Eyes and Eyelids: Structure and Function

• Over half the sensory receptors in the human body reside in the eyes
• A large part of the cerebral cortex processes visual information

Eyelids

• The eyelids (palpebrae) shade the eyes during sleep and protect them from sunlight and debris
• From superficial to deep, each eyelid consists of epidermis, dermis, subcutaneous tissue, fibers of the orbicularis oculi muscle, a tarsal plate, tarsal glands, and conjunctiva
• The tarsal plate provides form and support to the eyelids
• The tarsal (meibomian) glands secrete fluid (meibum) to prevent the eyelids from adhering to each other and the bulbar conjunctiva
• The conjunctiva is a thin mucous membrane, lining the inner aspect of the eyelids (palpebral) and reflecting onto the anterior surface of the eyeball (bulbar)

Vision

• Eyelashes & eyebrows protect from foreign objects, perspiration & sunlight
• Sebaceous glands are present at the base of eyelashes
  • Clogging and inflammation of sebaceous glands results in a sty
• The palpebral fissure is the gap between the eyelids
• The eyeball has a diameter of 1 inch
• 5/6 of the eyeball is inside the orbit & protected

Extraocular Muscles

• 6 muscles insert on the exterior surface of the eyeball
• These muscles are innervated by cranial nerves III, IV or VI (3, 4, & 6)
• The 4 rectus muscles are superior, inferior, lateral, and medial
• The 2 oblique muscles are inferior and superior

Lacrimal Apparatus

• About 1 ml of tears are produced per day to spread over the eye by blinking
• Tears contain the bactericidal enzyme lysozyme

Layers of the Eye

• The eye consists of three layers
  • Fibrous Tunic (outer layer)
  • Vascular Tunic (uvea) (middle layer)
  • Nervous Tunic (inner layer)

Fibrous Tunic - Cornea

• The cornea is transparent and colorless
• It helps focus light (refraction)
• The cornea has 5 layers of epithelial tissue, connective tissue, and endothelial tissue
• Cornea transplants are common & successful
• There are no blood vessels in the cornea, so there are no antibodies to cause rejection
• The cornea is nourished by tears & aqueous humor

Fibrous Tunic – Sclera

• The sclera is the "white" of the eye
  • it has a cooperative eye hypothesis
• Sclera consists of a dense irregular connective tissue layer of collagen & elastic fibers
• Sclera provides shape & support
• The junction of the sclera and cornea has an opening: the scleral venous sinus
• The optic nerve (CNII) pierces the sclera posteriorly

Vascular Tunic

• The choroid contains pigmented epithelial cells (melanocytes) & blood vessels
  • Provides nutrients to the retina
  • Melanin in melanocytes absorbs scattered light
• The ciliary body consists of ciliary muscle
  • The smooth muscle that alters the shape of the lens
  • Functions in accommodation
• Ciliary processes:
  • Folds on the ciliary body
  • Secrete aqueous humor

Iris

• The iris is the colored portion of the eye
• The iris is donut-shaped and is suspended between the cornea & lens
• Muscle fibers, pigment, connective tissue, and epithelial tissue makes up the iris
• The hole in the center is the pupil
• The function of the iris is to regulate the amount of light entering the eye by autonomic reflexes
  • Circular muscle fibers contract in bright light to constrict the pupil
  • Radial muscle fibers contract in dim light to enlarge the pupil
• Constrictor pupillae (circular) are innervated by parasympathetic fibers
• Dilator pupillae (radial) are innervated by sympathetic fibers
• Response to light varies with different levels of light

Lens

• The lens is not part of the Vascular Tunic and is avascular
• The lens contains crystallin proteins arranged like layers in an onion
• The clear capsule of the lens is perfectly transparent
• The lens is held in place by suspensory ligaments
• The lens focuses light on the retina
• Age-related loss of elasticity of the lens leads to presbyopia

Nervous Tunic

• The nervous tunic is the posterior 3/4 of eyeball

Optic disc

• The optic disc is the location of the optic nerve exiting the back of the eyeball

Central retina BV

• Central retina blood vessels fan out to supply nourishment to the retina
• These blood vessels also exit via the optic disc
• Blood vessels are visible for inspection and may indicate hypertension & diabetes

Detached retina

• Detached retina may be a result of trauma, diabetes, or AIDS
• Fluid between layers causes distortion or blindness

Photoreceptors

• Photoreceptor shapes differ
• Rods specialize in dim light
  • Allow discrimination between different shades of dark and light
  • Permit seeing shapes and movement
• Cones specialize in bright light scenarios and sharpness of vision (high visual acuity)
  • Most densely concentrated in the central fovea, a small depression in the center of the macula lutea
• The macula lutea is in the posterior portion of the retina, almost in the center, corresponding to the visual axis of the eye
• The central fovea, found within the macula lutea, is the area of sharpest vision because of the high concentration of cones
• Rods are absent from the fovea and macula but the density increases toward the periphery of the retina
• Rods are rod-shaped
  • Function in shades of gray in dim light, 120 million rod cells
  • Shapes & movement are noted
  • Distributed along periphery
• Cones are cone-shaped
  • Function in sharp, color vision
  • 6 million cones
  • Fovea of macula lutea
  • Densely packed region
  • At exact visual axis of eye
  • Sharpest resolution (acuity)

Retina

• The retina makes up the pigmented epithelium nonvisual portion
• Absorbs stray light & helps keep image clear
• Contains 3 layers of neurons (outgrowth of brain)
  • Photoreceptor layer
  • Bipolar neuron layer
  • ganglion neuron layer
• 2 other cell types (modify the signal)
• Horizontal and amacrine cells are inside the retina

Visual Pathway

• Light penetrates the retina
• Rods & cones transduce light into action potentials
• Rods & cones excite bipolar cells
• Bipolars excite ganglion cells
• Axons of ganglion cells form the optic nerve leaving the eyeball at the optic disc (blind spot)
• The signal heads to the thalamus (LGN)
• Third-order neurons end at the primary visual cortex

Intraocular Cavities

• The anterior cavity (anterior to lens) is filled with aqueous humour
  • Produced by the ciliary body
  • Continually drained
  • Replaced every 90 minutes
• The anterior cavity includes 2 chambers
  • Anterior chamber between the cornea and iris
  • Posterior chamber between the iris and lens
• The posterior cavity (posterior to lens)
  • Filled with vitreous humour (jellylike) that doesn’t get cycled
  • Formed once during embryonic life
  • Floaters (crystalized proteins) are debris in vitreous body of older individuals

Intraocular Pressure

• The pressure in the eye, called intraocular pressure, is produced mainly by the aqueous humor
• Intraocular pressure maintains the shape of the eyeball and keeps the retina smoothly applied to the choroid so the retina will form clear images

Glaucoma

• Increased intraocular pressure
• Problem with drainage of aqueous humor
• May produce degeneration of the retina and eventual blindness

Aqueous Humour

• Aqueous Humour is continuously produced by the ciliary body
• It flows from the posterior chamber into the anterior chamber through the pupil
• The scleral venous sinus includes:
  • Canal of Schlemm
  • Lymphatic-like drainage at the junction of cornea & sclera
  • Drainage of aqueous humor from eye to bloodstream

Imaging Formation

• Refraction of light occurs by the cornea & lens
• Light rays must fall upon the retina
• Accommodation of the lens occurs by changing the shape of the lens so light is focused
• Constriction of the pupil occurs when less light enters the eye

Refraction

• Refraction = bending of light
• Refraction occurs when light moves through substances with different refractive indices at a non-perpendicular angle