Motor-Unit Synchrony Within and Across Compartments of the Human Flexor Digitorum Superficialis

Abstract

An interesting feature of the muscular organization of the human hand is that the main flexors and extensors of the fingers are compartmentalized and give rise to multiple parallel tendons that insert onto all the fingers. Previous studies of motor-unit synchrony in extensor digitorum and flexor digitorum profundus indicated that synaptic input to motor neurons supplying these multitendoned muscles is not uniformly distributed across the entire pool of motor neurons but instead appears to be partially segregated to supply subsets of motor neurons that innervate different muscular compartments. Little is known, however, about the organization of the synaptic inputs to the motor neurons supplying another multitendoned finger muscle, the flexor digitorum superficialis (FDS). Therefore in this study, we estimated the extent of divergence of last-order inputs to FDS motor neurons by measuring the degree of short-term synchrony among motor units within and across compartments of FDS. The degree of synchrony for motor-unit pairs within the same digit compartment was nearly twofold that of pairs of motor units in adjacent compartments and more than fourfold that of pairs in nonadjacent compartments. Therefore like other multitendoned muscles of the hand, last-order synaptic inputs to motor neurons supplying the FDS appear to primarily supply subsets of motor neurons innervating specific finger compartments. Such an organization presumably enables differential activation of separate compartments to facilitate independent movements of the fingers.

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

“…A nonuniform activation of motor units within muscle regions as found in this study has also been observed in other human muscles, such as the extensor digitorum (Keen and Fuglevand 2004), the flexor digitorum profundus (Reilly et al 2004), the flexor digitorum superficialis (McIsaac and Fuglevand 2006), and the upper trapezius muscle (Falla and Farina 2008a). Furthermore, in the external intercostal muscles, a cranial-caudal (Hudson et al 2011) anddorsal-ventral (De Troyer et al 2003) gradient of activation has been observed.…”

Analysis of crosstalk in the mechanomyographic signals generated by forearm muscles during different wrist postures

“…19 Therefore, cross-correlations can represent crosstalk, but they can also represent other mechanisms such as common neural drive between muscles. 36,37 Because anatomical architecture of different muscles (i.e., fiber length, pennation angle, muscle length, tendon displacement) that influences MMG signal differs as a function of wrist actions, there is a small chance for a similar pattern of motor unit response between 2 muscles.…”

Limits to the Control of the Human Thumb and Fingers in Flexion and Extension

“…This coupling also fits with studies on short-term synchronization of motor units in the multitendon extrinsic hand muscles. Synchronization is strongest among the compartments for middle and ring fingers and also for ring and little fingers (in ED and FDP; Keen and Fuglevand 2004a;McIsaac and Fuglevand 2007). These (and other) studies also often calculated a "selectivity" index for each digit (e.g., Hager-Ross and Schieber 2000; Keen and Fuglevand 2004a).…”