How is the iris sphincter muscle innervated?

The iris sphincter muscle is primarily innervated by the parasympathetic nervous system. This is crucial for regulating the size of the pupil in response to light and accommodation. When light hits the retina, the parasympathetic fibers, which originate from the Edinger-Westphal nucleus, are activated and travel via the oculomotor nerve (cranial nerve III) to the iris sphincter muscle. This stimulation causes the muscle to contract, resulting in miosis, or pupil constriction, thus allowing for better visual acuity in bright light conditions.

In contrast to the iris sphincter, the dilator muscle of the iris is innervated by sympathetic fibers, which facilitate pupil dilation in low light conditions. Therefore, while both the sympathetic and parasympathetic systems play roles in controlling pupil size, the specific innervation of the iris sphincter is exclusively parasympathetic.