Responses of human motoneurons to la inputs: effects of background firing rate

Jones, Kelvin E.; Bawa, Parveen

doi:10.1139/y95-174

Cited by 35 publications

(14 citation statements)

References 21 publications

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“…When the motoneurone is firing tonically, computer simulations (Jones & Bawa, 1997;Matthews, 1999) and experiments (Kudina, 1988;Piotrkiewicz et al 1992;Jones & Bawa, 1995;Olivier et al 1995) have shown that for a test stimulus of given magnitude, the firing probability is greatest at the lowest firing frequencies. There exists some controversy regarding the range of firing frequencies over which this inverse relationship between firing rate and probability of firing will hold.…”

Section: Discussionmentioning

confidence: 99%

The sites of neural adaptation induced by resistance training in humans

Carroll¹,

Riek²,

Carson³

2002

J Physiology

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Section: Discussionmentioning

confidence: 99%

The sites of neural adaptation induced by resistance training in humans

Carroll¹,

Riek²,

Carson³

2002

J Physiology

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“…when the motoneurone membrane was nearing its firing threshold. During this period, motoneurone responsiveness to monosynaptic inputs of cortical or peripheral origin has been found to reach maximal values (Jones and Bawa 1995;Olivier et al 1995). The fact that the firing frequency could not be taken to be responsible for the differences between the motor units' responsiveness was established by the regression analyses between response probability and mean interspike interval performed on a group of slowly contracting, low-threshold motor units and a group of fast-contracting, high-threshold motor units tested at various frequencies in the same experiments.…”

Section: Contribution Of the Ia Monosynaptic Inputs To The Motor Unitmentioning

confidence: 99%

“…This issue has been reinvestigated recently by Jones and Bawa (1995) in the flexor carpi radialis muscle. When electrical stimulations of the muscle afferents were delivered with post-spike delays longer than 60 ms or at random, the responses became smaller as motor unit firing frequency increased.…”

Section: Contribution Of the Ia Monosynaptic Inputs To The Motor Unitmentioning

confidence: 99%

“…This would at least partly explain why the responsiveness tends to be lower at higher frequencies when the stimulation is delivered either randomly or towards the end of the post-spike hyperpolarization period, as observed in the present data and in previous studies (cf. Jones and Bawa 1995). If, however, the firing frequency increases in such way that the inter-spike intervals tend to become shorter than the time required for the afferent volleys to reach the mot 9 after the triggering spike (post- spike delay+reflex loop latency), many of the EPSPs might be immediately preceded by spontaneous action potentials, and this would greatly reduce motoneurone responsiveness.…”

Section: Contribution Of the Ia Monosynaptic Inputs To The Motor Unitmentioning

confidence: 99%

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The “size principle” and synaptic effectiveness of muscle afferent projections to human extensor carpi radialis motoneurones during wrist extension

Schmied

Morin

et al. 1997

Exp Brain Res

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The question of whether muscle spindle afferents might control human motoneurone activity on the basis of the "size principle" during voluntary contraction was investigated by recording the discharge of single motor units (n = 196) in wrist extensor muscles while stimulating the homonymous muscle spindles by means of tendon taps. The mechanical stimuli were delivered with a constant post-spike delay of 80 ms so that the resulting afferent volleys could be expected to reach the motoneurones towards the end of the inter-spike interval (mean +/- SD duration: 124.7 +/- 11.9 ms). In the six subjects tested, the response probability was found to be significantly correlated with the motor units' functional parameters. Differences in twitch rise times, twitch amplitudes, recruitment thresholds and macro-potential areas were found to account for 18%, 9%, 6% and 2% of the differences in the response probability observed within the whole population of motor units tested. These differences could not be due to differences in firing rate for two reasons: first, the motor units were found to discharge with a similar range of inter-spike intervals whatever their functional characteristics; secondly, the weak positive correlation observed between the response probability and the motor unit firing rate showed parallel regression lines between the late-recruited fast-contracting motor units and the first-recruited slowly contracting motor units, but the y-intercept was significantly higher in the latter case. This confirmed that the responses of the first-recruited slowly contracting motor units tended to be larger whatever the firing rates. In most of the pairs tested in the same experiment, the motor units which had the lowest recruitment thresholds, longest contraction times, smallest contraction forces or smallest motor unit macro-potentials tended to produce the largest responses, which also had the longest latencies. Taking the response latency to be an index of a motoneurone's conduction velocity and therefore of its size, the data obtained with this index and with other functional indices such as the twitch rise times and amplitudes, the macro-potential areas and the recruitment thresholds-can be said to be fully consistent with the "size principle", as previously found in anaesthetized animals. It can be inferred that the presynaptic inhibition which is liable to take action during voluntary contraction does not seem to alter the graded distribution of the muscle afferent projections to human wrist extensor motoneurones.

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“…The electrical stimuli were applied with a Grass S88 stimulator via a SIU5 stimulus isolation unit and a Grass CCU1A constant-current unit. Using a pulse duration of 0.5-1.0 ms, the placement of the electrode and the current were adjusted to obtain a pure H-reflex with no M-wave in the FCR at the lowest stimulus intensity possible (Jones and Bawa 1995). Once the optimal position was obtained, the electrode was secured in place.…”

Section: Methodsmentioning

confidence: 99%

Synaptic connections from large afferents of wrist flexor and extensor muscles to synergistic motoneurones in man

Chalmers

Bawa

1997

Exp Brain Res

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Short-latency excitatory Ia reflex connections were determined between pairs of human wrist flexor and extensor muscles. Spindle Ia afferents were stimulated by either tendon tap or electrical stimulation. The activity of voluntarily activated single motor units was recorded intramuscularly from pairs of wrist flexor or extensor muscles. Cross-correlation between stimuli and the discharge of the motor units provided a measure of the homonymous or heteronymous excitatory input to a motoneurone. Homonymous motoneurone facilitation was generally stronger than that of the heteronymous motoneurones. The principal wrist flexors, flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU), were tightly connected through a bidirectional short-latency reflex pathway. In contrast, the extensor carpi ulnaris (ECU) and the extensor carpi radialis (ECR) did not have similar connections. ECU motoneurones received no short-latency excitatory Ia input from the ECR. ECR motoneurones did receive excitatory Ia input from ECU Ia afferents; however, its latency was delayed by several milliseconds compared with other heteronymous Ia excitatory effects observed. The wrist and finger extensors were linked through heteronymous Ia excitatory reflexes. The reflex connections observed in humans are largely similar to those observed in the cat, with the exception of heteronymous effects from the ECU to the ECR and from the extensor digitorum communis (EDC) to the ECU, which are present only in humans. The differences in the reflex organization of the wrist flexors versus the extensors probably reflects the importance of grasping.

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Responses of human motoneurons to la inputs: effects of background firing rate

Cited by 35 publications

References 21 publications

The sites of neural adaptation induced by resistance training in humans

The sites of neural adaptation induced by resistance training in humans

The “size principle” and synaptic effectiveness of muscle afferent projections to human extensor carpi radialis motoneurones during wrist extension

Synaptic connections from large afferents of wrist flexor and extensor muscles to synergistic motoneurones in man

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