Background: The ciliary muscle in man serves two different functions, namely accommodation and regulation of aqueous outflow. It is still not known whether both functions are combined or whether they can also be fulfilled independently from each other. (The latter could provide the possibility of an isolated pharmacological influence on the outflow-related function which should certainly be of use e.g. in glaucoma treatment).
Material and methods: To investigate the presence of functionally different muscle portions within the ciliary muscle and its relation to accommodation we have studied the ciliary muscle of various species showing no (rat, rabbit), moderate (cattle) and good (cat, tupaia glis) accommodative activities. For that purpose enzyme histochemical methods were used which are normally applied for differentiation of skeletal muscle fibers into fast phasic type II-fibres and slow tonic type I-fibres. Additionally, electron microscopical studies were undertaken to evaluate the ultrastructure of the muscle cells.
Results: It was found that only those species showing accommodation, were characterized by slight (cattle) or pronounced (cat, tupaia glis) differences in histochemical staining and ultrastructure of muscle cells. Characteristically, the longitudinal portion showing structural affinity to the aqueous outflow system, was different from the inner reticular and circular portions showing more relation to accommodative functions.
Conclusions: These differences might indicate that two different functional systems within the ciliary muscle do exist which have been developed during evolution of higher accommodative mechanisms in the eye.