Autonomic Nervous System and the Eye

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Questions and Answers

Which of the following best describes the physiological role of the iris muscles in the eye?

  • They regulate the amount of light entering the eye by adjusting the pupillary aperture. (correct)
  • They facilitate the transmission of visual signals from the retina to the brain.
  • They produce aqueous humor, which is crucial for intraocular pressure.
  • They control accommodation of the lens by adjusting its curvature.

A drug that acts as a muscarinic agonist would directly cause which of the following?

  • Constriction of the pupil due to circular muscle relaxation.
  • Constriction of the pupil due to circular muscle contraction. (correct)
  • Dilation of the pupil due to radial muscle contraction.
  • Dilation of the pupil due to radial muscle relaxation.

What distinguishes multi-unit smooth muscle from single-unit smooth muscle in terms of their function in the eye?

  • Multi-unit smooth muscles have fewer gap junctions, allowing independent action of fibres; single-unit muscle contracts as a cohesive unit. (correct)
  • Multi-unit smooth muscles are poorly innervated, leading to slow contractions; single-unit muscles have rich innervation.
  • Multi-unit muscles are responsible for controlling intraocular pressure; single-unit muscles alter pupil diameter.
  • Multi-unit smooth muscles have more gap junctions, leading to synchronized contractions; single-unit muscles function independently.

Administration of a muscarinic antagonist would result in which of the following ocular effects?

<p>Increased pupillary dilation due to decreased circular muscle activity. (A)</p> Signup and view all the answers

A drug that stimulates the muscarinic receptors in the eye would be classified as a:

<p>Parasympathomimetic. (B)</p> Signup and view all the answers

An acetylcholinesterase inhibitor would have what effect on pupil size?

<p>Pupillary constriction due to increased acetylcholine levels at the receptor. (B)</p> Signup and view all the answers

Which of the following accurately describes the role of the ciliary body's smooth muscle in vision?

<p>It adjusts the lens's curvature to focus objects at different distances. (D)</p> Signup and view all the answers

Stimulation of α1-adrenoceptors in the iris would lead to which effect on the pupil?

<p>Pupillary dilation due to the contraction of radial muscles. (B)</p> Signup and view all the answers

If a drug is described as a sympatholytic, what effect would it most likely have on the eye?

<p>Decreased pupil dilation (miosis). (D)</p> Signup and view all the answers

Which of the following statements accurately describes accommodation in the eye?

<p>It's a reflex involving the ciliary muscle adjustment of lens for focusing at different distances. (B)</p> Signup and view all the answers

If a patient uses a combination of muscarinic antagonist and an α1-adrenoceptor agonist, what would be the combined effect on pupil size?

<p>Marked pupillary dilation due to combined blockade of parasympathetic and stimulation of sympathetic. (B)</p> Signup and view all the answers

Cocaine, known to inhibit noradrenaline re-uptake, would lead to what effect on pupillary size?

<p>Pupillary dilation as noradrenaline levels rise in the iris. (B)</p> Signup and view all the answers

A parasympatholytic drug would likely have which effect on the pupil?

<p>Mydriasis, causing pupil dilation. (B)</p> Signup and view all the answers

What effect would excessive stimulation of β2-adrenoceptors by a drug have on lens shape?

<p>The lens would flatten for far vision. (C)</p> Signup and view all the answers

Which statement correctly compares the roles of the sympathetic and parasympathetic nervous systems in regulating pupil size?

<p>Sympathetic stimulation predominantly causes pupillary dilation, while parasympathetic stimulation results in constriction. (B)</p> Signup and view all the answers

Which receptor subtype directly stimulates the production of aqueous fluid in the ciliary body?

<p>β2-adrenoceptors (A)</p> Signup and view all the answers

A drug which is a muscarinic antagonist would have what primary effect on focusing ability?

<p>Impaired ability to focus on near objects, enhanced ability to focus on far objects. (B)</p> Signup and view all the answers

An adrenergic agonist is likely, due to its action on the autonomic nervous system, to have which effect on the eye?

<p>Increased pupil size (mydriasis). (C)</p> Signup and view all the answers

If a drug blocks muscarinic receptors, what effect would it not result in, in the eye?

<p>Pupillary constriction. (B)</p> Signup and view all the answers

What is the main function of the aqueous fluid within the eye?

<p>To provide nutrients to non-vascular structures like the cornea and lens. (C)</p> Signup and view all the answers

A drug that increases the drainage of aqueous fluid by causing contraction of the iris smooth muscle is likely to be an agonist of which receptor type?

<p>M3 muscarinic receptors (A)</p> Signup and view all the answers

Which structure within the eye would directly contract in response to increased acetylcholine release from parasympathetic nerves?

<p>Ciliary muscles and pupillary constrictor muscles. (A)</p> Signup and view all the answers

Which of the following best describes a drug that acts as a sympathomimetic on the ciliary body?

<p>Stimulates aqueous humor production through activation of β2-adrenoceptors. (B)</p> Signup and view all the answers

A drug that acts as a sympatholytic at the trabecular meshwork might have what effect if administered systemically?

<p>Increase intraocular pressure by reducing aqueous fluid outflow. (A)</p> Signup and view all the answers

What is the expected effect of an agonist drug acting on M3 receptors within the iris?

<p>Pupillary constriction and decreased aqueous outflow. (B)</p> Signup and view all the answers

If a drug has high affinity for β2-adrenoceptors but does not induce any change in aqueous fluid production, it would be best described as:

<p>A β2-adrenoceptor antagonist (A)</p> Signup and view all the answers

A parasympatholytic drug is MOST likely to cause what effect on intraocular pressure if applied locally to the eye?

<p>Increased intraocular pressure due to reduced outflow through the trabecular meshwork. (D)</p> Signup and view all the answers

What would be the expected ocular effect of a muscarinic receptor antagonist?

<p>May cause an increase in intraocular pressure by opposing iris muscle constriction (C)</p> Signup and view all the answers

What is the primary mechanism of a drug that acts as a parasympathomimetic in decreasing intraocular pressure?

<p>Increased aqueous outflow through the trabecular meshwork with iris constriction (B)</p> Signup and view all the answers

What is the expected effect of a β2-adrenoceptor antagonist, applied topically to the eye?

<p>Decreased aqueous fluid production and decreased intraocular pressure. (D)</p> Signup and view all the answers

Flashcards

Autonomic Nervous System

The nervous system that controls involuntary functions like heart rate, digestion, and breathing.

Sympathetic Nervous System

The branch of the autonomic nervous system that prepares the body for 'fight or flight' responses.

Parasympathetic Nervous System

The branch of the autonomic nervous system that controls 'rest and digest' functions.

Sympathomimetic

A drug that stimulates the sympathetic nervous system.

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Sympatholytic

A drug that blocks the sympathetic nervous system.

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Parasympathomimetic

A drug that stimulates the parasympathetic nervous system.

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Parasympatholytic

A drug that blocks the parasympathetic nervous system.

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Sphincter Pupillae

The smooth muscle of the iris that controls pupil constriction.

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Dilator Pupillae

The smooth muscle of the iris that controls pupil dilation.

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Accommodation Reflex

The process by which the lens changes shape to focus on objects at different distances.

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Aqueous humor

The fluid that fills the space between the cornea and the lens of the eye. It is produced by the ciliary body and drains through a meshwork of channels.

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Trabecular meshwork

A network of small channels in the eye that helps drain aqueous humor.

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Canal of Schlemm

A canal located at the edge of the cornea that collects aqueous humor before it drains into the venous system.

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Intraocular pressure

The pressure inside the eye, caused by the presence of aqueous humor.

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Glaucoma

A condition where the intraocular pressure is abnormally high, potentially damaging the optic nerve.

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Chronic open angle glaucoma

The most common type of glaucoma, where the trabecular meshwork becomes less efficient with age, leading to increased pressure.

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Acute narrow angle glaucoma

A sudden increase in intraocular pressure caused by a blockage of the trabecular meshwork by the iris.

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Iris

The part of the eye that controls pupil size, and can contribute to narrowing the angle in acute glaucoma.

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Pupillary Constrictor Muscle

The smooth muscle of the iris that constricts the pupil, reducing the size of the pupil (miosis) and regulating light entry.

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Pupillary Dilator Muscle

The smooth muscle of the iris that enlarges the pupil, increasing the size of the pupil (mydriasis) and admitting more light.

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Muscarinic Agonist

A type of drug that activates muscarinic receptors, mimicking the effects of acetylcholine.

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Muscarinic Antagonist

A type of drug that blocks muscarinic receptors, preventing acetylcholine from binding and causing its usual effects.

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Acetylcholinesterase

An enzyme that breaks down acetylcholine, rapidly reducing its effects.

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Acetylcholinesterase Inhibitor

A type of drug that inhibits the breakdown of acetylcholine by acetylcholinesterase, increasing the effectiveness of acetylcholine.

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α1-Adrenoceptor Agonist

A type of drug that activates α1-adrenoceptors, triggering contraction of smooth muscles.

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Accommodation

The ability of the eye to focus on objects at different distances.

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Lens Function

The lens focuses the light on the retina, and its focal length is adjusted by the ciliary muscle.

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Study Notes

Autonomic Nervous System and the Eye

  • The autonomic nervous system (both parasympathetic and sympathetic branches) innervates the eye, regulating light entering, focusing on the retina, and intraocular pressure.
  • The eye's structure includes the cornea, pupil, lens, retina, ciliary muscle, and iris.
  • Light passes through the cornea, the pupil, and is focused onto the retina by the lens.
  • The smooth muscles of the iris regulate pupil size (constriction/dilation).
  • Radial dilator muscle increases pupil size (mydriasis) via alpha-1 adrenoreceptors.
  • Papillary constrictor muscle decreases pupil size (miosis) using cholinergic receptors.
  • The ciliary muscle controls lens shape for near/far vision.
  • Acetylcholine (Ach) activates muscarinic receptors, causing ciliary muscle contraction, resulting in near vision.
  • Noradrenaline (NA) activates beta-2 adrenoreceptors, causing ciliary muscle relaxation leading to far vision.
  • Aqueous humor nourishes the lens and cornea, while vitreous humor maintains the shape of the eye.
  • Aqueous humor production is influenced by sympathetic nerves (beta-2 adrenergic receptors).
  • Aqueous humor drainage is controlled by the trabecular meshwork/Schlemm's canal.
  • Glaucoma is a pathological increase in intraocular pressure.

Drug Effects on the Eye

  • Agonists and antagonists for adrenergic and cholinergic receptors have different effects on the eye/pupil;
    • Muscarinic agonists cause pupil constriction.
    • Muscarinic antagonists cause pupil dilation.
    • Beta-adrenoreceptor agonists cause dilation.
    • Beta-adrenoreceptor antagonists cause constriction.
    • Alpha-adrenoreceptor agonists cause dilation.
  • Cocaine inhibits norepinephrine re-uptake.
  • Acetylcholinesterase inhibitors prolong the effect of acetylcholine.

Glaucoma

  • Chronic open-angle glaucoma (90% of cases) is characterized by gradual increase in intraocular pressure due to reduced drainage of aqueous fluid.
  • Acute narrow-angle glaucoma is a sudden increase in pressure, potentially caused by the radial dilator muscle obstructing the trabecular meshwork.

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