Ocular physiology
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Questions and Answers

Which of the following is NOT a function of the eyelids?

  • Maintaining the precorneal tear film
  • Contributing the mucin to the precorneal tear film (correct)
  • Protecting the anterior segment from external trauma
  • Aiding movement of tears within the nasolacrimal system

    • What determines the shape and width of the palpebral fissure?

  • Nictitating membrane
  • Medial and lateral canthal ligamentous and muscle attachments (correct)
  • Accessory lacrimal glands
  • Meibomian glands

    • Which of the following breeds is likely to have a narrow, almond-shaped palpebral fissure?

  • Brachycephalic breeds
  • Dolichocephalic breeds (correct)
  • Both brachycephalic and dolichocephalic breeds
  • None of the above

    • Which metabolic pathway is primarily used by keratocytes for glucose metabolism?

    <p>Pentose phosphate shunt</p> Signup and view all the answers

    What is the main function of nicotinamide adenine dinucleotide phosphate (NADPH) produced by the corneal epithelium?

    <p>Free radical scavenging</p> Signup and view all the answers

    Which layer of the cornea has the highest glucose needs?

    <p>Corneal endothelium</p> Signup and view all the answers

    Which muscle is responsible for opening the upper eyelid in animals?

    <p>Levator palpebral superioris</p> Signup and view all the answers

    Which nerve innervates the levator palpebral superioris muscle?

    <p>Oculomotor nerve (CN III)</p> Signup and view all the answers

    Which reflexes are primitive reflexes with a purely subcortical course and are elicited by touch?

    <p>Corneal and palpebral reflexes</p> Signup and view all the answers

    Which reflex is cortically mediated and is initiated by a threatening gesture?

    <p>Menace response</p> Signup and view all the answers

    Which layer of the tear film is the thickest and performs the primary functions of the tear film?

    <p>The middle aqueous layer</p> Signup and view all the answers

    What is the primary purpose of the outer lipid layer in the tear film?

    <p>To prevent evaporation of the underlying layers</p> Signup and view all the answers

    Which gland is responsible for producing the outer lipid layer of the tear film?

    <p>Meibomian gland</p> Signup and view all the answers

    What is the approximate tear volume turnover rate in horses?

    <p>7 minutes</p> Signup and view all the answers

    Which of the following animals has been detected to have lysozyme activity in their tears?

    <p>Sheep</p> Signup and view all the answers

    Which of the following immunoglobulins is present in greater concentrations in the precorneal tear film (PTF) than immunoglobulins G and M?

    <p>IgA</p> Signup and view all the answers

    Which nerve provides the lacrimal gland with its parasympathetic and sympathetic fibers?

    <p>Trigeminal nerve</p> Signup and view all the answers

    Which cells in the lacrimal gland are primarily responsible for secretion of proteins in lacrimal gland fluid?

    <p>Acinar cells</p> Signup and view all the answers

    Which test is primarily used to evaluate the aqueous portion of tears in clinical settings?

    <p>Schirmer tear test (STT)</p> Signup and view all the answers

    Which layer of tears is composed of tear mucins produced by the apocrine conjunctival goblet cells?

    <p>Deep layer</p> Signup and view all the answers

    Which of the following is responsible for corneal scarring?

    <p>Changes to the light-scattering properties of keratocytes</p> Signup and view all the answers

    Which species has the highest density of goblet cells in the fornix?

    <p>Guinea pigs</p> Signup and view all the answers

    What is the primary role of the corneal endothelium?

    <p>To maintain stromal deturgescence</p> Signup and view all the answers

    Which type of mucin is associated with the corneal and conjunctival microvilli?

    <p>Membrane-bound mucin</p> Signup and view all the answers

    What is the function of the corneal epithelium?

    <p>To provide a physical barrier against aqueous humor</p> Signup and view all the answers

    What is the mechanism that maintains steady-state hydration in the cornea?

    <p>Pump-leak mechanism</p> Signup and view all the answers

    Which nerve is responsible for the involuntary blinking that occurs upon stimulation of the cornea?

    <p>Facial nerve</p> Signup and view all the answers

    Which skull type has the lowest corneal sensitivity in dogs?

    <p>Brachycephalic</p> Signup and view all the answers

    Which region of the cornea has the highest sensitivity?

    <p>Central cornea</p> Signup and view all the answers

    What is the function of the sphincter muscle in the iris?

    <p>To constrict the pupil</p> Signup and view all the answers

    Which nerve innervates the constrictor muscle in the iris?

    <p>Oculomotor nerve (CN III)</p> Signup and view all the answers

    What is the primary function of the dilator muscle in the iris?

    <p>To dilate the pupil</p> Signup and view all the answers

    Which nerves primarily innervate the dilator muscle in the iris?

    <p>Sympathetic nerves</p> Signup and view all the answers

    Which vessels in the eye are under the strong influence of sympathetic stimulation?

    <p>Choroidal vessels</p> Signup and view all the answers

    What is the primary cause of retinopathy of prematurity?

    <p>Exposure to higher oxygen concentrations</p> Signup and view all the answers

    Which type of autoregulation is present in the eye?

    <p>Metabolic and myogenous</p> Signup and view all the answers

    Which vessels have little to no autoregulatory mechanism?

    <p>Choroidal vessels</p> Signup and view all the answers

    Which blood vessels supply the cornea and lens in the embryo?

    <p>Uveal vessels</p> Signup and view all the answers

    Which structure is a heavily pigmented and highly vascularized source of nutrients for the inner retina in birds?

    <p>Pecten oculi</p> Signup and view all the answers

    Which blood vessels supply the iris and ciliary body in most species?

    <p>Uveal vessels</p> Signup and view all the answers

    Which blood vessels supply the choroid in most species?

    <p>Uveal vessels</p> Signup and view all the answers

    Which barrier prevents the movement of ions and fluid from the tears into the stroma and protects the cornea from pathogens?

    <p>Corneal epithelial barrier</p> Signup and view all the answers

    Which route is the most permeable for hydrophilic drugs and macromolecules?

    <p>Conjunctival/scleral route</p> Signup and view all the answers

    Which barrier prevents nearly all protein movement and is effective against low-molecular-weight solutes?

    <p>Blood-retinal barriers</p> Signup and view all the answers

    Which barrier is formed by the nonpigmented layer of the ciliary epithelium and the endothelium of the iridial vessels?

    <p>Blood aqueous barrier</p> Signup and view all the answers

    Which species have been demonstrated to have species differences in the α- and β-receptors?

    <p>Nonhuman primates, cats, and dogs</p> Signup and view all the answers

    Which receptor is primarily responsible for the constriction of feline pupils with the use of timolol?

    <p>β-receptor</p> Signup and view all the answers

    Which neurotransmitter is involved in enhancing adrenergic-induced contractions of the iris dilator muscles?

    <p>Neuropeptide Y</p> Signup and view all the answers

    Which photopigment has peak sensitivity at 480 nm in dogs?

    <p>Melanopsin</p> Signup and view all the answers

    Which layer forms a size-selective barrier for molecules entering the neuroretina?

    <p>Bruch’s Membrane</p> Signup and view all the answers

    Which cells form the inner blood-retinal barrier?

    <p>Endothelial cells</p> Signup and view all the answers

    What is the most permeable point of the blood-retinal barrier?

    <p>Optic nerve head</p> Signup and view all the answers

    What drives the transport of water from the retina to the choroid?

    <p>Active transport of chloride</p> Signup and view all the answers

    Which layer forms the blood-aqueous barrier in the eye?

    <p>Nonpigmented layer of the ciliary epithelium</p> Signup and view all the answers

    Which type of junctions are present in the layers forming the blood-aqueous barrier?

    <p>Tight junctions of the 'leaky' type</p> Signup and view all the answers

    Which part of the eye controls the flow of fluid into the posterior chamber?

    <p>Inner, nonpigmented ciliary epithelium</p> Signup and view all the answers

    Which pathway is dependent on IOP for outflow?

    <p>Traditional pathway</p> Signup and view all the answers

    What is the primary factor impacting uveoscleral outflow?

    <p>Ciliary body state</p> Signup and view all the answers

    Which medication can increase uveoscleral outflow?

    <p>Pilocarpine</p> Signup and view all the answers

    What is the main effect of cholinergic agonists on outflow resistance?

    <p>Decrease</p> Signup and view all the answers

    Which substance is found in higher concentrations in the aqueous humor (AH) compared to plasma?

    <p>Ascorbate</p> Signup and view all the answers

    Which substance has a lower concentration in the AH compared to plasma in dogs?

    <p>Amino acids</p> Signup and view all the answers

    Which substance has a concentration in the AH that is increased in diabetic patients?

    <p>Glucose</p> Signup and view all the answers

    Which substance is present in lower concentrations in the AH compared to plasma due to the blood-aqueous barrier (BAB)?

    <p>Urea</p> Signup and view all the answers

    Which animal species has the highest percentage of uveoscleral outflow?

    <p>Nonhuman primates</p> Signup and view all the answers

    What is ocular rigidity?

    <p>The resistance offered by the fibrous tunics of the eye to a change in intraocular volume</p> Signup and view all the answers

    What is the relationship between IOP and volume of the globe?

    <p>logP2 / P1 = k (V2 - V1)</p> Signup and view all the answers

    Which enzyme is abundantly present in the non-pigmented ciliary epithelium and plays a critical role in the active secretion of aqueous humor?

    <p>Na+-K+-ATPase</p> Signup and view all the answers

    What is the primary mechanism by which aqueous humor is formed in the eye?

    <p>Active secretion</p> Signup and view all the answers

    Which layer of the ciliary epithelium forms a barrier that inhibits paracellular diffusion?

    <p>Inner non-pigmented epithelium</p> Signup and view all the answers

    Which enzyme catalyzes the reaction of CO2 and H2O to produce bicarbonate, which is essential for the secretion of aqueous humor?

    <p>Carbonic anhydrase</p> Signup and view all the answers

    Which of the following is the primary route of aqueous humor outflow?

    <p>Conventional outflow through the trabecular meshwork</p> Signup and view all the answers

    What is the major site of aqueous outflow resistance?

    <p>The juxtacanalicular zone</p> Signup and view all the answers

    What is the primary cause of primary open angle glaucoma (POAG) in humans and likely dogs with the ADAMTS10 mutation?

    <p>Excessive deposition of extracellular matrix (ECM)</p> Signup and view all the answers

    What are the two routes of aqueous humor outflow in the iridocorneal angle (ICA)?

    <p>Conventional and nonconventional routes</p> Signup and view all the answers

    Study Notes

    Eyelid Functions and Anatomy

    • Eyelids protect the eye from foreign particles and excessive light.
    • The shape and width of the palpebral fissure are determined primarily by the tension of the orbicularis oculi and levator palpebrae muscles.
    • Breeds like the Siamese cat are likely to have a narrow, almond-shaped palpebral fissure.
    • The muscle responsible for opening the upper eyelid is the levator palpebrae superioris.
    • This muscle is innervated by the oculomotor nerve (CN III).

    Corneal Metabolism and Functions

    • Keratocytes primarily utilize the pentose phosphate pathway for glucose metabolism.
    • The corneal epithelium produces NADPH, which is crucial for antioxidant defense and maintaining corneal hydration.
    • The corneal epithelium has the highest glucose needs among corneal layers.
    • Primitive reflexes are triggered by touch and follow a subcortical pathway.
    • The blink reflex is cortically mediated and triggered by threatening gestures.
    • Involuntary blinking when the cornea is stimulated is managed by the trigeminal nerve (CN V).

    Tear Film Composition and Function

    • The aqueous layer of the tear film is thickest and performs major lubrication and nourishment functions.
    • The outer lipid layer of the tear film serves to reduce evaporation of the aqueous layer.
    • The meibomian glands are responsible for producing the outer lipid layer of the tear film.
    • Tear volume turnover in horses is approximately 7 to 8 times per hour.
    • Lysozyme activity has been detected in the tears of many species, including cats and dogs.
    • Immunoglobulin A (IgA) is present in greater concentrations in the precorneal tear film compared to IgG and IgM.

    Lacrimal Gland and Tear Evaluation

    • The lacrimal gland receives its innervation from both parasympathetic (facial nerve, CN VII) and sympathetic fibers.
    • Acinar cells in the lacrimal gland are primarily responsible for protein secretion.
    • The Schirmer test is commonly used to assess the aqueous portion of tears.

    Cornea and Conjunctiva

    • Goblet cells located in the conjunctiva are essential for syntheses of tear mucins.
    • Corneal scarring can result from various pathological conditions affecting corneal integrity.
    • The corneal endothelium regulates hydration and maintains corneal transparency.

    Autoregulation and Vascular Influence

    • Autoregulation in the eye helps maintain adequate blood flow despite varying pressures.
    • The ciliary and conjunctival blood vessels largely lack autoregulatory mechanisms.
    • The embryonic blood vessels supplying the cornea and lens are primarily the hyaloid artery and its branches.

    Blood-Retinal and Blood-Aqueous Barriers

    • The inner blood-retinal barrier is formed by retinal endothelial cells and maintains selective permeability.
    • The outer blood-aqueous barrier is formed by nonpigmented ciliary epithelial cells, preventing substance entry.
    • Tight junctions contribute to the selective barriers in the eye.

    Aqueous Humor Dynamics

    • Aqueous humor is formed primarily via the active secretion by ciliary epithelial cells.
    • The primary route of aqueous humor outflow is through the trabecular meshwork.
    • Cholesterol and high viscosity substances can influence the resistance to outflow, thus affecting intraocular pressure (IOP).

    Pathological Conditions

    • Retinopathy of prematurity is primarily caused by abnormal blood vessel growth in premature infants.
    • Primary open-angle glaucoma (POAG) in humans and potentially in dogs is related to aqueous humor outflow resistance.

    Uveoscleral Outflow

    • Uveoscleral outflow is influenced by IOP and can be increased by medications like prostaglandin analogs.
    • The highest percentage of uveoscleral outflow is seen in the dog.

    Additional Key Facts

    • Ocular rigidity refers to the resistance of the eye to changes in volume.
    • CO2 and H2O are catalyzed by carbonic anhydrase to form bicarbonate, essential for aqueous humor secretion.
    • The relationship between IOP and globe volume is critical in understanding ocular health.

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    Test your knowledge on Veterinary Medicine and Ophthalmology with this quiz! Covering topics such as small animal clinical sciences, surgical and radiological sciences, and absorption systems. Challenge yourself and see how well you know these important subjects in the field.

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