Retina Anatomy and Function Quiz

Episcleritis

• Episcleritis is inflammation of the episclera, a thin layer of vascular connective tissue between the conjunctiva and sclera.
• It's a benign, recurring inflammation affecting the Tenon's capsule but not the sclera itself.
• More common in young adult females than males.

Etiology of Episcleritis

• The exact cause isn't known.
• Associated with conditions like gout and psoriasis.
• Thought to potentially be a hypersensitivity reaction to endogenous tubular.
• There's congestion of the overlaying Tenon's capsule and conjunctiva.

Symptoms of Episcleritis

• Redness is a key symptom.
• Blurred vision, although less common, can occur.
• Mild ocular discomfort (described as gritty, burning, or foreign body sensation) is common.
• Often not accompanied by significant discomfort.
• Photophobia (light sensitivity) and lacrimation (eye watering) might occur.

Pathology of Episcleritis

• Localized lymphocytic infiltration of episcleral tissues, often accompanied by edema.
• Congestion of Tenon's capsule and conjunctiva is also present.

Signs of Episcleritis

• Episclera is acutely inflamed in the affected area.
• Diffuse episcleritis can involve the entire eye, with maximum inflammation limited to one or two quadrants.
• Nodular episcleritis presents as a pink or purple nodule, surrounded by injection, typically 2-3mm from the limbus.

Signs (continued)

• Nodules are firm, tender, and the overlying conjunctiva moves freely.
• Episcleritis typically has a limited 10-day course and resolves spontaneously.
• Recurrences are common and tend to occur in bouts.

Retinal Anatomy

• The retina is a delicate, transparent membrane, the innermost layer of the eye.
• It appears purplish-red, due to the rods and vascular choroid.
• Extends from the optic disc to the ora serrata, divided into the posterior pole & peripheral retina.
• The posterior pole includes the optic disc and macula lutea.

Retinal Anatomy (continued)

• Retinal equator is an imaginary line running through the exit of venae verticosae.
• Posterior pole is the area behind the equatorial line, comprising the distinct parts of the optic disc and macula lutea.
• Examination involves slit lamp indirect biomicroscopy using +78D and +90D lenses, and a direct ophthalmoscope.

Retinal Anatomy (continued)

• Optic disk is a pink area approximately 1.5mm in diameter where the retinal nerve fibers exit the eye.
• Physiological cup is a central depression in the optic disc.
• Macula lutea (yellow spot) is a highly sensitive area, containing the fovea.
• Fovea is a central pit, 0.35mm in diameter, 3mm temporal to the optic disc, the most sensitive part of the retina.

Microscopic Structure of Retina

• Pigmented epithelium is the outermost layer, single cell layer with pigment. Adheres to the choroid's basal lamina. Plays a role in photoreceptor renewal, vitamin A recycling, and mechanical support.
• Layer of rods and cones: Contains photoreceptor outer segments. These segments are arranged in a palisade manner.
• External limiting membrane: It's a fenestrated membrane, allowing passage of rods/cones.
• Outer nuclear layer: Contains the nuclei of rods/cones.
• Outer plexiform layer: Connects rods/cones to bipolar/horizontal cells.
• Inner nuclear layer: Contains nuclei of bipolar, horizontal, amacrine, and Muller cells.
• Inner plexiform layer: Joins bipolar/amacrine/horizontal cells to ganglion cells.
• Ganglion cell layer: Contains cell bodies of ganglion cells.
• Nerve fiber layer: Contains axons of ganglion cells heading out to the optic nerve.
• Internal limiting membrane: Innermost layer, separating the retina from the vitreous body.

Functions of Retina

• The retina is divided temporally and nasally by a midline. Optic nerve fibers from the temporal retina travel straight through the optic tract & terminate in the ipsilateral geniculate body. Optic nerve fibers from the nasal retina cross in optic chiasma and travel to the contralateral geniculate body.
• The retinal blood supply comes from the central retinal artery and its branches (superior/inferior nasal, temporal) that provide nutrition to the six inner layers.

Biochemistry of Retina

• Phototransduction: The process converting light into electrical signals via photoreceptor cells' (rods/cones) rhodopsin/cone opsins.
• Light converts 11-cis-retinal to all-trans-retinal, triggering a conformational change in rhodopsin.
• Activated rhodopsin activates transducin, a G-protein, which then activates phosphodiesterase and lowers cGMP levels.
• Decreased cGMP leads to closure of cGMP-gated sodium channels and membrane hyperpolarization.

Visual Pathway

• The visual cycle recreates 11-cis-retinal for continuous photoreceptor function. All-trans-retinal is converted to all-trans-retinol after phototransduction. This retinol is then transported to the retinal pigment epithelium (RPE).
• Isomerization: RPE enzymes (particularly RPE65) convert all-trans-retinol back to 11-cis-retinal for recycling back to the photoreceptors.

Energy Metabolism of Retina

• The retina has high energy demands and relies on glycolysis and oxidative phosphorylation.
• It uses aerobic glycolysis despite having sufficient oxygen, known as the Warburg effect.
• Mitochondria in the inner retina carry out oxidative phosphorylation for ATP generation.

Glucose Supply and Neurotransmitters

• Glucose is transported into the retina via GLUT1 transporters in the blood-retina barrier.
• The pentose phosphate pathway (PPP) provides NADPH which is essential for antioxidant defense.
• Neurotransmitters mediate signal transmission between retinal neurons.
• Glutamate acts as a primary excitatory neurotransmitter.
• GABA & Glycine act as inhibitory neurotransmitters.

Test your knowledge on the anatomy of the retina with this quiz. Explore key functions of the pigmented epithelium, the sensitivity of different retinal areas, and the properties of rods and cones. Match various retinal components with their characteristics to enhance your understanding of visual physiology.