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Biomedical subjects

P M Rack

Publications and source records attributed to P M Rack.

36 records · Page 2Linked to original sources
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↗
PubMed1974

The short range stiffness of active mammalian muscle and its effect on mechanical properties.

1. The tension in tetanized cat soleus and lateral gastrocnemius muscles was measured during alternating lengthening and shortening movements. Sinusoidal movements were sometimes used; on other occasions the movement was at a constant velocity but with periodic reversal of direction.2. With constant velocity movements of small amplitude the tension rose steeply during lengthening and fell during shortening in a relatively simple way. With longer movements the tension at first changed steeply as it had done with the smaller movement, but later in the movement the resistance of the muscles decreased so that the tension change became more gradual. The muscles resisted a small movement or the first part of a larger movement with a ;short range stiffness' which did not persist as the movement continued.3. So long as the constant velocity movement was not too slow the short range stiffness was independent of velocity though it lasted for more of a fast movement than of a slow one.4. In small movements the muscle was never extended beyond its short range stiffness, and the over-all peak-to-peak tension change was therefore large compared with the amplitude of movement. When, with larger movements, the muscle was stretched into a range in which it became more compliant, the peak-to-peak force fluctuation did not increase by an equivalent amount, and over the whole course of the movement the force change per unit extension was smaller.5. When the movement was confined to a short range, little work was expended in driving the muscle through a cycle of movement; its properties were essentially elastic. With larger amplitudes the muscle met the movement with a frictional resistance, the tension during lengthening then being greater than during shortening. A considerable amount of work had then to be done on the muscle to maintain the movement.6. The short range stiffness was also apparent in the response to sinusoidal movements.7. The short range stiffness was attributed to elastic properties of cross-bridges between thick and thin filaments in the myofibrils.8. The effect of the short range stiffness on the mechanical properties of the limb is discussed.

Animals↗
PubMed1974

The forces generated at the human elbow joint in response to imposed sinusoidal movements of the forearm.

1. The mechanical resistance of the human forearm has been measured during imposed sinusoidal flexion-extension movements of the elbow joint.2. The force required to move the limb can be divided into components required to move the mass, and components required to overcome the resistance offered by elastic and frictional properties of the muscles and other soft tissues.3. When during a vigorous flexing effort the limb was subjected to a small amplitude sinusoidal movement each extension was followed by a considerable reflex contraction of the flexor muscles. At low frequencies of movement this reflex provided an added resistance to extension, but at 8-12 Hz the delay in the reflex pathway was such that the reflex response to extension occurred after the extension phase of the movement was over and during the subsequent flexion movement. The reflex activity then assisted the movement whereas at other frequencies it impeded it.4. The reflex response to movement increased as the subject exerted a greater flexing force.5. Small movements generated a relatively larger reflex response than big ones.6. Even with large amplitudes of movement when the reflex activity was relatively small, the limb resisted extension with a high level of stiffness; this was comparable with the short range stiffness of muscles in experimental animals.7. The fact that at some frequencies the reflex response assisted the movement implies that with appropriate loading the limb could undergo a self-sustaining oscillation at those frequencies.

Adolescent↗
PubMed1974

The effects of load and force on tremor at the normal human elbow joint.

1. Spontaneous flexion-extension tremor of the elbow was recorded in normal human subjects while they exerted a flexing force against different loads.2. Increases in the flexing force up to (1/2) or (3/4) of the subject's maximum were accompanied by increases in the amplitude of the tremor. A further increase to very large forces was associated with a decrease in tremor.3. When the subject flexed against a spring, the frequency of the tremor was related to the properties of the spring; with increasingly stiff springs the tremor frequency increased up to about 12 Hz. With the stiffest springs, however, the tremor became irregular and very small in amplitude.4. Weights added to the limb at the wrist reduced the frequency of tremor.5. By using appropriate combinations of spring and mass, the principal tremor frequency could be adjusted between 2 and 12 Hz, higher in some subjects. Within this range the frequency(2) was approximately proportional to the spring stiffness/mass, just as it is in a spring-mass system, though the forearm provided some of this mass, and some of the spring-like resistance was in the flexor muscles.6. When by suitable loading the tremor frequency was brought between 8 and 12 Hz, and the subject forcibly flexed his elbow, the tremor became regular and large in amplitude. This was a form of limited instability in the stretch reflex which arose because a powerful reflex response to extension of the elbow acted back on the flexor muscles after a delay.7. When the limb was so loaded that its natural frequency was outside this 8-12 Hz range, it behaved as a filter, and in response to any disturbing noise it oscillated mainly at frequencies close to the natural frequencies of either the mechanical system or the stretch reflex.8. This filtering function of the limb with its stretch reflex probably plays an important part in the control of normal movement and tremor.9. The large flexing forces that were here associated with vigorous tremor normally occur when a limb supports a large mass. Such a mass would give the mechanical system within the limb a low natural frequency; inconveniently large oscillations in the 8-12 Hz range would therefore seldom occur.

Adolescent↗
PubMed1969

The effects of length and stimulus rate on tension in the isometric cat soleus muscle.

1. By subdividing ventral roots and supplying stimulating pulses to different groups of motor units in rotation, smooth contractions of soleus could be obtained with low rates of stimulation.2. Isometric tension was recorded with different rates of stimulation, and at different muscle lengths.3. Longitudinal histological sections were cut from muscles fixed at different lengths, and sarcomeres were measured. Mean sarcomere lengths in soleus could then be related to the angle at the ankle.4. At high rates of stimulation the maximum active tension was obtained at a length corresponding to an angle of about 60 degrees at the ankle, and a mean sarcomere length of about 2.8 mu. The isometric tension fell only slightly on shortening the muscle to a length equivalent to 100 degrees , and a mean sarcomere length about 2.3 mu. Further shortening caused a marked fall in tension.5. There was a reciprocal relationship between stimulus rate and muscle length; when the muscle was long low rates of stimulation gave near maximal tension, whereas at short lengths the maximum tension was reached only when the stimulus rate was very high. It is suggested that stimulating pulses activate the contractile machinery of the muscle more effectively at long than at short muscle lengths.6. When at low rates of stimulation pulses were distributed among the motor units in rotation to give a smooth contraction, the tension rose higher than during the unfused tetanus that accompanied synchronous stimulation of the same motor units at the same rate. It is suggested that in an unfused tetanus internal movement of the muscle reduces the tension below that developed in a truly isometric state.7. The rate of rise of tension in an isometric tetanus varied with both muscle length and rate of stimulation. At each stimulus rate there was a range of lengths in which the isometric tension developed slowly, this was the same length range in which, at that stimulus rate, the length tension curve was steep.

Animals↗
PubMed1969

The mechanical properties of cat soleus muscle during controlled lengthening and shortening movements.

1. By supplying pulses to different subdivisions of the ventral nerve roots in rotation, it was possible to obtain smooth contractions of cat soleus with low rates of stimulation.2. After contracting isometrically the muscle was subjected to constant velocity lengthening or shortening movements.3. During shortening the tension always fell below the isometric value. The fall in tension was usually greatest when low rates of stimulation were used.4. The effect of lengthening on tension depended on the rate of stimulation. At high rates of stimulation the tension during lengthening always rose above the isometric tension. At lower rates of stimulation (5-15 pulses/sec) the tension rose at the beginning of an extension, but decreased later in the movement to a level that was often less than the isometric tension corresponding to that muscle length. At these stimulus rates the tension during isometric contraction was usually higher than during a sustained movement in either direction.5. At low rates of stimulation longitudinal vibratory movements of more than 0.1 mm also reduced the tension far below the isometric value, whereas the reduction was quite slight when the rate of stimulation was high.6. The isometric tension during smooth contractions at low stimulus rates was remarkable in the following respects: it developed rather slowly, it was higher than the tension during or immediately after movements, and it was only slowly regained after movement had ceased.7. The results are discussed in relation to the sliding filament theory of muscle contraction, which, with certain assumptions, provides an explanation for many of the findings.

Animals↗
PubMed1969

Isotonic lengthening and shortening movements of cat soleus muscle.

1. By supplying pulses to different subdivisions of the ventral nerve roots in rotation it was possible to obtain smooth contractions of cat soleus with low rates of stimulation.2. After contracting isometrically the muscle was subjected to ;step' changes in tension after which it lengthened or shortened isotonically.3. Isotonic lengthening movements usually began relatively slowly but proceeded with increasing velocity; this acceleration was most conspicuous when low rates of stimulation were used.4. At low rates of stimulation the isotonic lengthening movement often continued beyond the length at which the muscle could have generated that tension in an isometric contraction. The muscle then shortened slowly back toward that length.5. Isotonic shortening movements began relatively rapidly, but as shortening continued the movement became slower, and often had an irregular oscillatory course.6. The isotonic movements are discussed in relation to the sliding filament theory of muscle contraction.7. The compliance of the series elastic elements was calculated from the relative amplitudes of the ;step' changes in tension and length. The stiffness of this component increased with increasing muscle tension.

Animals↗
PubMed1966

The behaviour of a mammalian muscle during sinusoidal stretching.

1. Tension changes have been recorded in tetanized and passive cat soleus muscles during sinusoidal stretching at frequencies between 0.25 and 12.5 c/s.2. During sinusoidal stretching the muscle tension varied in an approximately sinusoidal manner. The tension record showed an angular advance on muscle length. A method of measuring mean angular advance in nonlinear systems is described.3. In tetanized muscle, resistance to lengthening increased with increasing frequency of stretching. With increasing frequency the elastic stiffness increased more rapidly than the viscous stiffness, so that at the higher frequencies the angular separation between length and tension became smaller.4. With increasing stretch amplitude the elastic stiffness of the tetanized muscle declined, and the mean angular advance of tension became larger.5. In passive muscle the viscous stiffness increased at the higher frequencies, and the mean angular advance of tension on length was correspondingly increased.6. The functional significance of these findings and their relation to the findings of other workers are discussed.

Animals↗
PubMed1966

The reflex response to sinusoidal stretching of soleus in the decerebrate cat.

1. Soleus muscle in the decerebrate cat was stretched sinusoidally through various distances, at various frequencies while tension and e.m.g. activity were recorded.2. Two patterns of stretch reflex activity were seen. In one, slow stretching led to a large increase in tension, whereas in the other tension increased little during slow stretching, but rose steeply during a more rapid stretch. Intermediate states were also seen.3. Both these reflex patterns were abolished when the fusimotor fibres were blocked with Xylocaine.4. At low frequencies of stretching the e.m.g. activity was greatest at peak muscle length; at higher frequencies the greatest activity was found during lengthening. This angular advance of e.m.g. activity on length was greatest at 3.3-5 c/s. At the higher frequencies the e.m.g. activity was less in advance of muscle length.5. Angular advance of e.m.g. activity was greatest in the preparations that were also the most sensitive to slow stretching.6. Muscle contraction follows an electrical stimulus with a small delay. The delay between the end of stimulation and the end of muscle relaxation is longer, and may exceed 200 msec in cat soleus.7. The timing of the reflex tension during sinusoidal stretching depended on how far the angular advance of the e.m.g. combined with the damping properties of muscle offset the time delays in the reflex pathway.8. Changes in muscle tension generally preceded changes in muscle length. When, however, stretch amplitudes of 1 mm (peak to peak) were used, a phase delay of tension was sometimes seen. This only occurred in preparations that also showed little sensitivity to slow stretching.9. Phase delay of tension was usually found with frequencies of approximately 6-8 c/s, and this corresponds to the clonus frequency for soleus.10. The mechanisms of the two types of reflex behaviour, and the damping properties of the system are discussed.

Animals↗
PubMed1966

The effects of suxamethonium and acetylcholine on the behaviour of cat muscle spindles during dynamics stretching, and during fusimotor stimulation.

1. The responses of primary and secondary afferent fibres from muscle spindles in cat soleus were studied during constant velocity stretching.2. Intravenous Suxamethonium (SCh) caused a large increase in the response of the primary afferents to the dynamic phase of stretching, and a smaller increase in their response to static extension. The effects of SCh were similar to the effects of dynamic fusimotor stimulation.3. Increasing doses of SCh increased the response of primary afferents to dynamic stretching up to a point, but a peak discharge frequency was encountered beyond which the afferent fibre could not be induced to discharge.4. Suxamethonium increased the response of the secondary afferent fibres by a smaller amount than the primaries, and in particular caused a smaller increase in the response to dynamic extension.5. Acetylcholine given by close arterial injection had an effect similar to the effect of SCh.6. The effects of static and dynamic fusimotor stimulation on the response of primary afferents summed with the effects of small doses of SCh. When large amounts of SCh were used fusimotor stimulation sometimes had no further effect on the afferent discharge. It was not possible to say whether the fusimotor activity was then inhibited, or submerged in the SCh activity.7. The actions of SCh and of acetylcholine emphasized the differences in response of primary and secondary afferent endings to dynamic stretching. The use of these drugs enabled us to classify fibres of intermediate conduction velocity.8. Suxamethonium is known to activate slow muscle fibre systems with distributed nerve endings. The similarity between dynamic fusimotor activity and the effect of SCh suggests that the dyanimic fusimotor fibres act on slow intrafusal muscle fibres through multiple distributed endings.

Acetylcholine↗