Synchronization of motor unit firing during different respiratory and postural tasks in human sternocleidomastoid muscle.

Changing the posture of the human fingers can functionally ‘disengage’ the deep finger flexor muscle from its normal action on the terminal phalanx of the fourth (or third) finger. This enables the activity of the muscle to be studied both with and without its normal proprioceptive inputs. Spike trains of long duration from pairs of concurrently active motor units in this muscle were recorded in both the engaged and disengaged hand postures. Subjects voluntarily kept one of the motor units (the ‘controlled’ unit) discharging at the same target frequency in both postures. The strength of short‐term synchrony, the strength of common drive, and the variability of discharge of these pairs of motor units were determined in both postures. All subjects reported that the effort required to activate the motor units in the disengaged hand posture was substantially greater than in the normal engaged posture. Short‐term synchrony, which is a function of common corticospinal inputs to pairs of motor units, was similar in both hand postures. However, the strength of common drive was significantly decreased when the muscle was disengaged. Although the neural substrate for common drive is not known, this observation suggests that proprioceptive feedback is involved either directly or indirectly. Although the discharge rate of the ‘uncontrolled’ motor units increased when the muscle was disengaged, the variability of discharge of these and the ‘controlled’ motor units increased significantly. This supports the idea that the precision with which fine motor tasks can be performed is improved when proprioceptive feedback is intact.

Section: Short-term Synchronymentioning

confidence: 99%

Control of motor units in human flexor digitorum profundus under different proprioceptive conditions

Garland¹,

Miles

1997

“…In the present study we have turned our attention to the central mechanisms responsible for the generation of synchronization in terms of the organization of motoneurone synaptic input. The recording of short-term synchronization of cat intercostal motoneurones during normal respiration by Sears & Stagg (1976) and the careful analysis in terms of the underlying synaptic events (Tuck, 1977;Kirkwood & Sears, 1978;Kirkwood, Sears, Stagg & Westgaard, 1982 b) provides a sound basis for this approach and provides the investigator with a potentially powerful tool for the study of central synaptic organization in man in health and disease (Kirkwood, Sears & Westgaard, 1984;Datta et al 1985a, b;Davey, Ellaway & Friendland, 1986;Adams, Datta & Guz, 1989).…”

Section: Strength Of Synchronization and Recruitment Thresholdmentioning

confidence: 99%

Synchronization of motor unit activity during voluntary contraction in man.

Datta

Stephens

1990

Self Cite

199

167

SUMMARY1. Motor unit synchronization has been studied in human first dorsal interosseous muscle.2. Two needle electrodes were inserted into the muscle and the activity of pairs of motor units recorded.3. Pre-and post-stimulus histograms of the firing of unit pairs showed a narrow central peak of duration 1-3-9-3 ms (88% of sample in the range 1-6 ms; mode 3-0 ms), together with a variable amount of synchronization of somewhat longer duration.4. For the duration of the whole synchronization peak (85% sample in range 5-15 ms; mode between 6-1 and 8-0 ms (31 % of sample)), units fired between 8 and 485% times more often than would have been expected had the units been firing independently of one another. Amplitudes of the peak of the recorded histograms expressed as a proportion of control ranged from 1-8 to 10-9 (mean 3*9; bin width 160 ,ts).5. The strength of synchronization between the firing of motor unit pairs was inversely related to differences in recruitment threshold. The largest amount of synchronization was observed for pairs of units in which both had recruitment thresholds < 0 5 N or > 1P0 N. Less synchronization was found between pairs of units in which one had a recruitment threshold < 0-5 N and the other a threshold > 10ON.6. The time course of synchronization was well matched by the predictions of a theoretical model based on the hypothesis that underlying the observed synchronization is the joint arrival of EPSPs from branched last-order input fibres.

“…Neurophysiological and neuroanatomical studies in the cat have shown that the respiratory motoneurone pools in the spinal cord receive direct projections from the cerebral cortex (Aminoff & Sears, 1971;Lipski, Bektas & Porter, 1986; Rickard-Bell, Tork & Bystrzycka, 1986) as well as from the respiratory pattern generators in the brain stem (Merrill, 1971;Kirkwood & Sears, 1973;Cohen, Piercy, Gootman & Wolotsky, 1974;Hilaire & Monteau, 1976;Rickard-Bell, Bystrzycka & Nail, 1985;Davies, Kirkwood & Sears, 1985;Duffin & Lipski, 1987). Electrical stimulation of the motor cortex in human subjects has revealed that many respiratory muscles receive a powerful corticospinal projection (Gandevia & Rothwell, 1987;Gandevia & Plassman, 1988;Plassman & Gandevia, 1989; see also Gandevia, McKenzie & Plassman, 1990), and a recent study of motor unit synchronization suggests that the human sternomastoid muscle receives a corticospinal drive during voluntary respiration (Adams, Datta & Guz, 1989). In addition, the human somatosensory cortex receives a projection from respiratory muscles (Gandevia & Macefield, 1989).…”

Section: Introductionmentioning

confidence: 99%

The cortical drive to human respiratory muscles in the awake state assessed by premotor cerebral potentials.

Macefield

Gandevia

1991

101

SUMMARY1. We investigated the possibility of a cortical contribution to human respiration by recording from the scalp of awake subjects the premotor cerebral potentials that are known to precede voluntary limb movements.2. Electroencephalographic activity (EEG) was recorded from scalp electrodes and averaged for 1-8-2'0 s before the time at which airway pressure exceeded an inspiratory or expiratory threshold. Clear premotor cerebral potentials were recorded during brisk, self-paced nasal inhalations or exhalations. In ten subjects a slow cortical negativity (Bereitschaftspotential) was apparent in the averaged EEG, commencing 1P2+03 s before the onset of inspiratory (scalene) or expiratory (abdominal) muscle activity (EMG). It was maximal at the vertex, with a mean slope of 12-3 + 5-8 1tV/s, and was followed by a post-movement positivity.3. In four subjects the inspiratory premotor potential culminated in a large negativity, the motor potential, which began 24+ 15 ms before the onset of scalene EMG. It is argued that such a short latency is consistent with a volitionally generated respiratory command which travels relatively directly to the respiratory muscles, having a total central delay which is no longer than that for voluntary finger movements.4. That the respiratory premotor and motor potentials did not originate in subcortical structures was supported by their absence in a patient suffering from chronic reflexogenic hiccups, in whom cerebral activity was back-averaged from each brisk hiccup.5. During quiet breathing, in which subjects were relaxed and distracted from thinking about their respiration, no premotor cerebral potentials preceding inspiration could be detected. This failure was not due to the slow rate of rise of inspiratory activity during quiet breathing as compared with a brisk sniff, because premotor potentials were detected when subjects intermittently generated slow active expiratory efforts.