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PERSPECTIVES |
1 Department Physiology & Biophysics, Numa P.G. Adams Building, Howard University College of Medicine, 520 W Street N.W., Washington, DC 20059, USA
2 Lab. Neurobiologie Cellulaire et Moléculaire, INAF, Bât 32, CNRS, 1, ave. de la Terrasse, 91198 Gif-sur-Yvette, France
Email: wgraf{at}howard.edu and gabriella.ugolini{at}nbcm.cnrs-gif.fr
Saccadic eye movements are extremely rapid and jerk-like shifts of the eyes (up to 900 deg s–1) with which we explore our environment between periods of fixation, and other oculomotor tasks. We perform about 20 000–25 000 of such highly accurate eye movements per day to bring visual targets of interest onto our foveae. To achieve such precision, eye movements in general, and saccades in particular, need to be space–time coordinated. An article by Cromer & Waitzman (2006) in this issue of The Journal of Physiology suggests one mechanism of how space–time coordination of saccadic eye movements in the horizontal plane could be achieved by a little known and little studied area in the brainstem, the central mesencephalic reticular formation (cMRF). Although discovered years ago by the Cohen group (Cohen & Büttner-Ennever, 1984; Cohen et al. 1985, 1986), its existence had been largely neglected until the Waitzman group recently unearthed it.
The classically defined cMRF is located lateral to the oculomotor nucleus (Fig. 1), dorsal to the caudal pole of the red nucleus and ventral to the posterior commissure. Different cMRF regions receive projections from small and large saccade areas of the superior colliculus (Cohen & Büttner-Ennever, 1984; Cohen et al. 1986). Stimulation of the dorsal cMRF induces fixed vector saccades (‘retinocentric’), i.e. of constant amplitude and direction regardless of the initial eye position, whereas in the ventral cMRF saccade amplitude varies depending on eye position (Cohen et al. 1985).
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Neuroanatomical tract tracing results with a retrograde transneuronal marker have shown that the cMRF also innervates directly, i.e. monosynaptically, lateral rectus (Graf et al. 2000; Ugolini et al. 2001, and submitted) and medial rectus motoneurones (Graf et al. 2002). These direct projections may be part of the network involved in generation of combined eye and head movements, i.e. gaze control, since the cMRF also innervates the cervical spinal cord (Robinson et al. 1994; Scudder et al. 1996) and brainstem head movement centres (May et al. 2005; Perkins et al. 2005). Furthermore, the cMRF, like the superior colliculus, receives direct projections from the cortical frontal eye fields (FEFs) (Huerta et al. 1986; Stanton et al. 1988) and the supplementary eye fields (SEFs) (Huerta & Kaas, 1990; Shook et al. 1990) (Fig. 1). This makes the cMRF also a potential relay to transmit information related to saccade metrics from the FEFs and SEFs to horizontal extraocular motoneurones.
Finally, the cMRF may even play a role in maintaining eye position, gaze holding and fixation. Monosynaptic projections from the caudal (horizontal) cMRF to lateral rectus and medial rectus motoneurones support such an idea of participation in stabilizing eye position and fixation, particularly as such projections also target the so-called ‘slow’ lateral rectus motoneurones (Ugolini et al. 2001, and submitted), and bilateral stimulation of the cMRF promotes fixation (Cohen et al. 1985). The bilateral monosynaptic projections from the cMRF to lateral rectus motoneurones are also a potential substrate for the transmission to these motoneurones of vergence signals (Mays & Gamlin, 1995).
To summarize, the cMRF is embedded in an interconnected and complex network of eye movement circuits for the production of saccades, fixation, gaze holding and vergence (Fig. 1). It receives direct input from the cortical eye movement centres, the FEFs and SEFs, and it projects directly to horizontal, i.e. lateral and medial rectus, motoneurones. There it contacts both ‘fast’ and ‘slow’ motoneurones. Besides ocular motoneurones, the cMRF also contacts the horizontal saccade generator network (EBNs, IBNs, OPNs) and head movement related brainstem centres, and maintains reciprocal connections with the superior colliculus.
References
Cohen B & Büttner-Ennever JA (1984). Exp Brain Res 57, 167–176.[Medline]
Cohen B, Matsuo V, Fradin J & Raphan T (1985). Exp Brain Res 57, 605–616.[Medline]
Cromer JA & Waitzman DM (2006). J Physiol 570, 507–523.
Cohen B, Waitzman DM, Büttner-Ennever JA & Matsuo V (1986). Prog Brain Res 64, 243–256.[Medline]
Graf W, Dubayle D, Klam F, Biarnais T, Büttner-Ennever J & Ugolini G (2000). Soc Neurosci Abs 26, 363.5.
Graf W, Gerrits N, Yatim-Dhiba N & Ugolini G (2002). Euro J Neurosci 15, 1557–1562.[CrossRef][Medline]
Huerta MF & Kaas JH (1990). J Comp Neurol 293, 299–330.[CrossRef]
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May PJ, Perkins E, Zhou L & Warren S (2005). Soc Neurosci Abs 31, 858.4.
Mays LE & Gamlin PD (1995). Curr Opin Neurobiol 5, 763–768.[CrossRef][Medline]
Perkins E, May PJ & Warren S (2005). Soc Neurosci Abs 31, 858.3.
Robinson FR, Phillips JO & Fuchs AF (1994). J Comp Neurol 346, 43–62.[CrossRef][Medline]
Scudder CA, Moschovakis AK, Karabelas AB & Highstein SM (1996). J Neurophysiol 76, 332–352.
Shook BL, Schlag-Rey M & Schlag J (1990). J Comp Neurol 301, 618–642.[CrossRef][Medline]
Stanton GB, Goldberg ME & Bruce CJ (1988). J Comp Neurol 271, 473–492.[CrossRef][Medline]
Ugolini G, Büttner-Ennever J, Doldán M, Dubayle D, Klam F & Graf W (2001). Soc Neurosci Abs 27, 403.13.
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