Animals are highly visual beings and the eyes are needed to maintain focus of an object at both close and far distances. This action of changing optical power over various distances is called accommodation. Accommodation uses both the optic nerve (cranial nerve II) and the oculomotor nerve (cranial nerve III) to maintain the focus of the image on the fovea, which is the part of the retina that has the greatest resolution. Accommodation can be controlled (conscious action), but is more often a reflex (unconscious action). Thus it is called the accommodation reflex. Many animals, particularly mammals, birds, and reptiles, will change the optic power of their eye by altering the shape of the elastic lens. This is accomplished by using the ciliary body within the eye. The ciliary body is a tissue within the eye and near the lens. It is made up of the ciliary muscle and ciliary processes. As humans age, the elasticity of the lens decreases, which in turn makes the lens rigid. This means adults who are generally older than 50 years of age will have more difficulty in accommodating. Thus, most older adults will need to wear glasses to adjust their near vision for reading.
Anatomy and Physiology
The accommodation reflex allows the eye to change its focus from a near object to one that is far away and vice versa. This reflex is extremely fast and most people do not even realize that it is occurring. The speed of accommodation, however, slows down as a person ages because the lens becomes less flexible. The accommodation reflex changes the shape of the lens and pupil size during the action. The ciliary muscles alter the lens shape by changing the amount of muscle contraction, which in turn keeps the focus of the object on the retina. The ciliary muscles can make the lens flatten out for distance focus and then change to a very convex shape for close focus. Since it is a reflex, it is controlled by the autonomic nervous system, particularly the parasympathetic division. The reflex has three parts: pupil accommodation, lens accommodation, and convergence.
Pupil accommodation occurs with the changes in the amount of light entering the eye. For a single point of light in the distance, the pupil must dilate (enlarge) so that the greatest amount of light can enter, which produces a clear image on the retina (the photosensitive lining of the eye). At the same time, the ciliary muscle must relax, allowing the lens to have a long focal length. This process is the lens accommodation. However, when a single point of light enters the eye up close, the result is constriction of the pupil (gets smaller). This will divert light rays from entering the peripheral portions of the retina, allowing the image to be in focus. At the same time the ciliary muscle contracts so that the lens will have a shorter focal length. The optic nerve (the output of the retina) then takes this light and image information to the brain, specifically to the occipital lobe (the most posterior portion of the cerebrum). This is where vision is interpreted as well as the action of accommodation. The signal is then sent to the Edinger-Westphal nucleus in the midbrain and to the oculomotor nerve. The Edinger-Westphal nucleus is the accessory parasympathetic nucleus of cranial nerve III. Activation of the oculomotor nerve contracts and relaxes the ciliary muscle as well as the medial rectus and sphincter pupillae muscles of the eye.
Convergence is the process of both eyes moving to the center at the same time. This process is also called adduction. The left eye moves medially to the right and the right eye moves medially to the left. This helps in focusing objects just in front of the face, such as a computer monitor. Thus, for up-close vision, the eyes will adduct, the ciliary muscles will contract, increasing the curvature of the lens, and the pupils will constrict. For a video showing accommodation, please use this link: https://www.youtube.com/watch?v=p_xLO7yxgOk
Jennifer L. Hellier
See also: Cranial Nerves; Hubel, David H.; Nerves; Optic Nerve; Visual Motor System; Visual System
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Schachar, Ronald A., Barbara K. Pierscionek, Ali Abolmaali, & Tri Le. (2007). The relationship between accommodative amplitude and the ratio of central lens thickness to its equatorial diameter in vertebrate eyes. British Journal of Ophthalmology, 91(6), 812—817.