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It also changes the shape of the lens within the eye but not the size of the pupil [5] which is carried out by the sphincter pupillae muscle and dilator pupillae. The ciliary muscle, pupillary sphincter muscle and pupillary dilator muscle sometimes are called intrinsic ocular muscles [6] or intraocular muscles. [7]
The ciliary muscle controls accommodation by altering the shape of the lens to be able to see an object from near to far. [2] The pupillary sphincter muscle and pupillary dilator muscle control the iris to adjust the size of the pupil to adjust how much light enters into the eye.
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. [5]
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.
The actions of the six muscles responsible for eye movement depend on the position of the eye at the time of muscle contraction. [2] The ciliary muscle, pupillary sphincter muscle and pupillary dilator muscle sometimes are called intrinsic ocular muscles [3] or intraocular muscles. [4]
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. [1] The reflex, controlled by the parasympathetic nervous system, involves three responses: pupil constriction, lens accommodation, and convergence.
It explains the defining features of a tonic pupil: (1) The pupil does not react to light. The original light-reaction neurons have been destroyed. (2) Tonic constriction with attempted near vision. Aberrant regeneration of nerve fibers intended for the ciliary muscle causes abnormal, tonic contraction of the pupil with accommodation.
The size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' dilator and sphincter muscles. Light energy enters the eye through the cornea, through the pupil and then through the lens. The lens shape is changed for near focus (accommodation) and is controlled by the ciliary muscle.