Accommodation reflex

Accommodation Reflex

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

Mechanism

The accommodation reflex involves three main processes:

• Convergence: This is the inward movement of both eyes toward each other to maintain single binocular vision when viewing an object.
• Lens accommodation: The ciliary muscle contracts, causing the lens to become more convex, increasing its refractive power to focus on near objects.
• Pupil constriction: The pupil constricts to increase the depth of focus by reducing the size of the pupil opening, which limits the amount of light entering the eye and reduces spherical aberration.

Neural Pathway

The accommodation reflex is controlled by the parasympathetic nervous system. The oculomotor nerve (cranial nerve III) is primarily responsible for the reflex. The process begins when the retina detects a change in focus, sending signals to the visual cortex in the occipital lobe. From there, signals are sent to the Edinger-Westphal nucleus, which then sends parasympathetic fibers to the ciliary ganglion. Postganglionic fibers innervate the ciliary muscle and the sphincter pupillae muscle, facilitating lens accommodation and pupil constriction.

Clinical Significance

The accommodation reflex is an important diagnostic tool in neurology and ophthalmology. It is tested to assess the integrity of the oculomotor nerve and the midbrain. Abnormalities in the reflex can indicate neurological disorders such as Adie syndrome, Argyll Robertson pupil, or Horner's syndrome.

Related Pages

• Pupillary light reflex
• Ciliary muscle
• Oculomotor nerve
• Visual cortex

Gallery

• 

  Diagram illustrating the accommodation reflex.

• 

  Human eye showing pupil constriction in bright light.