Impaired reflex sensitivity cause and effect

Abstract

When a voluntarily contracting human muscle is stretched its surface reflex electromyographic response has both short-latency (M1) and long-latency (M2) components. The M1 component occurs at a latency compatible with monosynaptic reflex activation. The long-latency component results from stimulation of skin and other subcutaneous receptors. In the initial experiment, using the first dorsal interosseous muscle (FDI) of the hand, the reflex sensitivity in normal human subjects was studied by comparing the various components of the electromyographic response generated by briefly stretching the voluntarily contracting muscle in subjects of various ages. It was found that age-related changes occur in the reflex response of human subjects. Although it is already known that the reflex response in human muscle slows with age, the result of the experiment showed that the size of that reflex response becomes smaller. The evidence suggests an impaired reflex sensitivity in older people which could be reflected in other motor control systems within the body. A detailed investigation was then undertaken to discover the causes of this change. Although the reflex response (M1) was found to alter with age the M2 component did not. This would seem to rule out neuromuscular block (NMB). NMB as a cause of the change was investigated and was found not to occur in the paradigm employed in this investigation. Possible changes in the mechanical properties of muscles and joints were looked for in a series of experiments using an accelerometer. No changes large enough to account for the reflex impairment could be found. In a further series of experiments the effects of fatigue and the effects of training on the electrical response of the FDI were studied. In a final series of experiments the changes with age, in the effects of coffee upon blood pressure were investigated.