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The pupil constricts when the iris sphincter muscle is stimulated and contracts In humans, it functions to constrict the pupil in bright light ( pupillary light reflex ) or during accommodation . [ citation needed ] In lower animals, the muscle cells themselves are photosensitive causing iris action without brain input.
By analogy with a camera, the pupil is equivalent to aperture, whereas the iris is equivalent to the diaphragm. It may be helpful to consider the Pupillary reflex as an 'Iris' reflex, as the iris sphincter and dilator muscles are what can be seen responding to ambient light. [2] Whereas, the pupil is the passive opening formed by the active iris.
During the accommodation reflex, the pupil constricts to increase the depth of focus of the eye by blocking the light scattered by the periphery of the cornea. The lens then increases its curvature to become more biconvex, thus increasing refractive power. The ciliary muscles are responsible for the lens accommodation response. [4]
The pupillary dilator muscle increases the pupillary diameter and it is arranged radially, but the pupillary sphincter muscle is responsible for the constriction of the pupil's diameter and it encircles the pupil. [2] The pupillary dilation is also called mydriasis, and the constriction of the pupil is also called miosis. [1]
Although the pupillary response, in which the pupil dilates or constricts due to light is not usually called a "reflex", it is still usually considered a part of this topic. Adjustment to close-range vision is known as "the near response", while relaxation of the ciliary muscle to view distant objects is known as the "far response".
The iris consists of two layers: the front pigmented fibrovascular layer known as a stroma and, behind the stroma, pigmented epithelial cells.. The stroma is connected to a sphincter muscle (sphincter pupillae), which contracts the pupil in a circular motion, and a set of dilator muscles (dilator pupillae), which pull the iris radially to enlarge the pupil, pulling it in folds.
Dilation and constriction of the pupil. Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve.. A constriction response (), [1] is the narrowing of the pupil, which may be caused by scleral buckles or drugs such as opiates/opioids or anti-hypertension medications.
The iris dilator muscle (pupil dilator muscle, pupillary dilator, radial muscle of iris, radiating fibers), is a smooth muscle [2] of the eye, running radially in the iris and therefore fit as a dilator. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called myoepithelial cells.