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Biomedical subjects

A Prochazka

Publications and source records attributed to A Prochazka.

At least 19 recordsLinked to original sources
PubMed2006

Intraspinal microstimulation excites multisegmental sensory afferents at lower stimulus levels than local alpha-motoneuron responses.

Microstimulation within the motor regions of the spinal cord is often assumed to activate motoneurons and propriospinal neurons close to the electrode tip. However, previous work has shown that intraspinal microstimulation (ISMS) in the gray matter activates sensory afferent axons as well as alpha-motoneurons (MNs). Here we report on the recruitment of sensory afferent axons and MNs as ISMS amplitudes increased. Intraspinal microstimulation was applied through microwires implanted in the dorsal horn, intermediate region and ventral horn of the L(5)-L(7) segments of the spinal cord in four acutely decerebrated cats, two of which had been chronically spinalized. Activation of sensory axons was detected with electroneurographic recordings from dorsal roots. Activation of MNs was detected with electromyographic (EMG) recordings from hindlimb muscles. Sensory axons were nearly always activated at lower stimulus levels than MNs irrespective of the stimulating electrode location. EMG response latencies decreased as ISMS stimulus intensities increased, suggesting that MNs were first activated transsynaptically and then directly as intensity increased. ISMS elicited antidromic activity in dorsal root filaments with entry zones up to 17 mm rostral and caudal to the stimulation sites. We posit that action potentials elicited in localized terminal branches of afferents spread antidromically to all terminal branches of the afferents and transsynaptically excite MNs and interneurons far removed from the stimulation site. This may help explain how focal ISMS can activate many MNs of a muscle even though they are distributed in long thin columns.

Action Potentials↗
PubMed2005

Control of locomotor cycle durations.

In intact animals and humans, increases in locomotor speed are usually associated with decreases in step cycle duration. Most data indicate that the locomotor central pattern generator (CPG) shortens cycle duration mainly by shortening the durations of extensor rather than flexor phases of the step cycle. Here we report that in fictive locomotion elicited by electrical stimulation of the midbrain locomotor region (MLR) in the cat, spontaneous variations in cycle duration were due more to changes in flexor rather than extensor phase durations in 22 of 31 experiments. The locomotor CPG is therefore not inherently extensor- or flexor-biased. We coined the term "dominant" to designate the phase (flexion or extension) showing the larger variation. In a simple half-center oscillator model, experimental phase duration plots were fitted well by adjusting two parameters that corresponded to background drive ("bias") and sensitivity ("gain") of the oscillator's timing elements. By analogy we argue that variations in background drive to the neural timing elements of the CPG could produce larger variations in phase duration in the half-center receiving the lower background drive, i.e., background drive may determine which half-center is dominant. The fact that data from normal cats were also fitted well by the model indicates that sensory input and central drive combine to determine locomotor phase durations. We conclude that there is a considerable flexibility in the control of phase durations in MLR-induced fictive locomotion. We posit that this may be explained by changes in background excitation of neural timing elements in the locomotor CPG.

Analysis of Variance↗
PubMed2004

Coding of position by simultaneously recorded sensory neurones in the cat dorsal root ganglion.

Muscle, cutaneous and joint afferents continuously signal information about the position and movement of individual joints. How does the nervous system extract more global information, for example about the position of the foot in space? To study this question we used microelectrode arrays to record impulses simultaneously from up to 100 discriminable nerve cells in the L6 and L7 dorsal root ganglia (DRG) of the anaesthetized cat. When the hindlimb was displaced passively with a random trajectory, the firing rate of the neurones could be predicted from a linear sum of positions and velocities in Cartesian (x, y), polar or joint angular coordinates. The process could also be reversed to predict the kinematics of the limb from the firing rates of the neurones with an accuracy of 1-2 cm. Predictions of position and velocity could be combined to give an improved fit to limb position. Decoders trained using random movements successfully predicted cyclic movements and movements in which the limb was displaced from a central point to various positions in the periphery. A small number of highly informative neurones (6-8) could account for over 80% of the variance in position and a similar result was obtained in a realistic limb model. In conclusion, this work illustrates how populations of sensory receptors may encode a sense of limb position and how the firing of even a small number of neurones can be used to decode the position of the limb in space.

Action Potentials↗
PubMed2004

The sense of movement elicited by transcranial magnetic stimulation in humans is due to sensory feedback.

It has been claimed that transcranial magnetic stimulation (TMS) of the human motor cortex can produce a sense of movement of the contralateral hand, even when the hand is paralysed. The sense of movement was equated with a 'corollary discharge', a nulling mechanism originally posited for maintaining constancy of the visual field during eye movements. Our experiments were designed to test whether the sensation that accompanies TMS-evoked finger movements is generated centrally or whether it arises as a result of sensory feedback. Matched twitches of the left and right fingers were elicited either by bilateral electrical stimulation of forearm extensor muscles, or by a combination of TMS of left motor cortex (eliciting twitches of the right forefinger), and electrical stimulation of the left forearm muscles (eliciting twitches of the left forefinger). The time interval between stimuli activating left and right twitches was varied randomly (range +/- 90 ms) from trial to trial. Subjects reported whether they sensed that the left or the right movement occurred first, or if they could detect no difference. The left and right movements evoked by bilateral electrical stimulation of muscles were sensed as near simultaneous when there was zero delay between them. When TMS was applied in conjunction with unilateral muscle stimulation, the TMS-evoked movement was felt, on average, 20 ms after the movement evoked by muscle stimulation. Similar results were obtained when the skin under the cathodal electrodes was anaesthetized. Since the TMS-evoked movements were felt later rather than earlier than the electrically evoked movements, the results do not support the idea that the sensation of movement was elicited centrally by TMS. Rather, they favour sensory feedback as the source of the sense of movement. The earlier perception of electrically evoked versus TMS-evoked movements was probably due to earlier sensory responses in the periphery rather than a suppression of the excitability of somatosensory cortex.

Adult↗
PubMed2004

Contribution of stretch reflexes to locomotor control: a modeling study.

It is known that the springlike properties of muscles provide automatic load compensation during weight bearing. How crucial is sensory control of the motor output given these basic properties of the locomotor system? To address this question, a neuromuscular model was used to test two hypotheses. (1) Stretch reflexes are too weak and too delayed to contribute significantly to weight-bearing. (2) The important contributions of sensory input involve state-dependent processing. We constructed a two-legged planar locomotor model with 9 segments, driven by 12 musculotendon actuators with Hill-type force-velocity and monotonic force-length properties. Electromyographic (EMG) profiles of the simulated muscle groups during slow level walking served as actuator activation functions. Spindle Ia and tendon organ Ib sensory inputs were represented by transfer functions with a latency of 35 ms, contributing 30% to the net EMG profile and gated to be active only when the receptor-bearing muscles were contracting. Locomotor stability was assessed by parametric variations of actuator maximum forces during locomotion in open-loop ("deafferented") trials and in trials with feedback control based on either sensory-evoked stretch reflexes or finite-state rules. We arrived at the following conclusions. (1) In the absence of sensory control, the intrinsic stiffness of limb muscles driven by a stereotyped rhythmical pattern can produce surprisingly stable gait. (2) When the level of central activity is low, the contribution of stretch reflexes to load compensation can be crucial. However, when central activity provides adequate load compensation, the contribution of stretch reflexes is less significant. (3) Finite-state control can greatly extend the adaptive capability of the locomotor system.

Computer Simulation↗
PubMed2004

The role of neuromuscular properties in determining the end-point of a movement.

How does the activation of several muscles combine to produce reliable multijoint movements? To study this question, we stimulated up to six sites in muscles, nerves, and the spinal cord. Flexion and extension of the hip, knee, and ankle were elicited in anesthetized and decerebrate cats. The movements occurred largely in the sagittal plane against a constant spring load and covered most of the passive range of motion of the cat's limb. The movements of the end-point (foot) were compared with predictions based on vectorial summation of end-point movements elicited by stimulating single electrodes. The lengths of the movements produced by stimulating more than one site exceeded what was expected from linear summation for small movements (<3 cm) and showed a less than linear summation for large movements (>11 cm). The data were compared with muscle and limb models. Since the deviations from linearity were predictable as a function of distance, adjustments might easily be learned by trial and error. The summation was less complete for spinal stimulation, compared to nerve and muscle stimulation, so spinal circuits do not appear to compensate for the nonlinearities. Movements were elicited from positions of the limb not only in a neutral position, but also in front and behind the neutral position. A degree of convergence was seen, even with stimulation of some individual muscles, but the convergence increased as more muscles were stimulated and more joints were actively involved. This suggests that convergence to an equilibrium-point arises at least partly from muscle properties. In conclusion, there are deviations from linear vectorial summation, and these deviations increase when more muscles are stimulated. The convergence to an equilibrium-point may simplify the computations needed to produce movements involving many muscles.

Anesthesia↗
PubMed2002

Limb movements generated by stimulating muscle, nerve and spinal cord.

We have compared the movements generated by stimulation of muscle, nerve, spinal roots and spinal cord in anesthetized, decerebrate and spinalized cats. Each method produced a full range of movements of the cat's hind limb in the sagittal plane against a spring load, except for stimulation of the roots. Stimulation of the dorsal roots produced movements that were mainly up and forward, whereas stimulation of the ventral roots produced complementary movements (down and backward). Results from stimulation in the intermediate areas of the spinal cord were compared to predictions of the "movement primitives" hypothesis. We could not confirm that the directions were independent of stimulus amplitude or the state of descending inputs. Pros and cons of stimulating at some sites were provisionally considered for the reliable control of limb movements with functional electrical stimulation (FES) in clinical conditions.

Anesthetics↗
PubMed2001

Neural prostheses.

Assuming that neural regeneration after spinal cord injury (SCI) will eventually become a clinical reality, functional recovery will probably remain incomplete. Assistive devices will therefore continue to play an important role in rehabilitation. Neural prostheses (NPs) are assistive devices that restore functions lost as a result of neural damage. NPs electrically stimulate nerves and are either external or implanted devices. Surface stimulators for muscle exercise are now commonplace in rehabilitation clinics and many homes. Regarding implantable NPs, since 1963 over 40 000 have been implanted to restore hearing, bladder control and respiration. Epidural spinal cord stimulators and deep brain stimulators are routinely implanted to control pain, spasticity, tremor and rigidity. Implantable NPs have also been developed to restore limb movements using electrodes tunnelled under the skin to muscles and nerves. Spinal cord microstimulation (SC[mu]stim) is under study as an alternative way of restoring movement and bladder control. Improvement in bladder and bowel function is a high priority for many SCI people. Sacral root stimulation to elicit bladder contraction is the current NP approach, but this usually requires dorsal rhizotomies to reduce reflex contractions of the external urethral sphincter. It is possible that the spinal centres coordinating the bladder-sphincter synergy could be activated with SC[mu]stim. Given the large and growing number of NPs in use or development, it is surprising how little is known about their long-term interactions with the nervous system. Physiological research will play an important role in elucidating the mechanisms underlying these interactions.

Animals↗
PubMed2001

Adaptive changes in locomotor control after partial denervation of triceps surae muscles in the cat.

This report concerns a test of the hypothesis that gain in the stretch reflex pathway of cat medial gastrocnemius (MG) muscle during locomotion increases after denervation of its synergists, lateral gastrocnemius (LG), soleus (SOL) and plantaris (PL) muscles. In four cats, MG, tibialis anterior (TA) and vastus lateralis (VL) muscles were implanted with electromyogram (EMG) electrodes. The cats walked on a row of elevated pegs, some of which were spring-loaded and could be triggered to pop up at the moment of foot touchdown, rapidly dorsiflexing the foot. Pre-stretch EMG activity in MG as well as short-, medium- and long-latency responses to the dorsiflexions were compared before and after unilateral denervation of synergists. Short- and medium-latency responses of MG to perturbations increased in proportion to the increase in pre-stretch EMG in the days and weeks after partial denervation. This argues against an adaptive increase in stretch reflex gain independent of centrally generated extensor drive. Local anaesthesia of the skin of the paw did not significantly change the sizes of the stretch responses of MG before or after partial denervation. We conclude that pre-stretch EMG activity as well as stretch reflexes in MG muscle increased substantially after denervation of synergistic muscles. The data were consistent with an adaptive increase in central locomotor drive, causing more motoneuronal activity, which in turn resulted in an increase in the size of stretch reflexes.

Adaptation, Physiological↗
PubMed2001

Quantification of the UPDRS Rigidity Scale.

In the clinical setting, parkinsonian rigidity is assessed using subjective rating scales such as that of the Unified Parkinson's Disease Rating System (UPDRS). However, such scales are susceptible to problems of sensitivity and reliability. Here, we evaluate the reliability and validity of a device designed to quantify parkinsonian rigidity at the elbow and the wrist. The method essentially quantifies the clinical examination and employs small sensors to monitor forces and angular displacements imposed by the clinician onto the limb segment distal to the joint being evaluated. Force and displacement data are used to calculate elastic and viscous stiffnesses and their vectorial sum, mechanical impedance. Interexaminer agreement of measures of mechanical impedance in subjects with Parkinson's disease was comparable to that of clinical UPDRS scores. Examiners tended to overrate rigidity on the UPDRS scale during reinforcement manoeuvres. Mechanical impedance was nonlinearly related to UPDRS ratings of rigidity at the elbow and wrist; characterization of such relationships allows interpretation of impedance measurements in terms of the clinical rating scales.

Biomechanical Phenomena↗
PubMed2000

Spinal cord microstimulation generates functional limb movements in chronically implanted cats.

Spinal cord injuries disrupt the communication between the brain and peripheral nerves, but leave motoneurons and networks of interneurons below the level of the lesion intact. It is therefore possible to restore some function following injury by providing an artificial stimulus to the surviving neurons below the level of the lesion. We report here on a novel approach for generating functional movements by electrically stimulating the spinal cord through chronically implanted ultrafine, hair-like electrodes. Six to 12 microwires were implanted in the lumbar enlargement of intact cats for 6 months. Twice a week, trains of stimuli were delivered through each microwire and the evoked electromyographic and torque responses were recorded. Strong coordinated hindlimb movements were obtained by stimulating through individual electrodes. The joint torques elicited were capable of supporting the animals' hindquarters. The responses were stable over time and the contractions caused no apparent discomfort to the animals. No obvious motor deficits were seen throughout the 6-month duration of implantation. The results demonstrate that microwires implanted in the spinal cord remain stably in place and stimulation through these electrodes produces strong, controllable movements. This provides a promising basis for the use of spinal cord neuroprostheses in restoring mobility following spinal cord injury.

Animals↗
PubMed2000

What do reflex and voluntary mean? Modern views on an ancient debate.

Are the words reflex and voluntary useful scientific concepts, or are they prescientific terms that should be discarded? Physiologists use these words routinely in their publications, in laboratory experiments and, indeed, like most lay people, in their daily lives. The tacit assumption is that we all know, more or less, what they mean. However, the issue has a rich history of philosophical and scientific debate; and, as this article demonstrates, present-day researchers still cannot reach a consensus on the meaning of the words and on whether it is possible to draw a scientific distinction between them. The five authors present five quite different analyses. In broad terms, they split into two camps: those who equate voluntary behaviours with consciousness and suppressibility and those who view all behaviours as sensorimotor interactions, the complexity of which determines whether they are reflexive or voluntary. According to the first view, most movements of daily life are neither purely reflex nor purely voluntary. They fall into the middle ground of automatic motor programs. According to the second view, as neuroscience advances the class of reflex behaviours will grow and the class of voluntary behaviours will shrink.

Animals↗
PubMed2000

Isometric muscle length-tension curves do not predict angle-torque curves of human wrist in continuous active movements.

In this study we tested the hypothesis that during steady contractions of human wrist extensors or flexors, the torque-angle relationship during movements imposed about the wrist is predicted by the classical isometric muscle length-tension curve, with ascending, descending and ascending limbs. Angle-torque relationships were measured during steady muscle activation (10% of maximal voluntary contraction: MVC), elicited either by electrical stimulation or voluntary regulation of the electromyogram (EMG). Flexion-extension movements of constant speed (+/-10 degrees /s) were imposed on the subjects' hands with a servo actuator, either through the full physiological range of motion +/-50 degrees, or through +/-10 degrees. During extensor contractions, angle-torque curves in +/-50 degrees movements had ascending, descending and ascending limbs, as in isometric contractions. However, in +/-10 degrees movements, torque always increased with increasing muscle length and decreased with decreasing length, even over angles corresponding to the descending limb of isometric curves. For flexor activation, angle-torque curves had similar properties, though descending limbs were less obvious or absent. During imposed movements, hysteresis was observed in the angle-torque curves. This was attributed to non-linearities of the active muscles. Hysteresis reached a maximum at intermediate wrist angles and declined at maximal muscle length, contradicting the recent hypothesis that sarcomere non-uniformity is responsible for the hysteresis. We conclude that the classical isometric length-tension curve, with its prominent descending limb, does not predict angle-torque curves of human wrist muscles in continuous movements. A more appropriate model is one in which stiffness about the wrist is always positive and hysteresis is a significant factor.

Adult↗
PubMed1999

Voluntary and reflex control of human back muscles during induced pain.

1. Back pain is known to change motor patterns of the trunk. The purpose of this study was to examine the motor output of the erector spinae (ES) muscles during pain in the lumbar region. First, their voluntary activation was assessed during flexion and re-extension of the trunk. Second, effects of cutaneous and muscle pain on the ES stretch reflex were measured, since increased stretch reflex gain has been suggested to underlie increased muscle tone in painful muscles. 2. The trunk movement and electromyographical (EMG) signals from the right and left ES during pain were compared with values before pain. Controlled muscle pain was induced by infusion of 5 % saline into the right lumbar ES. Cutaneous pain was elicited by mechanical or electrical stimulation of the dorsal lumbar skin. The stretch reflex was evoked by rapidly indenting the right lumbar ES with a servo-motor prodder. 3. The results from the voluntary task show that muscle pain decreased the modulation depth of ES EMG activity. This pattern was associated with a decreased range and velocity of motion of the painful body segment, which would normally serve to avoid further injury. Interestingly, when subjects overcame this guarding tendency and made exactly the same movements during pain as before pain, the EMG modulation depth was still reduced. The results seem to reconcile the controversy of previous studies, in which both hyper- and hypoactivity of back muscles in pain have been reported. 4. In the tapped muscle, the EMG response consisted of two peaks (latency 19.3 +/- 2.1 and 44.6 +/- 2.5 ms, respectively) followed by a trough. On the contralateral side the first response was a trough (26.2 +/- 3.2 ms) while the second (46.4 +/- 4.3 ms) was a peak, similar to the second peak on the tapped side. Cutaneous pain had no effect on the short-latency response but significantly increased the second response on the tapped side. Surprisingly, deep muscle pain had no effect on the stretch reflex. A short-latency reciprocal inhibition exists between the right and left human ES. 5. It is concluded that deep back pain does not influence the stretch reflexes in the back muscles but modulates the voluntary activation of these muscles.

Adult↗
PubMed1999

The effect of wrist angle on electrically evoked hand opening in patients with spastic hemiplegia.

This paper studied the effect of wrist angle on the amount of hand opening achieved by electrical stimulation in people with spastic hemiplegia. With their forearm in pronation, subjects were asked to relax while their affected wrist was passively moved in steps of about 15 degrees from full flexion into extension. Trains of stimuli were applied to the long finger extensor muscles through surface electrodes on the forearm. At each wrist position stimulation was turned on for a few seconds until hand opening equilibrated. Wrist angle and fingertip positions were recorded using a three-dimensional (3-D) motion analysis system. Maximal displacements between thumbtip and each fingertip occurred when the wrist was fully flexed. As the wrist was extended, hand aperture achieved by electrical stimulation progressively declined, reaching zero at 40 degrees of wrist extension. We conclude that electrical stimulation can significantly increase the grasp aperture of the hemiplegic hand, but this is strongly dependent on wrist posture and accompanying voluntary effort.

Adolescent↗
PubMed1999

Tremor suppression using functional electrical stimulation: a comparison between digital and analog controllers.

In this study, we compared digital and analog versions of a functional electrical stimulator designed to suppress tremor. The device was based on a closed-loop control system designed to attenuate movements in the tremor frequency range, without significantly affecting slower, voluntary movements. Testing of the digital filter was done on three patients with Parkinsonian tremor and the results compared to those of a functional electrical stimulation device based on an analog filter evaluated in a previous study. Additional testing of both the analog and digital filters was done on three subjects with no neurological impairment performing tremor-like movements and slow voluntary movements. We found that the digital controller provided a mean attenuation of 84%, compared to 65% for the analog controller.

Adult↗
PubMed1999

Multi-method assessment of access to primary medical care in rural Colorado.

The objectives of this study include conducting an analysis of access to primary medical care in rural Colorado through simultaneous consideration of primary care physician-to-population and distance-to-nearest provider indices. Analyses examined the potential development and implications of excessively large, perhaps unmanageable patient caseloads that might result from every rural Coloradoan's exclusive use of the nearest generalist physician as a regular source of care. Using American Medical Association Physician Masterfile data for 1995 and coordinates for latitude and longitude from U.S. Census files (Census of Population and Housing, 1990), the authors calculated distance to the nearest primary care physician for residents of each of the 1,317 block groups in Colorado's 52 rural counties. Caseloads for each generalist physician were computed assuming the population used the nearest provider for care. Straight-line mileage to primary medical care was modest for rural Coloradoans--a median distance of 2.5 miles. Almost two-thirds (65 percent) of the population resided within 5 miles, and virtually all residents (99 percent) were within 30 miles of a generalist physician. However, had everyone traveled the shortest possible distance to care, demand for service from many of the 343 primary care doctors in rural regions of the state would have been overwhelming. The results of simultaneous application of distance-to-care and provider-to-population techniques unrestricted by geographic boundaries depict access to primary medical care and corresponding consumer difficulty more fully than in previous studies. Further combination of methods of needs assessment such as those used in this analysis may better inform the future efforts of organizations mandated to address health care underservice in rural areas.

Colorado↗