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Biomedical subjects

P M Rack

Publications and source records attributed to P M Rack.

8 recordsLinked to original sources
PubMed1992

Changes in the stretch reflex of the human first dorsal interosseous muscle during different tasks.

1. Subjects flexed the interphalangeal joint of the index finger against a lever which was mounted on the shaft of a torque motor. 2. There were two different tasks. In one, the subject attempted to maintain a constant finger position in the face of changing forces, whereas in the other the subject attempted to maintain a constant force while the motor moved the lever. 3. Each of the tasks was interrupted by ramp extensions. These evoked stretch reflexes which were recorded in the first dorsal interosseous (FDI) muscle electromyogram (EMG). 4. Long-latency (55-90 ms) reflex responses were larger during the 'maintain position' task than during the 'maintain force' task, although the ramp extensions began from a similar finger position, a similar flexing force, and with a similar amount of FDI EMG activity. 5. It is concluded that the nature of the task has an effect on the magnitude of the long-latency stretch reflex.

Adult↗
PubMed1992

Task-dependent changes in the response of human wrist joints to mechanical disturbance.

1. Subjects sat with their right hands in a mould which was attached to the shaft of a servo-controlled motor. 2. There were two different tasks. In one, the subject attempted to maintain a constant wrist flexing force, while the motor imposed flexion-extension movements. In the other, the subject attempted to maintain a constant wrist position while the motor exerted variable flexion-extension forces. 3. The 'maintain force' task was punctuated by randomly timed ramp extension movements to test the stretch reflex activity. The 'maintain position' task was punctuated by force changes which were the same as those recorded during the ramp extensions mentioned above. In this way it was possible to test stretch reflexes with the same disturbance during the 'maintain force' and 'maintain position' tasks. 4. When, after some practice, subjects had become good at maintaining the constant force, later components of the stretch reflex (after 40 ms) were found to be smaller than when they were maintaining a constant position. 5. Attempts to maintain a constant position often involved co-activation of the wrist extensor muscles. This could be prevented by local anaesthesia of the radial nerve. The long-latency stretch reflex of the flexors was still task dependent after this had been done. 6. The roles of co-activation and of task-dependent reflex responses are discussed.

Adult↗
PubMed1976

Timing of the responses in the motor cortex of monkeys to an unexpected disturbance of finger position.

Monkeys were trained with food rewards to hold the wrist and fingers of their right hand in a flexed posture and maintain force with the finger tips against an isometric lever for a number of seconds. Once the animal had learned to produce a reliable performance of the task an assembly was attached to the skull through which microelectrodes could be introduced into the precentral cortex to record the activity of single neurones. Neurones whose activity was correlated with the force of finger flexion were studied; some of these could be identified as pyramidal tract neurones by their response to electrical stimulation in the medullary pyramids. While the monkey was flexing against it, the lever was sometimes suddenly released so that the fingers flexed without resistance. This unexpected disturbance was often followed by a change in the discharge of precentral neurones, although the monkey had not been trained to respond to the release in any particular way. On release of the lever the discharge of a given cortical neurone might either increase or decrease, and the direction of this change could not be predicted from the behaviour of the neurone during the isometric task. The most common response was an increase in cortical cell firing in neurones whose natural discharge was associated with the active development of force. The discharge of pyramidal tract neurones changed 25-50 msec after the sudden unexpected peripheral disturbance. Earlier changes were seen in some other neurones situated within the precentral gyrus and in the anterior bank of the central sulcus.

Animals↗
PubMed1976

Short-range stiffness of slow fibers and twitch fibers in reptilian muscle.

The semitendinous muscle of the lizard Tilique contains both slow and twitch fibers; by subdivision of its motor nerve, fibers of each type may be stimulated separately. When, during repetitive stimulation of nerve filaments, the muscle was lengthened or shortened, the tension changes included an initial short-range stiffness, followed by a later compliance. With increasing velocities of movement, the short-range stiffness increased toward a limiting value. For slow fibers this limiting value was reached with lower velocities of movement than for the twitch fibers. Provided that the same velocity of movement was used and the movements began from similar initial isometric tensions, the slow fibers resisted the movements with a greater stiffness than the twitch fibers. It is suggested that not all of the observed differences between the two fiber types can be interpreted simply in terms of differences in rates of formation and breakdown of cross-links.

Animals↗