The innervation of the monkey accessory lateral rectus muscle

Area of Science:

• Neuroscience
• Comparative Anatomy
• Ophthalmology

Background:

• The accessory lateral rectus muscle's innervation pattern in primates has been debated.
• Previous studies suggested mixed innervation and broader motoneuron localization.

Purpose of the Study:

• To re-investigate the source and pattern of innervation of the accessory lateral rectus muscle in Macaca fascicularis.
• To clarify the composition of muscle fibers and the location of motoneurons.

Main Methods:

• Utilized a tracing method with [125I] wheat germ agglutinin.
• Conducted morphological analysis of myo-neuronal junctions.

Main Results:

• The accessory lateral rectus muscle appears to be composed exclusively of singly innervated fibers.
• Motoneurons innervating this muscle were localized to the accessory abducens nucleus.
• Findings contrast with previous reports of singly and multiply innervated fibers and broader motoneuron distribution.

Conclusions:

• The macaque accessory lateral rectus muscle's organization aligns with the retractor bulbi muscle in other vertebrates.
• This muscle is vestigial in macaques, reflecting a trend of reduction in primate evolution.
• Absence of comparable motor units in humans suggests a loss of structure and function rather than integration into the lateral rectus system.