Common input to different regions of biceps brachii long head

The University of New South Wales School of Medical Sciences Sydney NSW 2052 Australia
Experimental Brain Research (Impact Factor: 2.04). 03/2008; 193(3):351-359. DOI: 10.1007/s00221-008-1631-2


The purpose of the experiment was to compare the level of synchronization exhibited by pairs of motor units located within
and between functionally distinct regions of the biceps brachii muscle. Pairs of single motor units were recorded from seven
subjects using separate electrodes located in the lateral and medial aspects of the long head of biceps brachii. Participants
were required to exert a combination of flexion and supination torques so that both motor units discharged at approximately
10pps for ≥200s and the level of motor unit synchronization could be quantified. When motor unit recordings were sufficiently
stable at the completion of this synchrony task, a series of ramp contractions with multiple combinations of flexion and supination
torques were performed to characterize the recruitment thresholds of the motor units. Common input strength (CIS) was significantly
greater (P<0.01) for the within-region pairs of motor units (0.28 extra sync. imps/s, n=26) than for the between-region pairs (0.13 extra sync. imps/s, n=18), but did not differ significantly for the 12 within-region pairs from the lateral head and 14 from the medial head
(0.27 vs. 0.29 extra sync. imps/s; P=0.83). Recruitment thresholds were measured for 33 motor units, but there was only a weak association between CIS and the
respective recruitment patterns for motor unit pairs (n=9). The present investigation provides evidence of a differential distribution of synaptic input across the biceps brachii
motor neuron pool, but this appears to have minimal association with the recruitment patterns for individual motor units.

Download full-text


Available from: Roger Enoka
  • Source
    • "Approximately twelve synaptic connections that mediate biceps brachii activity have been defined, for which many are to synapse with an equal distribution onto the target motor neurone pool (Naito 2004, Barry et al. 2008). However, electrophysiological studies (Burke 1968, Burke & Rymer 1976, Lev-Tov et al. 1983, Lucas et al. 1984, Burke & Glenn 1996) and at least one computer-simulated model (Segev et al. 1990) of the monosynaptic reflex in cats have also demonstrated that muscle spindle afferent input is unequally distributed among MUs of the homonymous motor neurone pool, as well as across synergist MUs (Lucas et al. 1984, Hamm et al. 1985, Vanden Noven et al. 1986). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Compartmentalized responses in motor unit (MU) activity of the short (SH) and long (LH) heads of the biceps brachii are observed following forearm position change. Differential muscle spindle afferent distribution has been proposed as a potential mechanism underlying this behavior. Tendon vibration is an effective, non-invasive method of increasing muscle spindle afferent activity of a target muscle group offering a paradigm in which this hypothesis may be investigated further.AimTo determine the effect of tendon vibration on MU recruitment and discharge rates of the SH and LH, muscle activity of the elbow flexors and triceps brachii, intermuscular coherence among the SH, LH, brachioradialis and triceps brachii, and force steadiness in young male and females during isometric elbow flexion.Methods Intramuscular electromyography (EMG) of the SH and LH, and surface EMG of the elbow flexors were recorded pre- and post-vibration during low force isometric contractions. Motor unit recruitment thresholds, MU discharge rates, and MU discharge variability; surface EMG amplitude, intermuscular coherence, and force steadiness were determined pre- and post-vibration.ResultsDifferential changes in all MU properties, EMG amplitude and intermuscular coherence were observed among elbow flexors. Although MU properties exhibited differential changes, they accounted for little variance in isometric force steadiness. However, intermuscular EMG coherence among all muscles investigated was reduced post-vibration.Conclusion Uncoupling of common oscillatory input as a result of differential muscle spindle afferent inputs to elbow flexors may be responsible for the reduction in force steadiness following tendon vibration and a forearm position change.This article is protected by copyright. All rights reserved.
    Full-text · Article · May 2014 · Acta Physiologica
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Evidence from human and animal studies suggests that motor neuron pool organization is not uniform for all motor tasks. Groupings of motor units within a muscle may be recruited differentially for a given task based on principles beyond anatomical or architectural features of the muscle alone. This study aimed to determine whether: (1) there was differential activation across locations of the first dorsal interosseous (FDI) muscle during a given task, (2) the differential activation was related to directional requirements and/or end goal of the task, and (3) there was an anatomical pattern to the differential activation. Twenty-six healthy right-handed participants carried out isometric finger/hand contractions in sitting while surface EMG was collected from 4 bipolar sites on the FDI muscle simultaneously. The tasks included: abduction, flexion, diagonal, 30% abduction + 30% flexion, 30% flexion + 30% abduction, key pinch, and power grasp. Mean peak integrated EMG for each task was normalized to site and task specific mean M waves. Differential activation was evident across FDI sites based on movement direction, order of directional components within a combination condition, and end goal of the task. There was greatest activation in the distal ulnar site for all tasks. Additionally there was a trend toward an ordering effect in the amount of activation at each site: distal ulnar > distal radial > proximal radial > proximal ulnar.
    Full-text · Article · Oct 2011 · Experimental Brain Research
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Motor units within human muscles usually exhibit a significant degree of short-term synchronization. Such coincident spiking typically has been attributed to last-order projections that provide common synaptic input across motor neurons. The extent of branched input arising directly from cortical neurons has often been suggested as a critical factor determining the magnitude of short-term synchrony. The purpose of this study, therefore, was to quantify motor unit synchrony in a variety of human muscles differing in the presumed extent of cortical input to their respective motor nuclei. Cross-correlation histograms were generated from the firing times of 551 pairs of motor units in 16 human muscles. Motor unit synchrony tended to be weakest for proximal muscles and strongest for more distal muscles. Previous work in monkeys and humans has shown that the strength of cortical inputs to motor neurons to also exhibit a similar proximal-to-distal gradient. However, in the present study, proximal-distal location was not an exclusive predictor of synchrony magnitude. The muscle that exhibited the least synchrony was an elbow flexor, whereas the greatest synchrony was most often found in intrinsic foot muscles. Furthermore, the strength of corticospinal inputs to abductor hallucis (AH) an intrinsic foot muscle, as assessed through transcranial magnetic stimulation, was weaker than that projecting to tibialis anterior (TA), even though AH had higher synchrony values compared to TA. We argue, therefore, that factors other than the potency of cortical inputs to motor neurons, such as the number of motor neurons innervating a muscle, significantly affects motor unit synchrony.
    Full-text · Article · Sep 2012 · Journal of Neurophysiology
Show more