Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

BSAVA Ophthalmology > Structure and Function of the Eye > Flashcards

Structure and Function of the Eye Flashcards

1
Q

What defines the shape of the palpebral aperture of the eyelids?

A

Defined by the connective tissue - medial and lateral palpebral ligaments attaching to the canthi of the orbital wall, inserting medially onto nasal bones and laterally via retractor oculi muscle into temporal fascia.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Which muscles close and open the eyelids?

A

Orbicularis Oculi - close lids
Levator palpebrarum superioris - open lids

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What does the levator palpebrarum superioris insert onto? What condition may this structure be associated with?

A

Tarsal plate - fibrous band of connective tissue

“Diamond eye” = of importance as defines abnormal shape of the lids

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Which lid contains eyelashes/cilia?

A

Upper eyelid - lowers rarely have in most species

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What glands are found on the eyelids?

A

Outer lid - glands of moll (modified sweat glands) and glands of zeiss (modified sebaceous glands)

Inner lid - meibomian glands (lipid production for tear film) - openings for glands at apex of the eyelid margin. Ducts lined with keratinised epithelium.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Discuss the structure of the conjunctiva.

A

Lines inner surface of the eyelids and runs to the fornix/cul de sac where it turns into the globe conjunctiva.

Conjunctiva of lid = stratified squamous epithelium
As it nears fornix = pseudo-stratified with number of goblet cells

Sustantia propria of conjunctiva = two layers - adenoid layer with lymphoid follicles and deeper fibrous layer with blood vessels and nerves

Conjuctiva = highly vascular tissue
Loose stroma of lid on which it lies is richly populated by fibroblasts, macrophages, mast cells and lymphocytes.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is a key feature of the eyelids?

A

Perfect apposition to the corneal surface - allows for protection and distribution of tear film.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What are the roles of the nictitating membrane?

A

Arises from medial canthus
Protection of corneal surface
Distribution of tear film across ocular surface every time the eye is drawn back in the orbit by the retractor oculi muscle complex.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Describe the structure of the nictitating membrane/third eyelid.

A

Supported internally by hyaline cartilage in a T shape
Inner surface = lymphoid follicles
Base of the cartilage = nictitans gland/lacrimal gland of third eyelid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What does the nicitans gland supply?

A

Serous and mucous secretions - up to 30% of aqueous portion of the tear film.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Where does the bulk of the tear film come from?

A

Lacrimal gland situated on the dorsolateral surface of the globe.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How do canine and feline orbits differ from humans?
Why is this the case?

A

Open orbits with sizeable proportion of their circumference formed by muscle rather than bone.

This anatomy allows for wide opening of the jaw for prehension and mastication of food. Does mean ramus of mandible impinges on the orbital contents when the jaw is opening wide - why we see pain with orbital cellulitis on opening of the jaw.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Which bones make up the orbit?

A

Frontal bone - medial
Sphenoid bone - caudal orbit
Zygomatic and maxillary bone - rostral and lateral orbit
Lacrimal bone - also rostral orbit

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is the function of the orbital liagment?

A

Open orbit laterally in dogs/cats
Ligament extends from the zygomatic process of frontal bone to frontal process of zygomatic bone

Bridges lateral incomplete area of orbit

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What does the medial wall of the orbit consist of? What is contained within the medial wall?

A

Medial wall - frontal bone with orbital wing of presphenoid forming part of the medial wall which contains the optic canal.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Which structures pass through the optic canal foramina? Which bone does this foramina occur in?

A

Pre-sphenoid/sphenoid bone

Optic nerve and internal ophthalmic artery

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Which structures pass through the orbital fissure foramina? Which bone does this occur in?

A

Pre-sphenoid/sphenoid bone

Occulomotor, trochlear, abducens and ophthalmic nerves + anastomatic branch of the external ophthalmic artery and ophthalmic venous plexus

Retractor bulbi muscle also originates from the orbital fissure.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Which structures enter the orbit through the ethmoidal forminae?

A

Ethmoidal artery and nerve

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What muscle is forms the floor of the orbit? Which structure is it associated and how may this be relevant clinically?

A

Medial pterygoid muscle
Zygomatic salivary gland associated - salivary adenitis or mucocoeles within this salivary gland can result in orbital sequelae.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Which artery and nerve cross the orbital floor?

A

Maxillary artery and nerve

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What structure is the major refractive agent in the eye?

A

The cornea

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What type of epithelium does the cornea have?

A

Corneal epithelium - stratified squamous

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Where does the stem cell population of corneal cells originate from?

A

Stem cells of cornea originate at the limbus - continual production of new basal cells which migrate to central cornea.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What type of cells do the basal stem cells of the cornea become once they reach the centre? What happens to them after this?

A

Wing cells - rise up the stratified layers of cells becoming more squamous losing cell organelles until they are eventually desquamated as the lids rub them off the corneal surface.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

What is the name of the hypothesis for corneal epithelial cell migration?

A

XYZ hypothesis

X = epithelial cell formation at the limbus
Y = movement to central cornea
Z = desquamation from the ocular surface

Explains how normal epithelial erosions heal rapidly.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

How do the basal epithelial cells of the cornea attach to the basement membrane and to each other?
Why is how they attach to one another important?

A

Link to basement membrane via hemidemosomes and link to one another with tight junctions.

Tight junctions = prevent movement of water through the epithelial layer (hydrophobic)
Only when defect present within epithelium we see water movement from the tears into the stroma - highly polar molecule fluorescein uptake.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

What property does the stroma have? What is it composed of?

A

Hydrophilic (unlike cornea which is hydrophobic)

Composed of collagen fibris which are held apart in regular structure by water molecules attached to negatively charged proteoglycans which in turn are attached to defined sites on the collagen fibrils.

In this way only just enough water is held in the cornea keeping it transparent. Disruption to this arrangement = loss of transparency.

28
Q

What is the function of the corneal endothelium?

A

Acts continually to pump excess water across and into the aqueous humour - preventing corneal oedema and translucency of the cornea.

29
Q

What is the key to corneal transparency?

A

Collagen fibrils exactly the same diameter and held at exactly the same distance apart at 22-25nm.

Wavelength of light 20x this measurment light passes unimpeded through the cornea.

30
Q

What is the basement membrane of the corneal endothelium called? What is it composed of

A

Descemet’s membrane - composed of collagen type IV, glycoprotein laminin and collagen VII filaments anchoring overlying cells.

31
Q

Where is the endothelium derived from and why is it unable to regenerate if damaged.

A

Endothelium derived from neural crest - post mitotic therefore unable to regenerate if damaged.

32
Q

How many layers thick is the endothelium? What type of junctions does it have - why is this important?

A

Single cell layer
Tight junctions between each of the hexagonal cells
Allows cells to control the movement of water and solutes with leakage of these solutes and nutrients from the aqueous humour into the stroma while at the same time pumping water out into the AH.

33
Q

What is the dual function of the endothelium referred to as?

A

Pump-leak hypothesis
Energy driven process requiring ATP consumption by Na/K ATPase pump and carbonic anhydrase.

34
Q

What are the 3 portions of the tear film?

A

Mucin - goblet cells of conjunctiva
Aqueous - lacrimal gland, nictitans gland
Lipid - meibomian glands

35
Q

What is the normal volume of the tear lake? What is the volume of a normal eye drop?

A

Typically around 5-8 ul

Normal eye drop - 25-40ul - considerable overflow of drug into nasolacrimal system/over eyelids

36
Q

What are the functions of the iris?

A

Diaphragm extending from ciliary body covering anterior surface of lens

Regulates amount of light entering posterior segment
Lymphoid aggregation with immunological functions
Communication structure - widening to denote emotional change

37
Q

What are the layers of the iris?

A

Anterior border layer - containing melanocytes and fibroblasts
Stroma - composed of loosely arranged collagen fibres and melanocytes, fibroblasts and chromatophores, within stroma = blood vessels, muscles (circumferential sphincter and axial dilator), lymphoid tissue and nerves (parasympathetic and sympathetic)
Posterior epithelial layer

38
Q

Discuss the immune function of the iris (ACAID) and other factors.

A

Anterior chamber acquired immune deviation

Normal immune response = infammation (risks loss of vision in eye and therefore try to avoid)
Aim of immune system in the eye = reduce bystander injury

Lymphocytes travel from the eye to the spleen - TH2 environment in spleen where lymphocytes produce antibodies that travel back up to the eye.

Other factors altering immune response in eye = cytokine tansforming growth factor beta - reduces cytotoxic responses

Fas system - Fas receptor = detah receptor which triggers apopotosis when interact with Fas ligand. T lymphocytes express the Fas receptor and the ligand is expressed on cells in the anterior chamber = active lymphocytes undergo apoptosis rather than attack the tissues in the eye

39
Q

Discuss the role of the iris as a sphincter.

A

Dilates and constricts to regulate entry of light to posterior segment.

Circumferential sphincter and axial dilator smooth muscles - innervated by the oculoparasympathetic and sympathetic nerves giving miosis and mydriasis respectively.

40
Q

Where does the parasympathetic innervation to the iris muscle originate from?

A

Parasympathetic = occulomotor nerve (CN 3)
Arises from Edinger-Westphal nucleus, synapses at the ciliary ganglion and enters the eye via the short ciliary nerves

41
Q

Where does the sympathetic innervation to the iris smooth muscle originate from?

A

Circuitous route
First order neuron - caudal hypothalamus and travel caudally to synapse in anterior thoracic spinal cord (cillosponal centre of Budge)
2nd order neuron exits spinal cord with brachial plexus and travels anteriorally with vagus nerve as passes in opposite direction with 2nd synapse in the superior cervical gangion.
3rd order postganglionic nerve travels through the middle ear and joins fibres of long ciliary nerve to innervate the dilator muscles of the iris and levator palpebrum oculi muscle

42
Q

How does the lens attach to scleral shell of the globe?

A

Suspended by zonules (set of protein filaments)

43
Q

What is the function of the lens?

A

Function = focus light onto the retina

44
Q

What is the lens formed from in embryology?

A

Lens formed from the ectodermal placode

45
Q

Describe the structure of the lens. What is key for the lens to be transparent?

A

Biconvex structure with equator and Y shaped suture lines

Capsule - thicker anteriorally with an epithelium from which it forms the basement membrane. Thinner posteriorally without epithelium for here it is the basement membrane for the lens fibre cells which fill the lens.

Central nucleus - formed in utero
Surrounded by cortex
Lens fibre cells produced at equator and laid down throughout life

Lens fibre cells are elongated and arranged in parellel closely interdigitating structure to ensure transparency
Crystallins - no nuclei and mostly protein, attract water molecules which holds proteins in liquid crystal formation allowing anti-oxidants and enzymes to move through lens and prevent oxidation.

46
Q

How do age related cataracts occur?

A

Continual exposure to light over time - photo-oxidation leading to oxidative degeneration.
Crystallins within lens have numerous thiol (SH) groups - negatively charged and attracting water. Oxidation of two thiol groups = disulphide bridge

Leads to aggregation of the crystallin lens fibres. Aggregated protein from disulphide bridge formation and cataract formation.

47
Q

What is the likely cause for juvenile cataracts?

A

Juvenile cataracts = gene mutation within specific crystalline molecules or mutations of heat shock proteins in lens.

48
Q

How does the lens remain clear until later on in life?

A

Major protein in lens = alpha crystalline
Dual action - structural molecule but also chaperone molecule
Heat shock proteins also present.

Chaperone molecule prevents oxidative damage and reverse protein misfoldng

Dietary anti-oxidants also prevent lens protein photo-oxidation

Enzymes also important in maintaining lens health (produced by lens epithelial cells) - glutathionine perioxidase (keeps glutathionine in reduced state)

Vital lens molecule glutathione - tripeptide acts as oxidation transfer system between proteins at risk of damage and anti-oxidants there to save them.

Enzymes need to be able to move through lens to work (needs to be in relatively fluid crystal state) - why ther is failure of this system as age

49
Q

Where are cortical cells of the lens produced? Why does the lens not continue to grow in size and what condition is seen as a result of this process?

A

Nucleus of lens = formed before birth
Cortex = grows continually after birth

Cortical cells produced at equatorial region of the lens and laid down continually over the nucleus.

Nucleus of the lens is continually constricted by cortical development eventually giving higher density of protein in the nucleus - harder centre to the lens = nuclear sclerosis

50
Q

What is the function of the aqueous humour? What can be found in AH?

A

Oxygen and nutrition for ocular structures (lack of vasculature to cornea and lens)
Keeps anterior portion of eye “inflated”

Ultrafiltrate of plasma - contains proteins such as immunoglobulins, enzymes, lipids (much lower level than plasma due to blood-aqueous barrier)

51
Q

What forms the blood-aqueous barrier?

A

Tight junctions of ciliary body epithelial cells

52
Q

Where is aqueous humour formed? What mechanisms are there to promote movement of fluid.

A

Formed from ciliary body through movement of fluid from the vasculature of the ciliary body across the epithelium.

Movement of fluid promoted by ionic flow in 3 mechanisms
Active transport of sodium ions
Ultrafiltration of chloride ions
Diffusion of bicarbonate produced at high concentrations from carbon dioxide and water through action of carbonic anhydrase.

53
Q

How does aqueous humour flow in the anterior chamber?

A

Flows downwards centrally and then upwards peripherally as moved by thermal gradients with the episcleral venous plexus warming it peripherally.

54
Q

How does aqueous humour drainage occur and where does this occur?

Which pathway is increased in the presence of prostaglandins?

A

Conventional route - through trabecular meshwork in the iridocorneal angle

Unconventional route - vessels in the choroid in the posterior segment (uveoscleral outflow)

Prostaglandins = increase in outflow via unconventional pathway - results in lower IOP in uveitis

55
Q

What does the vitreous consist of?

A

Vitreous = gel of hyaluronic acid

56
Q

How does the vitreous initially develop and what conditions can we see associated with these?

A

Vitreous body develops first as primary structure with blood vessel running from optic nerve head - the hyaloid artery.
Hyaloid artery surrounds the posterior portion of the lens and ramifies with the anterior vascular structures to give the tunica vasculosa lentis.

Persistence of these structures anteriorly = persistent pupillary membranes
Vessels behind the lens fail to regress = persistent hyperplastic primary vitreous or just persistence of hyaloid artery.

57
Q

Which portions of the optic cup develop which retinal structures?

A

Inner leaf of optic cup = neuroretina
Outer portion = retinal pigment epithelium

Potential space between the neuroretina and RPE (relevant for retinal detachments)

58
Q

What happens during retinal detachment?

A

Intra-retinal separation with the neuroretina separating away from the RPE

59
Q

Describe the cellular structure of the retina.

A

Closest to the choroid:

RPE - retinal pigment epithelium (nourishment of photoreceptors, absorption of scattered light, phagocytosis of photoreceptor outer membranes, continued immune priviledge of eye through tight junctions)
Photoreceptors - rods and cones
(Rods = dark, cones = bright + colour)
Outer nuclear layer (cell bodies of rods/cones)
Outer plexiform layer
Inner nuclear layer (cell bodies of integrating neurons, muller cells, horizontal, biopolar and amacrine cells)
Inner plexiform layer (synapses of integrating neurons and dendrites of optic tract neurons - ganglion cells)
Ganglion cell layer

Ganglion cell layer synapses with optic nerve fibre layer and internal limiting membrane.

60
Q

What happens when photon of light hits photoreceptors?

A

Photon absorbed by photoreceptor pigment rhodopsin
Stops dark current (continual movement of ions across photoreceptor outer segment)
Causes membrane hyperpolarisation via interactions of proteins in photoreceptor inner membrane by which phosphodiesterase activated producing cGTP which closes outer membrane channel
Initiates nerve impulse up in ganglion cells

61
Q

Which retinal cells would be affected by PRA?

A

PRA - photoreceptors, initially cones the progresses to rods. Decrease in photoreceptors and their cells bodies in the outer nuclear layer.

62
Q

Which retinal cells are affected in glaucoma?

A

Glaucoma = retinal ganglion cell layer
Atrophy of ganglion cells and inner nuclear layers occurs due to ischaemia of retinal blood vessels within these layers due to raised IOP.
Raised IOP can also cause kinking of ganglion cell axons as they exit eye in lamina cribrosa region (optic nerve head cupping)

63
Q

Through what structure do the ganglion cells of the retina send their axons to the optic nerve?

A

Lamina cribosa

64
Q

What percentage of the nerves decussate in dogs/cats at the optic chiasm?

A

75% in the dog
66% in the cat

65
Q

Which nerves controls motor functions to control the pupil and extra ocular muscles?

A

Occulomotor (CN3) - parasymapathetic innervation to iris via short ciliary nerves + extraocular muscle except…

Dorsal oblique - trochlear (CN 4)
Lateral rectus - abducens (CN 6)

66
Q

Where does the innervation to the lacrimal system originate from?

A

Parasympathetic supply - facial nerve (CN VII) but courses to the eye with the terminal trigeminal nerve (CN V), the bridge between the two provided by the nervus intermedius

Nerve endplate releases both acetylcholine and peptide hormone vasoactive intestinal peptide.
Same nerve which innervates the lacrimal gland innervates medial nasal gland which supplies moisture to external nares - neurogenic KCS.

BSAVA Ophthalmology (18 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions