Logan Steele
The pupil is the dark aperture in the center of the iris, the colored part of the eye. The iris contains two muscles: the sphincter pupillae and the dilator pupillae. The sphincter pupillae muscle, also known as the pupillary sphincter or iris sphincter muscle, is responsible for constricting the pupil. It is located in the central part of the iris and encircles the pupil. The constriction of the pupil occurs when the circular muscle, controlled by the parasympathetic nervous system, contracts, and also when the radial muscle relaxes.
| Characteristics | Values |
|---|---|
| Muscle Name | Sphincter pupillae, iris sphincter muscle, pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibres |
| Muscle Location | Central part of the iris, in the coloured part of the eye |
| Muscle Diameter | 0.75 mm wide by 0.15 mm thick |
| Muscle Function | Constricts the pupil in bright light or during accommodation |
| Muscle Control | Parasympathetic nerve fibres, preganglionic axons from the Edinger-Westphal nucleus, cholinergic axons, sympathetic innervation from the superior cervical ganglion |
| Muscle Response | Pupillary light reflex, miosis (pupil constriction) |
| Muscle Activation | Release of acetylcholine, activation of phospholipase C, production of inositol triphosphate, elevated intracellular Ca2+ levels |
| Related Disorders | Adie syndrome, iridoplegia, Argyll Robertson pupil, Horner syndrome |
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What You'll Learn
The iris sphincter muscle
The constriction of the pupil allows the iris to control the amount of light that reaches the back of the eye at the retina. A constricted pupil admits less light and also blocks light that has entered from the outer edges of the cornea, increasing the "depth of field". This is similar to the "aperture stop" in camera terminology.
In lower animals, the muscle cells of the iris sphincter muscle are photosensitive, causing iris action without brain input. In humans, some individuals have the rare ability to exert direct and voluntary control over their iris sphincter muscles, allowing them to dilate and constrict their pupils on command, regardless of lighting conditions.
Disorders such as Adie syndrome, also known as Holmes-Adie syndrome, are characterised by a lack of response to light and a pupil that is tonically dilated. This results from damage to postganglionic fibres of the parasympathetic innervation to the eye, leading to an inability to constrict the iris sphincter muscle. Iridoplegia, or the paralysis of the sphincter muscle of the iris, can also result from direct trauma to the orbit, inflammation, or the use of eyedrops.
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Pupillary response
The sphincter pupillae is a circular muscle controlled by the parasympathetic nervous system (PSNS). When this muscle contracts, the pupil constricts, reducing the amount of light entering the eye. This process is known as miosis and is important in photography to increase the "depth of field". Additionally, a constricted pupil can block light that has entered from the outer edges of the cornea, improving image quality by reducing spherical and chromatic aberrations.
The dilator pupillae, on the other hand, is located in the outer parts of the iris and is controlled by sympathetic fibres. When these muscles are activated, the pupil dilates, allowing more light to enter the eye. This response is known as mydriasis and is often observed in low light conditions or when an individual is interested in or excited by something.
The pupillary response can be influenced by various factors, including exposure to light, emotional states, and certain substances. For example, in low light conditions, the pupils naturally dilate to let more light into the eye. Additionally, stimulant drugs such as MDMA, cocaine, and amphetamines can cause pupil dilation, while opiates and opioids can lead to constriction.
The latency of pupillary response, or the time it takes for the response to occur, increases with age. Intensive studies of pupillary response are often conducted in ophthalmology using videopupillometry. Some individuals possess the rare ability to exert voluntary control over their iris sphincter and dilator muscles, allowing them to dilate and constrict their pupils on command regardless of lighting conditions.
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Parasympathetic nerve fibres
The parasympathetic nervous system (PSNS) is a network of nerves that relaxes your body after periods of stress or danger. It is part of the autonomic nervous system and is responsible for many automatic functions, including control of heart rate, blood pressure, digestion, urination, and sweating. The parasympathetic nervous system is associated with the "rest and digest" response, helping to run life-sustaining processes when the body feels safe and relaxed.
The parasympathetic nerve fibres that are involved in pupil constriction are the postganglionic fibres. These fibres leave the ciliary ganglion, a structure approximately 3mm in size, located 2-3mm posterior to the globe and lateral to the optic nerve. They travel into the orbit via the short ciliary nerves, a branch of the oculomotor nerve, and innervate the sphincter pupillae muscle in the iris. The sphincter pupillae muscle is one of two muscles in the iris, the other being the dilator pupillae muscle, which dilates the pupil.
The ciliary ganglion receives input from preganglionic neurons in the Edinger-Westphal nucleus, which lies in the rostral mesencephalon. Axons from these neurons travel in the oculomotor nerve (cranial nerve III) as preganglionic parasympathetic fibres to the ciliary ganglion, where they contact postganglionic cells. The ciliary ganglion is also innervated by parasympathetic fibres from the pterygopalatine ganglion, the largest of the four parasympathetic ganglia.
The parasympathetic nerve fibres play a crucial role in pupil constriction by innervating the sphincter pupillae muscle. This muscle contracts to narrow the pupil, reducing the amount of light entering the eye. The balance between the activation of the sphincter pupillae and dilator pupillae muscles dictates pupil diameter.
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The dilator pupillae muscle
The function of the dilator pupillae muscle is important for maintaining the balance between the activation of the dilator and sphincter pupillae muscles, which dictates pupil diameter. The constriction of the pupil, or miosis, occurs when the circular muscle, controlled by the parasympathetic nervous system (PNS), contracts, and also when the radial muscle relaxes.
While pupillary response is typically an involuntary reflex, some humans possess the rare ability to exert direct and voluntary control over their iris sphincter and dilator muscles, allowing them to dilate and constrict their pupils on command, regardless of lighting conditions.
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The sympathetic nervous system
The SNS has a thoracolumbar outflow, with sympathetic nerves arising from the intermediolateral nucleus of the lateral grey column near the middle of the spinal cord. Specifically, the SNS neurons arise from the thoracic and lumbar regions of the spinal cord (T1 to L2 or L3), with the cell bodies distributed in four regions of the grey matter in the spinal cord bilaterally and symmetrically. The SNS is composed of many pathways that perform a variety of functions across various organ systems.
Fibers from the SNS innervate tissues in almost every organ system, providing at least some regulation of functions such as pupil diameter, gut motility, and urinary system output and function. In the eye, sympathetic activation causes the radial muscle of the iris to contract, leading to mydriasis (pupil dilation) and allowing more light to enter. This occurs in conjunction with the relaxation of the ciliary muscle, which improves far vision. The SNS also affects the heart, where it increases the heart rate, force of contraction, and rate of conduction, resulting in increased cardiac output to supply the body with oxygenated blood.
The SNS is involved in the body's response to stress, also known as the sympatho-adrenal response. In response to stress, the SNS mediates the release of adrenaline (epinephrine) and, to a lesser extent, noradrenaline (norepinephrine) from the adrenal medulla. This response primarily affects the cardiovascular system and can lead to increased blood pressure and heart rate. Techniques to reduce SNS activity include exercise training, acupuncture, and dietary changes such as reducing caffeine intake.
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Frequently asked questions
The muscle that constricts the pupil is called the sphincter pupillae muscle, or the iris sphincter muscle.
The sphincter pupillae muscle is located in the central part of the iris, which is the coloured part of the eye.
The sphincter pupillae muscle controls the amount of light that reaches the back of the eye by constricting the pupil in bright light.
The sphincter pupillae muscle is controlled by the parasympathetic nervous system (PSNS). When the circular muscle contracts, and the radial muscle relaxes, the pupil constricts.
