.

Length-force relations, both active and passive, and twitch contraction characteristics were quantified for left medial gastrocnemius muscles of four young, four adult, and four old male Wistar rats. Muscle and bundle optimum length and muscle weight were also determined and subsequently used for calculation of a number of morphological characteristics of the muscles. Fiber optimum length was derived from muscle bundle optimum length. Generally, physiological characteristics remained constant during growth. There was no change either in active tension at muscle optimum length or in active working range relative to fiber optimum length, relative passive fiber stiffness, active force relative to passive force at optimum length, twitch contraction time and twitch half relaxation time at optimum length. A number of morphological changes, however, did take place in the medial gastrocnemius muscle during growth. Fiber optimum length increased but only by about 2 mm from youth to old age, whereas muscle optimum length increased by approximately 14 mm, presumably owing to extensive hypertrophy of the muscle fibers during growth. The priority for force of the medial gastrocnemius muscle (defined as the quotient of physiological cross-sectional area of a muscle and the cubed root of its volume, a measure independent of architecture and dimensions of muscles) increased during growth. This increase indicates that during growth the muscle shifts relatively more towards force generation than towards excursion generation. These findings are discussed in view of existing scaling theories.

Sarcomere length control in striated muscle.

A system that makes control of muscle length (ML), sarcomere length (SL), and force (F) possible in striated muscle preparations is described. SL was measured by light diffraction techniques and two diffractometers. Control was performed by influencing ML with a penmotor system with a frequency response of 190 Hz. SL or F could be controlled by interrupting the internal position (i.e., ML) feedback of the motor and by closing the respective loop. Velocity feedback of the motor through an internal velocity coil was maintained in all cases for optimal damping. Steady-state error of the system was minimized by an integrating loop filter. The feedback path was selected by means of potentiometers or analog switches. Electronic stops in the circuit protected the muscle against excessive stretch and load. A microprocessor-based average-response computer could be used for feedforward control to eliminate noise or to analyze longitudinal uniformity of the muscle. Responses of rat cardiac trabeculae during SL and F control are shown. Transient behavior of SL and F during control and measures to eliminate the transients are discussed.

Membrane current and noise measurements in voltage-clamped node of Ranvier.

In voltage-clamp configurations for nodes of Ranvier the axoplasm resistance functions as a voltage-current converter. In existing configurations this resistance cannot be measured directly. In the present arrangement the electrical resistances of the preparation (axoplasm, membrane and seals) can be measured only from two measurements. This allows us to: 1. calibrate the ionic current under voltage-clamp conditions, and 2. calculate the intensity of the current fluctuations, not arising from the membrane (background noise). The measured axoplasm resistances are considerably higher than the values calculated on the basis of fiber geometry and axoplasm resistivity. The difference is due to the presence of constrictions in the nerve fiber. Membrane current estimation based on geometrical parameters in the presence of wide seals may contain large errors. Variations in the axoplasm resistance for voltage-membrane current conversion were observed within 1.5 hr. In 68% of the fibers this resistance decreased with 30% of the original value. With our current calibration the values for the maximum sodium conductance gNa (at 0mV membrane potential), maximum potassium conductance gK and leakage conductance gL are 49.5 X 10(-8), 6.66 X 10(-8) and 1.71 X 10(-8) S. respectively. The contribution of the different noise sources to the total background noise was calculated at the holding potential. For frequencies below 10(3) Hz there is an excellent agreement between measured and calculated noise levels.