Non-invasive magnetic stimulation of human motor cortex

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for “central” fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal cha...

Spinal and Supraspinal Factors in Human Muscle Fatigue

Non-invasive electrical and magnetic stimulation of the brain, spinal cord and roots: basic principles and procedures for routine clinical application. Report of an IFCN committee

http://www.cognitiveneuroscience.it/wp-content/uploads/2015/10/Rossini_CliNeuro_2015.pdf

Non-invasive electrical and magnetic stimulation of the brain, spinal cord, roots and peripheral nerves: Basic principles and procedures for routine clinical and research application: An updated report from an I.F.C.N. Committee

To be successful, precision manipulation of small objects requires a refined coordination of forces excerted on the object by the tips of the fingers and thumb. The present paper deals quantitatively with the regulation of the coordination between the grip force and the vertical lifting force, denoted as the load force, while small objects were lifted, positioned in space and replaced by human subjects using the pinch grip. It was shown that the grip force changed in parallel with the load force generated by the subject to overcome various forces counteracting the intended manipulation. The balance between the two forces was adapted to the friction between the skin and the object providing a relatively small safety margin to prevent slips, i.e. the more slippery the object the higher the grip force at any given load force. Experiments with local anaesthesia indicated that this adaptation was dependent on cutaneous afferent input. Afferent information related to the frictional condition could influence the force coordination already about 0.1 s after the object was initially gripped, i.e. approximately at the time the grip and load forces began to increase in parallel. Further, “secondary”, adjustments of the force balance could occur later in response to small short-lasting slips, revealed as vibrations in the object. The new force balance following slips was maintained, indicating that the relationship between the two forces was set on the basis of a memory trace. Its updating was most likely accounted for by tactile afferent information entering intermittently at inappropriate force coordination, e.g. as during slips. The latencies between the onset of such slips and the appearance of the adjustments (0.06–0.08 s) clearly indicated that the underlying neural mechanisms operated highly automatically.

Roles of glabrous skin receptors and sensorimotor memory in automatic control of precision grip when lifting rougher or more slippery objects

Muscular movements are under the control of a servo similar in many ways to those used in engineering control systems but with a subtlety of design not found in man-made servo mechanisms—including an automatic gain compensation for altered load which must be useful, for example, for adjusting to reduced g on the Moon.

Servo Action in Human Voluntary Movement

1. In the long flexor of the thumb the latency of the stretch reflex and of other manifestations of servo action is some 45 msec, roughly double the latency of a finger jerk. 2. Tendon jerks are feeble or absent in the long flexor of the thumb even in subjects with brisk long-latency stretch reflexes in this muscle. This, and other facts, suggests that the nervous mechanism of the tendon jerk is different from that of the stretch reflex. 3. A muscle that has feeble tendon jerks may show a late component in the response to a tendon tap, with a latency similar to that of the long-latency stretch reflex. 4. On the hypothesis that the excess latency of the stretch reflex over that of a tendon jerk is because the stretch reflex employs a cortical rather than a spinal arc, the excess would be expected to be larger in magnitude for the long flexor of the big toe and smaller for the jaw closing muscles. This is confirmed, 5. An alternative hypothesis that the long latency of stretch reflexes in thumb and toe is because they are excited by slow-conducting afferents is made improbable by the finding that stretch reflexes with an equal or greater excess latency are also found in proximal arm muscles. 6. The long-latency stretch reflex in proximal muscles was seen most distinctly in a healthy subject who happened to have feeble or absent tendon jerks. In ordinary subjects there is often a large, short-latency, presumably spinal component of the stretch reflex in proximal muscles; and short-latency responses to halt and release are also seen, The significance of this spinal latency servo action in proximal muscles remains to be explored. 7. The Discussion argues that the available data on conduction time to and from the cerebral cortex are compatible with the hypothesis that the long-latency component of the stretch reflex uses a transcortical reflex arc, and that none of the experiments described in the present paper are inimical to this view.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1976.sp011481

Stretch reflex and servo action in a variety of human muscles.

Human postural responses

1. The amount of positional compensation afforded by the long-latency reflex in the flexor pollicis longus has been investigated in ten normal human subjects. 2. The interphalangeal joint of the thumb was extended by between 2 and 40 degrees at up to 900 deg/s by suddenly increasing the standing force applied to the lever against which the subject was pressing with the pad of the thumb. 3. Electromyographic (e.m.g.) responses at spinal-latency were very small or absent for stretches of this magnitude. The long-latency stretch reflex produced an average positional correction of about 50% for disturbances in the range of 5-25 degrees. The response began to saturate for disturbances of greater than 25 degrees. 4. The e.m.g. response was pulsatile, lasting only some 50 ms, even during continuously increasing disturbances; frequently it terminated despite a remaining positional error. 5. There was a large variation from subject to subject in the average amount of positional correction provided by the stretch reflex. Examination of single responses to the same stretch in individual subjects showed an even greater variation from trial to trial. 6. Variation in the compensation produced by the long-latency stretch reflex from trial to trial could not be explained by the slight variation in size or maximum velocity of the individual stretches.

Reliability and efficacy of the long‐latency stretch reflex in the human thumb.

Direct electrical stimulation of corticospinal pathways through the intact scalp in human subjects.

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