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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