Biomedical subjects

J M Gracies

Publications and source records attributed to J M Gracies.

21 records · Page 2Linked to original sources

PubMed1992

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

1. 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 system. Convergence was found between the different mixed nerves and between ulnar and superficial radial nerves. 6. The wide convergence found between different inputs onto common neurones and the finding that, during contraction of a given muscle, descending excitation reaches subsets of neurones projecting to motor nuclei of muscles operating at other joints suggest that the propriospinal-like system would be operative during complex multi-joint movements.

Adult↗

PubMed1992

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

1. 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 motoneurones was consistently reduced at the onset of contraction, and to a much lesser extent during a weak tonic contraction.

Electric Stimulation↗

PubMed1991

Pattern of propriospinal-like excitation to different species of human upper limb motoneurones.

1. The pattern of distribution of non-monosynaptic (propriospinal-like) excitation to various motor nuclei (deltoid, extensors and flexors of the elbow, the wrist and the fingers) was investigated. 2. Changes in the firing probability of individual voluntarily activated motor units were studied following conditioning stimuli. Conditioning volleys were evoked by weak electrical stimuli applied to various mixed nerves (circumflex, musculocutaneous, median, radial, ulnar) and to the skin. 3. In all investigated nuclei stimulation of the 'homonymous' nerve evoked a peak of increased firing probability with a latency which was 2-7 ms longer than the monosynaptic Ia latency. The average central delay of the late excitation, measured from monosynaptic latency, seems to depend only on the segmental level of the motor nucleus: the more caudal the nucleus the longer the latency. This strongly suggest a transmission through neurones located above the cervical enlargement, as are C3--C4 propriospinal neurones in the cat. 4. Both group I muscle and cutaneous afferents were shown to contribute to propriospinal-like excitation. It is argued that a spatial facilitation of the effects evoked by these two inputs might explain why the threshold of late excitation is always below that of the monosynaptic Ia excitation in motoneurones. 5. The pattern of distribution of propriospinal-like excitation was diffuse: stimulation of each mixed nerve was able to evoke excitation in all investigated motor nuclei. Similarly, stimulation of a given skin field could produce excitation of biceps and wrist flexor and extensor units. 6. Each motor nucleus therefore receives excitation from a multimodal and wide range peripheral input. However, it is argued that what appears as a diffuse pattern might simply reflect connections which are not used in each movement but appropriately selected by higher centres.

Adult↗