Recruitment of motor units in two fascicles of the semispinalis cervicis muscle

Abstract

Schomacher J, Dideriksen JL, Farina D, Falla D. Recruitment of motor units in two fascicles of the semispinalis cervicis muscle. J Neurophysiol 107: 3078 -3085, 2012. First published March 7, 2012 doi:10.1152/jn.00953.2011.-This study investigated the behavior of motor units in the semispinalis cervicis muscle. Intramuscular EMG recordings were obtained unilaterally at levels C2 and C5 in 15 healthy volunteers (8 men, 7 women) who performed isometric neck extensions at 5%, 10%, and 20% of the maximal force [maximum voluntary contraction (MVC)] for 2 min each and linearly increasing force contractions from 0 to 30% MVC over 3 s. Individual motor unit action potentials were identified. The discharge rate and interspike interval variability of the motor units in the two locations did not differ. However, the recruitment threshold of motor units detected at C2 (n ϭ 16, mean Ϯ SD: 10.3 Ϯ 6.0% MVC) was greater than that of motor units detected at C5 (n ϭ 92, 6.9 Ϯ 4.3% MVC) (P Ͻ 0.01). A significant level of short-term synchronization was identified in 246 of 307 motor unit pairs when computed within one spinal level but only in 28 of 110 pairs of motor units between the two levels. The common input strength, which quantifies motor unit synchronization, was greater for pairs within one level (0.47 Ϯ 0.32) compared with pairs between levels (0.09 Ϯ 0.07) (P Ͻ 0.05). In a second experiment on eight healthy subjects, interference EMG was recorded from the same locations during a linearly increasing force contraction from 0 to 40% MVC and showed significantly greater EMG amplitude at C5 than at C2. In conclusion, synaptic input is distributed partly independently and nonuniformly to different fascicles of the semispinalis cervicis muscle. motoneurons; synchronization WHEN THE SYNAPTIC INPUT is equally distributed among motoneurons, small-sized motoneurons are recruited before larger ones (Henneman 1957(Henneman , 1985. However, motoneurons innervating muscle fibers within the same muscle but with different mechanical action may receive different synaptic input that depends on the biomechanical demands. Those fibers that have a mechanical action with greater advantage for the task may be preferentially activated (English et al. 1993;Hudson et al. 2009), so that the recruitment order is task dependent within a muscle. For example, the recruitment of motor units in different regions of the long head of the biceps brachii varies with the relative amount of elbow flexion and forearm supination torque (ter Haar Romeny et al. 1982, 1984. Moreover, a series of studies on inspiratory muscles showed that the recruitment pattern of motor units across inspiratory motoneuron pools follows the mechanical advantage for respiration (Butler 2007; Butler and Gandevia 2008). Other muscles with complex mechanical actions, such as the trapezius, also display a location-dependent modulation of motor unit discharge rate, likely reflecting spatial dependence in the control of motor units (Falla and Farina 2008a).It may be expected that motor...

Motor unit recruitment by size does not provide functional advantages for motor performance

“…One possible explanation for this result may be muscle compartmentalization, which implies that some muscles consist of several subunits, each receiving separate synaptic input (English et al 1993). Presumably this phenomenon arises to improve the functional efficiency of the muscle and has been reported for arm (ter Haar Romeny et al 1984), neck (Schomacher et al 2012) and respiratory (Butler & Gandevia, 2008) muscles. In such cases, the optimization criteria would apply to each subunit rather than to the entire muscle, so that the total number of motor units in the entire muscle would be much greater than in current simulations.…”

Anatomic connections of the diaphragm influence of respiration on the body system

“…The thoracolumbar fascia is essential for muscles that involve the column,15,17 and diaphragmatic dysfunction will negatively affect this tissue, leading to central and peripheral symptoms. For instance, in a proper physiologic context, the semispinalis muscles of the neck are mainly activated at the C2 level; however, when anomalous tensions are registered, the muscular stress will burden C5 118. Obviously, the phrenic nerve and any connections with this region may be negatively affected.…”

Can neck exercises enhance the activation of the semispinalis cervicis relative to the splenius capitis at specific spinal levels?

“…However, measurements were performed at one spinal level only (C3), and given that synaptic input is distributed independently and non-uniformly to different fascicles of the semispinalis cervicis (Schomacher et al, 2012a), it is necessary to consider whether the activity of the semispinalis cervical can be enhanced relative to the splenius capitis at different spinal levels.…”