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Biomedical subjects

A Prochazka

Publications and source records attributed to A Prochazka.

At least 73 records · Page 4Linked to original sources
PubMed1989

Flexible fusimotor control of muscle spindle feedback during a variety of natural movements.

A refined version of an experimental iterative simulation method is described, which was used to infer, from chronic spindle afferent recordings, type and time course of static and dynamic fusimotor activation during a variety of voluntary movements. When used to estimate overall fusimotor drive (without distinction between static and dynamic action) the method provides unique solutions. However, when generating independent gamma s and gamma d activation profiles, the solutions no longer are strictly unique. Yet the boundary conditions imposed by the type specific characteristics of gamma-action nevertheless permit detection of powerful activation, especially of dynamic efferents. Extending the finding of selective dynamic fusimotor activation during unpredictably imposed and resisted stretches, evidence for powerful, often transient activation of dynamic efferents has now been obtained for three additional motor paradigms. First, initiation of walking was accompanied by mixed fusimotor action. Static drive was stepped up and then maintained, whereas dynamic drive declined after an initial abrupt peak. Second, corrective balancing on a narrow walk beam was characterized by largely maintained static background drive, whilst dynamic activation profiles often exhibited powerful surges or transients, when the animal crouched to regain balance. These preceded subsequent EMG bursts during the stretch phase of crouching by about 300 ms. Third, preparation for landing from rapid lowering featured prominent and possibly selective activation of dynamic fusimotor neurones, which peaked while the animal was in mid-air and declined upon landing, and which preceded the sharp onset of EMG after landing by several hundred milliseconds. In all cases the fusimotor activation profiles were unrelated to the parent muscle EMG and difficult to reconcile with the notion of alpha-gamma linkage or coactivation. These findings then clearly support the concept of flexible central control, particularly of dynamic gamma-motoneurones during certain motor tasks.

Animals↗
PubMed1989

Muscle afferent contribution to control of paw shakes in normal cats.

1. The discharge of various hindlimb muscle afferents was recorded during paw shakes in normal cats with the use of floating dorsal root electrodes. 2. Muscle spindle group Ia-afferents and tendon organ group Ib-afferents fired during muscle lengthening, reaching very high peak discharge rates and then silencing at or shortly after the onset of shortening. The timing of Ia firing was consistent with the predictions of a linear model as well as the responses of Ia endings subjected to identical length variations in separate anesthetized cats. 3. In the latter "reconstruction" experiments, waxing and waning dynamic fusimotor action straddling whole paw-shake sequences gave the most consistent matches with the data from the normal cats. The reproducibility of the inferred fusimotor action justifies the inclusion of paw shakes as a class of movement in which fusimotor set is high. 4. The peak ensemble Ia activity from single hindlimb muscles was estimated to be approximately 20 kiloimpulses/s (Kips). Ankle extensor and hamstrings length variations were nearly in phase in the first cycles of a paw-shake sequence. From published data on spindle populations in these muscles, this indicated that peak Ia input to the spinal cord exceeded 0.2 megaimpulses/s (Mips). 5. The phase relationship between origin-to-insertion muscle length and Ia firing during paw shakes was little affected by doubling or tripling the moment of inertia of the foot. We argue that this refutes the notion that in paw shakes phase reversals occur between muscle fibers and tendons in the muscles studied. 6. Inertial loading of the foot led to small but significant reductions in mean paw-shake frequency. This is consistent with an afferent contribution to the generation of these movements. 7. We conclude that in paw shakes in normal cats, the CNS "chooses" to sensitize Ia-afferents to muscle length variations by increasing dynamic fusimotor action. The resulting ensemble Ia input is very large and is likely to play a significant role in reflexly shaping the alpha-motoneuronal activity responsible for the paw shakes.

Action Potentials↗
PubMed1988

Instability in human forearm movements studied with feed-back-controlled muscle vibration.

1. Frequency-modulated vibration was applied to the elbow flexor and extensor tendons to produce reflex movements of the forearm in normal subjects. The modulating (command) signal caused equal and opposite deviations from the 40 Hz carrier frequency so that when flexor vibratory frequency increased, extensor frequency decreased, and vice versa. 2. It is argued that the movements resulted largely from the reflex action of muscle spindle primary afferents whose firing frequency had been 'taken over' and modulated by the vibration. 3. Bode plots relating forearm movements to command signal (modulating) frequency showed the transfer function of the Ia afferent-CNS-muscle-load system to have a low-pass filter characteristic. The phase lag of movement on command increased progressively with command signal frequency, exceeding 180 deg at 3-4 Hz. 4. The transduced forearm movements were fed back to provide the command signal to the vibrators (and thus indirectly to the spindle afferents) via a filter mimicking the dynamic responsiveness of muscle spindle primary endings. Our aim was to 'break into' the reflex arc, and re-route it so that we could artificially vary the gain without significantly altering the dynamics of the pathway. 5. Nearly all subjects developed forearm oscillations (tremor) when the gain exceeded a threshold value. Subjects varied widely in their threshold, though for a given subject the threshold remained fairly constant from day to day. The results suggest that reflexly active individuals may not have a large safety margin with respect to forearm instability. 6. The frequency range of the oscillations observed in seven subjects was 3-8 Hz. The frequencies depended upon the level of flexor-extensor co-contraction, and increased from 3 to 5 Hz at 10% co-contraction to 5-8 Hz at 100% co-contraction. An analysis of the mechanical impedance of the arm provided estimates of tremor frequencies consistent with these results. 7. These unexpectedly low tremor frequencies led us to propose that it may be erroneous to expect stretch reflexes to contribute to forearm tremor in the range 8-12 Hz (e.g. physiological and 'enhanced' physiological tremors). Rather, their contributions should be sought in the range 3-8 Hz (e.g. pathological tremors such as those of Parkinson's and cerebellar disease).

Action Potentials↗
PubMed1988

Instability in human forearm movements studied with feed-back-controlled electrical stimulation of muscles.

1. Amplitude-modulated electrical stimulation was applied to the elbow flexors and extensors to produce movements of the forearm in normal subjects. The parameters of the modulating (command) signal were set in isometric trials so as to produce equal and opposite background torques, and equal and supportive torque modulations. 2. Bode plots relating forearm movement to command signal (modulating) frequency showed the muscle-load to have a low-pass characteristic similar to that previously described in the cat, and a slightly larger bandwidth than described previously in man. 3. The transduced forearm signals were fed back to provide the command signal to the stimulators via a filter which mimicked the transfer function of muscle spindle primary endings. In effect this replaced the neural part of the reflex arc with an accessible model, but left the muscle-load effector intact. 4. All six subjects developed forearm oscillations (tremor) when the loop gain exceeded a threshold value. The mean tremor frequency at onset was 4.4 Hz, which was similar to that of the equivalent vibration-evoked tremor (previous paper, Prochazka & Trend, 1988). 5. With the linear spindle model, oscillations tended to grow rapidly in amplitude, and the stimuli became painful. The inclusion of a logarithmic limiting element resulted in stable oscillations, without significant alterations in frequency. This allowed us to study the effect on tremor of including analog delays in the loop, mimicking those associated with peripheral nerve transmission and central reflexes. In one subject, loop delays of 0, 20, 40 and 100 ms resulted in tremor at 4.0, 3.6, 3.0 and 2.1 Hz respectively, as quantified by spectral analysis. 6. By considering separately the phase contributions of the different elements of the reflex arc, including delays, it became clear that muscle-load properties were important in setting the upper limit of tremor frequencies which could conceivably be supported by reflexes. 7. The results support the conclusion of the related vibration study (Prochazka & Trend, 1988), that for moderate levels of background co-contraction, the contribution of stretch reflexes to tremor at the elbow should be sought in the 3-5 Hz range. Exaggerated long-latency reflexes would be expected to reduce these baseline frequencies by 1 or 2 Hz.

Adult↗
PubMed1987

An experimental simulation method for iterative and interactive reconstruction of unknown (fusimotor) inputs contributing to known (spindle afferent) responses.

A simulation technique, combining chronic recordings in freely moving alert cats with acute experiments on a nerve muscle preparation, has been designed to estimate fusimotor activity profiles underlying chronically recorded muscle spindle afferent responses to movements. Fusimotor stimulation patterns are iteratively generated and tested for their ability to simulate a target response during reproduction of the movement. The error between a simulated and the target response is incorporated into the current stimulation pattern, to generate the stimulation profile for the next cycle of iteration. The procedure is semi-automatic and offers a number of interactive features. For instance, the user has the choice to redraw manually critical segments of a given stimulation profile. The procedure converges rapidly, and solutions are unique, since target responses (first produced with known inputs) could be reconstructed by virtually identical, iterated, profiles. The method opens up the possibility of investigating complex transient adjustments of fusimotor drive, e.g. during adaptive motor performance.

Animals↗
PubMed1987

Relative displacements in muscle and tendon during human arm movements.

1. X-ray, cine and video recordings were made of the movement of radio-opaque markers injected into the musculo-tendinous junctions of biceps brachii muscle. 2. In strong isometric contractions, the distal tendon of the long head of biceps lengthened by about 2% of its estimated rest length. 3. During voluntary isotonic elbow flexion-extension movements at frequencies up to 5.5 Hz there was no detectable phase shift between intramuscular and joint displacements. 4. In the fastest alternating movements (5.5-6.7 Hz) small phase advances developed in the muscle. 5. We conclude that human tendons do stretch during muscle contraction, but not enough to cause intramuscular phase reversals in rapid unloaded movements. This in turn means that muscle spindles shorten and lengthen virtually in phase with joint movements under most conditions.

Arm↗
PubMed1986

Proprioception during voluntary movement.

In the last decade, a number of laboratories have accumulated data on the firing of single afferent fibres from muscle and skin during movement in awake cats, monkeys and human subjects. While there is general agreement on the firing behaviour of skin afferents and tendon organ (Ib) afferents during movement, there remains a significant divergence of opinion regarding the way in which the response of muscle spindle afferents (Ia and II) to length changes is modified by fusimotor action (e.g., alpha-gamma linkage versus "fusimotor set"). The controversies surrounding the fusimotor system have tended to overshadow the emergence of several important characteristics of proprioceptive behaviour, corroborated in separate laboratories. (i) Mean Ia firing rates during active movements are nearly always higher than at rest. Thus, activation of the fusimotor system is reserved for the control of, or preparation for, movement. In animals, there is now strong evidence that there is usually a tonic component of fusimotor action during rhythmical movements. (ii) During fast, unloaded movements (peak muscle speeds, 0.2 resting lengths/s or more), the firing of both Ia and II afferents usually increases during lengthening and decreases during shortening. Ib afferents fire during even the most rapid active shortening of their parent muscles. (iii) During powerful shortening contractions performed against significant loads, Ia firing is often appreciable, suggesting that there is at least some underlying alpha-gamma coactivation. (iv) During fast imposed muscle stretches, Ia afferents respond with segmented bursts of firing (threshold speed for segmentation, 0.5-1.0 resting length/s). Ib afferents show far less segmentation of discharge under similar circumstances.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent Pathways↗
PubMed1985

'Fusimotor set': new evidence for alpha-independent control of gamma-motoneurones during movement in the awake cat.

The discharge activity of single muscle spindle receptors was recorded in freely moving cats. Large changes in responsiveness to length variations was observed in different types of movement. In separate stimulations on anaesthetised cats, the activity of the gamma-fusimotor neurones responsible for these changes was reconstructed. The results suggested that fusimotor action on a given spindle afferent during the movements studied was not rigidly alpha-linked, but 'set' by the CNS to steady levels, and that it could switch from largely static (gamma s) to largely dynamic (gamma d) according to the motor tasks performed.

Animals↗
PubMed1985

Action of single dynamic fusimotor neurones on cat soleus Ia afferents during muscle shortening.

The ability of single dynamic fusimotor (gamma d) fibres to sustain the firing of muscle spindle primary (Ia) afferents during shortening was investigated in soleus muscles of anaesthetised cats. Of 11 gamma d fibres, 10 could maintain Ia firing during 10 mm/s shortening. Of the 7 tested at greater velocities, 5 could maintain Ia firing during shortening at velocities greater than 50 mm/s. This ability was, however, critically dependent upon the timing of the stimulation. In particular, it rapidly reduced with increasing duration of stimulation before the onset of shortening. Furthermore, if appreciable stretch occurred between the onset of gamma d stimulation and the onset of shortening, this could greatly reduce the ability of gamma d fibres to sustain Ia discharge. If gamma d neurones are on occasion phasically activated during voluntary shortening movements, their action could be an important determinant of Ia firing, even in the presence of weak gamma s action. Therefore in chronic recordings, observation of Ia firing during muscle shortening is not an adequate criterion for inferring gamma d activity.

Action Potentials↗
PubMed1985

Treatment of primary proliferative polycythaemia by venesection and low dose busulphan: retrospective study from one centre.

Sixty-five patients with primary proliferative polycythaemia (polycythaemia rubra vera) were followed during the period 1962-83 and analysed retrospectively. Primary control of PCV was by venesection only with low dose busulphan solely as required to keep the platelet count below 400 X 10(9)/l. Median survival was 11.1 years from diagnosis which is equal to or marginally better than with other reported regimens. Vascular causes of death were only a little higher than expected in a comparable normal population. Only deaths from acute leukaemia and myelofibrosis were significantly increased above the normal population incidence. There was no evidence to suggest that these transformations were busulphan induced. Analysis of the incidence of occlusive vascular lesions lends support to an earlier recommendation that the PCV level be maintained below 0.45. No support was found for the possible disadvantages of a predominantly venesection regimen, such as iron deficiency and reactive thrombocytosis. The case is put for this use of low dose busulphan. The data presented would warrant the future inclusion of this therapeutic regime as one limb of a controlled trial.

Adult↗
PubMed1984

Can fusimotor activity potentiate the responses of muscle spindles to a tendon tap?

Experiments on the cat soleus muscle have determined the effect of selective activation of the fusimotor system on the responses of muscle receptors to a simulated tendon tap. Primary endings of spindles responded in the passive muscle with an average 4.3 impulses at a mean instantaneous rate of 502 impulses/s. Static fusimotor stimulation at 100 pulses/s increased the number of impulses during the tap to 4.8 but dropped the mean instantaneous rate to 400 impulses/s. Dynamic fusimotor stimulation increased the number of impulses to 6.3 and the instantaneous rate to 557 impulses/s. Combined stimulation of the two axons gave intermediate values. We consider these effects as rather feeble. The tendon jerk in man shows a large increase in reflex amplitude following a reinforcement manoeuvre (Jendrassik manoeuvre). Based on our animal experiments we conclude that such increases cannot be accounted for simply in terms of selective engagement of the fusimotor system.

Action Potentials↗
PubMed1984

Tendon organ firing during active muscle lengthening in awake, normally behaving cats.

Recordings were obtained of the discharge of single tendon organ (Ib) and muscle spindle (Ia) afferents of the ankle extensor muscles during movement in normal cats. During very slow, smooth increases and decreases in muscle force, Ib afferents showed from one to five stepwise changes in firing rate, attributable to the recruitment of motor units inserting into the receptor capsule. These 'recruitment steps' in Ib firing rate became smoothed and tended to merge during faster variations in muscle force, and were rarely discernible in normal movements such as slow stepping. Rapid imposed stretches resulted in Ib firing patterns which fitted well a dynamic function of whole muscle force. Comparisons were made between the responses of Ib and Ia afferents during rapid, imposed muscle stretch. The segmentation of discharge typical of Ia afferents was not present in Ib afferents, despite segmentation of the e.m.g. of the receptor-bearing muscles. This would imply that Ib afferents exert a rapidly fluctuating reflex action against a relatively steady background of Ib input. Ankle extensor Ib firing during stepping was characterized by feeble firing during the swing phase and substantial, smoothly modulated firing during the stance phase. Taken together with previous chronic recordings, the data support the view that the ensemble of Ib afferents from a muscle signals a dynamic, non-linear function of whole muscle force over a wide range of normal movement.

Action Potentials↗
PubMed1984

The after-effects of stretch and fusimotor stimulation on the responses of primary endings of cat muscle spindles.

These experiments explore the after-effects of repetitive movements and dynamic fusimotor stimulation on the responses of primary endings of soleus muscle spindles in the anaesthetized cat. If immediately following a series of conditioning stretch and shortening movements, the muscle was held at the stretched length for 3 s before being returned to its rest length, the subsequent response to a brief dynamic fusimotor tetanus given during a slow test stretch produced only a small increase in spindle firing. If, on the other hand, the muscle was returned to its rest length immediately after the movements, the fusimotor tetanus evoked a much larger afferent burst. This difference in the size of the burst could only be observed if the fusimotor tetanus was given soon after onset of the test stretch. If it was delayed and given at a time when the test stretch passed through the length at which the muscle had been held stretched after the movements, there was no difference in the size of the afferent burst. If following the movements the muscle was held stretched for less than 3 s, the response to the subsequent tetanus was not fully depressed. Once the depressed condition had been achieved, the muscle had to be left undisturbed for up to half an hour before the response had recovered its original fully undepressed size. Conditioning repetitive stimulation of the fusimotor fibre was just as effective as using alternating movements in producing the effects. If the test tetanus, which was normally ten shocks in 50 ms, was made longer, the change in size of the impulse burst became a change in latency of onset of the response to the tetanus. Holding the muscle stretched at the end of the conditioning movements/tetanus produced a delay in onset of the response to the test tetanus without significantly altering its size. These observations have been interpreted as arising from development of stable cross-bridges between actin and myosin filaments in the intrafusal fibres. During repetitive movements or fusimotor stimulation, stable bridges become detached and during the subsequent 3 s they re-attach, at the length at which the muscle is being held after conditioning.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1983

A new simulation method to deduce fusimotor activity from afferent discharge recorded in freely moving cats.

Direct recordings from identified and classified fusimotor gamma-motoneurons in freely moving animals have not so far been achieved, and present knowledge on fusimotor activity during natural movements is based on qualitative inferences made from spindle afferent discharge. In order to put such deductions on a firmer basis, a simulation method has been developed, which provides quantitative estimates of the fusimotor drive that shaped the spindle afferent discharge, as recorded in chronically implanted cats during voluntary or imposed movements. Simulations are performed in acute experiments on anesthetized cats, whereby variations in muscle length and EMG envelopes are reproduced by an electromagnetic servo from digitally stored segments of the original records. The responses of spindle afferents to the simulated movements are then examined, both in the absence of fusimotor action and during concomitant stimulation of functionally single gamma-motoneurons, rate modulated according to a variety of stored functions (including the original EMG envelope).

Afferent Pathways↗
PubMed1982

Effect of fusimotor stimulation on ia discharge during shortening of cat soleus muscle at different speeds.

1. In barbiturate-anaesthetized cats, the L7 and S1 dorsal and ventral roots were dissected to isolate functionally single afferents identified as primary endings of soleus muscle spindles, and motor filaments which exerted a fusimotor action on the afferents with limited action on extrafusal muscle. Up to seven filaments, with an action on a given primary ending, could be isolated and each was classified as exerting either a predominantly dynamic or static action.2. Combined stimulation of these filaments, at rates up to 200 impulses/s could maintain afferent firing during muscle shortenings at speeds up to 200 mm/s.3. Fusimotor stimulation could also maintain afferent firing at a target frequency of 100 impulses/s during muscle shortenings up to 200 mm/s. The timing, in relation to the onset of shortening, and the rates of fusimotor stimulation were found to be critical in achieving the target frequency.4. Sinusoidal modulation of the frequency of fusimotor stimulation was used to study the conditions required to achieve constant afferent firing in the face of imposed sinusoidal length changes.5. For given depths of modulation, the phase advance of fusimotor stimulation needed to produce minimum modulation of afferent firing (best compensation) increased with increasing frequency of the sinusoids. The compensation deteriorated with an increase in the frequency of the sinusoids and a change in the mean muscle lengths, although in some cases it could be restored by adjustments to the depth of modulation of fusimotor rate. This suggests that for movements of varying speeds and amplitudes, settings which are appropriate for shortening at a given velocity and mean muscle length, do not apply if either of these two variables are altered.6. These findings demonstrate that the fusimotor system is potentially capable of eliciting constant afferent firing as envisaged in the ;servo-assistance' hypothesis (Matthews, 1964, 1972; Stein, 1974). This, and the fact that constant afferent firing is not seen during normal unobstructed shortenings at velocities greater than 0.2 resting length/s (Prochazka, 1981), are used to argue that it is by choice rather than necessity that ;servo-assistance' (as defined above) is not employed during normal movements. However, servo-assistance of a different form (involving modulated spindle afferent feed-back from both agonists and antagonists) remains a viable alternative.

Animals↗
PubMed1981

Muscle spindle function during normal movement.

1. Discharge of muscle spindles during voluntary movement Recent recordings from afferents in awake animals have re-emphasized the function of spindle endings as stretch receptors. The available evidence suggests that, in voluntary movements involving muscle velocities above 0.2 RL sec-1 the modulation of firing rate of both primary and secondary endings is closely related to the length variations. Below 0.2 RL sec-1, phasic changes in fusimotor action may sometimes dominate such modulation. The modulatory strength of the component of fusimotor action strictly linked to skeletomotor activity is generally low. On the other hand, there is good evidence that in most movements, at least in cats and monkeys, there is steady, low level fusimotor action, independent of homonymous skeletomotor activity. It is therefore probable that fusimotor neurons are controlled largely independently of skeletomotor neurons, in such a way as to set the sensitivity and bias of spindle endings to levels appropriate for whole sequences of movements. 2. Reflex excitation of motoneurons Skeletomotor reflex responses to length perturbations can have a powerful linearizing effect on muscle stiffness. Spindle afferents often play an important role in mediating such reflexes and probably also contribute significantly to skeletomotor excitation during tonic contractions. Transmission in the pathways from spindles to motoneurons, including those through supraspinal structures, can be modified by a number of identified segmental mechanisms. These mechanisms could well be involved in the volitional control of the amount of spindle afferent excitation reaching the motoneuron pool, and may act as a form of gain control at different levels of arousal. 3. Proprioceptive cues for coordination Stretch-evoked activity of spindle system to modify the rhythm of the spinal locomotor generator. In particular, the onset of flexion or extension depends in part on spindle afferent activity. 4. Kinesthesia There is now evidence that muscle spindles contribute to position and velocity sense, although deficits nevertheless occur if skin and joint afferents are paralyzed.

Afferent Pathways↗