Anatomy of the Eye - External Structures

Anatomy of the Eye - External Anatomy

• Eyebrows and eyelids deflect sweat and debris away from the eyes.
• They provide shade from sunlight.
• They also help with facial expression and nonverbal communication.
• Eyelids (palpebrae) protect the eye from injury and drying.
• They spread tears across the eye's surface.
• They regulate light entry.
• Conjunctiva is 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 gland produces tears.
• Lacrimal sac collects tears.
• Nasolacrimal duct drains tears into the nasal cavity.
• This apparatus produces, distributes, and drains tears
• Tears provide lubrication, nutrients, and antimicrobial protection.
• Extraocular muscles control eye movement, enabling gaze in different directions.

Anatomy of the Eye - Internal Anatomy

• The fibrous tunic is the outer layer
• It includes the sclera, which does NOT cover the cornea
• The white of the eye is from the collagen
• The episclera, a loose tissue layer, covers the sclera and protects it.
• The fibrous tunic gives structural support and maintains the shape of the eye
• It serves as an attachment point for the extraocular muscles
• It protects the internal components of the eye.
• The cornea is transparent and avascular (for clear vision).
• It is highly innervated
• Primary refractive surface of the eye that focuses light
• It protects the internal structures.
• Vascular tunic (middle layer)
• The choroid is highly vascular and pigmented
• It is located between the sclera and the retina.
• Melanin absorbs extra light.
• Choroid provides blood supply to the outer layers of the retina.
• It absorbs stray light to prevent internal reflections.
• Ciliary body is a ring-shaped structure located behind the iris.
• Smooth muscle fibres control lens curvature.
• Epithelial projections secrete aqueous humour.
• Zonular fibres attach the ciliary body to the lens, allowing changes in depth.
• Production of aqueous humour
• Lens shape is controlled via accommodation.
• Iris is pigmented with a circular structure between cornea and lens
• Pupil is the opening in the center of the iris.
• Sphincter pupillae muscle constricts the pupil (miosis).
• Dilator pupillae muscle dilates the pupil (mydriasis).
• Iris controls the size of the pupil, regulating the amount of light entering the eye.
• Iris consists of two smooth muscles
  • Sphincter pupillae, inner circular muscles cause constriction using muscarinic receptor agonists
  • Dilator pupillae, outer radial muscles dilation using muscarinic receptor antagonists
  • alpha 1 adrenoceptor agonists
• Bright light constricts the pupil narrowing it to 1.5 mm
• Dark conditions open the pupil widening it to 8 mm
• A 28-fold change in pupil area
• Changing the lens shape using zonular fibres during accommodation
• Lens changes shape to adjust refractive power and focus images on the retina
• Relaxed ciliary muscle leads an elongated lens and allows distant vision
• Contracted ciliary muscle leads to a rounded lens allows near vision (Accommodation)
• Mechanism of accommodation is under control of parasympathetic system.
• Acetylcholine goes through muscarinic receptors
• The nervous tunic (inner layer) includes the retina
• Macula is the central region of the retina for sharp, central vision
• Fovea centralis is a small depression in the macula with the greatest visual acuity
• It has a high concentration of cones for colored vision
• Optic disc is where the optic nerve exits and lacks photoreceptors.
• Light is converted into neural signals and begins the visual process.
• Photoreceptor cells in the retina comprise rods for low light vision; black-and-white vision
• Cones are sensitive to bright light for color vision, concentrated in the macula and fovea centralis.
• Retina, the inner layer, has three layers
  • ganglion cells
  • bipolar cell
  • outer containing photoreceptors, rods, and cones
• Light passes through the inner and middle layers and blood vessels before reaching the photoreceptors (in reverse order).
• Rods and cones differ in function:
  • Rods = black and white vision, 100x10^6 abundance, periphery of retina, low visual acuity.
  • Cones = color vision, 3x10^6 abundance, highest at fovea, high visual acuity
• Convergence of signals from rods increases sensitivity and allows detection in low light.
• Rods are highly sensitive but have low resolution.
• Cones require high stimulus levels.
• The ratio is three rods to every bipolar cell compared to one cone per bipolar cell
• The photopigment bleaching results from retinal absorbing light, conforming and being released from the opsin pigment
• Rhodopsin is detroyed as a result of light.
• Photoreceptors have to synthesize new disks containing photopigment

Phototransduction Cascade

• Photoreceptor (rod or cone) membrane process
• Retinal is the light-absorbing part of rhodopsin activated when light hits, stimulating transducin
• Transducin acitvates cyclic GMP phosphodiesterase (cGMP), the which binds to cation channels and keeps them open for calcium and sodium ions to enter
• Photopigment synthesis, rhodopsin in the dark where Vitamin A is combined with opsin
• Bleaching is when rhodopsin re-forms the all-trans Isomer and then degenerates
• Rhodopsin is regenerated by RPE for photoreceptors
• Aqueous humour and vitreous humour support the eye
• Vitreous humor is located in the back and provides shape (80% of volume), in additon to hyaluronic acid, salts, sugars, proteins, collagen, amino-acids.
• Aqueous humour is located in from and is between the cornea and lens and
  • Continuously produced and Recycled.
  • It regulates ocular pressure and provides nutrients such as amino acids electrolytes sodium and potassium, ascorbic acid, glutathione and immunoglobulins

Sensory physiology

• Sensation: sensing environment via physicochemical processes.
• Perception: interpretation of signals via psychological process.
• Sensory pathways project to the thalamus (translator, relay, processes inputs, and relays information to cerebral cortex)
• Pupil reflex with shine a light, consensual response is used to assess integrity of sensory and motor function
• In a normal reflex shining a light in one eye causes both to constrict
• In an abnormal, left doesn't respond when shining a light in the right is causes by nerve damage
• Information on visual fields are processed on opposite sides for projection
• The projected image is upside down on the retina.
• Visual processing in the brain reverses the image.
• Rubin's vase cognitive illusion from Rubin in 1915.
• Mental choice of two interpretations that the brain is unable to do at once
• Brain classifies depth and relations around surroundings
• Clear patterns brain tries faces

Corneal Repair and Regeneration

• "Eye cells heal quickly, often in 48 hours."
• Limbus forms border between corneal and conjunctival epithelium; LSCs maintain/repair the cornea.
• Inflammation causes scar formation and limbus zone destruction.
• Transparency/avascularity is essential; factors clarity are: inflammatory reactions
• Pathological angiogenesis during corneal wound healing, leading to transparency reduction and scar formation.
• Neovascularization follows inflammation and can be inhibited by anti-angiogenic pharmaceutical agents

Diseases of the Eye

• Examples of Eye Diseases includes
  • Diabetic retinopathy
  • Glaucoma
    • damage the optic disk including nerve including damage that causes visual field defects
    • related the loss of retinal ganglion cells (treated open-angle
    • can increase intra-ocular pressure
  • Migraine with aura that comes before headache and can cause visual
    • caused by pressure and flow of neurotransmitters