A central mesencephalic reticular formation projection to the supraoculomotor area in macaque monkeys

Abstract

The central mesencephalic reticular formation is physiologically implicated in oculomotor function and anatomically interwoven with many parts of the oculomotor system's premotor circuitry. This study in Macaca fascicularis monkeys investigates the pattern of central mesencephalic reticular formation projections to the area in and around the extraocular motor nuclei, with special emphasis on the supraoculomotor area. It also examines the location of the cells responsible for this projection. Injections of biotinylated dextran amine were stereotaxically placed within the central mesencephalic reticular formation to anterogradely label axons and terminals. These revealed bilateral terminal fields in the supraoculomotor area. In addition, dense terminations were found in both the preganglionic Edinger-Westphal nuclei. The dense terminations just dorsal to the oculomotor nucleus overlap with the location of the C-group medial rectus motoneurons projecting to multiply innervated muscle fibers suggesting they may be targeted. Minor terminal fields were observed bilaterally within the borders of the oculomotor and abducens nuclei. Injections including the supraoculomotor area and oculomotor nucleus retrogradely labeled a tight band of neurons crossing the central third of the central mesencephalic reticular formation at all rostrocaudal levels, indicating a subregion of the nucleus provides this projection. Thus, these experiments reveal that a subregion of the central mesencephalic reticular formation may directly project to motoneurons in the oculomotor and abducens nuclei, as well as to preganglionic neurons controlling the tone of intraocular muscles. This pattern of projections suggests an as yet undetermined role in regulating the near triad.

Keywords: Edinger–Westphal; Eye movements; Midbrain; Oculomotor; Saccades; Vergence.

Figure 1

A large BDA injection into the central mesencephalic reticular formation (cMRF) (insets B–E) produced dense terminal labeling (stipple) in the periaqueductal gray (PAG), supraoculomotor area (SOA) (A–E) and preganglionic Edinger-Westphal nucleus (EWpg) (A–D) and more limited terminal labeling in the oculomotor nucleus (III) (A–E) and its caudal central subdivision (CC) (E). Scattered terminals were also present in the trochlear (IV) (F) and abducens (VI) (G&H) nuclei. Terminals were also present dorsal to IV, as well as in and above the adjacent facial nerve (VIIn), and in the caudal linear raphe nucleus (RCL). All chartings arranged in rostral to caudal order, with insets to show level in this and other chartings. Injection sites are shown in the whole-section insets

Figure 2

A smaller BDA injection into the cMRF (insets C&D) produced considerable terminal labeling (stipple) in the periaqueductal gray (PAG), supraoculomotor area (SOA) and preganglionic Edinger-Westphal nucleus (EWpg) (A–E), and more limited terminal labeling in the oculomotor nucleus (III) (A–E). Terminals were also present in trochlear (IV) (E) and abducens (VI) (F&G) nuclei, as well as in and above the adjacent facial nerve (VIIn).

Figure 3

The pattern of axonal labeling from a BDA injection of the cMRF in a section located near the rostral pole of the oculomotor nucleus (III) (A). The box indicates the region shown in B, which includes the preganglionic Edinger-Westphal nucleus (EWpg) and the portion of the supraoculomotor area (SOA) between EWpg and III. The axons show few branches, display primarily en passant boutonal enlargements, and densely populate this region. They cross the EWpg border freely, but tail off when crossing the border of III. Cresyl violet stained cells are indicated by shading.

Figure 4

Photographic plates showing the pattern of BDA labeling after a cMRF injection illustrated in figure 2. A&F show low magnification views from rostral (A) and caudal (F) sections through the oculomotor nucleus (III) to define the location of the high magnification views (B–E and G–I, respectively). Close associations (arrowhead) between labeled boutons and counterstained cell bodies were observed in the supraoculomotor area (SOA) (D&H), and preganglionic Edinger-Westphal nucleus (EWpg) (B,E&G). Associations were also present in the regions containing the C-group (I) and S-group (C) motoneurons adjacent to the oculomotor nucleus (III). Fewer close associations were seen in the abducens nucleus (J–L). All scale bars = 50 μm. Scale in A=F, Scale in I=B,D,E,G,H&J–L. Z-planes combined: B=5, C=7, D=1, E=3, G=5, H=3, I=1, J–K=1.

Figure 5

A small PhaL injection into the cMRF (insets F&G) produced considerable terminal labeling (stipple) in the periaqueductal gray (PAG), supraoculomotor area (SOA) and preganglionic Edinger-Westphal nucleus (EWpg) (A–E), and much more limited terminal labeling in the oculomotor nucleus (III) (A–F) and caudal central subdivision (CC) (F). Almost no terminals were present in trochlear IV (G) and abducens (VI) (H) nuclei.

Figure 6

Photographic plates showing the pattern of PhaL labeling after the cMRF injection illustrated in figure 5. A. shows a low magnification view to define the location of the high magnification views (B–D). Close associations (arrowheads) between labeled boutons and counterstained cell bodies were observed in the high magnification views of ipsilateral (C) and contralateral (D) supraoculomotor area (SOA), and in the ipsilateral preganglionic Edinger-Westphal nucleus (EWpg) (B). Scale in D=B&C. Z axis planes: B=3, C=4, D=1.

Figure 7

The pattern of retrograde labeling (dots) observed in the peri interstitial nucleus of Cajal portion of the mesencephalic reticular formation (piMRF) (A) and the central mesencephalic reticular formation (cMRF) (B–F) after an injection of BDA into the oculomotor nucleus (III) and supraoculomotor area (SOA) (A–E). In the cMRF, the cells are arranged as a band stretching across the mediolateral breadth of the nucleus.

Figure 8

Relationship of the pattern of retrograde label (dots) following an oculomotor nucleus (III) injection (see Fig. 7) to the locations of four (1–4) BDA injection sites.

Figure 9

Photomicrographs showing the injection of BDA into the oculomotor nucleus (III) and supraoculomotor area (A), and the distribution of labeled cells (arrows) in the central mesencephalic reticular formation (cMRF) (A) that resulted from this injection. B&C show the morphology of the homogeneously labeled neurons within the cMRF. Note the poorly branched, tapering dendrites and range of cell sizes. Box in A shows location of B. Scale in B=C.

Figure 10

Illustration of the dendritic fields of central mesencephalic reticular formation (cMRF) cells projecting to the oculomotor nucleus (III) within a single section (C). Following the injection shown in A, retrogradely labeled cells (dots) extend across the cMRF. The dendritic fields of the labeled cells on the right side of this section are illustrated in B. Note the morphology of the homogeneously labeled neurons within the cMRF and the fact their dendrites are largely constrained within the mediolaterally oriented band of labeled cells.

Figure 11

Schematic showing the subdivisions (A) and possible perioculomotor targets (B) of the central mesencephalic reticular formation (cMRF). A. location of the cells that are the source of the cMRF projection to the supraoculomotor area (SOA) (hatching) in comparison to the sources of other cMRF projections to the superior colliculus, pons, the spinal cord and medullary reticular formation (blue hatch). B. Potential target populations within the SOA and oculomotor nucleus (III).