The accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object , comprising coordinated changes in vergence, lens shape and pupil size . It is dependent on cranial nerve II , superior centres and cranial nerve III.The change in the shape of the lens is controlled by the cilliary muscles inside the eye. Changes in contraction of the ciliary muscles alter the focal distance of the eye, causing nearer or farther images to come into focus on the retina; this process is known as accommodation. The reflex, controlled by the parasympathetic nervous system, involves three responses; pupil accommodation, lens accommodation, and convergence. A near object (for example, a computer screen) appears large in the field of vision, and the eye receives light from wide angles. When moving focus from a distant to a near object, the eyes converge. The ciliary muscle contracts making the lens more convex, shortening its focal length. The pupil constricts in order to prevent diverging light rays from hitting the periphery of the retina and resulting in a blurred image. Pathway Information from the light on each retina is taken to the occipital lobe via the optic nerve and optic radiation, where it is interpreted as vision. The peristriate area 19 interprets accommodation, and sends signals via the Edinger-Westphal nucleus and the 3rd cranial nerve to the ciliary muscle, the medial rectus muscle and (via parasympathetic fibres) the sphincter pupillae muscle. Pupil and lens accommodation During accommodation, the pupil increases the depth of focus of the eye by blocking the light scattered by the periphery of the cornea. The lens then increases its curvature thus increases refractive power. The ciliary muscles are responsible for the lens accommodation response Convergence Convergence is the ability of the eye to simultaneously demonstrate inward movement of both eyes toward each other. This is helpful in effort to make focus on near objects clearer. Three reactions occur simultaneously; the eyes adduct, the ciliary muscles contract, and the pupils become smaller. This action involves the contraction of the medial rectus muscles of the two eyes and relaxation of the lateral rectus muscles. The medial rectus attaches to the medial aspect of the eye and its contraction adducts the eye. The medial rectus is innervated by motor neurons in the oculomotor nucleus and nerve. Focus on near objects The refractive index of the eye's lens system allows the eye to produce sharply focused images. For example, geometrical optics show that as a distant object is brought closer to the eye, the focus of the object becomes blurrier in the plane behind the retina; however, as a result of the increase in the refractive power of the eye, this image becomes clear. The refractive power mainly resides in the cornea, but the overall refractive power is achieved by the actual lens changing its shape. In order to fixate on a near object, the ciliary muscle contracts around the lens to decrease its size. The suspensory ligaments relax and the radial tension around the lens is released. This causes the lens to form a more spherical shape achieving a higher level of refractive power. Focus on distant objects When the eye focuses on distant objects, the lens holds itself in a flattened shape by on the suspensory ligaments(zonal fibers). Ligaments pull the edges of the elastic lens capsule towards the surrounding ciliary body and by opposing the internal pressure within the elastic lens, keep it relatively flattened. Opposite of fixating on a near object, the ciliary muscle relaxes and the diameter of around the lens increases to increase the lens' size. The tension along the suspensory ligaments is increased to flatten the lens and decrease the curvature and achieve a lower refractive power. Neural circuit Three regions make up the accommodation neural circuit, the afferent limb, the efferent limb and the ocular motor neurons that are between the afferent and efferent limb. 1. The afferent limb of the circuit This limb contains the main structures; the retina that contains the retinal ganglion axons in the optic nerve, chiasm and tract, the lateral geniculate body, and the visual cortex. 2.The efferent limb of the circuit This limb includes Edinger-Westphal nucleus and the oculomotor neurons. The main function of the Edinger-Westphal nucleus is to send axons in the oculomotor nerve to control the ciliary ganglion which in turn, sends its axons in the short ciliary nerve to control the iris and the ciliary muscle of the eye. The oculomotor neurons functions to send its axons in the oculomotor nerve, to control the medial rectus, and converge the two eyes. Attribution: Article text available under CC-BY-SA sourced from: http://en.wikipedia.org/wiki/Accommodation_reflexPublic domain image sourced from http://wikipedia.org/wiki/File:Focus_in_an_eye.svg

• published: 18 Sep 2013
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