Athena Vale
The pupil is a highly controlled part of the eye, with abnormalities often indicating underlying diagnoses. Pupil dilation is controlled by the dilator pupillae, a group of muscles in the peripheral third of the iris. The iris dilator muscle is a smooth muscle that runs radially in the iris. It is innervated by the sympathetic nervous system and is considered a major regulator of pupil dilation. The iris sphincter muscle, on the other hand, is controlled by the parasympathetic nervous system and acts as a constrictor of the pupil.
| Characteristics | Values |
|---|---|
| Name | Iris dilator muscle, pupil dilator muscle, pupillary dilator, radial muscle of iris, radiating fibers, dilator pupillae muscle |
| Type of muscle | Smooth muscle |
| Location | Iris |
| Innervation | Sympathetic nervous system |
| Function | Pupil dilation, allowing more light to enter the eye |
| Controlled by | Superior cervical sympathetic ganglion |
| Stimulated by | Fight-or-flight response |
| Neurotransmitter | Noradrenaline |
| Receptors | α1-receptors |
| Pathway | Begins at the cortex, first synapse at the cilliospinal center, neurons travel through the brain stem, exit through the cervical sympathetic chain and superior cervical ganglion |
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What You'll Learn
The dilator pupillae muscle
When stimulated, the cells contract, widening the pupil and allowing more light to enter the eye. This process is known as mydriasis. The dilator pupillae muscle is innervated by the sympathetic system, which acts by releasing noradrenaline, which acts on α1-receptors. This stimulation causes the muscle to contract and dilate the pupil, increasing the amount of light that reaches the retina.
The iris also contains a sphincter muscle controlled by parasympathetic fibers. This muscle encircles the pupil and functions as a constrictor of the pupil, causing miosis or pupillary constriction. The balance between the stimulation of the dilator pupillae muscle and the sphincter muscle controls the size of the pupil in response to changes in light exposure and various brain states.
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The iris sphincter muscle
The muscle fibres of the iris sphincter originate from the Edinger-Westphal nucleus, travel with the oculomotor nerve (cranial nerve III) to the ciliary ganglion, and finally reach the iris sphincter muscle. The parasympathetic nervous system stimulates the iris sphincter muscle to contract, causing the pupil to constrict. This process is particularly important in bright light to protect the retina from excessive light exposure.
The neuromodulatory systems, particularly the locus coeruleus (LC)-noradrenergic system and the cholinergic neuromodulatory system, play a crucial role in pupil dilation and constriction. By interacting with the sympathetic and parasympathetic nervous systems, these systems help regulate the diameter of the pupil in response to changes in brain states and lighting conditions.
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The sympathetic nervous system
The iris dilator muscle, also known as the dilator pupillae, is a smooth muscle of the eye that runs radially in the iris. It is composed of a spokelike arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system.
In the context of pupil dilation, the sympathetic nervous system exerts its influence through the iris dilator muscle. When an individual encounters a threatening stimulus, the sympathetic nervous system is activated, releasing noradrenaline, which acts on α1-receptors. This stimulation causes the myoepithelial cells of the iris dilator muscle to contract, resulting in the widening of the pupil, or mydriasis. This dilation allows more light to enter the eye, enhancing the individual's visual perception of their surroundings.
The sympathetic innervation of the iris dilator muscle follows a specific pathway. It begins at the cortex, with the first synapse occurring at the cilliospinal center, also known as Budge's center. Post-synaptic neurons then travel through the brain stem and exit through the cervical sympathetic chain, ultimately synapsing at the superior cervical ganglion. From there, third-order neurons continue their journey, passing through the carotid plexus and entering the orbit through the first division of the trigeminal nerve.
While the sympathetic nervous system plays a key role in pupil dilation, it's important to note that the parasympathetic nervous system also contributes to the regulation of pupil size. The parasympathetic system controls the iris sphincter muscle, which encircles the pupil and functions as a constrictor, causing miosis or pupillary constriction in response to bright light. This dynamic balance between the sympathetic and parasympathetic systems allows for precise control over pupil size, ensuring optimal visual adaptation to varying light conditions.
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The parasympathetic nervous system
The pupil is under tight neurological control, and abnormalities in this control can indicate underlying diagnoses. Pupil dilation and constriction are the result of a balance between the sympathetic and parasympathetic nervous systems.
The iris sphincter muscle can be blocked by muscarinic antagonists, such as atropine, which is isolated from the plant Atropa belladonna. Blocking the iris sphincter muscle causes dilation of the pupil, which is utilized clinically to better visualize the posterior portions of the eye and treat conditions like anterior uveitis.
The parasympathetic pathway also plays a role in rapid pupil dilation, as evidenced by pharmacological manipulation of the pupil dilator and constrictor muscles separately. While the pupil dilator muscle is considered the primary regulator of pupil dilation, it is not involved in rapid dilation. Instead, the pupil constrictor muscle, controlled by the parasympathetic pathway, is the major controller of rapid pupil dilation upon brain arousal.
In summary, the parasympathetic nervous system, through its control of the iris sphincter muscle and its role in rapid pupil dilation, plays a crucial role in regulating pupil size and responding to changes in brain states and lighting conditions.
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The neuromodulator system
The iris sphincter muscle, controlled by the parasympathetic pathway, is rapidly modulated by the neuromodulator system and is key to quick pupil dilation. When this muscle is blocked with drugs like tropicamide, pupil dilation is delayed and reduced. This muscle is also influenced by the Edinger-Westphal nucleus (EWN) and the parasympathetic nervous system.
On the other hand, the neuromodulator system sends signals to the iris dilator muscle with a slower latency, contributing to sustained pupil dilation. This muscle is innervated by the sympathetic nervous system and is considered a major regulator of pupil dilation. It is stimulated by the sympathetic nervous system, which releases noradrenaline, acting on α1-receptors. This muscle is also influenced by the superior cervical sympathetic ganglion.
The locus coeruleus (LC) noradrenaline neurons are believed to be the core region inducing pupil dilation. Neural activation in the LC precedes pupil dilation, and stimulation at this site induces dilation. The noradrenergic projections to the cortex also increase with pupil dilation, as seen in mice. Similarly, neural activity in the LC correlates with pupil diameter in humans.
Additionally, the cholinergic neuromodulatory system has been linked to pupil dilation in mice, and pupil dilation is associated with cortical arousal mediated by the release of noradrenaline and acetylcholine.
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Frequently asked questions
The iris dilator muscle, also known as the dilator pupillae, controls pupil dilation. It is a smooth muscle of the eye, running radially in the iris.
The iris dilator muscle is stimulated by the sympathetic nervous system. When stimulated, the muscle contracts, widening the pupil and allowing more light to enter the eye.
The opposite action of the iris dilator muscle is performed by the iris sphincter muscle, which is controlled by the parasympathetic nervous system. This muscle constricts the pupil in response to bright light.
