Physiology and Nervous System Quiz

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)
  • 2. Photoreceptor layer
  • 3. Outer limiting membrane (OLM) / External limiting membrane (ELM)
  • 4. Outer Nuclear Layer (ONL)
  • 5. Outer Plexiform Layer (OPL)
  • 6. Inner Nuclear Layer (INL)
  • 7. Inner Plexiform Layer (IPL)
  • 8. Ganglion cell layer (GCL)
  • 9. Retinal nerve fibre layer (RNFL)
  • 10. 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.

Blood Supply

• The blood supply to the brain branches from the internal carotid artery and vertebral artery.
• The posterior cerebral artery (PCA) supplies the medial surface of the visual cortex, while the middle cerebral artery supplies the lateral surface.
• The blood supply is important for understanding localized vascular damage caused by stroke or other obstructions.

Feedback Projections

• Direct feedback projections to V1 originate from V2, V3, V4, V5, Mid Temporal (MT), Frontal eye field (FEF), and infero-temporal cortex.
• Feedback projections from V1 extend to Superior colliculus (SC), LGNd, pulvinar, and pons.