Accommodation reflex
Encyclopedia
The accommodation reflex is a reflex action
of the eye
, in response to focusing
on a near object, then looking at distant object (and vice versa), comprising coordinated changes in vergence
, lens
shape and pupil
size. It is dependent on cranial nerve II
(afferent limb of reflex), superior centres and cranial nerve III
.
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.
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..
Reflex action
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
, in response to focusing
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
on a near object, then looking at distant object (and vice versa), comprising coordinated changes in vergence
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 projection of the image is in the centre of the retina in both...
, lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...
shape and pupil
Pupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...
size. It is dependent on cranial nerve II
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...
(afferent limb of reflex), superior centres and cranial nerve III
Oculomotor nerve
The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...
.
Field of view
The field of view is the extent of the observable world that is seen at any given moment....
, and the eye receives light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
from wide angles. When moving focus from a distant to a near object, the eyes converge. The ciliary muscle
Ciliary muscle
The ciliary muscle is a ring of striated smooth muscle in the eye's middle layer that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It changes the shape of the lens within the eye not the size of the pupil which is...
contracts making the lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...
more convex, shortening its focal length. The pupil
Pupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...
constricts in order to prevent diverging light rays from hitting the periphery of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
and resulting in a blurred image.
Pathway
Information from the light on each retina is taken to the occipital lobeOccipital lobe
The occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...
via the optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...
and optic radiation
Optic radiation
The optic radiation is a collection of axons from relay neurons in the lateral geniculate nucleus of the thalamus carrying visual information to the visual cortex along the calcarine fissure.There is one such tract on each side of the brain.-Parts:A distinctive...
, where it is interpreted as vision. The peristriate area 19 interprets accommodation, and sends signals via the Edinger-Westphal nucleus
Edinger-Westphal nucleus
The Edinger-Westphal nucleus is the accessory parasympathetic cranial nerve nucleus of the oculomotor nerve , supplying the constricting muscles of the iris...
and the 3rd cranial nerve
Oculomotor nerve
The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...
to the ciliary muscle
Ciliary muscle
The ciliary muscle is a ring of striated smooth muscle in the eye's middle layer that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It changes the shape of the lens within the eye not the size of the pupil which is...
, the medial rectus muscle and (via parasympathetic fibres) the sphincter pupillae muscle..
External links
- Accommodation at Georgia State UniversityGeorgia State UniversityGeorgia State University is a research university in downtown Atlanta, Georgia, USA. Founded in 1913, it serves about 30,000 students and is one of the University System of Georgia's four research universities...