Gating of the afferent volley of the monosynaptic stretch reflex during movement in man.

212

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.

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

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

Nielsen¹,

Kagamihara²

1993

212

144

“…Experiments were performed with a very weak tonic contraction involving only one motor unit, or at the onset of a phasic anisometric contraction, of relatively weak strength. With the phasic contractions the first stimulus to be delivered was triggered by the onset of EMG activity, which was recorded with a higher amplification than the reflex, rectified and connected to a triggering circuit (see Meunier & Pierrot-Deseilligny, 1989). In most cases the test stimulus also evoked an M response in FCR.…”

Section: Methodsmentioning

confidence: 99%

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

Burke

Graciès

Mazevet

et al. 1992

Self Cite

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:

“…Changes in presynaptic inhibition of afferents synapsing with propriospinal neurones have never been investigated in the cat. Nevertheless the possibility must be considered because, in the human lower limb, there is a decrease in presynaptic inhibition of monosynaptic Ia projections to motoneurones at the onset of voluntary contractions (Hultborn, Meunier, Pierrot-Deseilligny & Shindo, 1987 b;Meunier & Pierrot-Deseilligny, 1989). The present study was therefore undertaken to determine whether primary afferents projecting to propriospinal-like neurones are subject to presynaptic inhibition and whether changes in presynaptic inhibition contribute to the increase in excitability of the propriospinal-like system associated with a voluntary contraction.…”

Section: Introductionmentioning

confidence: 99%

Changes in presynaptic inhibition of afferents to propriospinal‐like neurones in man during voluntary contractions.

Burke

Graciès

Meunier

et al. 1992

Self Cite

SUMMARY1. The possibility was investigated that the facilitation of the transmission in the propriospinal-like system during voluntary contraction, documented in the companion paper (Burke, Gracies, Mazevet, Meunier & Pierrot-Deseilligny, 1992), is due to a decrease in presynaptic inhibition of afferents projecting to propriospinal-like neurones.2. The radial nerve was stimulated to evoke presynaptic inhibition of the monosynaptic Ia projections to forearm flexor motoneurones (Berardelli, Day, Marsden & Rothwell, 1987) and, hopefully, of the afferent input to propriospinal-like neurones projecting to these motoneurones.3. The propriospinal-like excitation of forearm motoneurones evoked from mixed afferent inputs was depressed by radial nerve stimulation, and this depression was long-lasting (200 ms). Despite the convergence of mixed nerve and cutaneous afferents onto common propriospinal-like neurones, the radial stimulation did not depress the cutaneous-induced excitation. This differential effect and the time course of the depression suggest that it results from presynaptic inhibition of mixed nerve afferents (presumably large muscle afferents) projecting to propriospinal-like neurones.4. With voluntary contractions, phasic or tonic, the radial-induced depression of the propriospinal-like excitation evoked by mixed nerve afferents was much greater than at rest, but the cutaneous-evoked excitation was unchanged. Thus, with voluntary contractions, there was no evidence of decreased gating of the afferent input to propriospinal-like neurones whether the input was of muscle or cutaneous origin and it is concluded that changes in presynaptic inhibition cannot account for the facilitation of the transmission in the propriospinal-like system during voluntary contraction.5. By contrast, presynaptic inhibition of the monosynaptic Ia projections to