Physiology and Nervous System Quiz

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Questions and Answers

What is the primary purpose of the bulk movement of blood plasma into the ciliary stroma?

To regulate blood pressure

To form the plasma ultrafiltrate reservoir (correct)

To transport selected substances against a concentration gradient

To facilitate active secretion

What percentage of total aqueous formation is contributed by active secretion?

90% (correct)

100%

70%

50%

Which type of transporters are involved in the transfer of NaCl from stroma to PE cells?

Secondary active transporters

Electroneutral transporters (correct)

Primary active transporters

Facilitated diffusion transporters

What is the function of the Na+/K+ activated ATPase in aqueous production?

To release Na+ and Cl- into the posterior chamber Signup and view all the answers

What is the direction of water movement in the third step of aqueous production?

Into the posterior chamber Signup and view all the answers

What is the primary purpose of the gap junctions between PE and NPE cells?

To allow free movement of NaCl Signup and view all the answers

What is the sequence of steps in aqueous production?

Uptake, passage, release, primary active transport Signup and view all the answers

What is the primary function of the Cl- channels in aqueous production?

To release Cl- into the posterior chamber Signup and view all the answers

Which type of transport is involved in the release of HCO3- into the posterior chamber?

Secondary active transport Signup and view all the answers

What is the role of the primary active transporters in aqueous production?

To transport Na+ and K+ out of the posterior chamber Signup and view all the answers

What percentage of aqueous drainage occurs through the uveoscleral route?

10% Signup and view all the answers

Which muscle's state affects the uveoscleral drainage?

Ciliary muscle Signup and view all the answers

What is the effect of ciliary muscle contraction on trabecular outflow?

Increases trabecular outflow Signup and view all the answers

What is the final destination of some aqueous humor that passes through the suprachoroidal space?

Vortex veins Signup and view all the answers

What is the primary component of the anterior chamber angle?

Trabecular meshwork Signup and view all the answers

Which of the following is NOT a route of aqueous humor outflow?

Vitreous humor Signup and view all the answers

What is the effect of ciliary muscle relaxation on trabecular outflow?

Increases uveoscleral outflow Signup and view all the answers

What is the site of aqueous humor drainage into the episclera?

Uveal trabeculae Signup and view all the answers

Which of the following is NOT a component of the outflow pathway of aqueous humor?

Retina Signup and view all the answers

What is the primary purpose of the uveoscleral pathway?

To remove excess aqueous humor Signup and view all the answers

What is the location of Schwalbe's line?

The junction between the cornea and iris Signup and view all the answers

What is the main function of the trabecular meshwork?

To drain the aqueous humour from the anterior chamber Signup and view all the answers

What is the name of the space between the posterior surface of the iris and the anterior ciliary body?

Ciliary sulcus Signup and view all the answers

What is the function of the juxtacanalicular meshwork?

To offer resistance to aqueous humour flow Signup and view all the answers

What is the name of the canal that receives aqueous humour from the trabecular meshwork?

Schlemm's canal Signup and view all the answers

What is the function of the endothelial cells in the trabecular meshwork?

To maintain the patency of the trabecular meshwork Signup and view all the answers

What is the name of the structure that prevents Schlemm's canal from collapsing?

Scleral spur Signup and view all the answers

What is the route of aqueous humour drainage from the eye?

Trabecular meshwork → Schlemm's canal → collector channel → episcleral plexus Signup and view all the answers

What is the name of the layer of cells that lines the inner wall of Schlemm's canal?

Endothelial cells Signup and view all the answers

What is the function of the trabecular cells in the trabecular meshwork?

To phagocytose debris and maintain the trabecular meshwork Signup and view all the answers

What is the name of the muscle that drives the eye's closure?

Orbicularis oculi Signup and view all the answers

Which branch of the facial nerve innervates the lacrimal gland?

Zygomatic branch Signup and view all the answers

What is the function of the orbicularis oculi muscle around the eye?

Drives the eye's closure Signup and view all the answers

Which nerve is responsible for the sensation of hearing?

Vestibulocochlear nerve Signup and view all the answers

Which muscle is innervated by the marginal branch of the facial nerve?

Muscles of the lower lip Signup and view all the answers

What is the function of the buccal branch of the facial nerve?

Innervates the muscles of the cheek Signup and view all the answers

Which muscle is responsible for the movement of the frontalis?

Frontalis Signup and view all the answers

What is the function of the zygomatic branch of the facial nerve?

Innervates the lacrimal gland Signup and view all the answers

Which muscle is innervated by the zygomatic branch of the facial nerve?

Gomatic muscle Signup and view all the answers

What is the function of the marginal branch of the facial nerve?

Innervates the muscles of the lower lip Signup and view all the answers

Which artery branches off the ophthalmic artery and supplies the lacrimal gland?

Lacrimal artery Signup and view all the answers

What is the function of the muscular branch of the lacrimal artery?

Supplies the lateral and superior rectus muscles Signup and view all the answers

What is the terminal branch of the lacrimal artery?

Lateral palpebral artery Signup and view all the answers

Which artery supplies the episclera and conjunctiva?

One branch of the ophthalmic artery Signup and view all the answers

What is the function of the recurrent branch of the lacrimal artery?

Anastomoses with the meninges Signup and view all the answers

Which artery is a branch of the lacrimal artery and supplies the orbicularis oculi muscle?

Zygomatic artery Signup and view all the answers

How many branches does the lacrimal artery have?

5 Signup and view all the answers

What is the function of the glandular branch of the lacrimal artery?

Supplies the lacrimal gland Signup and view all the answers

Which artery anastomoses with the medial palpebral artery?

Lateral palpebral artery Signup and view all the answers

What is the function of the lateral palpebral artery?

Supplies the lower and upper lids Signup and view all the answers

What is the origin of sympathetic nerve activity that mediates relaxation?

Diencephalon Signup and view all the answers

What is the main contributor to presbyopia?

Increase in stiffness of crystalline lens Signup and view all the answers

What is the anatomical basis underpinning accommodation?

Lens, Capsule, Zonules, and Ciliary muscle Signup and view all the answers

Where do post-ganglionic fibres travel to?

Ciliary muscle via nasociliary nerve Signup and view all the answers

What is the age of onset of presbyopia?

40 years old Signup and view all the answers

What is the result of increased lens thickness with age?

Change in zonule insertion Signup and view all the answers

What is the function of the short ciliary nerves?

Innervate the ciliary muscle Signup and view all the answers

What is the primary mechanism underlying presbyopia?

Lens hardening Signup and view all the answers

What is the role of the ciliary ganglion?

Innervate the ciliary muscle Signup and view all the answers

What is the effect of presbyopia on accommodation?

Decreased accommodation Signup and view all the answers

What percentage of the globe's volume is made up of the vitreous body?

80% Signup and view all the answers

What is one of the functions of the vitreous body?

To support the lens anteriorly Signup and view all the answers

What is the function of vitamins in the vitreous?

To promote intraocular wound healing Signup and view all the answers

What change occurs in the vitreous with aging?

Liquefaction Signup and view all the answers

What is the function of collagen fibrils in the vitreous?

To provide mechanical strength Signup and view all the answers

What is the function of hyaluronic acid in the vitreous?

To maintain vitreous hydration Signup and view all the answers

What is the effect of aging on the collagen fibril network in the vitreous?

It becomes free-floating bundles Signup and view all the answers

What is the function of the vitreous as a molecule repository?

To receive metabolites from the ciliary body and retina Signup and view all the answers

What is the function of the vitreous as a blood-ocular barrier?

To prevent movement of macromolecules and cells Signup and view all the answers

What is the overall function of the vitreous?

To maintain optical clarity and provide structural support to the globe Signup and view all the answers

What is the characteristic of the fovea in the retina?

It has no overlying retinal nerve fibre layer Signup and view all the answers

Where do ganglion cell axons travel within the retina?

Within the retinal nerve fibre layer Signup and view all the answers

What is the function of microglia in the retina?

To phagocytose degenerating retinal neurons Signup and view all the answers

What is the shape of the optic nerve head?

Oval Signup and view all the answers

What is the location of the optic nerve head in relation to the fovea?

Nasal to the fovea Signup and view all the answers

What is the region between the optic cup and the optic disc?

Neural retinal rim Signup and view all the answers

What is the function of Müller cells?

To support retinal neurons Signup and view all the answers

What is the characteristic of the central region of the optic disc?

It is the optic cup Signup and view all the answers

How do ganglion cell axons travel within the retina?

In an arcuate pattern Signup and view all the answers

What is the function of astrocytes in the retina?

To support retinal neurons Signup and view all the answers

What is the main blood supply to the outer retina?

Choroid Signup and view all the answers

What is the main function of the tight junctions between the lining glial cells and the adjacent RPE?

To block the entry of proteins from the ONH into the retina Signup and view all the answers

What is the structure that forms a circular arterial anastomosis at the scleral level?

Circle of Zinn-Haller Signup and view all the answers

What is the main function of the astrocytes in the optic nerve?

To support the optic nerve fibers Signup and view all the answers

What is the region of the optic nerve that carries ganglion cell axons from the globe?

Surface nerve fibre layer Signup and view all the answers

What is the blood supply to the inner retina?

Central retinal artery Signup and view all the answers

What is the layer of cells that lines the inner wall of the optic nerve?

Glial cells Signup and view all the answers

What is the main function of the blood-retina barrier?

To block the entry of proteins from the ONH into the retina Signup and view all the answers

What is the structure that separates the optic nerve from the sclera?

Lamina cribrosa Signup and view all the answers

What is the region of the optic nerve that is responsible for the myelination of nerve fibers?

Retrolaminar region Signup and view all the answers

What is the primary function of the choroid?

To maintain retinal homeostasis Signup and view all the answers

What is the main advantage of OCTa?

It detects movement of RBCs Signup and view all the answers

What is the characteristic feature of Vogt-Koyanagi-Harada's disease in ICGA?

Presence of choroidal granuloma Signup and view all the answers

What is the characteristic feature of Age-related Macular Degeneration (AMD)?

Accumulation of waste material in Bruch's membrane Signup and view all the answers

What is the number of layers in the choroid?

5 Signup and view all the answers

What is the purpose of ICGA?

To monitor choroidal blood flow Signup and view all the answers

What is the significance of OCTa in clinical assessment of the choroid?

It is a non-invasive technique that detects movement of RBCs Signup and view all the answers

What is the significance of choroidal imaging techniques in the clinic?

They are used to assess choroidal health Signup and view all the answers

What is the characteristic feature of normal OCTa in the outer retina?

Presence of bright areas Signup and view all the answers

What is the significance of Bruch's membrane in AMD?

It becomes thicker Signup and view all the answers

What is the main function of the blood-aqueous barrier (BAB) and blood-retinal barrier (BRB)?

To separate the blood from the neural retina and regulate the secretion of aqueous humor Signup and view all the answers

Which cells form the inner blood-retinal barrier (BRB)?

Retinal capillary endothelial cells Signup and view all the answers

What is the primary function of the tight junctions in the blood-retinal barrier (BRB)?

To form a barrier between the blood and the neural retina Signup and view all the answers

What happens to the tight junctions in the blood-retinal barrier (BRB) during inflammation?

They become weaker and more permeable Signup and view all the answers

What is the purpose of the blood-ocular barrier?

To separate the blood from the neural retina and regulate the secretion of aqueous humor Signup and view all the answers

What is the role of the RPE in the blood-ocular barrier?

To act as a barrier between the blood and the neural retina Signup and view all the answers

What is the function of the choroid?

To provide oxygen and nutrients to the retina Signup and view all the answers

What is the result of the breakdown of the blood-ocular barrier during inflammation?

The blood-retinal barrier becomes more permeable Signup and view all the answers

What is the function of the non-pigmented ciliary epithelial cells?

To regulate the secretion of aqueous humor Signup and view all the answers

What is the significance of the blood-ocular barrier?

It protects the neural retina from unwanted substances Signup and view all the answers

What happens to the dark current in the photoreceptor when photons activate the phototransduction cascade?

It decreases Signup and view all the answers

What is the effect of the closure of cGMP-Na+ channels on the photoreceptor membrane?

It becomes more negative Signup and view all the answers

What happens to the K+ channels in the photoreceptor during the phototransduction cascade?

They open Signup and view all the answers

What is the result of the hyperpolarisation of the photoreceptor membrane?

Decrease in glutamate release Signup and view all the answers

What is the role of PDE in the phototransduction cascade?

It converts cGMP to GMP Signup and view all the answers

What is the first step in the phototransduction cascade?

Light photon activates rhodopsin Signup and view all the answers

What is the effect of the closure of cGMP-Na+ channels on the photoreceptor?

It decreases the dark current Signup and view all the answers

What happens to the K+ channels during the hyperpolarisation of the photoreceptor membrane?

They remain the same Signup and view all the answers

What is the result of the decrease in glutamate release from the photoreceptor?

Decrease in the signal transmission Signup and view all the answers

What is the final result of the phototransduction cascade?

Hyperpolarisation of the photoreceptor membrane Signup and view all the answers

What is the main characteristic of Midget ganglion cells?

Smaller receptive field Signup and view all the answers

Which type of ganglion cells detect low contrast and are movement sensitive?

Parasol ganglion cells Signup and view all the answers

What is the function of Horizontal cells?

Provide feedback to photoreceptors Signup and view all the answers

Which type of Horizontal cells connect primarily to S cones and rods?

HII Signup and view all the answers

What is the characteristic of Bistratified ganglion cells?

S cone pathway Signup and view all the answers

Where are the dendrites of Horizontal cells located?

Outer plexiform layer Signup and view all the answers

What is the main role of Horizontal cells?

Provide feedback to photoreceptors Signup and view all the answers

Which type of ganglion cells have a larger receptive field than Midget ganglion cells?

Parasol ganglion cells Signup and view all the answers

What is the characteristic of Parasol ganglion cells?

Input to magnocellular layers in the LGN Signup and view all the answers

Which type of Horizontal cells connect to L and M cones?

HIII Signup and view all the answers

Which part of the visual cortex receives branches from the posterior cerebral artery (PCA)?

Medial surface Signup and view all the answers

What is the primary function of understanding the blood supply to the brain?

To identify the source of neuro-ophthalmological problems Signup and view all the answers

Which of the following arteries forms the anastomotic branches at the circle of Willis?

Internal carotid artery Signup and view all the answers

What is the origin of direct feedback projections to V1?

V2, V3, V4, V5, or Mid Temporal (MT) Signup and view all the answers

Which part of the visual cortex receives cortical branches from the middle cerebral artery?

Lateral surface Signup and view all the answers

What is the function of the posterior cerebral artery (PCA)?

To supply the posterior part of the brain Signup and view all the answers

Which of the following is a branch of the posterior cerebral artery?

Posterior temporal artery Signup and view all the answers

What is the role of the branches that penetrate directly into the visual cortex?

To supply the visual cortex with blood Signup and view all the answers

Which of the following is NOT a target of direct feedback projections from V1?

Frontal eye field-FEF Signup and view all the answers

What is the primary function of the blood supply to the brain?

To supply the visual cortex with blood Signup and view all the answers

Study Notes

Accommodation and Pupil Reflex

Accommodation: the process of focusing on objects that are close
Accommodation/near reflex: consists of accommodation, pupil constriction, and convergence
Pupil reflex: controlled by the parasympathetic pathway via the ciliary ganglion and CNIII

Pathways and Muscles

Parasympathetic pathway: stimulates the ciliary muscle and constricts the pupil
CNIII (oculomotor nerve): controls the medial rectus muscle for convergence
Ciliary muscle receptors: dominated by muscarinic M3 receptors with adrenergic β2 receptor input
Iris: contains dilator muscle with adrenergic receptors and sphincter muscle with muscarinic receptors

Diagnostic Drugs

Tropicamide: an anticholinergic that blocks muscarinic receptors, causing pupil dilation and impaired accommodation
Cyclopentolate: a diagnostic drug used to induce cycloplegia

Ciliary Body

Ciliary stroma: supports the ciliary body, contains loose connective tissue and vessels
Anterior surface of the iris: has no epithelial covering
Substances released into the ciliary body: diffuse into the aqueous humor through the anterior surface of the iris
Ciliary body: lies over the ciliary muscle, approximately 5 mm anterior to the equator

Ciliary Muscle

3 muscle groups: longitudinal/meridional/Brucke's muscle, radial, and circular/Muller's muscle
Longitudinal muscle: closest to the sclera, tendon attaches to the scleral spur
Radial muscle: closest to the lens
Circular muscle: closest to the lens, works in unison with the parasympathetic innervation

Accommodation Mechanism

Ciliary muscles move anterior to the lens, reducing zonular fiber contraction, allowing the lens to bulge (accommodate)
Parasympathetic innervation: stimulates the ciliary muscle to contract and move towards the lens
Sympathetic innervation: does not directly affect the ciliary muscle

Aqueous Production

Bulk movement of blood plasma into the ciliary stroma: via fenestrated capillaries
Formation of plasma ultrafiltrate reservoir: in the stroma
Active secretion: requires energy, contributes to the chemical composition and volume of the posterior chamber aqueous
Steps of aqueous production:
- Step 1: NaCl is transferred from stroma to PE cells by electroneutral transporters
- Step 2: Passage of NaCl from PE to NPE cells through gap junctions
- Step 3: Primary active transport (Na+K+ ATPase) and secondary active transport (Cl- and HCO3- via Cl- exchange)

Anterior Chamber

Acknowledgement of Noongar land and people

Anterior Chamber Angle

Junction between cornea and iris
Drainage of aqueous humour

Structures in Anterior Chamber Angle

Schwalbe’s Line
Anterior TM
Posterior TM
Scleral Spur
Ciliary Body
Iris

Gonioscopy

Visualization of anterior chamber angle structures

Histology of Iris Insertion

Iris root inserts into anterior border of ciliary body
Ciliary sulcus: space between posterior surface of iris base and anterior ciliary body
Sulcus = depression, furrow

Histology of Scleral Sulcus

Shallow indentation posterior to limbus
Bridged by trabecular meshwork (posteriorly) and forms Schlemm’s canal (anteriorly)

Histology of Schwalbe's Line

Termination of Descemet’s membrane
Most anterior structure of angle
Corneal endothelium transitions to trabecular meshwork (TM)

Histology of Trabecular Meshwork

Connective tissue beams enclosed by trabecular cells
Avascular ‘lamellated meshwork’
Three regions: uveal, corneoscleral, and juxtacanalicular/corniform meshwork
Trabecular cells phagocytose debris

Histology of Trabecular Meshwork Regions

Uveal meshwork: closest to anterior chamber
Corneoscleral meshwork: middle layer
Juxtacanalicular/corniform meshwork: loose CT, sieve-like plates, main source of resistance to aqueous flow

Phenotype of Trabecular Cells

Endothelial: maintain passageway patency, neutralize reactive oxygen species
Macrophage: biological filter/phagocytosis, immune mediation
Fibroblastic: ECM turnover/tissue repair
Smooth muscle: contractile tone, mechanotrasduction

Histology of Schlemm's Canal

Outer Wall: endothelial cells, well-developed basement membrane
Inner Wall: endothelial cells, incomplete sub-endothelial layer, tight junctions

Histology of Scleral Spur

Posterior wall of Schlemm’s canal
Site of anterior ciliary muscle attachment
Prevents Schlemm’s canal from collapsing under ciliary muscle contraction

Venous Drainage of Aqueous

TM → Schlemm’s canal → collector channel → episcleral plexus → ophthalmic veins
Direct and indirect routes to episcleral plexus

Uveoscleral Drainage

10% drainage
Fluid passes between uveal trabeculae and ciliary muscle bundles → episclera
Affected by drugs and state of accommodation (ciliary muscle)

Ciliary Muscle and Aqueous Flow

Relaxed ciliary muscle: ↓ trabecular outflow, ↑ uveoscleral outflow
Contracted ciliary muscle: ↑ trabecular outflow, ↓ uveoscleral outflow

Cranial Nerve III - Oculomotor Nerve

Innervates superior, inferior, and medial rectus muscles, as well as inferior oblique and levator palpebrae superioris muscles
Divides into superior and inferior divisions before entering the superior orbital fissure
Superior division innervates superior rectus and levator palpebrae superioris muscles
Inferior division innervates medial and inferior recti, inferior oblique, ciliary, and iris sphincter muscles
Enters the orbit through the superior orbital fissure, traveling along the lateral cavernous sinus
Synapses in the ciliary ganglion, and then travels as short ciliary nerves to the sphincter pupillae and ciliary muscle

Cranial Nerve IV - Trochlear Nerve

Innervates the superior oblique muscle
Leaves the posterior brain stem, decussates, and runs forward along the lateral cavernous sinus
Enters the orbit through the superior orbital fissure, superior to the common tendinous ring

Cranial Nerve V - Ophthalmic Division

Passes through the superior orbital fissure inside the common tendinous ring
Branches into the nasociliary nerve, anterior ethmoidal nerve, and infratrochlear nerve
Provides sensory innervation to the ciliary body, iris, sclera, and cornea
Also provides sensation to the lateral side of the forehead and skin of the medial lower eyelid and lateral nose

Cranial Nerve VII - Facial Nerve

Has sensory, parasympathetic, and motor functions
Provides proprioception from facial muscles, taste from the anterior 2/3 of the tongue, and sensation to the lateral side of the forehead
Innervates the lacrimal gland, mucous membranes of the nasal and oral cavities, and salivary glands
Has six main branches: temporal, zygomatic, buccal, marginal, cervical, and posterior auricular branch
Innervates muscles of facial expression, including orbicularis oculi and orbicularis oris

Cranial Nerve VIII - Vestibulocochlear Nerve

Contributes to the innervation of the eye
Does not directly innervate the eye, but is involved in the sensation of hearing and balance

Blood Supply to the Orbit and Globe

Blood to the eye:
- Right internal carotid artery → right ophthalmic artery → orbit
- Left internal carotid artery → left ophthalmic artery → orbit
Internal carotid artery:
- Runs through neck, enters cranial vault via carotid canal
- Bifurcates into ophthalmic artery as it exits cavernous sinus
Ophthalmic artery:
- Enters orbit via optic canal (along with optic nerve)
- Bifurcates into multiple arteries

Blood Supply to the Globe

Central retinal artery:
- 1st branch of the ophthalmic artery
- Pierces the dura ~ 1 cm from globe and enters optic nerve
- Many collateral branches serve the ON fibres prior to ONH
- Exits optic nerve, visible when viewing fundus
Cilioretinal artery:
- Short posterior ciliary artery
- Radial peripapillary capillary plexus
- Superficial vascular plexus
- Intermediate capillary plexus
- Deep capillary plexus

Venous Return from the Globe

Inner retina venous return:
- Post capillary venules → branched retinal veins → central retinal vein
- CRV empties into superior ophthalmic vein or cavernous sinus
Short posterior ciliary arteries:
- Pierce sclera in an annulus around the ONH
- Supply choroid and outer retina
- Form circle of Zinn-Haller to supply optic nerve
Long posterior ciliary arteries:
- Pierce posterior sclera to supply choroid and ciliary body
- Join major circle of the iris to supply ciliary body and iris

Other Arteries

Muscular arteries:
- From ophthalmic artery, 2 branches
- Superior branch supplies levator, superior rectus, superior oblique, and lateral rectus
- Inferior branch supplies inferior rectus, inferior oblique, and anterior ciliary arteries
Anterior ciliary arteries:
- Branch from muscular arteries
- One branch joins the major circle of iris to supply iris and ciliary body
- One branch supplies episclera and conjunctiva
Lacrimal artery:
- 2nd branch off ophthalmic artery
- Runs along orbital wall with lateral rectus
- Muscular branch supplies lateral and superior rectus muscle
- Glandular branch supplies lacrimal gland
- Recurrent branch anastomoses with meninges
Zygomatic artery:
- Branch off the lacrimal artery before lacrimal gland
- Supply orbicularis and other facial muscles
Lateral palpebral artery:
- Terminal branch of the lacrimal artery
- Supply lower and upper lids
- Anastomose with the medial palpebral artery

Accommodation

Accommodation involves a dioptre change in the optical power of the eye, which allows the eye to focus on objects up close.
The lens becomes thicker and more curved, increasing its refractive power.

Ocular Structures

Ciliary muscle:
- A smooth muscle controlled by the autonomic nervous system.
- Arranged in 3 bundles: outer longitudinal, middle radial, and inner circular.
- Attaches anteriorly to the scleral spur and trabecular meshwork (TM) and posteriorly to the elastic network of Bruch's membrane and the choroid.
Zonules:
- Arise from the pars plana.
- Non-collagenous glycoprotein secreted by the ciliary epithelium.
- Primary function is to stabilize the lens and facilitate accommodation.
- Three groups of fibers: anterior, equatorial, and posterior zonules.
Lens capsule:
- Covers the whole lens.
- A continuous basement membrane with elastic properties that envelops the lens.
- Serves as the point of attachment of the lens zonules.
- Regional variations in thickness.

Accommodation Mechanism

Ciliary muscle contraction:
- During accommodation, the outer longitudinal fibers contract, decreasing the muscle area.
- Main mass of ciliary muscle moves forward along the curved inner wall of the sclera towards the scleral spur.
- This forces the inner radial fibers and overlying ciliary processes to bulge inwards towards the posterior chamber, increasing the muscle area.
Zonules:
- During accommodation, the zonules relax, allowing the lens to become more curved.
Lens capsule:
- The lens capsule draws the lens into a more accommodated form.
- Decapsulation results in flattening and an increase in focal length.

Lens Shape and Displacement

During accommodation, the lens becomes more curved and the anterior pole is displaced towards the anterior chamber.
The most displacement occurs in the nucleus, not the cortex.
Posterior pole remains unchanged.
Forward displacement leads to shallowing of the anterior chamber, increased lens convexity, and closer proximity of the cornea to the anterior lens.

Neural Control of Accommodation

The neural control of accommodation originates from the retina.
The blur signal is sent to the primary visual cortex, then to the pretectal area, and finally to the Edinger-Westphal nucleus.
Two signals are then sent: one via the parasympathetic pathway to the ciliary muscle (accommodation) and the other via the CN III to the medial rectus (convergence).
Parasympathetic nerves dominate the ciliary muscle, causing muscarinic-mediated contraction.
Sympathetic nerves have a minor role in relaxation.

Presbyopia

Presbyopia is a loss of accommodation with age.
Common presentation in clinic, onset around 40 years old.
Gradual inability to accommodate due to increased stiffness of the crystalline lens and changes in zonule insertion with age.
Optical correction is necessary.

Vitreous Anatomy

The vitreous is a flattened sphere that takes up approximately 80% of the volume of the globe.
It is a transparent, gel-like structure surrounded by a thin collagenous membrane.
The vitreous has an anterior indentation (also known as the hyaloid/patellar fossa) that surrounds the lens.
It has a firm attachment to the retina.

Vitreous Structure

The vitreous has three anatomical zones: central/medullary, cortex, and basal.
The central/medullary zone is cell-free, contains collagen fibrils and hyaluronic acid, and is in a gel state.
The cortex zone is approximately 100 µm thick, has a higher collagen concentration, and is the metabolic center of the vitreous.
The basal zone is dense, thickened, and has the highest hyalocyte density.

Anatomical Zones

The anterior hyaloid membrane is the anterior border of the vitreous, has high collagen density, and is attached to the posterior lens capsule.
The posterior hyaloid membrane separates the vitreous from the retina, is not a typical membrane, and has densely packed type II collagen.

Site of Attachments

The vitreous has attachment points to the inner limiting membrane of the retina, ora serrata, optic nerve head (ONH), posterior lens capsule, and ciliary epithelium.
Strong attachment sites include the vitreous base, ONH, blood vessels, and around the foveola.
Attachments weaken with age.

Vitreous Components

The vitreous is composed of 99% water, 1% structural proteins, and a few hyalocytes in the periphery.
The structural proteins include type II, IX, and XI collagen, as well as hyaluronic acid.
Hyaluronic acid is involved in modulating immune responses.

Vitreous Biochemistry

Collagen provides mechanical strength to the vitreous, with type II collagen forming fibrils, type IX collagen providing structural support, and type V/XI collagen co-assembling fibrils with type II.
Hyaluronic acid is a large glycosaminoglycan that maintains vitreous hydration, ensures proper collagen spacing, and provides vitreal viscoelastic properties.

Vitreous Biochemistry - Hyalocytes

Hyalocytes are mononuclear cells that produce hyaluronic acid and collagen.

Vitreous Function

The vitreous provides optical clarity due to its biochemical structure, which includes type II collagen, hyaluronan, and glycoproteins.
The vitreous maintains the shape of the globe, supports the lens anteriorly, and keeps the retina adherent to the choroid/sclera.
The vitreous absorbs external forces and reduces mechanical deformation of the globe.
The vitreous acts as a molecule repository, receiving metabolites from the ciliary body and retina, and contains ions and organic molecules.
The vitreous promotes intraocular wound healing via vitamin C.

Vitreous Function - Blood-Ocular Barrier

The vitreous acts as a barrier preventing the movement of macromolecules and cells between the anterior and posterior segments.

Aging Changes

Liquefaction of the vitreous occurs in the central region, leading to changes in hyaluronic acid conformation and the formation of floaters.
Collagen fibril networks become free-floating bundles, leading to a decrease in mechanical strength.

Macular Pigment

Found in the macula lutea, a generic term describing yellow pigments composed of three dietary carotenoids: lutein, zeaxanthin, and meso-zeaxanthin.
Hypothesized functions: reduce light scatter and chromatic aberration, absorb blue light, decrease oxidative damage on photoreceptors, and have antioxidant properties.

Fovea

Centre of the macula, responsible for high special vision.
No rod photoreceptors, only maximum cone packing.
Centre of the fovea: foveal pit, no retina overlying the foveal pit.

Retinal Layers

10 layers in the retina, from outermost to innermost:
- 1. Retinal pigmented epithelium (RPE)
- 1. Photoreceptor layer
- 1. Outer limiting membrane (OLM) / External limiting membrane (ELM)
- 1. Outer Nuclear Layer (ONL)
- 1. Outer Plexiform Layer (OPL)
- 1. Inner Nuclear Layer (INL)
- 1. Inner Plexiform Layer (IPL)
- 1. Ganglion cell layer (GCL)
- 1. Retinal nerve fibre layer (RNFL)
- 1. Inner Limiting membrane (ILM)

Photoreceptor Layer

Contains the outer and inner segments of the rod and cone photoreceptors.
Photoreceptors are tightly packed to ensure sharp central vision.
Highest concentration of photoreceptors in the central retina.

Processes of Visual Signal

Through pathway: Photoreceptors → Bipolar cells → Ganglion cells.
Lateral pathway: Horizontal and amacrine cells, local feedback, optimizing through pathway.

Rod vs Cone Convergence

126 million rods and cones converge to 1 million ganglion cells.
Higher convergence of rods than cones.
Average of 120 rods to one ganglion cell, average of 6 cones to one ganglion cell.
Cones in the fovea have a 1:1 connection to ganglion cells.

Central vs Peripheral Retina

Central retina: thicker, more photoreceptors, dominated by cones.
Peripheral retina: thinner, fewer photoreceptors, dominated by rods.

Optic Nerve Head (ONH)

Located approximately 17° (4.5 to 5mm) or 2.5 disc diameters nasal to the fovea.
Pinkish, oval area measuring 2 * 1.5 mm.
Optic cup: white central region, does not contain ganglion cell axons.
Neural retinal rim: contains ganglion cell axons.

Glial Cells

Support cells for retinal neurons.
3 types: Müller cells, Astrocytes, and Microglia.
Microglia function as macrophages and engage in phagocytosis of degenerating retinal neurons in response to trauma.

Optic Nerve Structure

No ganglion cells are found within the optic nerve head/optic disc.
The optic cup is bigger, with less neuroretinal ring and fewer ganglion cells.

Organization of Fibres

Fibres are organized into bundles that converge towards the disc.
There is no overlap between upper and lower halves of the retinal fibres.
The horizontal raphe of nerve fibres divides the superior and inferior retina.
The arcuate pattern of nerve fibres serves to ensure that there is no overlapping.

Cell Bodies and Astrocytes

Cell bodies are located in the nerve fibre layer.
Astrocytes are supporting cells (glial cells) that provide physical support for neurons, repair, and maintenance of the blood-retina barrier.
There are two types of astrocytes: Type 1 astrocytes line the periphery of the optic nerve, and Type 2 astrocytes are primarily located in the interior of the optic nerve.

Laminar Region

The laminar region has a specialized extracellular matrix, the lamina cribrosa, which consists of a band of dense, sieve-like fenestrated sheets of connective tissue and occasional elastic fibers.
The lamina cribrosa bridges the scleral canal and has 3-10 sheets of connective tissue with approximately 400 fenestrations.
The lamina cribrosa allows the passage of nerve bundles.

Blood Supply

The optic disc surface is supplied by the central retinal artery (CRA) and choroid.
The outer retina is supplied by the choroid.
The prelaminar and laminar regions are supplied by short posterior ciliary arteries.
The retrolaminar region has an independent blood supply.
The circle of Zinn-Haller is a circular arterial anastomosis at the scleral level.

Blood-Retina Barrier

Plasma proteins readily leak from the choroid into the ONH directly and through the sclera.
The entry of proteins from the ONH into the retina is blocked by a series of tight junctions between the lining glial cells and the adjacent RPE.

Choroid Anatomy

Located between the sclera and retina
Thins with age, from 200 μm at birth to 80 μm by 90 years old
Highly vascularized, supplied by short posterior ciliary arteries (SPCA) and long posterior ciliary arteries (LPCA), and drains via vortex veins

Choroid Functions

Supplies nutrients and oxygen to the outer retina
Regulates thermoregulation via heat dissipation due to high blood flow
Modulates intraocular pressure (IOP) via blood flow control
Aqueous humour drainage via the uveoscleral pathway, accounting for 10-15% of drainage in humans

Choroid Layers

5 layers, including Haller's and Sattler's layers, which are histologically defined but do not have precise borders in imaging
Bruch's membrane, a 2-4 μm thick avascular layer, undergoes age-related changes such as fibre calcification, increased fibre cross-linkage, and accumulation of fat and advanced glycation end products

Choriocapillaris

A thin sheet of capillaries, thickest at the fovea (10 μm) and thinning to 7 μm peripherally
Feeder arteriole from Sattler's layer supplies a hexagonal area

Sattler's and Haller's Layers

Sattler's layer: 100 μm thick, contains medium to small-sized vessels
Haller's layer: 200 μm thick, contains large-sized vessels

Suprachoroid

A 30 μm thick layer, transition between the choroid and sclera
Contains collagen fibres, fibroblasts, and melanocytes, of pharmacological interest as a drug delivery target

Clinical Assessment of the Choroid

Indocyanine green (ICG) angiography: uses ICG dye to investigate blood flow in the choroid, appearing as generalized hyperfluorescence
Optical coherence tomography angiography (OCTa): a non-invasive, relatively new technique that detects movement of red blood cells and produces enface imaging of retinal and choroid vessels

Diseases Affecting the Choroid

Vogt–Koyanagi–Harada's disease: characterized by choroidal granuloma and absence of blood vessels on ICGA
Age-related macular degeneration (AMD): characterized by thickening of Bruch's membrane and formation of drusen due to accumulation of waste material

Inflammation

Inflammation is the body's immune system response to an irritant, which can be acute or chronic
Involves cells, proteins, and other mediators from both innate and adaptive immune systems
Initiated by signals from injured cells or cells of the immune system
Causes 'controlled' destabilization of blood vessels and associated exudation of blood contents, and altered blood flow in the local area

Purpose of Inflammation

Protection response: eliminates the cause of injury, limits the geographic extent of consequences, and prevents sequelae of injury
Initiates the healing and regeneration process
Inflammation is a good thing, but if it's too aggressive, it can do more damage than the injury

Clinical Signs of Inflammation

Signs of acute inflammation: Rubor (redness), Calor (heat), Tumor (swelling), and Dolor (pain)
Lead to loss of function

Cells of Inflammation

Innate immune cells: part of the removal of tumor cells and present antigens to the adaptive immune cell
Adaptive immune cells: do not generate antibodies, but activated B-lymphocytes do
Cells involved in inflammation: eosinophil cells, neutrophil cells, and B-lymphocytes

Inflammation in the Eye

Ocular surface allergy: eosinophil cells and serous fluid
Corneal infection: neutrophil cells inside the anterior chamber and in corneal tissue
Anterior chamber reaction: cells and proteins (fibrin) in the anterior chamber
Chorioretinal inflammation: cells and proteins in the posterior segment of the eye

Ocular Immune Privilege

Cell and protein response in inflammation inside the eye can reduce vision (both short-term and lead to permanent visual loss)
System exists to ensure that excessive immune response inside the eye is limited – 'ocular immune privilege'
Evidence for ocular immune privilege: success of corneal transplantation, unrestricted growth of non-self tumor cells inside the eye, and aqueous and vitreous fluids inhibit inflammatory cells in vitro
Signaling protein TGF-β2 is high in the eye, produced by RPE, and pigment epithelium of ciliary body and iris, which 'disarms' inflammatory cells that cross from the blood stream into the eye

Blood Ocular Barriers

3 Main sites of barriers: iris, ciliary body, and retina
Iris: barrier to prevent passage of macromolecules, capillaries not fenestrated, tight junctions (and related junctions) form the cell-cell bond
Ciliary body: blood vessels in ciliary stroma leak, no barrier to macromolecules, but no leakage outside the ciliary stroma – barrier within the epithelial cells
Retina: 2 separate locations – inner retinal vessels (tight junctions in capillary endothelium, non-fenestrated) and choroidal vessels (fenestrated, target delivery of blood products to retina)

Blood Ocular Barrier Breakdown

Inflammatory cells and proteins mix with aqueous when iris and ciliary body inflamed
Tight junctions break down in inflammation allowing this to happen
Proteins usually mainly fibrin, cells depend on what is causing the inflammation

Anatomy of Phototransduction

The University of Western Australia acknowledges that its campus is situated on Noongar land.

Photoreceptor

Found in the outer retina, made up of cones and rods, with no rods at the fovea.
Function: Convert light energy into electrical energy.

Photoreceptor Anatomy

Inner segment: responsible for synthesizing all proteins required for photoreceptors.
Maintained by Na+/K+ ATPases.
Constant release of the excitatory neurotransmitter glutamate.
Requires energy via Na+/K+ ATPases.

Park Current

Refers to the flow of ions, particularly sodium, into the photoreceptor cell when not exposed to light.
Rod photoreceptors maintain a relatively depolarized state (-40mV) due to the inflow of sodium ions through CGMP-gated channels.

Role of Sodium-Potassium Pump

Actively transports sodium out and potassium into the cell to maintain ionic balance and cell potential.

Rod Activation

Photons activate the phototransduction cascade, leading to:
- Closure of cGMP-Na+ channels, reducing dark current.
- K+ channels still open, allowing K+ to flow out of the photoreceptor.
Resulting in hyperpolarization of the photoreceptor membrane (-70mV).
Decrease in glutamate release.

Phototransduction Cascade

Step 1: Light photon activates rhodopsin.
Activated PDE converts cGMP to GMP, causing closure of cGMP-gated channels.

Photoreceptor/Bipolar Cell Synapse

The junction between rods or cones and bipolar cells is a specialized organelle called a ribbon synapse.
Located in the outer plexiform layer, it is responsible for transmitting signals from photoreceptors to bipolar cells.

Neurotransmitter Release

The main neurotransmitter for signal propagation in the retina is glutamate.
Glutamate release requires high rates of neurotransmitter release in the synapse.

Bipolar Cell Subtypes

ON bipolar cells tend to stratify in sublamina 3-5.
OFF bipolar cells tend to stratify in sublamina 1-2.
Some bipolar cells have multiple dendrites to connect to multiple photoreceptors.

Ganglion Cells

ON and OFF ganglion cells synapse at different sublamina in the inner plexiform layer (IPL).
Rod bipolar cells do not contact ganglion cells.
ON bipolar cells and ON ganglion cells synapse at IPL sublamina b, while OFF bipolar cells and OFF ganglion cells synapse at IPL sublamina a.

Ganglion Cell Subtypes

Parasol ganglion cells have a larger receptive field, input to magnocellular layers in the LGN, and detect low contrast and movement.
Midget ganglion cells have a smaller receptive field, input to parvocellular layers in the LGN, and are involved in perception of color and fine detail.
Bistratified ganglion cells are involved in the S cone pathway and proprioception.

Horizontal Cells

Horizontal cells provide feedback to photoreceptors and have a lateral pathway.
They have dendrites in the outer plexiform layer and a cell body in the inner nuclear layer.
There are three subtypes of horizontal cells: HI, HII, and HIII, which connect to different types of cones and rods.

Visual Pathway II: LGN to Visual Cortex

The visual pathway involves the transmission of information from the retina to the lateral geniculate nucleus (LGN) and then to the visual cortex.

Lateral Geniculate Nucleus (LGN)

The LGN is located in the thalamus, just above the optic tract.
It has a dorsal (upper) and ventral (lower) part, with the dorsal part receiving retinal information.
The LGN consists of six layers: 2 magnocellular, 2 parvocellular, and 2 koniocellular layers.
The layers have specific spatial relationships to surrounding structures.

Optic Radiations

The optic radiations carry information from the superior and inferior fields of each eye.
Lesions involving the optic radiations often result in field defects in one quadrant.
The optic radiations have different spatial relationships to surrounding structures, with Meyer's loop and Baum's loop being specific parts of the optic radiations.

Visual Cortex

The visual cortex receives input from the optic radiations and is organized in a retinotopic map, with each point in the visual field corresponding to a specific location in the visual cortex.
The visual cortex consists of several areas, including V1, V2, V3, V4, and V5, each with specific functions and organization.
V1 is also known as the primary visual cortex, striate cortex, or Brodmann area 17.

Receptive Field

A receptive field is the area in which stimulation leads to a response in a particular sensory neuron.
The receptive field is preserved in the spatial organization of the neurons within the LGN layers.
The arrangement of ganglion cells in the retina is preserved in the LGN layers.

Organization of V1

V1 is organized in a laminar arrangement, with specific layers receiving input from the LGN.
The layers have specific anatomical and functional features.
The cortex is also arranged in a columnar arrangement, with cells being organized in vertical columns.
The organization of V1 is similar to other neocortical areas.