Sources of direct excitatory and inhibitory inputs from the medial rhombencephalic tegmentum to lateral and medial rectus motoneurons in the cat

Abstract

The paramedian pontine and bulbar tegmentum was explored by microstimulation to outline the sites of origin of direct excitatory and inhibitory inputs to lateral rectus (LR) and medial rectus (MR) motoneurons (MNs). In order to avoid activation of fibers of passage and axon reflexes originating outside the stimulation sites, experiments were carried out 4--22 days after brain stem transections causing degeneration of vestibulo-ocular pathways. Additionally, in some experiments the paramedian tegmentum was isolated from the contralateral side by midline transections. Mapping of stimulus sites from which monosynaptic EPSPs and IPSPs were elicited brought out the following preoculomotor reticular regions: 1. LR-MNs received monosynaptic IPSPs from the contralateral reticular formation corresponding to Nucl. reticularis points caudalis (R.p.c.) and the rostral part of Nucl. reticularis gigantocellularis (R.gc.). 2. Monosynaptic inhibitory input to MR-MNs could only be demonstrated after degeneration of excitatory pathways ascending from the internuclear neurons of the VIth nucleus and from the ipsilateral vestibular nuclei. Monosynaptic IPSPs originated in the ipsilateral dorso-medial tegmentum through the entire extent of the Nucl. reticularis pontis oralis and rostral R.p.c. including the region of the ipsilateral VIth nucleus. 3. Monosynaptic excitation of LR-MNs was induced by stimulation of the ipsilateral R.p.c. and the rostral half of the paramedian bulbar tegmentum (R.gc.). 4. The sites from which monosynaptic EPSPs were evoked in MR-MNs were confined to the contralateral VIth nucleus and its immediate vicinity. No evidence could be obtained for direct excitatory inputs to MR-MNs from the ipsilateral paramedian tegmentum. It is concluded that the paramedian rhombencephalic reticular formation contains four pools of premotor neurons related to coordination of conjugate horizontal eye movements. Two of them are excitatory for LR- and MR-MNs with ipsilateral ON-directions, the other two mediate reciprocal inhibition of the antagonistic motor nuclei.

Abduction paresis with rostral pontine and/or mesencephalic lesions: Pseudoabducens palsy and its relation to the so-called posterior internuclear ophthalmoplegia of Lutz

“…This projection originates from neurons in the pontine reticular formation between the level of the 4th and 6th cranial nerve nuclei [88] ascending adjacent but separate from the MLF [76,85], and approaching medial rectus motoneurons [87]. Stimulation of these neurons in animals with bilaterally destructed MLFs was followed by monosynaptic inhibitory potentials in ipsilateral 3rd nerve nucleus neurons and ipsilateral medial rectus motoneurons [88-90]. If this connection mediates medial rectus inhibition, its lesion would be followed by an abduction paresis with normal lateral rectus activation but impaired medial rectus inhibition.…”

Neural Control of Saccadic Eye Movements

“…The PPRF projects ipsilaterally to the abducens nucleus [27][28][29][30], to the prepositus hypoglossi nucleus (NPH) [27,28,30,31], to the median vestibular nucleus (MVN) [32] and to the posterior vermis [33]. The PPRF receives input from the SC, the posterior vermis via the fastigial nuclei and the FEF.…”

Functional Diversity of Neurotrophin Actions on the Oculomotor System

“…Their axons cross the midline at its exit from the nucleus and project contralaterally through the medial longitudinal fascicle to innervate medial rectus motoneurons. Both abducens motor and internuclear neurons share the same threefold input system, that convey signals associated to either saccades (pontomedullary reticular formation), slow phases of the vestibular nystagmus (medial vestibular nucleus) or eye fixations (prepositus hypoglossi nucleus [28,30,31,32,33,34,35], see Figure 1). …”