Maximus Gage
The human eye is a complex organ that relies on a variety of muscles and nerves to function properly. One of the key muscle groups involved in eye function is the intrinsic muscles of the eye, which include the circular and radial muscles of the iris, also known as the sphincter pupillae and dilator pupillae, respectively. These muscles control the amount of light that enters the eye and the depth of field. The sphincter pupillae is innervated by the parasympathetic nervous system, while the dilator pupillae is innervated by the sympathetic nervous system. Additionally, the ciliary muscles of the eye, which control the shape of the lens and, therefore, the focus of the eye, are innervated by both parasympathetic and sympathetic fibers. The sympathetic nervous system also plays a role in tear production and overall eye health.
| Characteristics | Values |
|---|---|
| Controlled by | Sympathetic nervous system (SNS) |
| Part of | Autonomic nervous system (ANS) |
| Function | Controls pupil diameter, ocular accommodation, ocular blood flow, and intra-ocular pressure (IOP) |
| Innervation | Superior cervical ganglion |
| Muscles controlled | Sphincter pupillae, ciliary, and dilator pupillae |
| Effect on pupil | Pupil dilation (mydriasis) |
| Effect on ciliary muscle | Relaxation for far vision |
| Effect on iris | Contraction of radial muscle, allowing more light to enter |
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What You'll Learn
Sympathetic innervation of the iris
The vertebrate eye receives innervation from both the sympathetic and parasympathetic nervous systems. The sympathetic innervation of the iris is involved in controlling pupil size and ocular accommodation, or the process by which the lens "bulges" to increase its refractive power. The sympathetic nervous system also plays a role in tear secretion and ocular blood flow.
The sympathetic nervous system acts in conjunction with the parasympathetic nervous system to regulate pupil size. While the sympathetic system controls pupil dilation, the parasympathetic system controls pupil constriction. This antagonistic action between the two systems is often used as a didactic paradigm to demonstrate their opposing roles in autonomic regulation.
The sympathetic innervation of the iris is supplied by postganglionic fibres from the superior cervical ganglion. These fibres project to the dilator pupillae muscle of the iris, causing pupil dilation. In contrast, the parasympathetic system supplies the sphincter pupillae muscle of the iris, leading to pupil constriction.
The sympathetic nervous system also contributes to ocular accommodation, which is the process of adjusting the eye's focus. It acts over a longer period of time (10-40 seconds) compared to the parasympathetic system, which has a rapid onset (one second) and generates a greater positive accommodation.
Additionally, the sympathetic nervous system influences tear secretion through two mechanisms: alteration of blood flow and increased secretion of sympathetic neurotransmitters. However, the parasympathetic nervous system plays a more significant role in tear production, as evidenced by the decreased tear secretion observed in individuals with parasympathetic nerve lesions.
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Sympathetic activation and pupil dilation
The Ocular Autonomic Nervous System (OANS) is responsible for the physiological functions of the eye, including pupil diameter, ocular accommodation, and circulation within the eyes. The pupil is controlled by the autonomic nervous system, which has sympathetic and parasympathetic branches. The sympathetic nervous system is responsible for pupil dilation, while the parasympathetic nervous system is responsible for pupil constriction.
The basic autonomic mechanism controlling the pupil is as follows: the parasympathetic nervous system activates the circular sphincter pupillae muscle, causing the pupil to constrict, while the sympathetic nervous system activates the radial dilator pupillae muscle, resulting in pupil dilation. The sympathetic preganglionic neurons involved in pupil dilation are located in the "ciliospinal center" of the cervico-thoracic spinal cord (segments C8-T2). These neurons project to the superior cervical ganglia, which innervate the dilator pupillae muscle.
Pupil dilation is influenced by various factors, including luminance and arousal. In low light conditions, the pupil dilates to optimize retinal illumination and enhance visual perception. On the other hand, in bright light, the pupil constricts in a response known as the pupillary light response. Changes in arousal levels also impact pupil diameter, with sympathetic activation leading to dilation during heightened sensory responsiveness and perception.
Additionally, the sympathetic nervous system affects tear secretion by altering blood flow and increasing the secretion of sympathetic neurotransmitters. However, the parasympathetic nervous system primarily controls tear secretion, and lesions in the parasympathetic nerves can lead to decreased tear production.
Understanding the autonomic functions of the eye and the interplay between the sympathetic and parasympathetic nervous systems is crucial for managing various physiological and pathological conditions, such as Parkinson's disease, and exploring the potential for restoring function after injury.
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Sympathetic nervous system and tear production
The Ocular Autonomic Nervous System (OANS) is responsible for the physiological functions of the eye, including the regulation of intraocular pressure (IOP), pupil dimensions, lens accommodation, and circulation within the eyes. The sympathetic and parasympathetic nervous systems play opposite but complementary roles in the OANS.
The sympathetic nervous system is part of the body's "fight-or-flight" response to stress, increasing the heart rate and putting the body on alert. It influences tear production in two ways: altering blood flow and increasing the secretion of sympathetic neurotransmitters. However, the exact role of sympathetic nerves in the lacrimal gland is uncertain, and tear secretion is primarily controlled by the parasympathetic nervous system.
The parasympathetic nervous system, often referred to as "rest-and-digest," manages the body's calm and relaxed processes, such as digestion and heart rate regulation. It is connected to the lacrimal glands, and activation of parasympathetic receptors by the neurotransmitter acetylcholine leads to tear production.
The sympathetic and parasympathetic systems work together to maintain balance in the body. For example, in the context of tear production, the sympathetic nervous system may initially stimulate tear secretion through its actions on blood flow and neurotransmitter release. However, the parasympathetic nervous system then takes over to produce the tears through the activation of the lacrimal glands.
Additionally, the sympathetic and parasympathetic systems influence ocular accommodation, which is the adjustment of the lens to focus on objects at different distances. The sympathetic system acts over a longer period (10-40 seconds) to cause hypermetropia, while the parasympathetic system acts rapidly (within one second) to generate positive accommodation.
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Sympathetic innervation of the ciliary muscle
The ciliary muscle is an intrinsic muscle of the eye that controls the accommodation for viewing objects at varying distances. It is formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). The ciliary muscle is supplied with blood by the branches of the ophthalmic artery.
The ciliary muscle is innervated by the autonomic nervous system (ANS), which includes the sympathetic and parasympathetic nervous systems. The sympathetic nervous system's role in the ciliary muscle's innervation has been a topic of investigation for many researchers over the last 150 years. While it is well-established that autonomic control of ocular accommodation is predominantly parasympathetic, there is evidence to suggest that sympathetic innervation also plays a role. This is supported by the presence of sympathetic nerve terminals in the dilator pupillae muscle, which receives input from the ciliary ganglion.
The ciliary muscle's function is to change the shape of the lens within the eye, which is crucial for the accommodation reflex. When the ciliary muscle contracts, it loosens the zonular fibers, increasing the convexity of the lens and allowing for near vision. This contraction also regulates the pore size of the trabecular meshwork, which is important for the drainage of aqueous humor into the canal of Schlemm and the regulation of intraocular pressure.
The sympathetic innervation of the ciliary muscle has been found to act on two subclasses of postsynaptic receptors: the inhibitory α1 and β2 adrenoceptors. The activation of these receptors by noradrenaline influences the accommodative response, refractive error, and mental effort. Additionally, the sympathetic innervation provides inhibitory impulses, inhibiting the accommodation reflex and influencing ocular accommodation during sustained near-vision tasks.
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Sympathetic ganglia and Horner's syndrome
The human eye is innervated by the sympathetic, parasympathetic, and trigeminal sensory nerve fibers. The sympathetic nervous system is a division of the autonomic (or involuntary) nervous system. The autonomic nervous system (ANS) regulates the function of the majority of the intraocular muscles in the eyes. The sympathetic nerves also affect tear secretion by altering blood flow and increasing the secretion of sympathetic neurotransmitters.
Horner's syndrome is a rare condition that occurs when there is a complete sympathetic denervation of an upper limb, reaching the stellate ganglia. It is primarily an acquired condition secondary to systemic/local diseases or iatrogenic causes. However, in rare cases, it can be congenital and purely hereditary. Horner's syndrome may result from several nervous conditions: a lesion of the primary neurone, a brainstem stroke, or trauma to the brachial plexus. It is characterized by partial ptosis (drooping or falling of the upper eyelid), miosis (constricted pupil), and facial anhidrosis (loss of sweating) due to a disruption in the sympathetic nerve supply. In children, Horner's syndrome sometimes leads to heterochromia, a difference in eye color between the two eyes, due to a lack of sympathetic stimulation interfering with melanin production.
The superior tarsal muscle, which helps raise the upper eyelid, has a sympathetic nerve supply. Denervation of this muscle causes ptosis, milder than oculomotor (CN III) palsy, which supplies the levator palpebrae superioris. Horner's syndrome can be identified by medical imaging and response to particular eye drops.
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Frequently asked questions
The sympathetic muscle in the eye is the dilator pupillae muscle, also known as the radial muscle of the iris.
The sympathetic muscle in the eye controls how much light enters by dilating the pupil. This is known as mydriasis. It also relaxes the ciliary muscle, allowing for improved far vision.
The sympathetic muscle in the eye is controlled by the sympathetic nervous system (SNS), one of the two divisions of the autonomic nervous system (ANS). The SNS and ANS work unconsciously to regulate the body.
