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The morphology and ultrastructure of medial rectus subgroup motoneurons in the macaque monkey

Erichsen, Jonathan Thor, Wright, Nicholas Fraser and May, Paul J. 2014. The morphology and ultrastructure of medial rectus subgroup motoneurons in the macaque monkey. The Journal of Comparative Neurology 522 (3) , pp. 626-641. 10.1002/cne.23437

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There are two muscle fiber types in extraocular muscles: those receiving a single motor end plate, termed singly innervated fibers (SIFs), and those receiving multiple small terminals along their length, termed multiply innervated fibers (MIFs). In monkeys, these two fiber types receive input from different motoneuron pools: SIF motoneurons found within the extraocular motor nuclei, and MIF motoneurons found along their periphery. For the monkey medial rectus muscle, MIF motoneurons are found in the C-group, while SIF motoneurons lie in the A- and B-groups. We analyzed the somatodendritic morphology and ultrastructure of these three groups of macaque medial rectus motoneurons to better understand the structural determinants controlling the two muscle fiber types. The dendrites of A- and B-group motoneurons lay within the oculomotor nucleus, but those of the C-group motoneurons were located outside the nucleus, and extended into the preganglionic Edinger-Westphal nucleus. A- and B-group motoneurons were very similar ultrastructurally. In contrast, C-group motoneurons displayed significantly fewer synaptic contacts on their somata and proximal dendrites, and those contacts were smaller in size and lacked dense-cored vesicles. However, the synaptic structure of C-group distal dendrites was quite similar to that observed for A- and B-group motoneurons. Our anatomical findings suggest that C-group MIF motoneurons have different physiological properties than A- and B-group SIF motoneurons, paralleling their different muscle fiber targets. Moreover, primate C-group motoneurons have evolved a special relationship with the preganglionic Edinger-Westphal nucleus, suggesting these motoneurons play an important role in near triad convergence to support increased near work requirements. J. Comp. Neurol., 2013. © 2013 Wiley Periodicals, Inc.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Optometry and Vision Sciences
Subjects: R Medicine > RE Ophthalmology
Uncontrolled Keywords: Oculomotor; Vergence; Extraocular; Motor Neurons; Gaze; Eye Muscles; Eye Movements
Additional Information: Online publication date: 29 July 2013.
Publisher: Wiley-Blackwell
ISSN: 0021-9967
Date of Acceptance: 18 July 2013
Last Modified: 15 Feb 2019 16:00

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