Anatomy and Physiology of the Human Eye

Accessory Structures of the Eye

Definition: Accessory structures are found outside of or attached to the eyeball and assist the eye in its functioning by providing protection and support.
Eyebrows: Consist of coarse hairs on the arches above the eye that help keep debris out of the eye.
Eyelids (Palpebrae): * Upper and lower eyelids are separated by the Palpebral fissure. * They meet at the medial and lateral angles (canthi). * Medial canthus: The inner corner of the eye closer to the nose. * Lateral canthus: The outer corner of the eye further from the nose.
Conjunctiva: Thin, transparent mucous membranes that line the whites of the eye and the inner surface of the eyelids. * Function: Produces a lubricating mucous secretion that prevents the eye from drying out. * Palpebral conjunctiva: Lines the inner surface of the eyelids. * Bulbar conjunctiva: The transparent membrane that covers the anterior surface of the eyeball, specifically over the sclera, but not over the cornea. * Conjunctival sac: The space between the palpebral conjunctiva lining the eyelids and the bulbar conjunctiva covering the eyeball.

The Lacrimal Apparatus

Definition: A system responsible for producing, distributing, and draining tears to moisten, lubricate, cleanse, and protect the eye.
Function of Tears: * Help prevent the eye from drying out. * Wash away irritants and debris. * Possess immune functions.
Components and Pathway: 1. Lacrimal Gland: Produces lacrimal fluid (tears). 2. Excretory Lacrimal Ducts: Convey the fluid to the surface of the eye. 3. Surface of the Eye: Fluid is moved across the eye as the eyelids blink. 4. Pores (Lacrimal Puncta): Tears enter these pores to reach the lacrimal sac. 5. Lacrimal Sac: Receives tears from the surface of the eye and conveys them to the nasolacrimal duct. 6. Nasolacrimal Duct: Conveys lacrimal fluid into the nasal cavity.

Extrinsic Eye Muscles

General Characteristics: There are six skeletal muscles that attach the eyeball to the bones of the orbit. They originate on the bones and insert onto the outer surface of the eyeball to control movement.
The Six Muscles: 1. Lateral Rectus: Moves the eye laterally (away from the nose); controlled by the Orabducens nerve. 2. Medial Rectus: Moves the eye medially (towards the nose). 3. Superior Rectus: Moves the eye upward. 4. Inferior Rectus: Moves the eye downward. 5. Superior Oblique: Moves the eye at an angle; passes through a pulley-like structure called the trochlea; controlled by the trochlear nerve. 6. Inferior Oblique: Moves the eye at an angle.

Internal Fluids of the Eye

Aqueous Humor: * A yellowish fluid continuously formed as a blood filtrate. * Production: Produced by structures called the Ciliary Body in the posterior chamber. * Circulation: Circulates through the anterior chamber (the space between the lens and cornea). * Drainage: Drained through the scleral venous sinus, where it moves back into general circulation. * Function: Supplies nutrients to the avascular lens and cornea.
Vitreous Humor: * A jelly-like substance found in the posterior segment (from the lens to the back of the eye). * Characteristics: Thicker than aqueous humor; humans are born with it and keep the same amount throughout life (not produced or drained). * Functions: 1. Transmits light to the retina. 2. Supports the posterior surface of the lens. 3. Helps maintain intraocular pressure. 4. Helps keep the retina in place against the choroid. * Floaters: Occur when the vitreous humor breaks down.

The Three Tunics of the Eyeball

Fibrous Tunic (Outermost Layer): * Composed of dense connective tissue in two regions. * Sclera: The "whites of the eye"; opaque dense connective tissue that maintains eye shape. * Cornea: The transparent anterior portion; lacks vascularization but is highly innervated. It allows light to enter and helps bend (refract) light rays to focus them on the retina. It is nourished by tears on the superficial side and aqueous fluid on the deep side.
Vascular Tunic (Middle Layer / Uvea): * Choroid: The back four-fifths of the eyeball. It is a vascular, darkly pigmented membrane (due to melanocytes). The pigmentation absorbs excess light to prevent scattering inside the eye. * Ciliary Body: A thickened ring of tissue encircling the lens. * Ciliary Muscle: Smooth muscle that contracts and relaxes to change the shape of the lens for accommodation. * Ciliary Processes: Found on the posterior surface; produce aqueous humor. * Suspensory Ligaments (Ciliary Zonules): Collagen fibers that connect the ciliary muscles to the lens. * Iris: The visible colored portion composed of rings of smooth muscle and melanocytes. It contains the pupil.
Neural Tunic (Innermost Layer / Retina): * Pigmented Layer: A single layer of melanocytes that absorb excess light. * Neural Layer: A sheet of nervous tissue containing neurons and neuroglial cells. It is responsible for responding to light and generating electrical signals.

Pupillary Anatomy and Control

Pupil: The central opening of the iris that regulates light entry.
Pupillary Sphincter (Constrictor): Contracts to narrow the pupil, reducing light entry. This is under parasympathetic innervation.
Pupillary Dilator: Contracts to enlarge the pupil, increasing light entry. This is under sympathetic innervation.

Cellular Organization of the Retina

Signal Transmission Pathway: Light $\rightarrow$ Photoreceptors $\rightarrow$ Bipolar cells $\rightarrow$ Ganglion cells $\rightarrow$ Optic Nerve.
Photoreceptor Cells: * Rods: * More numerous than cones; located toward the periphery. * Highly sensitive to light; allows vision in dim light (night vision). * Provide monochromatic, low-detail vision. * Cones: * Operate best in bright light; enable high-acuity color vision. * Located near the center of the retina. * Three types: red light, green light, and blue light cones.
Bipolar Cells: Transmit electrical signals from photoreceptors to ganglion cells.
Ganglion Cells: The output cells of the retina. Their axons form the Optic Nerve, which sends sensory information to the visual centers in the brain.

Specialized Regions of the Retina

Ora Serrata: The anterior boundary of the retina with a scalloped/serrated appearance; where the retina stops at the ciliary body.
Macula Lutea: A yellowish region at the back of the retina with a high concentration of cones.
Fovea Centralis: A tiny pitted area in the center of the macula containing only cones. Light hits these cones directly, making it the region of highest visual acuity.
Optic Disc: The area where ganglion cell axons exit the eye to form the optic nerve. It contains no photoreceptors and is known as the blind spot.

The Visual Pathway

Photoreceptors: Activated by photons.
Bipolar Cells: Relay the signal.
Ganglion Cells: Generate the output signal.
Optic Nerve: Transmits information from the eye.
Optic Chiasm: Allows the right side of the occipital lobe visual cortex to process information from the left visual field (and vice versa).
Optic Tract: Path from the chiasm to the brain.
Thalamus: Serves as a relay station.
Visual Cortex: Found in the occipital lobe for processing.

Physiology of Phototransduction

Photopigments: Embedded in the outer segments of rods (cylindrical) and cones (conical).
Rhodopsin: The photopigment in rods consisting of: * Opsin: A G-protein coupled receptor. * Retinal: A $\text{Vitamin A}$ based pigment.
Mechanism: When light hits the pigment, it activates a cascade involving phosphodiesterase (PDE). PDE hydrolyzes cyclic guanosine monophosphate (cGMP), leading to changes in cell polarization.

The Lens and Accommodation

Lens Anatomy: Transparent, biconvex, and encapsulated in an elastic capsule that allows it to expand and recoil.
Distant Vision (Unaccommodated): * Ciliary muscles are relaxed. * Suspensory ligaments are tight/stretched. * Lens is thin/flat and weakly refractive.
Near Vision (Accommodation): * Ciliary muscles contract. * Suspensory ligaments become loose. * The elastic lens recoils into a short, thick (round) shape. * Lens becomes strongly refractive to focus light from close objects.