Accommodation reflex

Accommodation reflex is a reflex action of the eye, occurring in response to focusing on a near object, then looking at a distant object (and vice versa), comprising coordinated changes in vergence, crystaline lens shape (accommodation) and pupil size.

Mechanism

The accommodation reflex is triggered when looking at a near object. The ciliary muscle contracts, causing the lens to become more convex, increasing its refractive power. This allows the eye to focus on the near object. The pupil also constricts to increase the depth of focus of the eye by blocking the light scattered by the periphery of the cornea.

When looking at a distant object, the ciliary muscle relaxes, allowing the lens to become less convex, decreasing its refractive power. The pupil dilates to allow more light into the eye.

Neural control

The accommodation reflex is controlled by the parasympathetic nervous system via the oculomotor nerve. The Edinger-Westphal nucleus in the midbrain is responsible for controlling the contraction and relaxation of the ciliary muscle and the sphincter muscle of the pupil.

Clinical significance

Abnormalities in the accommodation reflex can indicate a variety of conditions. For example, Adie syndrome is characterized by a pupil that is slow to constrict in response to light, but has a normal or near-normal response to accommodation. Argyll Robertson pupils constrict with accommodation but do not react to light, indicating potential neurosyphilis.

See also

• Near response
• Pupillary light reflex
• Refraction (optics)