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Biomedical subjects

S C Gandevia

Publications and source records attributed to S C Gandevia.

At least 19 recordsLinked to original sources
PubMed1992

Voluntary discharge frequencies of human motoneurons at different muscle lengths.

The relationship between the discharge frequencies of motoneurons and the voluntary force at high contraction strengths (50% to 100% maximum) was determined for tibialis anterior with the ankle: (i) at 90 degrees (control length); and (ii) this angle reduced by about 15 degrees (short length), an amount sufficient to reduce the maximal voluntary force by about 35%. In the shortened position, twitch contraction and half-relaxation times were reduced by 11% and 18%, respectively. At both muscle lengths, the ability to activate the muscle fully by voluntary effort was confirmed by twitch interpolation. Motor unit firing rates were recorded during isometric voluntary contractions lasting 10 seconds, performed at 50%, 75%, and 100% maximal force. At each length, discharge rates varied in proportion to the relative force exerted. Despite the difference in twitch contractile speed at the two lengths, no significant differences were found between motoneuron discharge rates recorded at each muscle length during maximal or submaximal contractions.

Adult↗
PubMed1992

Independent control of the digits: changes in perceived heaviness over a wide range of force.

Perceived heaviness of a weight lifted by flexion of the distal joint of one digit increases when an adjacent digit concurrently lifts a weight. The present study confirmed this finding for relatively low weights (representing 3-5% maximal voluntary force) and a method was adapted to show that this effect occurs for much larger weights (20-25% maximal force). Thus, the increase in perceived heaviness is likely to operate over a wide range of muscle force generated by the hand. As perceived heaviness is biased by the magnitude of the central motor commands, these findings may reflect a lack of complete independence of motor commands to the long flexor muscles acting on the digits.

Fingers↗
PubMed1992

Selective temporal shift in the somatosensory evoked potential produced by chronic stimulation of the human index finger.

The present study determined whether the cortical potential from the human index finger changed with chronic nerve stimulation. Cerebral potentials were repeatedly recorded to stimulation of the ulnar nerve and the digital nerves of thumb, index and middle fingers, before and during a 7-day period in which the index was electrically stimulated (80 Hz) for 8-10 h daily. Cerebral potentials were recorded at three scalp sites over the contralateral "hand" area. Chronic stimulation produced no significant changes in the amplitudes or distribution of the cerebral potentials from the individual digits or the ulnar nerve. However, for the stimulated index finger there was a significant, progressive increase in latency of N20 and P25 without a detectable change in conduction velocity of distal peripheral axons. Timing in human central somatosensory pathways may be altered by the previous pattern of peripheral nerve inputs.

Adult↗
PubMed1992

Kinaesthetic signals and muscle contraction.

Signals generated both peripherally and centrally contribute to the group of sensations termed kinaesthesia. Many experiments report sensations of position and movement under passive relaxed conditions without muscle contraction. However, kinaesthetic acuity is probably of greater functional value when subjects are active rather than passive and, accordingly, movement detection is markedly improved by muscular contraction. One mechanism contributing to this enhancement is likely to involve muscle spindle volleys. When identical microstimulation techniques are applied to skin, joint and muscle spindle endings innervating the hand, some cutaneous afferents and some joint afferents elicit a sensation, but activation of certain other cutaneous afferents and muscle spindle afferents rarely does. Activity in more than one muscle spindle afferent may be required for kinaesthetic sensations, whereas some single cutaneous and joint afferents may have a more 'secure' central projection.

Animals↗
PubMed1992

Peripheral and central delays in the cortical projections from human truncal muscles. Rapid central transmission of proprioceptive input from the hand but not the trunk.

In contrast to the cortical connections to and from the muscles of the hand, the transmission of an afferent volley from the intercostal muscles to the cerebral cortex takes approximately 10 ms longer than it takes a cortical motor volley to reach the muscle. This disparity in afferent and efferent cortical transmission times could be due to a slower peripheral conduction velocity of intercostal muscle afferents or a slower afferent conduction within the central nervous system. The present study derived peripheral and central conduction times for the truncal muscles from the onsets of the mechanically evoked intercostal and abdominal spinal reflexes and the onsets of the cortical sensory potentials. Mean latencies of the ipsilateral intercostal and abdominal reflexes (evoked and recorded in the mid-clavicular line) were 11.9 +/- 0.7 (SEM) ms and 13.7 +/- 0.9 ms, respectively; calculated peripheral conduction velocities were 69.4 +/- 4.1 m/s and 56.2 +/- 2.3 m/s (assuming equal velocities for the sensory and motor axons and an intraspinal delay of 1 ms). Central sensory conduction time (spinal cord to cortex) was calculated by subtracting the peripheral conduction times for the intercostal and abdominal afferents (5.5 +/- 0.3 ms and 6.4 +/- 0.4 ms) from the onsets of the cortical sensory potentials (19.4 +/- 0.8 ms and 25.3 +/- 12.3 ms); central sensory conduction times (14.2 +/- 1.7 ms and 18.6 +/- 2.3 ms) were 8-11 ms longer than central motor conduction times. These results demonstrate that peripheral conduction velocities of intercostal and abdominal afferents are not slow, and that, when compared with the extremities, there is a relatively long central conduction time for proprioceptive information from the trunk to the cerebral cortex.

Adult↗
PubMed1992

Central and peripheral fatigue of human diaphragm and limb muscles assessed by twitch interpolation.

1. This study used a sensitive modification of the twitch interpolation technique to compare the extent of voluntary neural drive to the diaphragm and the elbow flexors during fatigue. For the diaphragm both inspiratory and expulsive efforts were tested, and fatigue was induced by expulsive efforts which were either maximal voluntary contractions (MVCs, 10 s duration, 50% duty cycle) or submaximal contractions (50% MVC, 3 s duration, 60% duty cycle). 2. Over the series of thirty MVCs peak elbow torque declined to 57.9 +/- 3.0% (mean +/- S.E.M.) of the initial value while maximal inspiratory pressure declined to 78.7 +/- 7.3% (P < 0.05). For the diaphragm the relative decline in voluntary peak inspiratory (and expulsive) force was similar to the decline in twitch responses to single and twin (10 ms interval) stimuli. However, for the elbow flexors the decline in twitch force was disproportionately greater than the decline in maximal voluntary force. The decline in twitch force for the diaphragm could not be attributed to failure at the neuromuscular junction. 3. At the start of the exercise, twitch potentiation (following three brief MVCs) was significantly less for the diaphragm than for the elbow flexors (20% versus 61%, P < 0.01). 4. In the unfatigued state maximal voluntary efforts by subjects activated 98.4 +/- 0.4% of the stimulated elbow flexors compared with 95.0 +/- 1.5% of the diaphragm (P < 0.05). During the exercise period there was a progressive failure in the ability to activate the limb muscle ('central fatigue'; voluntary drive declined from 98.4 +/- 0.4 to 86.8 +/- 2.2%, P < 0.01) whereas the decline in voluntary activation during inspiratory contractions was not significant (from 95.0 +/- 15 to 91.5 +/- 2.5%). 5. Voluntary activation during attempted maximal efforts was less complete for both muscles when stimuli were delivered without warning. The index of voluntary activation for unwarned stimuli was lower for the diaphragm (performing expulsive efforts, 81.0 +/- 2.8%) than for the limb muscle (89.9 +/- 1.5%, P < 0.01). 6. During repeated submaximal expulsive efforts we confirmed that subjects develop a marked inability to contract the diaphragm voluntarily, but when the diaphragm performed inspiratory manoeuvres at the same level of contractile fatigue, the index of voluntary drive was greater than 94%. 7. In conclusion, when tested with inspiratory efforts the diaphragm developed less central fatigue than the limb muscle over the same exercise period.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1992

Coherence between the sympathetic drives to relaxed and contracting muscles of different limbs of human subjects.

1. This study was undertaken to quantify the simultaneous sympathetic drives to muscles in the two legs of human subjects, and to elucidate the extent to which a common drive determines sympathetic outflow to different limbs at rest, during apnoea and during voluntary contractions. 2. Sympathetic efferent activity was recorded simultaneously from fascicles of both peroneal nerves, innervating the pretibial flexor muscles. At rest the similarity was quantified for a sample of records by manual measurement of equivalent bursts in the two recordings, and for all records by cross-correlation and power spectral analysis of the two recordings. During contractions, only the latter method was used. 3. At rest the correlation coefficient for the relationship between the burst amplitudes for the two recordings was 0.72 (S.D. 0.1). For the same sequences, the computed coherence between the two recordings was 85.6% (S.D. 6.7%) at the cardiac period. There was a statistically significant linear relationship between these two measures of similarity, and this was stronger when data from sequences recorded during apnoea were included in the analysis. At rest the mean difference in coherence between consecutive sequences with no intervening manoeuvre (apnoea, contraction, change in recording site) was 4.2% (S.D. 4.3%). In only two of forty-nine such instances was the difference in coherence > 10%. 4. Apnoea at end-expiration increased the amplitude and frequency of sympathetic bursts and increased the similarity between the two recordings. The correlation coefficients increased from a mean of 0.72 at rest to 0.89 during apnoea. Coherence increased from a mean of 82.1% at rest to 91.9% during apnoea. 5. On the right side, graded voluntary contractions were performed at 5, 10, 20 or 30% maximal force using the muscle innervated by the fascicle from which the recording was made. The coherence between the recordings made from the right and left legs decreased by > 10% at each contraction level. Pooling the data for all contractions, there was a significant decrease in power at the cardiac frequency in the sympathetic recording from the contracting leg. Contraction of a synergist or antagonist at 10% maximum produced negligible changes in coherence. 6. It is concluded that, at rest, homologous muscles of the lower limbs are subject to a common drive and that, during apnoea, this common drive can dominate the sympathetic outflow to the virtual exclusion of regional drives. During voluntary activity, the importance of this common drive is lessened, presumably because of regionally specific changes involving the contracting muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1992

Dynamic changes in human diaphragm length: maximal inspiratory and expulsive efforts studied with sequential radiography.

1. The maximal voluntary pressure generated by the diaphragm (transdiaphragmatic pressure, Pdi) is about 50% greater during maximal expulsive efforts than during maximal inspiratory efforts against a closed airway. However, these pressures cannot be increased by interpolated phrenic stimuli in trained subjects. This suggests that variable neural drive is not responsible for the difference in voluntary pressure. To investigate whether dynamic changes in diaphragm length during inspiratory and expulsive efforts could account for this difference, we used digital sequential radiography at 6 frames per second. 2. During the development of peak Pdi in inspiratory efforts, total diaphragm length decreased by about 20% in the antero-posterior and lateral projections. During maximal expulsive efforts (with glottis open), the diaphragm shortened slightly in the early stage of pressure development but then lengthened due to contraction of abdominal muscles before peak pressure was achieved. 3. Given that force increases when a contracting muscle is lengthened (expulsive effort) and decreases during shortening (inspiratory effort), this study provides a definitive explanation for the difference in maximal voluntary pressure between pure inspiratory and expulsive efforts.

Adult↗
PubMed1992

Postural proprioceptive reflexes in standing human subjects: bandwidth of response and transmission characteristics.

1. This study investigated the reflex control of postural sway during human bipedal stance. The experiments were designed to: (i) find evidence for the operation of 'stretch reflex' pathways during quiet stance, (ii) determine the bandwidth of the reflex response, (iii) describe the reflex transmission characteristics in standing subjects, and (iv) assess the ability of subjects to make a task-dependent change in the reflex. 2. A continuous random perturbation that did not threaten stability was applied at waist level to nine standing subjects. The effects of the perturbation on ankle torque, ankle movement and soleus electromyographic activity (EMG) were identified by cross-correlation. The bandwidth of the reflex response and the transmission characteristics of reflexes that respond to ankle movement were identified by spectral analysis. Changes in these reflex responses were investigated when subjects attempted to stand as still as possible, had their eyes closed, or balanced a load equivalent to their own body in a situation in which neither visual nor vestibular reflexes would be activated. 3. When standing, a reflex response coherent with the perturbation was seen in soleus EMG at frequencies up to 5 Hz, with maximal coherence at 1.0-2.0 Hz. Reflex gain increased with frequency, and there was a frequency-dependent phase advance of soleus EMG on ankle movement reaching 135 deg at 3 Hz. When attempting to minimize sway, subjects produced a more coherent reflex response and significantly increased reflex gain. 4. The response and transmission characteristics of the lower limb proprioceptive reflex in freely standing subjects were similar to those in subjects balancing a load at the ankle, a situation in which vestibular and visual inputs could not contribute. 5. It is concluded that reflex feedback related to ankle movement contributes significantly to maintaining stance, and that much of the reflex response originates from lower limb mechanoreceptors stimulated by ankle rotation. Although reflex gain may be relatively low during quiet stance it can be increased when necessary to maintain stability.

Adult↗
PubMed1992

Inspiratory muscle strength and endurance during hyperinflation and histamine induced bronchoconstriction.

BACKGROUND: This study investigated whether the inspiratory muscles are susceptible to fatigue during acute airway narrowing because of increased airway resistance and hyperinflation. METHODS: Asthmatic subjects performed up to four series (on separate days) of 18 maximal static inspiratory efforts of 10 seconds' duration with 10 second rest intervals (50% duty cycle; total duration six minutes): at functional residual capacity (FRC) (control); after histamine induced bronchoconstriction, which decreased forced expiratory volume in one second (FEV1) to a mean of 55% (SD 11%) of the initial value; at a voluntarily increased lung volume (initial volume held at 140% control); and after inhalation of histamine at a voluntarily increased lung volume. RESULTS: For the group of subjects the mean (SD) maximal inspiratory pressure (MIP) in the control experiments was 114 (22) cmH2O and the initial volume was 3.5 (1.2) 1. After histamine inhalation the initial lung volume for contractions increased to 118% (5%) of the control volume. In the high lung volume experiments initial volumes were 140% (12%) of the control (volume without histamine) and 140% (15%) (with histamine). The relation between MIP and initial absolute lung volume was determined for each subject before fatigue developed. When the inspiratory pressures for each contraction in the endurance test were normalised to the pressure expected for that lung volume, no significant differences were found between the four experimental conditions for MIP, or between pressures sustained over the 18 contractions. CONCLUSIONS: Histamine induced bronchoconstriction and hyperinflation had no detectable effect on inspiratory muscle strength or endurance in these asthmatic subjects.

Adult↗
PubMed1992

Reflex responses in active muscles elicited by stimulation of low-threshold afferents from the human foot.

1. Reflex responses were elicited in muscles that act at the ankle by electrical stimulation of low-threshold afferents from the foot in human subjects who were reclining supine. During steady voluntary contractions, stimulus trains (5 pulses at 300 Hz) were delivered at two intensities to the sural nerve (1.2-4.0 times sensory threshold) or to the posterior tibial nerve (1.1-3.0 times motor threshold for the intrinsic muscles of the foot). Electromyographic (EMG) recordings were made from tibialis anterior (TA), peroneus longus (PL), soleus (SOL), medial gastrocnemius (MG), and lateral gastrocnemius (LG) muscles by the use of intramuscular wire electrodes. 2. As assessed by averages of rectified EMG, stimulation of the sural or posterior tibial nerves at nonpainful levels evoked a complex oscillation with onset latencies as early as 40 ms and lasting up to 200 ms in each muscle. The most common initial responses in TA were a decrease in EMG activity at an onset latency of 54 ms for sural stimuli, and an increase at an onset latency of 49 ms for posterior tibial stimuli. The response of PL to stimulation of the two nerves began with a strong facilitation of 44 ms (sural) and 49 ms (posterior tibial). With SOL, stimulation of both nerves produced early inhibition beginning at 45 and 50 ms, respectively. With both LG and MG, sural stimuli produced an early facilitation at 52-53 ms. However, posterior tibial stimuli produced different initial responses in these two muscles: facilitation in LG at 50 ms and inhibition in MG at 51 ms. 3. Perstimulus time histograms of the discharge of 61 single motor units revealed generally similar reflex responses as in multiunit EMG. However, different reflex components were not equally apparent in the responses of different single motor units: an individual motor unit could respond slightly differently with a change in stimulus intensity or background contraction level. The multiunit EMG record represents a global average that does not necessarily depict the precise pattern of all motor units contributing to the average. 4. When subjects stood erect without support and with eyes closed, reflex patterns were seen only in active muscles, and the patterns were similar to those in the reclining posture. 5. It is concluded that afferents from mechanoreceptors in the sole of the foot have multisynaptic reflex connections with the motoneuron pools innervating the muscles that act at the ankle. When the muscles are active in standing or walking, cutaneous feedback may play a role in modulating motoneuron output and thereby contribute to stabilization of stance and gait.

Adult↗
PubMed1992

Some central and peripheral factors affecting human motoneuronal output in neuromuscular fatigue.

Fatigue may be defined as a reduction in the maximal force-generating capacity of a muscle. It may result from peripheral processes distal to the neuromuscular junction and from central processes controlling the discharge rate of motoneurons. When assessed with a sensitive test using twitch interpolation, most 'maximal' voluntary contractions approach but do not attain optimal muscle output. During fatigue, reflex inputs from intramuscular receptors may contribute to a decline in motor unit discharge rate--a decline which optimises force production during maximal efforts. Further studies should investigate how the central nervous system controls the discharge rate of motor units during fatigue produced by different forms of exercise.

Exercise↗
PubMed1991

Radiation risks.

Explore the source record for details and available documents.

Dose-Response Relationship, Radiation↗
PubMed1991

Recovery from fatigue of human diaphragm and limb muscles.

This study was designed to compare the recovery from fatigue of human inspiratory and limb muscles using repeated maximal static contractions. Series of 18 maximal contractions of 10 sec duration were performed with a duty cycle of 50% for maximal inspiratory efforts (against a shutter at FRC), and with duty cycles of 5%, 10%, 20% and 50% for the elbow flexors in repeated studies on 6 subjects. The peak inspiratory pressure at the end of the series declined to 86.7% +/- 5.3% (mean +/- S.D.) of its initial value: maximal force of the elbow flexors declined to 83.5% +/- 7.0% (5% duty cycle), 80.0% +/- 5.5% (10% duty cycle), 70.0% +/- 9.3% (20% duty cycle), and 66.4% +/- 8.0% (50% duty cycle). Thus, the elbow flexors required approximately a 10-fold reduction in duty cycle to maintain over a series of contractions a force generating capacity comparable to that of the diaphragm. A small degree of 'central' fatigue developed progressively during all series of contractions but did not correlate with duty cycle. Fatigue-induced changes in twitch contraction properties varied with changes in duty cycle. Our major conclusions are that the human diaphragm has a marked capacity to recover from fatigue and that this may have been underestimated in previous studies from this and other laboratories.

Adult↗
PubMed1991

The cortical drive to human respiratory muscles in the awake state assessed by premotor cerebral potentials.

1. We investigated the possibility of a cortical contribution to human respiration by recording from the scalp of awake subjects the premotor cerebral potentials that are known to precede voluntary limb movements. 2. Electroencephalographic activity (EEG) was recorded from scalp electrodes and averaged for 1.8-2.0 s before the time at which airway pressure exceeded an inspiratory or expiratory threshold. Clear premotor cerebral potentials were recorded during brisk, self-paced nasal inhalations or exhalations. In ten subjects, a slow cortical negativity (Bereitschaftspotential) was apparent in the averaged EEG, commencing 1.2 +/- 0.3 s before the onset of inspiratory (scalene) or expiratory (abdominal) muscle activity (EMG). It was maximal at the vertex, with a mean slope of 12.3 +/- 5.8 microV/s, and was followed by a post-movement positivity. 3. In four subjects the inspiratory premotor potential culminated in a large negativity, the motor potential, which began 24 +/- 15 ms before the onset of scalene EMG. It is argued that such a short latency is consistent with a volitionally generated respiratory command which travels relatively directly to the respiratory muscles, having a total central delay which is no longer than that for voluntary finger movements. 4. That the respiratory premotor and motor potentials did not originate in subcortical structures was supported by their absence in a patient suffering from chronic reflexogenic hiccups, in whom cerebral activity was back-averaged from each brisk hiccup. 5. During quiet breathing, in which subjects were relaxed and distracted from thinking about their respiration, no premotor cerebral potentials preceding inspiration could be detected. This failure was not due to the slow rate of rise of inspiratory activity during quiet breathing as compared with a brisk sniff, because premotor potentials were detected when subjects intermittently generated slow active expiratory efforts. 6. These observations suggest that during quiet breathing the cerebral cortex does not contribute to respiratory drive on a breath-by-breath basis. Conversely, the presence of clear premotor cerebral potentials when subjects performed self-paced inspiratory or expiratory manoeuvres illustrates the powerful cortical projection to human respiratory muscles.

Adult↗
PubMed1991

Decline in spindle support to alpha-motoneurones during sustained voluntary contractions.

1. To address whether the muscle spindle support to alpha-motoneurones is maintained during prolonged isometric voluntary contractions, the discharge of eighteen muscle spindle afferents, originating in the dorsiflexors of the ankle or toes, was recorded from the common peroneal nerve in eight subjects. Isometric contractions were generally sustained for 1 min, usually below 30% of the maximal voluntary dorsiflexion force. 2. Once the afferent had been identified, subjects were instructed to dorsiflex the foot slowly to recruit the spindle ending, to continue the ramp contraction until a predetermined target force was reached, and then to hold that force until requested to relax. 3. Five muscle spindle afferents maintained a constant discharge frequency during the hold phase of the isometric contraction. Following relaxation of the contraction two spindle afferents from tibialis anterior, exhibited a post-contraction discharge despite the absence of detectable electromyographic activity (EMG). 4. The discharge frequency of most of the spindle afferents (72%) declined progressively during the isometric contraction. The mean firing rates had declined to two-thirds of those at the onset of the contraction by 30 s, and to half after 1 min. The decline in spindle firing rate commenced during the ramp phase of the contraction and was statistically significant by 10 s, when force was held constant. The extent of the decline was greater for those units with the higher initial firing rates and for those units studied after many preceding contractions. 5. In the same contractions a progressive increase in EMG was required to maintain force and consequently the change in EMG was inversely related to the change in spindle discharge. While many mechanisms may contribute to the decline in spindle discharge during a sustained isometric contraction, it is argued that the result will be a progressive disfacilitation of alpha-motoneurones, which may contribute to the decline in motor unit firing rates during a sustained contraction.

Adult↗
PubMed1991

Independent digit control: failure to partition perceived heaviness of weights lifted by digits of the human hand.

1. A weight-matching task was used to investigate the ability to estimate heaviness when weight lifting was isolated to the extrinsic flexor muscles (or portions thereof) that act on the digits of the hand. 2. Subjects matched a reference weight (200 g) lifted by a digit on the right with a variable weight lifted by the left thumb; a concurrent weight was simultaneously lifted by a digit on the right (reference) side. The reference and concurrent weights were lifted either by the same muscle (digital portions of flexor digitorum profundus), or anatomically separate but functionally related muscles (flexor digitorum profundus and flexor pollicis longus). Anaesthesia of the radial nerve and/or posturing of the hand was used to eliminate any small forces generated by co-contraction of the extensor muscles of the digits. 3. When the concurrent weight was equal to or greater than the reference weight, the perceived heaviness of the reference weight increased significantly from control trials (in which no concurrent weight was lifted). Although perceived heaviness of the reference weight increased progressively as the concurrent weight increased, reproducibility (expressed as the coefficient of variation) did not deteriorate when a weight was lifted concurrently. These findings were qualitatively similar when the reference and concurrent weights were lifted by two digital portions of flexor digitorum profundus or when the weights were lifted by flexion of the thumb and index finger. Also, anaesthesia of the digits which lifted the reference and concurrent weights did not alter the changes in perceived heaviness. 4. Perceived heaviness of the reference weight lifted by flexor digitorum profundus did not change when subjects lifted the concurrent weight with a remote muscle group (ankle dorsiflexors). 5. This study shows that signals of heaviness are systematically overestimated whenever more than one portion of the extrinsic muscles which flex the different digits are simultaneously active. Given that estimates of heaviness are biased by the central motor command, one explanation is that when the total motor drive increases, the central nervous system is unable to partition precisely the destination of motor commands to functionally related 'muscles'.

Electromyography↗