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Parasympathetic Effect on the Iris of the Eye

The iris, the colored part of the eye, controls the size of the pupil, the hole in the center of the iris, through actions of the autonomic nervous system. The parasympathetic nervous system causes pupillary constriction, or miosis, and also controls accommodation.

  1. Mechanism

    • The autonomic nervous system includes the sympathetic and PNS nerves.The PNS controls involuntary smooth muscle action of circular muscle in the eye, including the papillary sphincter muscle and the ciliary body, which are attached to the iris.

    Constriction

    • The papillary sphincter muscle constricts the pupil when signaled by the PNS that less light is needed.

    Accommodation

    • The ciliary body controls accommodation under the control of the PNS by contracting the iris and changing of the shape of the lens in the eye to focus on close-up objects, according to Dartmouth Medical Center.

    Anticholinergic Effects

    • Certain medications called anticholenergics can change the pupillary PNS response by blocking transmission of acetylcholine to the brain; this can paralyze the ciliary body and papillary sphincter and prevent them from constricting the pupil.

    PNS Abnormalities

    • The PNS controls the cranial III, or oculomotor nerve. Damage to this nerve can cause permanent papillary dilation.

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