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The muscle that dilates the pupil is called the dilator pupillae muscle, also known as the iris dilator muscle. It is located in the outer parts of the iris and consists of a spoke-like arrangement of modified contractile cells called myoepithelial cells. Pupillary dilation is primarily controlled by the sympathetic nervous system, which releases noradrenaline to stimulate the muscle and cause it to contract and dilate the pupil. This response allows more light to enter the eye, especially in low-light conditions. The dilation of pupils can also be caused by certain substances such as adrenaline, stimulant drugs, and hallucinogenics.
| Characteristics | Values |
|---|---|
| Muscle Name | Dilator pupillae, iris dilator muscle, pupillary dilator, radial muscle of iris, radiating fibers |
| Muscle Location | Outer parts of the iris |
| Muscle Function | Dilates the pupil |
| Muscle Innervation | Sympathetic nervous system, postganglionic sympathetic nerves |
| Muscle Cells | Smooth muscle, myoepithelial cells |
| Muscle Response | Pupillary dilation, mydriasis |
| Muscle Control | Voluntary and involuntary |
| Muscle Disruption | Horner syndrome, anisocoria |
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What You'll Learn
The dilator pupillae muscle
The dilation of the pupil can be influenced by various factors, including changes in lighting conditions, emotional states, and certain substances. For example, in low light conditions, the dilator pupillae muscle contracts to let more light into the eye. Additionally, physical or emotional stress that triggers the autonomic sympathetic nervous system can lead to pupillary dilation.
Disruptions along the three-neuron pupillary dilation pathway can result in impaired pupillary dilation or a condition known as Horner syndrome. This syndrome is characterised by a triad of symptoms, including an abnormally small pupil that may be unable to dilate due to the disruption of the cervical sympathetic nerves.
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Pupillary dilation pathway
The pupillary dilation pathway is a sympathetically driven response that involves the relaxation of the iris sphincter and the contraction of the iris dilator, or the dilator pupillae muscle. This muscle is located in the outer parts of the iris and is composed of modified contractile cells called myoepithelial cells. When stimulated, these cells contract, causing the pupil to dilate and allow more light to enter the eye.
The pupillary dilation pathway begins in the hypothalamus and ends with the contraction of the dilator pupillae muscle. The hypothalamus is part of the autonomic pathways regulating the pupil, which are organized hierarchically, from the spinal cord to the cerebral cortex. The first-order neuron of the pupillary dilation pathway originates in the hypothalamus and descends through the midbrain to synapse onto the spinal cord's ciliospinal center of Budge, found between C8 to T2.
The second-order neuron, the preganglionic sympathetic neuron, exits the spinal cord through the ventral roots and ascends through the thorax, near the lung apex and subclavian vessels, onto the superior cervical ganglion. The third-order postganglionic neurons then travel along the periarterial carotid plexus through the cavernous sinus and enter the orbit through the short and long ciliary nerves. These axons synapse on the dilator pupillae muscle, causing pupillary dilation.
Disruptions along the three-neuron pathway can result in a loss of pupillary dilation and the development of Horner syndrome, a condition characterized by ptosis, miosis, and anhydrosis of the face due to a loss of sympathetic innervation.
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Cocaine's impact on dilation
Cocaine use leads to dilated pupils, a condition commonly known as "cocaine eyes" or "cocaine pupils". The dilation occurs because cocaine stimulates the sympathetic nervous system, which is responsible for the "'fight or flight' response. This stimulation causes pupil dilation to improve vision and focus. However, the dilated pupils become sensitive to light, causing painful light sensitivity and requiring the use of sunglasses.
In addition to pupil dilation, cocaine use can cause bloodshot or red eyes due to the expansion of blood vessels in the eye. The eyes may also appear watery. These effects can last from a few hours to several days after cocaine use, depending on factors such as the amount used, the presence of contaminants, and the user's overall health.
Cocaine has various short-term and long-term effects on the eyes and overall eye health. Short-term effects include pupil dilation, bloodshot eyes, and increased eye pressure. Long-term effects can include serious damage to eye health, such as:
- Glaucoma: Cocaine increases eye fluid pressure, which is a risk factor for glaucoma, a leading cause of preventable blindness.
- Keratitis: Smoking cocaine can cause irritation and dryness in the eyes, increasing the risk of keratitis, a serious condition that can lead to blurred vision, scarring, and ulcers.
- Retinal vascular occlusive disease: Prolonged cocaine use can lead to blood clots that block blood flow from the retina, causing vision loss.
- Optic neuropathy: Cocaine use can reduce blood flow to the eyes, leading to ischemic optic neuropathy and potentially permanent blurred vision.
It is important to note that while cocaine eyes can be an indication of cocaine use, there are other factors that can cause similar eye symptoms, such as lighting changes, other stimulants, medications, allergies, or underlying health conditions. Therefore, it is not a reliable indicator of cocaine use on its own.
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Mydriasis (dilation response)
Mydriasis is the dilation of the pupil, which is usually a non-physiological response, although it can sometimes be a physiological pupillary response. Non-physiological causes of mydriasis include disease, trauma, or the use of certain types of drugs. It may also occur without a clear reason. Physiological causes include light changes and emotional variables.
The dilation of the pupil is caused by two antagonistic muscles: the dilator pupillae, which is located in the outer parts of the iris and dilates the pupil, and the sphincter pupillae, located in the central parts, which constricts it. The dilator pupillae muscle is also referred to as the iris dilator muscle, the pupil dilator muscle, the radial muscle of the iris, and the pupillary dilator. The sphincter pupillae is also referred to as the iris sphincter muscle.
The iris dilator muscle is innervated by the sympathetic nervous system, which acts by releasing noradrenaline, causing the muscle to contract and the pupil to dilate. This allows more light to enter the eye. The sphincter pupillae muscle is controlled by the parasympathetic nervous system.
Mydriasis can be induced by blocking acetylcholine receptors, which reduces the pupillary muscles' ability to constrict and causes dilation. This is critical in eye surgery procedures such as cataract surgery, where uninterrupted access to the inner eye is required. Mydriasis can also be induced via modulation of adrenergic or cholinergic signalling.
Mydriasis can affect one pupil or both at the same time. Anisocoria is the term for when mydriasis affects only one eye, causing the pupils to be different sizes. This condition affects about 20% of the population and is usually considered non-threatening.
Mydriasis can be a symptom of an injury to the eyes or head, a problem within the brain, or a serious condition such as a tumour or stroke. If mydriasis occurs after an injury or is accompanied by symptoms such as headaches, dizziness, or confusion, medical attention should be sought immediately.
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Anisocoria (unequal pupil dilation)
Anisocoria is a term derived from two Greek words: "aniso-" meaning unequal, and "kore" meaning pupil. The suffix "ia" comes from the Latin word for an abnormal condition. Thus, anisocoria refers to unequal pupil sizes. It is sometimes a sign of a life-threatening underlying condition.
The pupil dilator muscle, also known as the dilator pupillae muscle, is responsible for dilating the pupil. It is located in the outer parts of the iris and consists of a spoke-like arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system, which acts by releasing noradrenaline, causing the muscle to contract and the pupil to dilate, allowing more light to enter the eye.
Anisocoria can be physiological or pathological. Physiological anisocoria is caused by something malfunctioning inside the body, whereas pathological anisocoria is caused by a health condition. It can occur congenitally, but it usually happens due to a health condition or another issue affecting the eyes. Anisocoria can also be a side effect of certain medications, although this is less common. Tonic pupil, or Adie pupil, is a well-known cause of anisocoria, where the pupil is large and demonstrates a poor constriction response to light.
Anisocoria that is more pronounced in the dark suggests a lesion in the sympathetic pathway, resulting in a smaller pupil that is unable to dilate in response to the removal of a light stimulus. This can be caused by Horner syndrome, iritis, mechanical anisocoria, or pharmacologic anisocoria from miotics, narcotics, or insecticides. Horner syndrome is characterised by a triad of ptosis, miosis, and anhidrosis, and can be confirmed by administering 4%-10% cocaine to both eyes, which will result in the dilation of the normal pupil but no movement of the Horner pupil.
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Frequently asked questions
The dilator pupillae muscle, also known as the iris dilator muscle, is responsible for dilating the pupil.
The dilator pupillae muscle is located in the outer parts of the iris and is responsible for widening the pupil, allowing more light to enter the eye.
The dilator pupillae muscle is controlled by the sympathetic nervous system. When stimulated, the muscle contracts, leading to pupil dilation.
Pupil dilation, or mydriasis, can be caused by various factors, including adrenaline, anticholinergic agents, stimulant drugs (such as MDMA, cocaine, and amphetamines), and certain hallucinogenic drugs (such as LSD). Emotional or physical stress can also trigger the sympathetic nervous system, leading to pupil dilation.
Yes, one condition associated with pupil dilation is Horner syndrome, which can result from disruptions along the oculosympathetic pathway or damage to the superior cervical ganglion during cervical surgery. It is characterized by a triad of symptoms, including ptosis (drooping eyelid), miosis (constricted pupil), and anhydrosis (decreased sweating) on the affected side of the face.
