Raphe nucleus of the pons containing omnipause neurons of the oculomotor system in the monkey, and Its homologue in man

Abstract

Omnipause neurons take part in the generation of saccadic eye movements. They lie around the midline in the caudal pontine reticular formation, in an area usually ascribed to the nucleus raphe pontis (rp). In this study of the monkey (Macaca fascicularis and M. mulatta), we describe four series of experiments aimed at establishing that omnipause neurons lie within a distinctive cytoarchitectonic entity, which we call the nucleus raphe interpositus (rip): (1) cytoarchitectural study, (2) recording-lesion experiments to establish in which cell group omnipause neurons lie, (3) cytochrome oxidase distribution in the omnipause region and neighboring structures, and (4) neuroanatomical tracing experiments to demonstrate afferents to the omnipause region. In the detailed cytoarchitectural study of the midline structures in the caudal pons and rostral medulla, a distinctive group of neurons (rip) adjoining the ventrocaudal border of rp and dorsal to the nucleus raphe magnus (rm) is described. The striking features of rip are the uniformly arranged, narrow row of the cells either side of the midline, and the extensive horizontally oriented dendritic trees of its neurons. The abducens rootlets (NVI) pass through the reticular formation at the same rostrocaudal level as rip and form a reliable landmark for its location. Cytochrome-oxidase-stained sections demonstrated additional differences between rip and adjacent cell groups: in rip the neurons and their extensive dendrites stained strongly, but not the surrounding neuropile, whereas in rp both the neurons and the neuropile stained darkly, so that individual neurons were difficult to see. Unlike rp, rip coincides with the location of omnipause neurons, and lesions marking the sites of individual omnipause units lay within its boundaries. Tritiated leucine was injected into superior colliculus (sc), which is known to have monosynaptic connections with omnipause neurons. Labelled axons and patterns of silver grains taken to indicate the presence of terminal branching were found in and around rip, but no significant labelling was seen in rp or rm. It is concluded that the omnipause neurons lie within the rip in the monkey. These functional and morphological differences between rip and the adjacent raphe nuclei are used to justify its characterization as an independent cell group in the monkey. In order to relate these findings to man, cytochrome oxidase experiments were carried out on the human brainstem, and the pattern of staining at the level of the abducens rootlets was correlated with the cytoarchitecture.(ABSTRACT TRUNCATED AT 400 WORDS)

Pontine Omnipause Activity During Conjugate and Disconjugate Eye Movements in Macaques

“…With regard to versional eye position, our static data are in agreement with previous reports (Büttner-Ennever et al 1988;Luschei and Fuchs 1972) stating that OPN firing rates during fixation do not change with static eye position. These previous studies did not manipulate viewing distance and therefore the static vergence angle of the animal was always the same, so that the positional changes were restricted to the versional components alone.…”

“…Pontine omnipause neurons (OPNs), located in the nucleus raphe interpositus (Büttner-Ennever et al 1988) are a critical component of the primate saccadic eye movement system. These neurons fire tonically in alert animals during fixation and pause just before and during saccades or quick phase eye movements of any size and direction (Everling et al 1998;Raybourn and Keller 1977).…”

Neurotransmitter profile of saccadic omnipause neurons in nucleus raphe interpositus

“…F. Spencer, personal communication). We considered the PAV-immunoreactive neurons in rip as OPNs for following reasons: virtually all neurons composing the bilaminar cell column of rip are PAV immunoreactive, and physiological studies indicate that OPNs are arranged in a compact cell group, which corresponds to rip (Biittner-Ennever et al, 1988) and that rip consists mainly of OPNs (Langer and Kaneko, 1990). The PAV expression is in line with the elevated energy consumption shown by the strong cytochrome oxidase activity in OPNs (Biittner-Ennever et al, 1988) and their high level of tonic activity, which often exceeds 100 spikes/set in the monkey.…”

“…At the location corresponding to that in monkey and cat, PAV immunocytochemistry revealed a group of horizontally oriented medium-sized neurons [(O,i, + 0,,,)/2 = 33.55 pm; SD = 5.27pm; N = 1341 adjacent to the midline, that we consider as the homolog rip (Biittner-Ennever et al, 1988). These PAV-immunoreactive neurons lay scattered within I mm of the midline, and Cxtended 2.6 mm in the anterior-posterior direction.…”

The brainstem switch for gaze shifts in humans

“…Saccadic burst neurons are inhibited by omnipause neurons, which lie in the pons of the brainstem [4]. Electrical stimulation of omnipause neurons stops saccades in mid-flight [5].…”