The accommodation reflex (or ''accommodation-convergence reflex'') is a

reflex action In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

of the

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

, in response to focusing on a near object, then looking at a distant object (and vice versa), comprising coordinated changes in

vergence A vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision. When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the proj ...

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

shape ( accommodation) and

pupil The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black becau ...

size. It is dependent on

cranial nerve II In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived from ...

(

afferent limb Afferent nerve fibers are axons (nerve fibers) of sensory neurons that carry sensory information from sensory receptors to the central nervous system. Many afferent projections ''arrive'' at a particular brain region. In the peripheral nervou ...

of reflex), superior centers (

interneuron Interneurons (also called internuncial neurons, association neurons, connector neurons, or intermediate neurons) are neurons that are not specifically motor neurons or sensory neurons. Interneurons are the central nodes of neural circuits, enab ...

) and cranial nerve III (

efferent limb Efferent nerve fibers are axons (nerve fibers) of efferent neurons that exit a particular region. These terms have a slightly different meaning in the context of the peripheral nervous system (PNS) and central nervous system (CNS). The efferen ...

of reflex). The change in the shape of the lens is controlled by ciliary 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 The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The autonomic nervous system is responsible for regulat ...

, involves three responses: pupil constriction, lens accommodation, and convergence. A near object (for example, a computer screen) subtends a large area in the

visual field The visual field is "that portion of space in which objects are visible at the same moment during steady fixation of the gaze in one direction"; in ophthalmology and neurology the emphasis is mostly on the structure inside the visual field and it i ...

, i.e. the eyes receive

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

from wide angles. When moving focus from a distant to a near object, the eyes converge. The

ciliary muscle The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscleSchachar, Ronald A. (2012). "Anatomy and Physiology." (Chapter 4) . in the eye's middle layer, the uvea ( vascular layer). It controls accommodation for vie ...

constricts making the

thicker, shortening its focal length. The

constricts in order to prevent strongly diverging light rays hitting the periphery of the

cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...

and the lens from entering the eye and creating a blurred image.

Pathway

Information from the light on each retina is taken to the

occipital lobe The occipital lobe is one of the four Lobes of the brain, major lobes of the cerebral cortex in the brain of mammals. The name derives from its position at the back of the head, from the Latin , 'behind', and , 'head'. The occipital lobe is the ...

via the

optic nerve In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual system, visual information from the retina to the brain. In humans, the optic nerve i ...

and

optic radiation In neuroanatomy, the optic radiation (also known as the geniculocalcarine tract, the geniculostriate pathway, and posterior thalamic radiation) are axons from the neurons in the lateral geniculate nucleus to the primary visual cortex. The optic ...

(after a synapse in the lateral geniculate body of the posterior thalamus), 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

, the medial rectus muscle and (via parasympathetic fibres) the sphincter pupillae muscle.

Pupil constriction and lens accommodation

During the accommodation reflex, the

constricts to increase the depth of focus of the eye by blocking the light scattered by the periphery of the

. The lens then increases its curvature to become more biconvex, thus increasing refractive power. The ciliary muscles are responsible for the lens accommodation response.

Convergence

Convergence Convergence may refer to: Arts and media Literature *''Convergence'' (book series), edited by Ruth Nanda Anshen *Convergence (comics), "Convergence" (comics), two separate story lines published by DC Comics: **A four-part crossover storyline that ...

is the ability of the eye to simultaneously demonstrate inward rotation 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 In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

of the eye's cornea-lens system allows the eye to produce sharply focused images on the retina. The refractive power resides mainly in the

, but the finer changes in refractive power of the eye are achieved by the lens changing its shape. As a distant object is brought closer to the eye, the image moves behind the retina, producing blurring at the retina. This blurring is minimized by squeezing the lens to a more spherical shape, which again moves the image back to the plane of the retina. In order to fixate on a near object, the ciliary muscle contracts around the lens to decrease its diameter and increase its thickness. The suspensory zonules of Zinn relax and the radial tension around the lens is released. This causes the lens to form a more spherical shape achieving greater refractive power.

Focus on distant objects

When the eye focuses on distant objects, the lens holds itself in a flattened shape due to traction from the suspensory ligaments. Ligaments pull the edges of the elastic

lens capsule The lens capsule is a component of the Globe (human eye), globe of the Human eye, eye. It is a clear elastic basement membrane similar in composition to other basement membranes in the body. The capsule is a very thick basement membrane and the t ...

towards the surrounding

ciliary body The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the ciliary ...

and by opposing the internal pressure within the elastic lens, keep it relatively flattened. When viewing a distant object, the

relaxes, the diameter of the lens increases and its thickness decreases. 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

, the

and the ocular motor neurons that are between the afferent and efferent limb. ; The afferent limb of the circuit : This limb contains the main structures; the retina that contains the retinal ganglion axons in the

, chiasm and tract, the lateral geniculate body, and the

visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalam ...

. ; 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 The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements o ...

to control the

ciliary ganglion The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. It is 1–2 mm in diameter and in humans contains approximately 2,500 neurons. The ganglion contains postganglionic parasympathetic neurons. T ...

which in turn, sends its

axons An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see spelling differences) is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action pot ...

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 The medial rectus muscle is a muscle in the orbit near the eye. It is one of the extraocular muscles. It originates from the common tendinous ring, and inserts into the anteromedial surface of the eye. It is supplied by the inferior division of th ...

, and converge the two eyes. ; Ocular motor control neurons : Neurons that are interposed between the afferent and efferent limbs of this circuit and include the

visual association cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus ...

, which determines the image is "out-of-focus, and sends corrective signals via the

internal capsule The internal capsule is a paired white matter structure, as a two-way nerve tract, tract, carrying afferent nerve fiber, ascending and efferent nerve fiber, descending axon, fibers, to and from the cerebral cortex. The internal capsule is situate ...

and

crus cerebri The cerebral crus (crus cerebri. ''crus'' means ‘leg’ in Latin.) is the anterior portion of the cerebral peduncle which contains the motor tracts, traveling from the cerebral cortex to the pons and spine. The plural of which is cerebral crura ...

to the supraoculomotor nuclei. It also includes the supraoculomotor nuclei (located immediately superior to the oculomotor nuclei) that generates motor control signals that initiate the accommodation response and sends these control signals bilaterally to the oculomotor complex.

References

External links

Accommodation
at

Georgia State University Georgia State University (Georgia State, State, or GSU) is a Public university, public research university in Atlanta, Georgia, United States. Founded in 1913, it is one of the University System of Georgia's four research universities. It is al ...

* {{DEFAULTSORT:Accommodation Reflex Oculomotor nerve Eye Ophthalmology Reflexes