A central mesencephalic reticular formation projection to the Edinger–Westphal nuclei

Abstract

The central mesencephalic reticular formation, a region associated with horizontal gaze control, has recently been shown to project to the supraoculomotor area in primates. The Edinger–Westphal nucleus is found within the supraoculomotor area. It has two functionally and anatomically distinct divisions: (1) the preganglionic division, which contains motoneurons that control both the actions of the ciliary muscle, which focuses the lens, and the sphincter pupillae muscle, which constricts the iris, and (2) the centrally projecting division, which contains peptidergic neurons that play a role in food and fluid intake, and in stress responses. In this study, we used neuroanatomical tracers in conjunction with immunohistochemistry in Macaca fascicularis monkeys to examine whether either of these Edinger–Westphal divisions receives synaptic input from the central mesencephalic reticular formation. Anterogradely labeled reticular axons were observed making numerous boutonal associations with the cholinergic, preganglionic motoneurons of the Edinger–Westphal nucleus. These associations were confirmed to be synaptic contacts through the use of confocal and electron microscopic analysis. The latter indicated that these terminals generally contained pleomorphic vesicles and displayed symmetric, synaptic densities. Examination of urocortin-1-positive cells in the same cases revealed fewer examples of unambiguous synaptic relationships, suggesting the centrally projecting Edinger–Westphal nucleus is not the primary target of the projection from the central mesencephalic reticular formation. We conclude from these data that the central mesencephalic reticular formation must play a here-to-for unexpected role in control of the near triad (vergence, lens accommodation and pupillary constriction), which is used to examine objects in near space.

A central mesencephalic reticular formation projection to medial rectus motoneurons supplying singly and multiply innervated extraocular muscle fibers

“…There was some spread of tracer rostrally into the piMRF (Figure 1b; inset), dorsally into the nucleus of the posterior commissure (Figure 1e; inset), and medially to the edge of the MLF (Figure 1b,c; insets). Consistent with previous reports (Bohlen et al, 2016; May et al, 2016), there was a bilateral terminal field (stipple) present within the SOA (Figure 1a–f), and terminals were particularly dense around the preganglionic Edinger-Westphal nucleus (EWpg) (Figure 1a–e). The terminals within SOA included a set that completely overlapped the distribution of C-group medial rectus motoneurons (Figure 1a–d).…”

“…Our work on cMRF projections supports this contention, as we have observed bilateral inputs with similar densities and morphologies terminating on preganglionic motoneurons in EWpg and on medial rectus MIF motoneurons (May et al, 2016, present results). Furthermore, medial rectus MIF motoneuron dendrites receive considerable input within EWpg (Erichsen et al, 2014), so the axosomatic contacts from cMRF terminals described here may simply be an extension of the cMRF projection they receive within this adjacent nucleus.…”

“…The cMRF provides a large (thick line), bilateral projection to the medial rectus motoneurons in the C-group (present work) and to preganglionic motoneurons in the preganglionic Edinger-Westphal nucleus (EWpg) (May et al, 2016). In addition, there is a modest (thin line) projection to the medial rectus motoneurons within the ipsilateral A- and B-groups.…”

“…Due to the fact that the cMRF’s SOA projection was bilateral (Bohlen et al, 2016), in separate animals, we utilized muscle injections ipsilateral and contralateral to the side of the cMRF injection. Portions of this work have appeared in abstract form (Bohlen, Warren, & May, 2015; May, Horn, Mustari, & Warren, 2011). …”

Internal organization of medial rectus and inferior rectus muscle neurons in the C group of the oculomotor nucleus in monkey

“…In monkey another monosynaptic bilateral input to the MR C‐group neurons and preganglionic neurons arises from the central mesencephalic reticular formation (CMRF; Horn et al, ; May et al, ). The CMRF is considered part of a circuitry for control of the horizontal saccades presumably providing a feedback signal to the SC (Cohen and Büttner‐Ennever, ; Wang et al, ; Zhou et al, ).…”