Muscle conduction velocity and morphology after prolonged hypoxemia and diabetes in rats.

One of the electrophysiological abnormalities in the experimental rat model of chronic hypoxia (10% O2) and in the experimental rat model of diabetes is an increase in jitter in the stimulated single fibre EMG, which is thought to result from a primary disorder of the axon with its terminal branches. But muscle fibre alterations that influence the propagation of muscle action potentials can also increase jitter. The contribution of possible changes in muscle conduction velocity and muscle morphology to jitter were investigated in the present study. Muscle conduction velocities were determined and compared with the morphological properties of muscle fibers in muscles of control, chronic hypoxic and terminal-stage diabetic rats. The mean muscle conduction velocities were in the same range in the three groups. The muscle fibre type composition and the mean muscle fibre diameters were about the same in the hypoxic and the control rats, whereas the muscles of the diabetic rats showed a higher percentage of intermediate type muscle fibres, which is suggestive of muscle degeneration, and a smaller mean muscle fibre diameter in comparison with muscles of the hypoxic and the control rats. It is concluded that the similarities between the electrophysiological properties of the muscles despite differences in their morphology, indicate that there is primary axonal degeneration in diabetic hypoxic rats.

Long term functional improvement of dystrophic mouse leg muscles upon early immobilization.

The long term effects of immobilization of one hindleg, during the second postnatal week, were investigated in dystrophic ReJ 129 dy/dy strain of mice. The muscles of the immobilized limb were compared with those of the contralateral, non-treated side and with those of naturally dystrophic age-mates, after 1, 2 and 3 months of remobilization. It appeared that the experimental animals made better use of their remobilized leg than of the contralateral leg for locomotion. The remobilized muscles were significantly less atrophic than the contralateral muscles and they also contained more muscle fibres. It is concluded that during postnatal growth and differentiation the dystrophic muscle fibres pass through a sensitive period. Immobilization during this period prevents the majority of the muscle fibres from becoming affected. Remobilization induces pathological features in the muscles, but they remain less damaged than the non-immobilized muscles.

The different intracellular action potentials of fast and slow muscle fibres.

The time course of the intracellular action potential was studied quantitatively, because it is an important factor in the generation of electromyographic signals. In in vivo preparations of the m. EDL and m. soleus of the rat single motor units were stimulated and intracellular action potentials were recorded in muscle fibres belonging to those motor units. In this arrangement it was possible to relate the intracellular action potential to the fibre type. The intracellular action potentials of fast twitch glycolytic (FTG) EDL and of slow soleus fibres were described, using 8 characteristics. All characteristics but one differed significantly between the two fibre populations. Comparing characteristics of intracellular action potentials of FTG fibres with slow fibres, it is concluded that: the resting membrane potential is more negative; the amplitude of the action potential is larger; the maximum rates of depolarization and repolarization are higher; and the shape of the repolarization phase is more variable.

An improved technique for the demonstration of glycogen depleted skeletal muscle fibres.

Glycogen depleted skeletal muscle fibres can be distinguished from non-depleted fibres with the periodic acid Schiff (PAS) reaction. In this paper the method of Meijer (1968) for the histochemical demonstration of phosphorylase activity is described as an efficient technique to increase the contrast between both groups of fibres.

Calculation and registration of the same motor unit action potential.

In order to increase insight into the electrical phenomena of active motor units, a computer simulation model has been developed. With this model motor unit action potentials (MUAPs) have been calculated. The model has been based on the superposition of the muscle fibre potentials of the fibres of one motor unit. For verification, calculated MUAPs have been compared with the matching recorded MUAPs. During experiments one motor unit was stimulated and the MUAP of this unit was measured with intramuscular wire electrodes. After the experiments the positions of the activated fibres of this unit and of the electrodes were determined by means of histochemical techniques. Other parameters were derived from other experiments or the literature. Using the obtained set of parameters in the model MUAPs were calculated. These MUAPs were compared with the measured MUAPs. From this comparison it has been concluded that the model predicts the MUAP to an appreciable degree. The results clearly show the dominating effect of muscle fibres in close vicinity of the electrode and the important effect of the activation moment of those fibres on the shape of the MUAP.

Force development of fast and slow skeletal muscle at different muscle lengths.

It is known that the behavior of fast and slow skeletal muscles differs, but a comparison between fast and slow muscles with different stimulation patterns at various muscle lengths is lacking. The twitch, tetanus, and double pulse responses with a number of interval times have been investigated for a fast (EDL) and a slow (soleus) muscle of the rat at three muscle lengths (in vivo, at 37 degrees C, pentobarbital sodium anesthesia). The twitches of EDL and soleus change with muscle length, but the relative form of the ascending phase is independent of muscle length. The tetanus amplitude is independent of the muscle length over a considerable length range below the optimum twitch length. The form of the ascending phases of EDL-tetanus and double pulse responses (with certain interval times between the stimuli) varies with muscle length in contrast with the comparable soleus force patterns. The maximum slope in the descending phase of all contraction patterns decreases with increasing muscle length above the optimum twitch length.