Behaviour of single motor units in human skeletal muscle

Abstract

In the first part it was shown that the firing frequencies of motor units at particular grades of isometric contraction depend on the total number of units available for recruitment and so are regulated by length and tension receptors in the muscle. Procainisation showed the importance of the gamma- regulated length receptors in voluntary contraction. In hypertonie Parkinsonism, however, increments of voluntary contraction are presumably largely dependent on recruitment of extra units. Certainly the units recorded in the present study showed a very marked tendency to fire at unusually slow rates. They did so with a regular rhythm suggesting strong tonic inhibition. This is in contrast to the evidence reported in Chapter 8 (Paired response) pointing to the presence of motoneurone facilitation. This paradox would be solved if two antagonistic mechanisms were in action - for instance a supraspinal facilitation and a peripheral as well as supraspinal inhibitory one. Hufschmidt's (1959) work has already been discussed. He has shown that inhibitory reflexes from muscle and tendon receptors are overactive in Parkinsonism. It is now submitted that the mechanism described by him is only one of at least two important factors regulating spinal motoneurones in Parkinsonism, and it may well be that the peripheral inhibitory overaction is reactive, a regulatory reflex called into premature activity owing to the absence of the gamma regulation of the length servo (see Chapter 9).

Further work immediately suggests itself. My colleague, R. Levy, has shown that supraspinal facilitation may be temporarily reduced by anticonvulsants and other drugs. Injection of procaine into muscle tendons will paralyse the Golgi tendon organs. Repetition of the observations reported here are planned in Parkinsonian patients after each of these procedures. It has not been a purpose of this thesis to discuss the mechanism of tremor in that disease but it may be pointed out that an interplay of two antagonistic tonic activities in spinal inter-neurones might lead to "oscillation of the servo -system ", causing a rhythmical tremor which would not require presence of the same rhythmicity in a descending extra- pyrimidal outflow.

For normal physiology this work emphasizes the importance of peripheral regulatory systems in the performance of voluntary movement, and is in complete accord with the suggestions of Granit and his school. The Unexpected findings in Parkinsonism and in Friedreich's ataxia suggest a further parameter of control which could not be detected by workers with animals in which the activity investigated can scarcely be termed "voluntary ". It has been accepted since the work of Adrian and Bronk (1929) that the tension of voluntary contracted muscle is raised by increasing the firing rates of motor units to a certain limiting value (possibly related to tetanus fusion frequency: see Chapter 3) and also by recruitment of new units. The present study shows that these controls are affected differentially by disease. Simpson (personal communication) suggests that supraspinal facilitation may control the recruitment of alpha motoneurones while the gamma loop regulates their firing frequency. This hypothesis should be tested by animal experiment.