Iris is a part of the eyeball that serves the regulation of the amount of light that reaches the retina, as it features an opening called the pupil through which the light can pass through. The diameter of the pupil is dynamic and it is regulated by a pair of antagonistic muscles found in the iris; the sphincter pupillae and the dilator pupillae.
Both of these muscles are smooth muscles, functioning beyond the control of our will. Their function is to change the diameter of the pupil during two reflexive events;
- Pupillary light reflex
- Accommodation reflex
In these reflexes, sphincter pupillae constricts the pupil (miosis), while dilator pupillae dilates it (midriasis).
|Origin and insertion||Pupillary margin of iris|
|Action||Constriction of pupil (miosis)|
|Innervation||Parasympathetic fibers of oculomotor nerve (CN III) via short ciliary nerves|
|Blood supply||Long posterior ciliary arteries, anterior ciliary arteries (via minor arterial circle)|
This article will discuss the anatomy and explain the function of the sphincter pupillae muscle.
Origin and insertion
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.
Being located in the stromal layer of iris, sphincter pupillae is cushioned in the connective tissue of the stroma and stromal neurovasculature, fibroblasts and melanocytes. The outer margin of sphincter pupillae is related to the myoepithelial cells of dilator pupillae muscle.
As it is found in the most anterior part of the iris, this muscle is covered anteriorly by a mash of fibroblasts and melanocytes tied together by numerous collagen fibers. Posteriorly to the muscle is the anterior epithelial layer of iris.
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.
To completely understand the logic behind the types of fibers within cranial nerves, take a look at our articles, video tutorials, quizzes, and labelled diagrams.
The arterial sources for the sphincter pupillae are the anterior and long posterior ciliary arteries, both being the branches of ophthalmic artery. These arteries anastomose at the level of ciliary body and reach the sphincter pupillae via two successive arterial circles;
- The major arterial circle found in the stroma of ciliary body
- The minor arterial circle located in the stroma of iris
The veins follow the arteries and converge to a minor venous circle in the iris. This circle empties into the vorticose veins, that belong to the system of ophthalmic veins.
When contracting, the sphincter pupillae constricts the pupil, which is called miosis. This action happens during accommodation and pupillary light reflexes.
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).