Anatomy of the Eye and Vision

• The lecture discusses the eye and vision.
• Wesley Ward can be contacted at [email protected].

Learning Outcomes

• Describe the anatomy of the eye.
• An introduction to the concept of sensory physiology.
• Trace the light path to the retina.
• Overview to corneal repair
• Introduction to eye diseases

Anatomy of the Eye

• Three major anatomical regions are external anatomy and internal anatomy, which consists of layers, compartments, and structures.

Anatomy of the Eye - External Anatomy

• Eyebrows and eyelids deflect sweat and debris and provide shade from sunlight, contributing to facial expression and nonverbal communication.
• Eyelids (palpebrae) protect the eye from injury and drying.
• Eyelids spread tears across the eye's surface and regulate light entry.
• The conjunctiva is a thin, transparent mucous membrane.
• Palpebral conjunctiva lines the inner surface of the eyelids.
• Bulbar conjunctiva covers the sclera (white of the eye).
• The fornix is the area where the palpebral and bulbar conjunctiva meet.
• Conjunctiva lubricates the eye's surface and provides a barrier against infection
• Lacrimal glands produces tears and lacrimal sacs collect tears.
• The nasolacrimal duct drains tears into the nasal cavity.
• Tears provide lubrication, nutrients, and antimicrobial protection.
• Extraocular muscles control eye movement and enable gaze in different directions.
• Superior rectus raises eye
• Inferior rectus lowers eye
• Medial and lateral rectus move the eye sideways

Anatomy of the Eye - Internal Anatomy

• The eye's layers are the fibrous tunic (outer), vascular tunic (middle), and nervous tunic (inner).
• Sclera does not cover cornea
• The sclera is a dense, fibrous connective tissue, the "white of the eye" due to its collagen content.
• The sclera is relatively avascular and contains some vessels.
• The episclera (loose connective tissue layer) covers the sclera and protects from major damage.
• The sclera offers structural support, maintains the eye's shape, and serves as an attachment point for extraocular muscles.
• Sclera protects the internal components of the eye.

Cornea

• The cornea is transparent and avascular, facilitating clear vision.
• It is highly innervated and sensitive to touch.
• The cornea is composed of five layers and the epithelium is the outermost layer, providing a barrier.
• Bowman's layer has a collagen layer, providing strength.
• Stroma is the thickest layer, mostly collagen.
• Descemet's membrane is a basement membrane.
• Endothelium is the innermost layer and maintains corneal hydration.
• The primary refractive surface of the eye (focuses light) and protects the internal structures.
• The choroid is highly vascular, pigmented, located between the sclera and retina.
• The structure provides blood supply to the retina's outer layers and absorbs stray light to prevent internal reflections
• The ciliary body is a ring-shaped structure behind the iris.
• Ciliary muscles are smooth muscle fibers that control lens curvature, allow changed in depth.
• The ciliary processes are epithelial projections that secrete aqueous humor (provides rigid shape of the eye).
• Zonular Fibers (Suspensory Ligaments) connect the ciliary body to the lens.

Iris

• The iris is pigmented, with a circular structure between the cornea and lens.
• The pupil is an opening in the center of the iris.
• Sphincter pupillae Muscle is a circular muscle that constricts the pupil (miosis).
• The dilator pupillae Muscle is a radial muscle that dilates the pupil (mydriasis).
• The iris controls pupil size, regulating the amount of light entering the eye.
• Iris consists of two smooth muscles.
• Sphincter pupillae muscle (inner circular) constriction acts through muscarinic receptor agonists.
• Dilator pupillae muscle (outer radial muscle) dilation acts through muscarinic receptor antagonists.
• Pupil size modulation before light reaches the retina: in bright light, the pupil narrows; in dark, it opens.
• Bright light constricts the pupil to ~1.5 mm.
• Dark conditions opens the pupil to ~8 mm.
• This produces a 28-fold change in pupil area.
• Accommodation changes lens shape.
• During accommodation, the lens changes shape to adjust refractive power to focus the image on the retina.
• Ciliary muscle (relaxed) lens elongated indicates visualization of distant objects (relaxed eye).
• Ciliary muscle (contracts) lens rounded indicates visualization of close objects (accommodation).
• The process is under the control of the parasympathetic system.
• Acetylcholine helps act through muscarinic receptors (parasympathetic stimulation of the ciliary muscle).

Retina

• The retina houses the macula, the central area responsible for sharp, central vision.
• The fovea centralis is a small depression in the macula and contains the highest concentration of cones for greatest visual acuity.
• The optic disc is where the optic nerve exits ("blind spot", lacks photoreceptors).
• Converts light into neural signals
• Initiates visual process
• Light-sensitive inner layer of the eye
• Photoreceptor Cells: Rods & Cones are present
• Rods are sensitive to low light levels for black-and-white vision.
• Cones are sensitive to bright light levels for color vision and concentrated in the macula and fovea centralis.

Retina Structure

• Composed of three layers: inner (ganglion cells), middle (bipolar cells), and outer (photoreceptors).
• The layers are in 'reverse order' and light must pass through inner and middle layers, and blood vessels before photoreceptors.
• Rods give black and white vision, while cones give colour vision.
• The abundance of rods is 100 x 10^6 /retina and the concs is in the periphery
• The abundance of cones is 3 x 10^6 /retina and the concs is in the fovea
• Convergence of Rod signals increases sensitivity and allows detection in low light.
• Low light stimulates Rods and the information is pooled.
• Rods have high sensitivity but low resolution, leading to lower visual acuity.
• Cones require you to have high stimulus levels.
• There are more rods to bipolar cells, than cones to bipolar cells.
• Pigmented epithelial layer, as well as light sensitive visual pigments are present.
• First step of Phototransduction:Light is Absorbed by Photopigments
• Rods contain Rhodopsin
• Retinal (light-absorbing part in all pigments) Opsin (varies for every pigment)
• Cones have pigments that closely related to rhodopsin
• Cone cells consist of Red, Green & Blue Light
• (Rhodopsin) destroysed by bright light
• Photoreceptors have to synthesise new disks (containing photopigment)
• Phototransduction cascase is present in outer section of photoreceptor. This is where Rhodopsin, G-Protein, CGMP, Phosphodiesterase are.
• CGMP gated sodium channels are present.
• When Light hits photoreceptor, the retinal section of rhodopsin is activated, stimulating transducin.
• Transducin helps activate cyclic GMP which then activates phosphodiesterase.
• When it is dark, cGMP binds to the cation channels of the outer segments of the photoreceptor cells and keeps them open. Light then activates phosphodiesterase to close them.
• In synthesis, rhodopsin forms, increases in dark. Vitamin A is isomerized. Vitamil A is then combined with opsin forms rhodopsin
• Pigment synthesises when retina reconverts back to cis isomers and then returns to photoreceptor cells outer sections.

Cornea

• In bleaching, rohdopsin absorbs light and conforms to all-trans isomer.
• Pigment synthesis and bleaching

Additional

• Fluid-filled eyes maintained by vitreous humor in the back and aqueous humor in the front between the cornea and lens.
• Vitreous humor helps to maintain eye shape.
• Aqueous humor, continuously produced/recycled, regulates ocular pressure and provides nourishment (amino acids, electrolytes).
• Sensory physiology has sensations, that sense environment.
• Perceptions help interprelation of signals.
• Thalamus relays information to the visual cortex, is linked to functions.
• Multiple functions occurs with translators, inputs and transferred information.
• The pupil reflex assesses sensory and motor function: shine light in one eye, both pupils should constrict.
• Information from one side of the visual field is processed on the opposite side of the brain.
• The projected image is upside down on the retina and the visual processing in the brain reverses the image.
• Figure-ground concept is a optical illusion (Rubin's vase).
• Impossible to interprete both figure and ground simultaneously

Corneal Repair and Regeneration

• Eye cells in the cornea heal in about 48 hours.
• The limbus of the cornea forms a border between corneal and conjunctival epithelium.
• Limbal stem cells are essential for maintenance and repair.
• Factors threatenng the corneal clarity are inflammatory reactions, neovascularization, and limbal deficiency
• Corneal neovascularization is preventable with anti-angiogenic pharmaceutical agents
• The main events during the phototransduction cascade include both synthesis and bleaching.

Diseases of the Eye

• Examples of eye diseases include Diabetic retinopathy or cataracts which cause Mass protein production
• Retinal detachment helps cause retinal damage or detachments
• Retinoblastoma is a cancer of the eye
• Retinitis pigmentosa is a genetical conditions to be aware of
• Diabetic retinopathy is the disruction of retinal vessecles and the effects can be seen
• This is done through a loss of photoreceptor and affects the vison
• Glaucoma is the damage/loss of optic nerve
• This pressure is caused by a damage/loss in the optic disc
• Pressure build up cases vision issues (narrowed Vison "side blinkers"
• Occurs when drainage is obstructed
• Happens due to pressure in cranisoum (high)
• Sensoty problems such as pins and needles or vision loss can occur
• 5HT1-receptor agonists, Sumatriptan and Seratonin