Gamma loop contributing to maximal voluntary contractions in man.

Hagbarth, K.‐E.; Kunesch, E.; Nordin, Magnus; Schmidt, Roland; Wallin, Elisabeth

doi:10.1113/jphysiol.1986.sp016303

Cited by 97 publications

(59 citation statements)

References 28 publications

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“…This effect is likely mediated by presynaptic inhibition of the group Ia afferents rather than by direct inhibitory effects onto the motor neuron pool (Butler et al 2003;Duchateau and Hainaut 1993;Duchateau et al 2002;Ellrich et al 1998;Pettorossi et al 1999;Rossi et al 1999). Additionally, the discharge rate of motor units becomes more variable when local anesthesia presumably blocks fusimotor axons (Hagbarth et al 1986), which might correspond to the greater disfacilitation of group Ia feedback during the position task. The differential adjustments in discharge rate exhibited by the low-and moderate-threshold units during the two tasks suggest significant differences in the synaptic input to the motor neuron pool during these conditions.…”

Section: Discussionmentioning

confidence: 99%

Motor-Unit Activity Differs With Load Type During a Fatiguing Contraction

Mottram¹,

Jakobi²,

Semmler³

et al. 2005

Journal of Neurophysiology

144

180

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Mottram, Carol J., Jennifer M. Jakobi, John G. Semmler, and Roger M. Enoka. Motor-unit activity differs with load type during a fatiguing contraction. J Neurophysiol 93: [1381][1382][1383][1384][1385][1386][1387][1388][1389][1390][1391][1392] 2005. First published October 13, 2004; doi:10.1152/jn.00837.2004. Despite a similar rate of change in average electromyographic (EMG) activity, previous studies have observed different rates of change in mean arterial pressure, heart rate, perceived exertion, and fluctuations in motor output during the performance of fatiguing contractions that involved different types of loads. To obtain a more direct measure of the motor output from the spinal cord, the purpose of this study was to compare the discharge characteristics of the same motor unit in biceps brachii during the performance of two types of fatiguing contractions. In separate tests with the upper arm vertical and the elbow flexed to 1.57 rad, the seated subjects maintained either a constant upward force at the wrist (force task) or a constant elbow angle (position task) for a prescribed duration. The force and position tasks were performed in random order at a target force equal to 3.5 Ϯ 2.1% (mean Ϯ SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of the isolated motor unit. Each subject maintained the two tasks for an identical duration (161 Ϯ 96 s) at a mean target force of 22.2 Ϯ 13.4% MVC (range: 3-49% MVC). The dependent variables included the discharge characteristics of the same motor unit in biceps brachii, fluctuations in motor output (force or acceleration), mean arterial pressure, heart rate, and rating of perceived exertion. Despite similar increases in the amplitude of the averaged EMG (% MVC) for the elbow flexor muscles during both tasks (P ϭ 0.60), the rates of increase in mean arterial pressure (P Ͻ 0.001), rating of perceived exertion (P ϭ 0.023), and fluctuations in motor output (P ϭ 0.003) were greater during the position task compared with the force task. Consistent with these differences, mean discharge rate declined at a greater rate during the position task (P ϭ 0.03), and the coefficient of variation for discharge rate increased only during the position task (P ϭ 0.02). Furthermore, more motor units were recruited during the position task compared with the force task (P ϭ 0.01). These findings indicate that despite a comparable net muscle torque, the rate of increase in the motor output from the spinal cord was greater during the position task.

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Section: Discussionmentioning

confidence: 99%

Motor-Unit Activity Differs With Load Type During a Fatiguing Contraction

Mottram¹,

Jakobi²,

Semmler³

et al. 2005

Journal of Neurophysiology

144

180

View full text Add to dashboard Cite

show abstract

“…If the nett effect were inhibition (or disfacilitation due to muscle spindle unloading) of the motoneurones involved in the reference lift, then the results could be explained. Such 'inhibition' would need to be gradable with contraction strength and to be more evident with muscle fatigue when reflex inhibition or disfacilitation occurs (BiglandRitchie, Dawson, Johansson & Lippold, 1986;Hagbarth, Kunesch, Nordin, Schmidt & Wallin, 1986;Gandevia, Macefield, Burke & McKenzie, 1990). Such an organization of the peripheral reflex machinery is clearly possible given the complexity of interneuronal circuits (for review see Baldissera, Hultborn & Illert, 1981;Fournier & Pierrot-Deseilligny, 1989), but may not provide the full explanation.…”

Section: Discussionmentioning

confidence: 99%

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

Kilbreath

Gandevia

1991

The Journal of Physiology

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SUMMARY1. 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 cocontraction 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 MS 8902 S. L. KILBREATI AND S C GANDEVIA increases, the central nervous system is unable to partition precisely the destination of motor commands to functionally related 'muscles'.

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“…Demyelination and nerve conduction abnormalities were well established in the relevant lumbar segments on the day of onset of hindlimb weakness. As the γ-loop is necessary for maximal skeletal muscle contraction (Hagbarth et al 1986), demyelination of the smaller diameter motor (β and γ) fibres as well as of the larger (α) fibres is likely to cause hindlimb weakness. Demyelination of the lumbar dorsal roots would also interrupt the γ loop and may therefore also contribute to the hindlimb weakness.…”

Section: Discussionmentioning

confidence: 99%

The pathophysiology of myelin basic protein-induced acute experimental allergic encephalomyelitis in the Lewis rat

Pender

1988

Journal of the Neurological Sciences

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Histological and electrophysiological studies were performed on Lewis rats with acute experimental allergic encephalomyelitis (EAE) induced by inoculation with guinea-pig myelin basic protein (MBP) and Freund's adjuvant. The histological studies showed demyelination in the lumbar, sacral and coccygeal dorsal and ventral spinal roots and to a lesser extent in the spinal cord, including the dorsal root entry and ventral root exit zones. The electrophysiological studies demonstrated reduced conduction velocities between the lumbar ventral roots and sciatic nerve. Conduction block was demonstrated at the ventral root exit zone of the lumbar spinal cord but was less severe than in rats with whole spinal cord-induced acute EAE. Recordings of the M wave and H reflex elicited in a hindfoot muscle by sciatic nerve stimulation showed a normal M wave, indicating normal peripheral nerve motor conduction, but a markedly reduced H reflex. The reduction in the H reflex is accounted for by demyelination-induced nerve conduction block in the dorsal and ventral spinal roots, intramedullary ventral roots and at the dorsal root entry and ventral root exit zones of the spinal cord. Demyelination and nerve conduction abnormalities were well established in the relevant lumbar segments on the day of onset of hindlimb weakness. It is concluded that demyelination in the lumbar ventral roots and to a lesser extent in the lumbar spinal cord, including the ventral root exit zone, is an important cause of hindlimb weakness in myelin basic protein-induced acute EAE in the Lewis rat.

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Gamma loop contributing to maximal voluntary contractions in man.

Cited by 97 publications

References 28 publications

Motor-Unit Activity Differs With Load Type During a Fatiguing Contraction

Motor-Unit Activity Differs With Load Type During a Fatiguing Contraction

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

The pathophysiology of myelin basic protein-induced acute experimental allergic encephalomyelitis in the Lewis rat

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