Sphincter pupillae

 Sphincter pupillae is a circular muscle, about 1 millimeter wide. It is located in the pupillary zone of stromal layer of the iris, attaching to and encircling the pupillary margin of iris. The muscle itself consists of six to eight circles of smooth muscle fibers, between of which are found the nerves and blood vessels that supply each fiber. 

Development : ectoderm 

Innervation

The sphincter pupillae receives parasympathetic innervation from the postganglionic fibers of ciliary ganglion, called short ciliary nerves, which are the branches of oculomotor nerve (CN III). Note that these particular nerve fibers originate from the accessory nucleus of oculomotor nerve (Edinger-Westphal) - a general visceral efferent nucleus - which provides autonomic (involuntary) control to this muscle.

Function

When contracting, the sphincter pupillae constricts the pupil, which is called miosis. This action happens during accommodation and pupillary light reflexes.

 

Accommodation

Accommodation is a reflexive event that adapts the eyes for observing close objects after being focused on distant ones. The reflex pathway includes optic nerve (CN II), visual and frontal cortex, oculomotor and accessory oculomotor nuclei and oculomotor nerve (CN III).

 

Once the eyes need to focus on a near object, the optic nerve sends signals to the visual cortex, which then stimulates the eye field within frontal cortex. The frontal neurons then send fibers to the ipsilateral oculomotor nucleus (unicortical innervation), and to both ipsi- and contralateral accessory oculomotor nuclei (bicortical innervation). The oculomotor nerve sends these signals to the ciliary ganglion, which then outsources the final stimuli via its two sets of branches;

 

Short ciliary nerves that cause the contraction of sphincter pupillae and miosis

Long ciliary nerves which stimulate the medial recti and ciliary muscles; the former causes the convergence of the eyeballs, while the latter thickens the lens increasing its refractive power

Pupillary light reflex

The pupillary light reflex happens when the eyes are exposed to bright light and the amount of light that falls onto the retina needs to be decreased in order to maintain clear vision. The reflex arc includes optic nerve (CN II), pretectal nucleus of midbrain, accessory oculomotor nucleus and oculomotor nerve (CN III).

 

Once the retinal photoreceptors detect light, the optic nerve sends a neural impulse to the pretectal nucleus. The pretectal nucleus stimulates the accessory oculomotor nucleus, which in turn innervates and constricts the sphincter pupillae via the short ciliary nerves. As pretectal nucleus connects to both accessory oculomotor nuclei, stimulation and miosis of one eye (direct pupillary light reflex) will always be followed by miosis on non-stimulated eye (indirect pupillary light reflex).