Abstract
Gaze changes involving the eyes and head are orchestrated by brainstem gaze centers found within the superior colliculus (SC), paramedian pontine reticular formation (PPRF), and medullary reticular formation (MdRF). The mesencephalic reticular formation (MRF) also plays a role in gaze. It receives a major input from the ipsilateral SC and contains cells that fire in relation to gaze changes. Moreover, it provides a feedback projection to the SC and feed-forward projections to the PPRF and MdRF. We sought to determine whether these MRF feedback and feed-forward projections originate from the same or different neuronal populations by utilizing paired fluorescent retrograde tracers in cats. Specifically, we tested: 1. whether MRF neurons that control eye movements form a single population by injecting the SC and PPRF with different tracers, and 2. whether MRF neurons that control head movements form a single population by injecting the SC and MdRF with different tracers. In neither case were double labeled neurons observed, indicating that feedback and feed-forward projections originate from separate MRF populations. In both cases, the labeled reticulotectal and reticuloreticular neurons were distributed bilaterally in the MRF. However, neurons projecting to the MdRF were generally constrained to the medial half of the MRF, while those projecting to the PPRF, like MRF reticulotectal neurons, were spread throughout the mediolateral axis. Thus, the medial MRF may be specialized for control of head movements, with control of eye movements being more widespread in this structure.
Highlights
The midbrain reticular formation (MRF) has been implicated as an intermediary in the brainstem circuits underlying the control of the eye and head components associated with a gaze change
Intracellular studies in squirrel monkeys have demonstrated that the activity in a subset of long lead burst neurons in the MRF is tightly coupled to the activity in the intermediate gray layer (SGI) of the superior colliculus (SC; Moschovakis et al, 1988a)
A fluorogold (FG) injection was made into the left SC and diamidino yellow (DY) was injected into the right SC
Summary
The midbrain reticular formation (MRF) has been implicated as an intermediary in the brainstem circuits underlying the control of the eye and head components associated with a gaze change. Physiological support for this contention came initially from experiments in which electrical stimulation of the MRF in macaque monkeys produced horizontal contraversive saccades (Bender and Shanzer, 1964; Cohen and Büttner-Ennever, 1984; Cohen et al, 1985). Recordings from head-free macaques demonstrate that the discharges of a subset of cMRF neurons are better correlated
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