Neuroanatomical identification of mesencephalic premotor neurons coordinating eyelid with upgaze in the monkey and man

Except during blinks, movements of the upper eyelid are tightly coupled to vertical eye movements. The premotor source for the coordination of lid and eye movements is unknown. The present paper provides the anatomical identi fication of a new premotor cell group in the rostral mesencephalon of the m onkey and human, which lies in close proximity to the premotor center for v ertical saccades and is thought to participate in lid-eye coordination. Aft er injections of a retrograde transsynaptic tracer (tetanus toxin fragment C or BIIb) into the levator palpebrae (LP), the superior rectus (SR), or th e inferior oblique (TO) muscle of macaque monkeys, a small circumscribed gr oup of premotor neurons was labeled in the central gray of the rostral mese ncephalon, but not after superior oblique or inferior rectus muscle injecti ons. This group lies immediately rostral to the interstitial nucleus of Caj al and medial to the rostral interstitial nucleus of the medial longitudina l fasciculus, each of which contain premotor neurons for vertical saccades, and was termed the M-group. Injections of tritiated leucine into the M-gro up led to efferent labeling primarily over LP motoneurons. In addition, lab el was present over the SR- and IO-motoneuron subgroups in the oculomotor n ucleus and frontalis muscle motoneurons in the facial nucleus. This project ion pattern of the M-group suggests a role in the coordination of the upper eyelid and eyes during upgaze. Double-labeling experiments in macaque monk eys revealed that the M-group is strongly parvalbumin immunoreactive and co ntains high levels of cytochrome oxidase activity. With these two histochem ical markers, the homologue of the M-group was identified in the human brai n as well. J. Comp. Neurol. 420:19-34, 2000. (C) 2000 Wiley-Liss, Inc.