Reticulospinal neurons in the pontomedullary reticular formation of the monkey (Macaca fascicularis)

Abstract

Recent neurophysiological studies indicate a role for reticulospinal neurons of the pontomedullary reticular formation (PMRF) in motor preparation and goal-directed reaching in the monkey. Although the macaque monkey is an important model for such investigations, little is known regarding the organization of the PMRF in the monkey. In the present study, we investigated the distribution of reticulospinal neurons in the macaque. Bilateral injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were made into the cervical spinal cord. A wide band of retrogradely labeled cells was found in the gigantocellular reticular nucleus (Gi) and labeled cells continued rostrally into the caudal pontine reticular nucleus (PnC) and into the oral pontine reticular nucleus (PnO). Additional retrograde tracing studies following unilateral cervical spinal cord injections of cholera toxin subunit B revealed that there were more ipsilateral (60%) than contralateral (40%) projecting cells in Gi, while an approximately 50:50 ratio contralateral to ipsilateral split was found in PnC and more contralateral projections arose from PnO. Reticulospinal neurons in PMRF ranged widely in size from over 50 microm to under 25 microm across the major somatic axis. Labeled giant cells (soma diameters greater than 50 microm) comprised a small percentage of the neurons and were found in Gi, PnC and PnO. The present results define the origins of the reticulospinal system in the monkey and provide an important foundation for future investigations of the anatomy and physiology of this system in primates.

Figure 1

A line drawing of a parasagittal section through primate PMRF showing the relative location and nuclear boundaries of PnO, PnC and Gi. The section is approximately 1.5 mm from midline. Gray boxes indicate the regions shown in high magnification photomicrographs below. A. Nissl stained photomicrograph of PnO. Note the sparse distribution of both multipolar and fusiform cells. B. Nissl stained photomicrograph of PnC showing giant cells scattered among a mixture of cells. C. Nissl stained photomicrograph of DPGi. D. Nissl stained photomicrograph of Gi.. E. Nissl-stained low-magnification photograph for a sagittal section through the PMRF. (Cresyl violet stain) F. Substance-P immunoreacted section adjacent to the section in E. Scale bar = 50 μm (A-D), 1.0 mm (E-F). Arrows in E and F show a blood vessel used to align the sections.

Figure 2

A low power darkfield photomicrograph through PMRF in case M121-12 (A). Note the electrode tracks passing obliquely through the section. The greatest density of HRP labeled cells is in Gi. Boxed area in PnC is shown at higher power in B. Note the labeled cells located along the track ventral to abducens nucleus (6N). Arrows denote electrode track and associated gliosis. (neutral red counterstain) Scale bar = 1.0 mm (A) and 50μm (B).

Figure 3

Line drawings of a series of coronal sections through the PMRF in case M121-12. Multiple bilateral injections of WGA-HRP were made into the spinal cord at mid-cervical levels. Black circles indicate WGA-HRP labeled cells. Gray lines indicate electrode tracks from cell recordings made during a reaching task (Davidson and Buford, 2004; 2006). Note that the greatest density of cell labeling was found in Gi as shown in sections 39-57. The distribution of spinal cord projection neurons extends from Gi dorsally into PnC just ventral to abducens nucleus (section 53). Approximate stereotaxic coordinates ranging from +2.5 - −2.5 mm are indicated next the section numbers. The scale bar indicates 5-mm with 1-mm increments. Bottom inset shows a drawing of coronal section through the cervical spinal cord showing the spread of the bilateral WGA-HRP injections. The blackened area indicates the needle track and the injection site core. The lighter gray regions denote lighter tracer staining corresponding to the injection halo. Scale bar= 1.0 mm

Figure 4

Line drawings of a series of sagittal sections through the PMRF in case M0502. Sections are arranged from lateral to medial; sections contralateral to the spinal cord injections are on the left and ipsilateral is shown on the right. Approximate distance from midline is noted below the section numbers. Red triangles indicate each reticulospinal cell greater than 50 μm in diameter. Black dots indicate all other labeled CTB cells. Bottom inset contains a drawing of a horizontal section showing the extent of the CTB injections into the cervical spinal cord at levels C4. The dense core of the injection sites is shown in black and halo is shown in gray. The halo of the injection site spread to include part of the contralateral spinal cord. Scale bar = 2 mm.

Figure 5

Line drawings of a series of sagittal sections through the PMRF in case M0503. Details follow that described in Fig. 2

Figure 6

Photomicrographs showing the CTB labeled reticulospinal cells. A low power photomicrograph through the ipsilateral PMRF showing CTB labeled cells in Gi in a sagittal section (A). Note that the greatest density of cell labeling is found in ventral Gi overlying the inferior olive (IO). Boxed region is shown at higher power in C. High power photomicrographs of the same section showing the labeled cells in PNc (B) and in Gi (C). Note the wide range of sizes and types of reticulospinal neurons in both B and C. Scale bar = 1.0 mm (A) and 50 μm (B & C).

Figure 7

Proportions of retrogradely labeled cells found in Gi, PnC, and PnO ipsilateral and contralateral to unilateral CTB injections in the spinal cord for four cases, and combined across all cases. The numbers in the inset show the percentage of ipsilateral cells as a proportion of the total number found in that nucleus. For example, across all cases, a total of 837 cells were found in PnO, and 34% of these were ipsilateral to the side of the CTB injection.