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Biomedical subjects

A Prochazka

Publications and source records attributed to A Prochazka.

82 records · Page 5Linked to original sources
PubMed1980

Neuromuscular responses to gait perturbations in freely moving cats.

If an obstacle impedes the forward swing of a cat's foot, the animal responds by rapidly lifting the foot over the obstacle. In freely moving cats, the electrical activity of hindlimb flexors and extensors was recorded during such reactions elicited both mechanically and electrically. The sequencing of muscle activity was more complex and longer in duration in the mechanically elicited reactions. Anaesthesia of the foot dorsum abolished responses in ankle extensors and knee flexors, and converted the responses of ankle flexors to simple stretch reflexes. Although our findings closely resemble those reported for chronic spinal kittens, there are interesting points of difference, which should be taken into account if the notion of a purely spinal mediation of the placing reaction during stepping is to be accepted.

Afferent Pathways↗
PubMed1980

Tendon organ discharge during voluntary movements in cats.

1. The discharge activity of tendon organ afferents was recorded during voluntary movements in cats. 2. The eight tendon organ afferents in our sample all fired during isotonic movements involving active muscle shortening. 3. Firing rates usually exceeded 100 sec-1, even up to the highest muscle shortening velocity observed, 1.8 resting lengths per second (lr/sec). 4. We suggest that during voluntary, isotonic movements involving muscle shortening at velocities exceeding 0.2 lr/sec, the net action of muscle afferents on homonymous motoneurones is often inhibition. 5. These observations on tendon organs, taken together with previous findings on muscle spindles, indicate that in normal fast movements the role of the large muscle afferents is to signal dynamic functions of muscle length and force.

Action Potentials↗
PubMed1979

Muscle spindle discharge in normal and obstructed movements.

1. The discharge activity of muscle spindle endings located in tail and hind limb muscles was recorded during voluntary movements in the cat. 2. During active shortening of the receptor-bearing muscles, both primary and secondary endings tended to fall silent. This was more pronounced, the higher the rate of muscle shortening. We suggest that in unobstructed movements in which muscle velocities exceed 0.2 resting lengths per second (lr/sec), the firing patterns of spindle afferents are dominated by their responses to the length variations. At velocities lower than 0.2 lr/sec, fusimotor action may predominate. 3. When active muscle shortening was unexpectedly halted, both primary and secondary endings resumed firing, but the increases in discharge rate were not as abrupt as might have been expected had there been strong co-activation of fusimotor and skeletomotor neurones. Rather, for the types of movements studied, fusimotor action appears to have been quite modest.

Action Potentials↗
PubMed1977

Ia afferent activity during a variety of voluntary movements in the cat.

1. Implanted dorsal root electrodes were used to record discharge trains of single spindle primary afferents (Ia's) of the cat's hind limb during different types of movement.2. The length of the ipsilateral ankle extensors was continuously monitored by an implanted length gauge. Length changes occurring during active stepping were subsequently passively reproduced during brief anaesthesia.3. A comparison of the Ia responses in active and simulated step cycles revealed that moderate fusimotor drive to ankle extensor spindles probably occurred mainly, if not exclusively, during the E(1), E(2) and E(3) phases of active stepping.4. A temporal advance in the Ia response to passive stretching in the F-phase was attributed to the after-effects of fusimotor activity in the extension phases.5. Light thrust applied to the animal's back evoked a potent fusimotor response. This load compensation effect may provide an explanation for the apparently higher degree of alpha-gamma co-activation seen in the mesencephalic locomotor preparation.6. Ankle extensor Ia discharge decreased during falls, despite an increase in extensor e.m.g. This is seen as a clear example of independent alpha and gamma control.7. Placing reactions during walking were consistent with the notion that cutaneous inputs dominate over proprioceptive inputs in these movements.8. alpha and Ia discharge during paw-shaking showed many of the characteristics of that in decerebrate and spastic clonus. The present results suggest that movements resembling clonus may be part of the animal's normal repertoire.9. Isometric co-contraction of agonists and antagonists was found to involve alpha-gamma co-activation.10. Hamstring Ia discharge behaviour during stepping further highlighted the increases in firing rate which normally occur during passive muscle stretching in ;pre-programmed' movements.

Action Potentials↗
PubMed1977

Reflexes in cat ankle muscles after landing from falls.

1. Electrical activity and length of ankle muscles were recorded by telemetry during free fall and landing in cats. 2. After foot contact, there was a delay in onset of stretch of ankle extensors of between 8 and 11 ms. High-speed cinematography showed the delay to be associated with rapid initial dorsiflexion of the toes. 3. Electromyograms (e.m.g.) from lateral gastrocnemius increased in amplitude prior to landing. An early depression of lateral gastrocnemius e.m.g. commenced at 8 ms after foot contact, and was followed by a large peak of activity commencing some 8 ms after the first increase in lateral gastrocnemius length. 4. Local anaesthesia of the plantar cushion did not alter this pattern of response. 5. The early inhibition of lateral gastrocnemius was attributed to the action on lateral gastrocnemius motoneurones of non-cutaneous afferents responding to the initial toe dorsiflexion. Additional autogenetic inhibition may also have contributed. 6. The subsequent peak of e.m.g. was at a latenty consistent with a rapid stretch reflex, and occurred soon enough for the resulting active tension to contribute significantly to the extensor force during body deceleration.

Animals↗
PubMed1976

Discharges of single hindlimb afferents in the freely moving cat.

1. Implanted semimicroelectrodes were used to record single afferent fiber discharges from L7 dorsal roots during unrestrained walking in the conscious cat. 2. A series of tests were used to identify an afferent during a short period of anesthesia following each recording session. The majority of afferents were from muscle spindle primary endings in hindlimb muscles. 3. Ankle extensor spindle primaries generally showed their highest firing rates during that phase of stepping in which they were passively stretched. During active muscle contraction there was evidence of fusimotor drive, although this was not usually sufficient to entirely overcome the unloading effect of rapid muscle shortening. The variability of firing rate from cycle to cycle was considerably greater for the phase of active muscle contraction. The EMG response to brisk stretches of the ankle extensor muscle indicated a rapid (disynaptic or trisynaptic) reflex arc in the conscious animal. 4. Knee flexor spindle primaries showed similarly higher firing rates during passive muscle stretching in the step cycle. The shorter periods of presumed alpha-gamma coactivation corresponded to the much more phasic role of these muscles in stepping. 5. Tendon organs in the physiological extensors of the toes were mainly active during stance, although some discharges were usually seen during the swing phase. It is suggested that previous experiments on mesencephalic preparations may have led to an exaggerated view of the degree of alpha-gamma coactivation during normal stepping movements.

Action Potentials↗
PubMedDate not supplied

Movements generated by intraspinal microstimulation in the intermediate gray matter of the anesthetized, decerebrate, and spinal cat.

The intermediate laminae of the lumbosacral spinal cord are suggested to contain a small number of specialized neuronal circuits that form the basic elements of movement construction ("movement primitives"). Our aim was to study the properties and state dependence of these hypothesized circuits in comparison with movements elicited by direct nerve or muscle stimulation. Microwires for intraspinal microstimulation (ISMS) were implanted in intermediate laminae throughout the lumbosacral enlargement. Movement vectors evoked by ISMS were compared with those evoked by stimulation through muscle and nerve electrodes in cats that were anesthetized, then decerebrated, and finally spinalized. Similar movements could be evoked under anesthesia by ISMS and nerve and muscle stimulation, and these covered the full work space of the limb. ISMS-evoked movements were associated with the actions of nearby motoneuron pools. However, after decerebration and spinalization, ISMS-evoked movements were dominated by flexion, with few extensor movements. This indicates that the outputs of neuronal networks in the intermediate laminae depend significantly on descending input and on the state of the spinal cord. Frequently, the outputs also depended on stimulus intensity. These experiments suggest that interneuronal circuits in the intermediate and ventral regions of the spinal cord overlap and their function may be to process reflex and descending activity in a flexible manner for the activation of nearby motoneuron pools.

Anesthesia↗