Adrenergic excitatory and cholinergic inhibitory innervations in the human iris dilator

To investigate the pattern of innervation in the human iris dilator muscle, effects of field stimulation on the isometric tension of dilator muscle were investigated in vitro. Throughout the experiments, spontaneous contractions did not occur. Application of repetitive field stimulations evoked biphasic mechanical responses (i.e. an initial phasic contraction followed by long lasting relaxation) of the isolated human iris dilator. These mechanical responses were abolished by 10(-7) M tetrodotoxin (a nerve poison), thereby indicating that the mechanical responses were neurogenic. Furthermore, the initial phasic contractions and following muscle relaxation were selectively blocked by phentolamine (10(-5) M) and atropine (10(-5) M), respectively. The degree of the muscle relaxation to that of contraction was much larger than that assumed by other investigators who used tissues from different species. The present results indicate that the human iris dilator muscle is innervated by adrenergic excitatory and cholinergic inhibitory nerves, and that cholinergic inhibitory innervations in this muscle may support the cholinergic miosis in the iris sphincter muscle.