Cell groups of the medial longitudinal fasciculus and paramedian tracts.

The aim of this article is to introduce the reader to a continuum of cell clusters which may play an important role in the maintenance of eye position. They lie interspersed between the fascicles of the medial longitudinal fasciculus (MLF) and paramedian tracts in the caudal pons and medulla, and they also constitute the rostral part of the classical abducens nucleus. Previous workers showed that these 'cell groups of the paramedian tracts' (pmt cell groups) project to the flocculus, and receive afferents from several horizontal premotor cell groups. Results of neuroanatomical tracer experiments reported here demonstrate that they also receive a direct input from the vertical premotor gaze neurons in the mesencephalon (rostral iMLF and the interstitial nucleus of Cajal), as well as from some groups of oculomotor internuclear neurons. The projecting fibres descend to the cell groups of the paramedian tracts in the MLF. It is suggested that deficits in gaze-holding seen in internuclear ophthalmoplegia, for example, may result from damage to afferents of this paramedian cell continuum.