Modulation of motor cortical excitability by electrical stimulation over the cerebellum in man.

1. We have stimulated over the cerebellum of intact human subjects by applying single electrical stimuli through electrodes placed on the back of the head, approximately at the level of the inion. The intensity of stimulation used was below that required to produce direct EMG responses in pre-activated muscles of the hand. 2. In ten subjects the effect of the stimulus over the cerebellum was to reduce the size of the EMG response in first dorsal interosseous muscle evoked by a magnetic stimulus to the cerebral cortex. In all subjects the onset of the period of suppression occurred when the test magnetic cortical shock followed the conditioning cerebellar shock by 5 ms. The duration of the suppression lasted from 3 to 7 ms. 3. The amount of suppression was related to the intensity of stimulation over the cerebellum. At 15% below the threshold for direct motor activation there was no effect; increasing suppression was evident at 10, 5 and 0% below motor threshold. 4. With a conditioning-test interval of 5-6 ms the suppression was the same whether the target muscle was relaxed or active. With longer conditioning-test intervals (12 and 15 ms) the amount of suppression was greater in active than relaxed muscles. 5. The short-latency suppression was greatest when the stimulating anode was ipsilateral to the target muscle and contralateral to the stimulated sensorimotor cortex. The later period of suppression was insensitive to the polarity of stimulation. When the stimulating electrodes were moved 2 cm caudally or cranially the short latency suppression disappeared whereas the longer latency suppression was still observed with the electrodes in the lower position. 6. Different results were obtained when the test EMG response was produced by an electrical (rather than magnetic) stimulus over the sensorimotor cortex. The short latency effect was no longer visible whereas the longer latency effect was the same as when testing with a magnetic cortical stimulus. 7. We suggest that a single electrical stimulus across the base of the skull (particularly with the anode over one cerebellar hemisphere) produces a short latency (5-6 ms) disfacilitation of the contralateral motor cortex through activation of cerebellar structures. A later (12 and 15 ms), less specific suppression which is present when testing in active muscles is thought to be mediated by a different mechanism and probably produces its effect at the level of the spinal cord.

Scope of a technique for electrical stimulation of human brain, spinal cord, and muscle.

Brief high-voltage electrical shocks from a special low-output-resistance stimulator, delivered through electrodes on the skin, can excite human muscle directly (not by way of the nerves) and can also excite the motor cortex, the visual cortex, and the spinal cord. Possible applications of the technique include measurement in muscle disorders of the latency relaxation and of the excitability and contractility of muscle without the interposition of nerve fibres or the neuromuscular junction; measurement of conduction velocity in the pyramidal tract; and the detection of neuropathy in the nerves to the external sphincter ani.

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

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.

Fatigue of long duration in human skeletal muscle after exercise.

1. After severe muscular contraction in man recovery of force is largely complete in a few minutes, but is not wholly so for many hours. The long-lasting element of fatigue is found to occur primarily for low frequencies of stimulation (e.g. 20/sec), and is much less pronounced, or absent, at high frequencies (80/sec). The twitch force is an unreliable measure of the state of fatigue. 2. The long-lasting element of fatigue is not due to depletion of high-energy phosphate nor is it due to failure of electrical activity as recorded from surface electrodes. It is probably the result of an impairment of the process of excitation-contraction coupling. Its practical importance for man could be significant as an explanation of the subjective feelings of weakness following exercise.

The effect of lesions of the sensorimotor cortex and the capsular pathways on servo responses from the human long thumb flexor.

Lesions of the sensorimotor cortex, or of the capsular pathways beneath it, caused (with one exception out of 14 cases) diminution "r loss of the servo responses in the thumb, which are based on the long-latency stretch reflex. When not absent the long-latency stretch reflex tended to be late in onset. When absent it was often replaced by a large early reflex response at spinal latency. In general the results are consistent with the transcortical theory of the long-latency stretch reflex for the thumb, but, in detail, they indicate that the theory will require elaboration.

The effect of posterior column lesions on servo responses from the human long thumb flexor.

Lesions of the posterior column pathways, in which muscle spindle afferents run towards the brain, are associated with loss of servo responses in the long flexor of the thumb, in the absence of motor weakness and with tendon jerks preserved. This evidence is consistent with the hypothesis that the long-latency stretch reflex (on which servo responses are based) uses a supraspinal, possibly a transcortical, reflex arc.

The sensory mechanism of servo action in human muscle.

1. Anaesthesia of the thumb suppresses servo action in the long flexor of the thumb during movements of the terminal phalanx. 2. It needs a greater subjective effort to flex an anaesthetized thumb than a normal one. 3. Anaesthesia of other digits is without effect on the thumb flexor. 4. In an anaesthetic thumb without servo responses the changes in force exerted when a mechanical perturbation is applied which are due purely to the passive mechanical properties of the muscle can be measured. Subtraction of these gives the active components of servo action in the normal thumb and thus an estimate of the mechanical gain of the servo. 5. Giving the subject a visual tracking task can partially restore servo action when the thumb is anaesthetic. 6. After some years subjects become resistant to the effect of peripheral anaesthesia. 7. Peripheral anaesthesia has no detectable effect on servo responses in the long flexor of the great toe, in infraspinatus or in pectoralis major. Servo action in these muscles is presumably based predominantly on muscle receptors. 8. Tendon jerks are not apparently influenced by peripheral anaesthesia. 9. The Discussion considers the possibility that, for the thumb, muscle afferents co-operate with a somatic and a visual input in a servo loop via the cerebral cortex.