Evidence for interneuronally mediated Ia excitatory effects to human quadriceps motoneurones.

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SUMMARY1. The patterns of excitation and convergence by peripheral afferents on propriospinal-like neurones projecting to forearm flexor carpi radialis (FCR) motoneurones in human subjects were determined at rest and during various voluntary contractions, using H reflex testing.2. At rest, the FCR H reflex could be facilitated by mixed nerve (ulnar, musculocutaneous) and cutaneous (afferents from both sides of the hand) inputs. The characteristics of this facilitation (low threshold, long central latency, short duration) were compatible with those of the propriospinal-like system. Quantitatively this facilitation was rare and weak.3. Voluntary contraction increased the extent of the propriospinal-like facilitation of the FCR H reflex. It is shown in the companion paper (Burke, Gracies, Meunier & Pierrot-Deseilligny, 1992) that this increase results not from a decrease in presynaptic inhibition of afferents to propriospinal-like neurones, but from increased excitation of these neurones. It is argued that at the onset of contraction this excitation is purely descending in origin, whereas the contraction-induced afferent discharge is probably the major factor during weak tonic contraction.4. The distribution of the increased facilitation of the FCR H reflex depended on the muscles involved in the contraction: ulnar nerve-evoked facilitation was increased much more at the onset of voluntary wrist flexion than voluntary elbow flexion, and vice versa for the musculo-cutaneous-induced facilitation. This finding is consistent with the view that there are subsets of propriospinal-like neurones, specialized with regard to afferent input, and indicates that descending excitation is directed preferentially to the subset of neurones which receives excitatory feedback from the contracting muscle.5. To investigate the convergence of different afferent inputs onto common neurones the spatial facilitation technique was used. When present the convergence had a threshold and time course compatible with those of the propriospinal-like * Present address:

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Convergence of descending and various peripheral inputs onto common propriospinal‐like neurones in man.

Burke

Graciès

Mazevet

et al. 1992

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“…Measurements at rest were alternated with measurements during contraction. Post-stimulus time histograms A method described by Fournier, Meunier, Pierrot-Deseilligny & Shindo (1986) was used. In this method the time of the stimulation is set with respect to the discharges of the single unit.…”

Section: General Experimental Arrangementmentioning

confidence: 99%

The regulation of presynaptic inhibition during co‐contraction of antagonistic muscles in man.

Nielsen¹,

Kagamihara²

1993

215

144

SUMMARY1. The size of the soleus, tibialis anterior and medial gastrocnemius H reflexes were measured at different levels of EMG activity in ankle plantar-and dorsiflexors. At similar levels of EMG activity in the agonist muscle, the soleus and tibialis anterior H reflexes were found to be smaller during co-contraction than during an isolated agonist contraction. In contrast to this, the medial gastrocnemius H reflex had the same size during co-contraction as during an isolated plantarflexion at comparable EMG levels.2. Peaks occurred at monosynaptic reflex latency in the post-stimulus time histogram (PSTH) of firing of soleus and tibialis anterior motor units following stimulation of their respective motor nerves. These homonymous Ia peaks were found to be smaller during co-contraction than during isolated agonist contraction, whereas that of medial gastrocnemius motor units was unchanged. As the monosynaptic peak in the PSTH may give an indirect measure of the size of the monosynaptic EPSP in the motoneurone, it is suggested that these findings reflect a differential control of presynaptic inhibition of soleus and tibialis anterior motor units with respect to medial gastrocnemius motor units.3. The monosynaptic Ia facilitation of the soleus H reflex evoked by a stimulation of the femoral nerve, was also found to be smaller during cocontraction of tibialis anterior and soleus than during isolated plantarflexion. This suggests that presynaptic inhibition of heteronymous I a fibres from the quadriceps muscle projecting to soleus motoneurones is regulated in the same way as presynaptic inhibition of homonymous I a fibres during co-contraction.4. The femoral nerve-induced monosynaptic facilitation of the soleus H reflex was found to decrease with increasing tonic levels of both co-contraction and isolated dorsi-and plantarflexion. The amount of facilitation decreased to the same degree with increasing levels of dorsiflexion and co-contraction. For the same level of EMG in the dorsiflexors there was thus no difference in the amount of facilitation whether the dorsiflexors were activated separately or in a co-contraction.

“…This implies that the number of triggers necessary to reveal obvious peaks in the PSTH can be much lower than if nerve stimulation is given without reference to the motor unit discharge. The conditioning stimulation of the nerves was therefore triggered at a fixed delay after the preceding voluntary motor unit discharge (Fournier et al 1986). The latency of the motor unit potential following the conditioning stimulus was then computed.…”

Section: Study Of Single Motor Unitsmentioning

confidence: 99%

Reciprocal Ia inhibition between elbow flexors and extensors in the human.

Katz

Pénicaud

Rossi

1991

113

SUMMARY1. Reciprocal inhibition between elbow flexor and extensor muscles (biceps and triceps brachii) has been investigated in nine healthy subjects. Two techniques were used to assess changes in motoneurone excitability after stimulation of antagonist muscle afferents: (1) monosynaptic reflexes elicited by a mechanical stimulation of the distal muscle tendon (tendon tap); (2) post-stimulus time histograms (PSTH) of voluntarily activated motor units.2. Electrical stimulation of the antagonist muscle nerve produced a short-latency and short-lasting inhibition of the flexor and extensor motoneurones. The amount of this inhibition was found to be similar in both motor nuclei.3. The inhibition could be evoked with conditioning electrical stimuli as low as 0 7 x motor threshold (MT) or by very weak tendon taps applied to the antagonist tendon. In the former case the threshold of this inhibition was found to be consistently increased after raising the threshold of Ia afferent fibres by a longlasting muscle vibration. Since a contribution from cutaneous afferent fibres was ruled out, it is concluded that this inhibition was Ia in origin.4. Post-stimulus time histograms of voluntarily activated triceps and biceps motor units were made following electrical stimulation of homonymous and antagonist muscle afferents. This enabled an estimate of the central synaptic delay of the inhibitory process. An average central delay of 0-94 ms in excess of that of monosynaptic facilitation was found, thus suggesting that the inhibitory process could be mediated by only one interneurone. 5. A conditioning reflex discharge elicited in the antagonist muscle by a tendon tap depressed or suppressed this inhibition. This depression was maximal when the reflex discharge was elicited 10-20 ms before the conditioning stimulus for the inhibition and never lasted more than 30 ms. It is argued that the only mechanism compatible with such a depression is the inhibitory activity of Renshaw cells acting on the pathway mediating reciprocal inhibition.6. We conclude that group Ia afferent fibres from elbow extensor and flexor