Compartmentalized Innervation of Primate Lateral Rectus Muscle

Abstract

Consistent segregation of intramuscular CN6 arborization in humans and monkeys suggests functionally distinct superior and inferior zones for the LR. Since the LR is shaped as a broad vertical strap, segregated control of the two zones could activate them separately, potentially mediating previously unappreciated but substantial torsional and vertical oculorotary LR actions.

Magnetic resonance imaging demonstrates compartmental muscle mechanisms of human vertical fusional vergence

“…The foregoing compartmental analysis should be considered capable of demonstrating differential compartmental function in EOMs, but incapable of excluding it for several reasons. The present analysis parsed rectus EOMs into two compartments having equal transverse dimensions corresponding to the average location of anatomical demarcations occasionally observable by MRI in the LR , and to average proportions consistently demonstrated by histological reconstruction in a small number of serially sectioned orbits for LR (da Silva Costa et al 2011;Peng et al 2010) and MR (da Silva Costa et al 2011). The present analysis parsed the SO into equal medial and lateral regions, again based upon histological observations consistently made in a small number of serially sectioned orbits (Le et al 2014b).…”

“…Absence of contribution of the MR to VFV stands in contrast to MRI evidence for significantly less MRs, but not MRi, contractility during horizontal convergence than during adduction . Selective compartmental control of LR is possible because the abducens nerve bifurcates into inferior and superior divisions prior to entry into the LR belly (da Silva Costa et al 2011;Peng et al 2010) and then arborizes within non-overlapping compartments of fibers that have only weak transverse mechanical coupling among generally parallel muscle (Shin et al 2012 and tendon fibers (Shin et al 2013). …”

“…Because human rectus EOM tendons are wide (Apt 1980;Apt and Call 1980;Demer 2009), their intrinsic parallel fibers individually insert at substantially differing scleral sites that have correspondingly different oculorotary actions. The motor nerve to each horizontal rectus EOM divides into superior and inferior branches supplying non-overlapping distributions, forming segregated neuromuscular compartments maintained throughout their entire lengths (da Silva Costa et al 2011;Peng et al 2010). These neuromuscular compartments contain roughly parallel EOM fibers having only sparse lateral interconnections Lim et al 2007) and thus minimal transverse mechanical coupling.…”

Independent Passive Mechanical Behavior of Bovine Extraocular Muscle Compartments

“…However, given the observed mechanical independence of these transverse grouping of EOM fibers, they could be coordinated into physiologically integrated compartments by appropriate local innervation whose distribution is limited only to the transverse extent of each compartment. Such compartmental segregation of intramuscular innervation has been demonstrated for the horizontal rectus EOMs of humans, monkeys, and other mammals, 8,9 but not in the vertical rectus EOMs. In other words, intramuscular motor nerve arborizations in the horizontal rectus EOMs do not cross transverse compartmental boundaries anywhere along the lengths of these EOMs, so the groups of EOM fibers separately innervated by the superior and inferior motor nerve trunks can constitute functionally independent superior and inferior muscle actuators whose tensions are delivered to separate insertions on the ocular sclera.…”

“…It has been suggested that extraocular muscle (EOM) has more motor units than apparently required by known ocular motility mechanisms. 6,7 Peng et al 8 traced the intramuscular arborization of the abducens nerve within the lateral rectus (LR) EOMs of monkeys and humans, and observed that the nerve bifurcated into non-overlapping superior and inferior territories that remained segregated throughout the EOM's length. Costa et al 9 subsequently confirmed Peng's 8 report of compartmentalized LR innervation, and extended the study by tridimensional reconstruction of nerve arborizations to the inferior (IR), medial (MR), and superior rectus (SR) EOMs of humans and monkeys.…”