ELECTROMYOGRAPHIC STUDIES ON THE FUNCTION OF THE SKELETAL MUSCLES : VI. CHARACTERS OF THE DISCHARGE-INTERVAL-TIME-SERIES OF A SINGLE NMU PART 4. THE τ^^--S CURVE OF THE DOG

Abstract

It is already a well-known fact that the NMU of the skeletal muscles in man is composed of two groups of quite different properties, that is, the K- and T-NMU. In animals, however, numerous studies have been made without reaching any decisive differentiation of the NMU, because of the difficuties in inducing the animals to a voluntary muscular movement. In maintaining a certain standing posture, which is regarded as one of the fundamental stances in the research on the function of the skeletal muscles in animals, the NMU obtained has always presented the T-NMU. The present study has been attempted to clarify the fact that the muscular movement of animals is maintained not only under the action of the T-NMU, controlled by the subcortical or spinal nervous system, but also under that of the K-NMU, which has been evoked by the activity of the motor area in the cerebral cortex. The procedure is as follows. 1.The sciatic nerves of young dogs and goats were deafferentated by cutting the dorsal root and ganglions on both sides of the spinal segment of L₆, L₇ and S₁. After the serious extension of the hind limbs caused by the operation wore off, the animals were exercised to support the hind half of the body on their feet. 2.Observation of the action potential of the gastrocnemius muscle was repeated every two weeks for period of 2 or 3 months. Using the numerous time-series of the discharge intervals of single NMUs, the average τ^^- and the standard deviation S were culculated, and then the τ^^--S points were graphed for differentiation of the two kinds of NMU. The following results were obtained. 1). The exercise, for two or three weeks after the forced extention of the hind limbs was abolished, enabled the dogs to stand for a few minutes on their tarsal joints with the help of the trainer. The τ^^--S points then scattered between the K- and T-curve, which is defined at the end of this experiment. 2.) After continuation of the exercise for a further period of 2 or 3 weeks, the animals could stand voluntarily on their hind limbs, touching the tarsal joints firmly on the floor. The τ^^--S points of the time were clearly divided into two groups, excluding the location of the K-NMU they were markedly biased to the left side. 3). 1 or 2 months later, the dogs learned to stand on their feet, keeping the tarsal joints away from the ground, and to walk and to run around. The τ^^--S points then distributed along the K- and T-curve without overlapping. 4). From the results above-mentioned, it was ascertained that the NMU of the skeletal muscles in the dog is composed of the K- and T-NMU. 5). Nevertheless, it was impossible to discriminate between the two NMU in the case of the goat, in which the legs remained inactive up to the time of their death without any improvement in the mechanical use of the limbs, despite carefull management and constant exercise.