Eye Anatomy and Physiology

1. Gross Anatomy - Layers of the Eyeball

Outer Fibrous Layer

• Sclera - the tough, opaque white outer shell that gives the eye its shape and protects internal structures. Light cannot pass through it.
• Cornea - the transparent anterior modification of the sclera. It is the primary refracting surface of the eye, accounting for ~75% of total refractive power. It is avascular and nourished by the aqueous humor and tear film. The lateral margin of the cornea is continuous with the conjunctiva, a clear mucous membrane covering the sclera.

Middle Vascular Layer (Uvea)

• Choroid - highly vascular layer just inside the sclera; supplies oxygen and nutrients to the outer retina.
• Ciliary body - contains circular and longitudinal smooth muscle fibers that control lens shape (accommodation). It also produces aqueous humor by active transport from plasma.
• Iris - the pigmented, opaque diaphragm in front of the lens. It has two muscles:
  • Sphincter pupillae (parasympathetic) - constricts the pupil (miosis)
  • Dilator pupillae (sympathetic) - dilates the pupil (mydriasis)
  • Variations in pupil diameter can produce up to a 16-fold change in the amount of light reaching the retina.

Inner Neural Layer

• Retina - the light-sensitive neural tissue lining the posterior two-thirds of the choroid. Contains photoreceptors and multiple neural processing layers (see Section 4 below).

2. Internal Compartments and Fluids

• Aqueous humor - a clear, protein-free fluid produced by the ciliary body. It nourishes the avascular cornea and iris, then drains through the trabecular meshwork into the canal of Schlemm at the filtration angle. Obstruction of this canal raises intraocular pressure (IOP), the primary risk factor for glaucoma.
• Vitreous humor - a viscous, gel-like fluid that fills the posterior chamber and keeps the eyeball spherical.

3. The Lens and Image Formation

• The crystalline lens is a transparent biconvex structure held in place by zonule fibers (suspensory ligament) attached to the ciliary body.
• At rest (distant vision), ciliary muscle is relaxed, zonules are taut, and the lens is flattened (reduced refractive power).
• For near vision, the ciliary muscle contracts, zonule tension reduces, and the lens becomes more convex (increased refractive power) - this is called accommodation. In young individuals, this can add up to 12 diopters of refractive power.

Refractive errors:

• Myopia (nearsightedness) - eyeball too long; image focuses in front of the retina. Corrected with biconcave (diverging) lens.
• Hyperopia (farsightedness) - eyeball too short; image focuses behind the retina. Corrected with biconvex (converging) lens.
• Presbyopia - age-related hardening of the lens reducing accommodative ability.

4. The Retina - Structure and Cell Layers

1. Nerve fiber layer - axons of ganglion cells traveling to optic nerve
2. Ganglion cell layer - output neurons of the retina
3. Inner plexiform layer - synapses between bipolar/amacrine cells and ganglion cells
4. Inner nuclear layer - bipolar, horizontal, and amacrine cells
5. Outer plexiform layer - synapses between photoreceptors and bipolar/horizontal cells
6. Outer nuclear layer - photoreceptor cell bodies (nuclei of rods and cones)
7. Photoreceptor outer segments - the transduction compartment
8. Retinal pigment epithelium (RPE) - absorbs stray light, regenerates photopigments

Key retinal landmarks:

• Fovea centralis - a small pit (~300-700 µm) at the center of the macula where visual acuity is highest. Contains only cones packed at maximal density (~0.05 µm center-to-center). Inner neural layers are displaced laterally to minimize light scattering.
• Optic disc (blind spot) - where ganglion cell axons exit as the optic nerve. Contains no photoreceptors, hence a physiological blind spot.
• Macula - the central area surrounding the fovea, responsible for fine-detail and color vision.

5. Photoreceptors - Rods vs. Cones

6. Phototransduction - Converting Light to Neural Signals

• Synaptic terminal - releases glutamate onto bipolar cells
• Inner segment - contains nucleus, mitochondria, and synthesizes photopigments
• Outer segment - the transduction site; a highly modified cilium with stacked disc membranes packed with photopigment

The Dark Current

Light Response - The Cascade

1. Photon absorption - light strikes rhodopsin (rod) or cone opsin, converting retinal from 11-cis to all-trans configuration.
2. Transducin activation - activated rhodopsin (metarhodopsin II) activates the G-protein transducin.
3. Phosphodiesterase (PDE) activation - transducin activates PDE, which hydrolyzes cGMP → GMP.
4. Channel closure - falling cGMP closes the outer-segment cation channels.
5. Hyperpolarization - the cell hyperpolarizes (to ~-70 mV), reducing glutamate release.
6. Signal to bipolar cells - decreased glutamate signals downstream neurons.

This is counterintuitive: photoreceptors are most active in the dark and reduce activity (hyperpolarize) in response to light.

7. Visual Pathway - From Retina to Cortex

Photoreceptors
    ↓
Bipolar cells (+ Horizontal cells for lateral inhibition)
    ↓
Ganglion cells (+ Amacrine cells)
    ↓
Optic nerve (CN II)
    ↓
Optic chiasm (nasal fibers cross, temporal fibers stay ipsilateral)
    ↓
Optic tract
    ↓
Lateral Geniculate Nucleus (LGN) of thalamus
    ↓
Optic radiation
    ↓
Primary visual cortex (V1) - occipital lobe (calcarine sulcus)

8. Protective and Accessory Structures

• Orbit - bony socket of the skull providing mechanical protection
• Eyelids and blinking - keep the cornea moist and protected
• Lacrimal gland - produces tears that cleanse and moisturize the corneal surface; tears drain via the nasolacrimal duct into the nose
• Extraocular muscles - three pairs (superior/inferior rectus, medial/lateral rectus, superior/inferior oblique) innervated by CN III, IV, and VI; move the eyeball in the orbit
• Conjunctiva - mucous membrane lining eyelids and covering anterior sclera

Summary Table - Key Structures at a Glance