NITRERGIC NERVE-CELLS IN THE PRIMATE CILIARY MUSCLE ARE ONLY PRESENT IN SPECIES WITH A FOVEA CENTRALIS

Nerve cells positive for NADPH diaphorase (D)/nitric oxide synthase in the human ciliary muscle appear to be involved in relaxation of the m uscle during disaccommodation. To study whether similar cells might me diate disaccommodation of the primate ciliary muscle in general, seria l sections of the ciliary muscle of 5 cynomolgus monkeys (Macaca fasci cularis) and 2 owl monkeys (Aotes trivirgatus) were stained for NADPH- D. Both monkey species have a ciliary muscle system and an accommodati ve amplitude comparable to that in humans. Positively stained cells we re frequently observed in the ciliary muscle of all cynomolgus monkeys , a diurnal species with a fovea, but never in owl monkeys, a nocturna l species without a fovea. The results indicate that NADPH-D-positive and probably nitrergic ganglion cells in the ciliary muscle are not pr esent in all primate species, but only in those with high requirements for visual acuity. They might smoothen the ciliary-ganglion-mediated contraction of the ciliary muscle or contribute to the small fluctuati ons or oscillations of accommodation that are observed under steady vi ewing conditions.