Fisiologi Penglihatan

Dr. Daniel H.Tjahjono MS.

Sense of Sight

• Eye: organ containing visual receptors– Provides vision with assistance of accessory

organs

• Visual accessory organs– Eyelids, lacrimal apparatus (protects eye),

and a set of extrinsic muscles (move eye)

Orbital Cavity

• Location of eye and accessory organs

• Pear-shaped

• Lined with the periosteum of various bones and contains fat, blood vessels, nerves, and connective tissues.

Eyelid (4 layers)

• Skin: thinnest skin of body. Covers the lids outer surface.

• Muscle: – Orbicularis oculi: acts as a sphincter and closes lid

when it contracts.– Levator palpebrae superioris: raises the upper lid

• Connective tissue• Conjunctiva: mucous membrane that lines the

inner surfaces of the eyelids and anterior surface of eyeball except for the central portion (cornea).

Sagittal section of the closed eyelids and anterior portion of the eye.

Lacrimal Apparatus

• Lacrimal Gland: secretes tears continuously. Located in the orbit and series of ducts that carry tears into nasal cavity.– Tears exit lacrimal gland through tiny tubules and flow

downward and medially across the eye.– Superior and inferior canaliculi collects tears into

lacrimal sac located in groove of lacrimal bone nasolacrimal duct which empties into nasal cavity

– Moistens and lubricates surface of the eye and lining of lids

– Tears contain lysozome – antibacterial agent reducing risk of eye infections.

The lacrimal apparatus consists of a tear-secreting gland and a series of ducts.

Extrinsic Muscles

• Arise from bones of the orbit and attach (insert) by broad tendons on the eye’s tough outer surface.

• 6 extrinsic muscles move the eye in various directions. Eye movements may utilize more than one muscle.

Extrinsic muscles of the right eye (lateral view)

Diplopia

• Double vision caused by one eye deviating from the line of vision.

• If condition persists, brain must suppress image from deviated eye

Suppression Amblyopia

• Turning eye becomes blind.

• Treatment early in life with exercises, eyeglasses and surgery can prevent monocular blindness (one eye)

Structure of the Eye

• Hollow, spherical structure 2.5cm in diameter

• 3 layers: fibrous outer tunic, vascular middle tunic, nervous inner tunic

• Spaces within eye filled with fluids that support its wall and internal parts and help maintain its shape.

Outer Tunic – Fibrous Tunic

• Cornea

• Sclera

• Optic Nerve: in the back of the eye

• Blood Vessels: pierce sclera

Cornea

• Anterior 6th of outer tunic• Bulges forward• Transparent window of the eye (contains few cells, no

blood vessels, cells and collagenous fibers form unusually regular patterns)

• Helps focus entering light rays• Composed largely of connective tissue with a thin layer

of epithelium on its surface.• Continuous with the sclera (white portion of the eye)

Transverse section of the right eye (superior view)

Sclera

• White portion of the eye

• Posterior 5/6th of the outer tunic

• Opaque due to many large, disorganized collagenous and elastic fibers.

• Protects the eye and is an attachment for the extrinsic muscles

Most Common Cause of Blindness

• Loss of transparency of the cornea• Corneal Transplant (penetrating

keratoplasty): treat condition by replacing central 2/3 of the defective cornea with similar-sized portion of cornea from a donor eye– Corneal tissues lack blood vessels,

transplanted tissue usually not rejected– Success rate of procedure is very high.

Middle Tunic – Vascular Tunic

• Choroid Coat

• Ciliary Body

• Iris

Choroid Coat

• Posterior 5/6th of globe of the eye• Loosely joined to the sclera• Honeycombed with blood vessels which

nourish surrounding tissues• Contains many pigment-producing

melanocytes which absorbs excess light and helps keep the inside of the eye dark.

Ciliary Body

• Thickest part of the middle tunic• Extends forward from the choroid coat and forms

an internal ring around the front of the eye.• Many radiating folds called ciliary processes• Groups of muscle fibers called ciliary muscles.• Suspensory ligaments: extend inward from the

ciliary processes and hold transparent lens and capsule in position.

Lens and ciliary body viewed from behind

Lens

• Lies directly behind the iris and pupil

• Composed of differentiated epithelial cells called lens fibers.

Lens Capsule

• Surrounds the lens

• Clear, membrane-like structure composed largely of intercellular material

• Elastic nature keeps it under constant tension. Can assume a globular shape.

• Suspensory ligaments attached to margin of capsule and the ciliary muscles. Changing tension changes the shape of the capsule and lens for focusing.

• Accommodation: the ability of the lens to adjust shape to facilitate focusing. Close objects= lens thickens; distant objects= thinner, less convex

In accommodation, A.) the lens thins as ciliary muscle fibers relax. B.) The lens thickens as ciliary muscle fibers contract.

Cataract

• Common eye disorder in older people• Lens or capsule slowly becomes cloudy and

opaque.• Without treatment it eventually causes blindness• Treatment:

– In past, surgical procedure with 2 week recovery– Now, laser treatment on out-patient basis.

Iris

• Thin diaphragm composed mostly of connective tissue and smooth muscle fibers

• The colored portion of the eye• Extends forward from the periphery of the ciliary

body and lies between the cornea and the lens.• Divides the space (anterior cavity) into the

anterior chamber (between the cornea and the iris) and posterior chamber (between iris and vitreous body containing the lens)

Aqueous Humor

• Watery fluid secreted by the epithelium on the inner surface of the ciliary body into posterior chamber.

Pupil

• Circular opening in the center of the iris. Fluid flows through from posterior to anterior chamber.

• Aqueous humor fills space between cornea and lens. Nourish these parts and aids in maintaining shape of the front of the eye.– Leaves anterior chamber through veins and

special drainage canal – scleral venous sinus (canal of Schlemm)

Glaucoma

• Eye disorder that develops when the rate of aqueous humor formation exceeds the rate of its removal.

• Fluid accumulates in anterior chamber of the eye, fluid pressure rises and is transmitted to all parts of the eye.

• Building pressure squeezes shut blood vessels that supply the receptor cells of the retina.

• Cells robbed of nutrients and oxygen may die and permanent blindness can result.

• Early diagnosis allows successful treatment with drugs, laser therapy, or surgery to promote outflow of aqueous humor

• Early stages typically produce no symptoms. Discovery of the condition by tonometer, an instrument that measures intracellular pressure.

Smooth Muscle Fibers of Iris

• Control size of the pupil through which light passes as it enters the eye.

• 2 groups– Circular set: acts as a sphincter. Contracts

smaller, less light enters.– Radial set: contracts to increase diameter of

pupil allowing more light to enter.

Dim light stimulates the radial muscles of the iris to contract, and the pupil dilates. Bright light stimulates the circular muscles of the iris to contract, and the pupil constricts.

Inner Tunic

• Consist of the retina which contains visual cells (photoreceptors)

• Nearly transparent sheet of tissue that is continuous with the optic nerve in the back of the eye and extends forward as the inner lining of the eyeball

• Ends just behind the margin of the ciliary body

Retina

• Thin and delicate. Complex structure with a number of distinct layers.– Macula lutea: central region. Yellowish spot– Fovea centralis: depression in its center.

Region that produces sharpest vision.

– Optic disc: medial to fovea centralis. Nerve fibers from the retina leave the eye and join the optic nerve.

• Central artery and vein also pass through optic nerve and vessels are continuous with the capillary networks of the retina and with vessels in the underlying choroid coat.

• Supply blood to cells of inner tunic• Known as the blind spot of the eye. Lacks receptor

cells.

The retinal consists of several cell layers.

Note the layers of cells and nerve fibers in this light micrograph of the retina.

Retina. A.) Nerve fibers leave the eye in the area of the optic disc (arrow) to form the optic nerve. B.) Major features of the retina.

Posterior Cavity

• Space bounded by the lens, ciliary body, and retina is the largest compartment of the eye.

• Filled with transparent, jelly-like fluid called vitreous humor, along with collagenous fibers it comprises the vitreous body.

• Vitreous body: supports internal parts of the eye and helps maintain its shape.

Floaters

• Specks or clumps of gel or deposits of crystal-like substances that form in the vitreous humor. Cast shadows on the retina.

• Person sees small, moving specks in the field of vision. Most apparent when looking at a plain background.

• More numerous as a person ages.

Light Refraction

• When a person sees something, the object is giving off light or light waves are reflected from it.

• Light waves enter the eye and an image of the object is focused on the retina.

• Refraction: bending of light waves to focus them. • Occurs when light waves pass at an oblique

angle from a medium of one density into a medium of another density.

A lens with a convex surface causes light waves to converge.

• Convex surface causes light waves to refract and converge (cornea, lens, fluids)

• If eye shape is normal, light waves focus sharply on the retina. – Image is upside down and reversed from left

to right.– Visual cortex interprets the image in its proper

position.

Visual Receptors

• Visual receptor cells are modified neurons of two distinct kinds. (rods and cones)

Rods Cones

*Long, thin projections at their ends.*Human eye has 125 million*Hundreds of times more sensitive to light than cones.*Can provide vision in dim light*Produce colorless vision*General outlines of objects. Less precise images because nerve fibers from many rods converge their impulses and transmit them to the brain on the same nerve fiber.

*Short, blunt projections.*Human eye has 7 million*Detect color*Provide sharp images (higher visual acuity)*Convergence of impulses less common. Brain can pinpoint stimulation more accurately.*Fovea centralis: Area of sharpest vision.

*Lacks rods but contains densely packed cones with few to no converging fibers.*To view something in detail, a person moves the eyes so that the important part of an image falls on the fovea centralis.

Rods and Cones. A.) A single sensory nerve fiber transmits impulses from several rods to the brain. B. Separate sensory nerve fibers transmit impulses from cones to the brain. C.) Scanning electron micrograph of rods and cones.

• Rods and cones are located in a deep portion of the retina, closely associated with the layer of pigmented epithelium.

• Epithelial pigment absorbs waves not absorbed by receptor cells.

• Along with pigment of choroid coat, it keeps light from reflecting off surfaces inside the eye.

• Projections from receptors are loaded with light-sensitive visual pigments and extend into this pigmented layer.

• Visual receptors are stimulated only when light reaches them. A light image focused on an area of the retina stimulate some receptors and sends impulses to the brain. This provides only a fragment of info required for the brain to interpret a total scene.

Pigmented epithelium and receptor cells

Visual Pigments

• Both rods and cones contain light-sensitive pigments that decompose when they absorb light energy.

• Decomposition of the pigments triggers a complex series of reactions that initiate a nerve impulse

• Pigments are synthesized from vitamin A.• Night blindness: poor vision in dim light results from

vitamin A deficiency which reduces the supply of retinal (a type of visual pigment) causing low rod sensitivity. Treated with Vitamin A supplements.

• Color vision comes from 3 sets of cones containing different light-sensitive pigments

• Each type of pigment is sensitive to different wavelengths (colors- red, green, blue) of light.

• The color a person perceives depends on which set of cones or combination of sets the light in a given image stimulates. (all three =white; none=black)

• Different forms of color blindness result from lack of different types of cone pigments.

Visual Nerve Pathway

• Axons of retinal neurons leave eyes to form optic nerves.• X-shaped optic chiasma. Some fibers cross over• Right and left optic tracts• A few nerve fibers enter nuclei that function in various

visual reflexes• The rest enter the thalamus• Visual impulses enter nerve pathways called optic

radiations which lead to the visual cortex of the occipital lobes.

The visual pathway includes the optic nerve, optic chiasma, optic tract, and optic radiations.

Clinical Terms Related to the Senses

Amblyopia

• Dim vision due to a cause other than a refractive disorder or lesion

Amblyopia is the medical term for poor development of vision in one eye. The word comes from the Greek. [ambly- (dull) + -opia (vision)] Amblyopia is often referred to as "lazy eye." It affects just two to three percent of the population. Central vision does not develop properly, usually in one eye, which is called amblyopic. The eye is anatomically normal, but visual acuity is reduced even with glasses. Amblyopia develops sometime between birth and 8 or 9 years of age, the critical period of time when the visual system develops and matures. Amblyopia causes more visual loss in the age group under 40 than all the injuries and diseases combined.

Anopia

• Absence of an eye

Audiometry

• Measurement of auditory acuity for various frequencies of sound waves

Blepharitis

• Inflammation of the eyelid margins

Causalgia

• Persistent, burning pain usually associated with injury to a limb

Also called complex regional pain syndrome.Most common between ages 40-60. Diagnosis through observation, thermography, and radiography.Treatment includes physical therapy, corticosteriods, local anesthetic, vasodilaters and antidepressants.

Conjunctivitis

• Inflammation of the conjunctiva

Viruses, bacteria, irritating substances (shampoo, dirt, smoke, pool chlorine), sexually transmitted diseases (STDs) or allergens (substances that cause allergies) can all cause conjunctivitis. Pink eye caused by bacteria, viruses or STDs can spread easily from person to person but is not a serious health risk if diagnosed promptly; allergic conjunctivitis is not contagious.

Diplopia

• Double vision

Emmetropia

• Normal condition of the eyes; eyes with no refractive defects.

Enucleation

• Removal of the eyeball

Exophthalmos

• Abnormal protrusion of the eyes

Associated with hyperthyroidism and Grave’s disease. In the case of Graves Disease, the displacement of the eye is due to abnormal connective tissue deposition in the orbit and extraocular muscles (Epstein et al, 2003) which can be visualized by CT or MRI.If left untreated, exophthalmos can causes the eye lids to fail to close during sleep leading to corneal damage. The process that is causing the displacement of the eye may also compress the optic nerve or ophthalmic artery leading to blindness

Hemianopsia

• Defective vision affecting half of the visual field

Hyperalgesia

• Heightened sensitivity to pain

Caused by injury, or allergic/inflammatory reaction. One unusual cause is platypus venom (venomous ankle spurs)

Iridectomy

• Surgical removal of part of the iris

Treatment for one type of glaucoma where the iris sags and blocks normal drainage.

Iritis

• Inflammation of the iris

Also called anterior uveitis. It is the 3rd leading cause of blindness in the developed world. White blood cells are shed into the anterior chamber of the eye in the aqueous humor. These cells can accumulate and cause adhesions between the iris and the lens. Iritis is associated with over 90 different pathogens and autoimmune disorders. Some treatments include antibiotics and steroids.

Keratitis

• Inflammation of the cornea

Symptoms include pain, and profuse tearing. Can be caused by infection, trauma, dry eyes, UV exposure, contact lens over-wear, degeneration.

Herpes simplex keratitis

Labyrinthectomy

• Surgical removal of the labyrinth

Labyrinthitis

• Inflammation of the labyrinth

Usually caused by a viral infection, occasionally bacterial. Symptoms include reduced hearing or distortion, ringing in the ear, dizziness, imbalance, nausea and vomiting. Often follows the common cold. Viral form improves on its own within a few weeks. Anti-nausea medication can be prescribed.

Meniere’s Disease

• Inner ear disorder that causes ringing in the ears, increased sensitivity to sounds, dizziness, and hearing loss

Meniere’s disease is a problem with the inner ear, the part of the ear responsible for balance as well as hearing. When you have Meniere’s disease, too much endolymph (fluid) backs up in the canals, a condition called endolymphatic hydrops. Extra fluid causes pressure to build up, so the canals swell and can’t work right. This leads to problems with the ear’s hearing and balance systems.

Neuralgia

• Pain resulting from inflammation of a nerve or a group of nerves.

Trigeminal neuralgia

Neuritis

• Inflammation of a nerve

Optic neuritis

Optic neuritis is acute visual loss owing to demyelination of the optic nerve. It may be an isolated autoimmune condition or part of multiple sclerosis. Fortunately, vision recovers to normal or near normal in over 90% of patients within six months. No treatment improves those chances.

Otitis media

• Inflammation of the middle ear

Bacterial or viral infection occurs in the fluid buildup after a respiratory illness

Otosclerosis

• Formation of spongy bone in the inner ear, which often causes deafness by fixing the stapes to the oval window

TreatmentIn the early stages of otosclerosis, or when the condition is mild, you might not need any treatment. Hearing aids are very useful initially. However, as the calcium buildup on the stapes progresses you will gradually lose your hearing. Sodium fluoride tablets have been shown to help prevent the progression of otosclerosis, but only if the condition has also affected the inner ear. At some point, most people usually have an operation - a stapedectomy or stapedotomy - where a tiny piston replaces the stapes so that sound can travel to the inner ear. This operation has a high success rate.

Pterygium

• Abnormally thickened patch of conjunctiva that extends over part of the cornea

Pterygium occurs more often in people who spend a great deal of time outdoors, especially in sunny climates. Long-term exposure to sunlight, especially ultra-violet (UV) rays, and chronic eye irritation from dry; dusty conditions seem to play an important causal role. When a pterygium becomes red and irritated, topical eye-drops or ointment may be used to help reduce the inflammation. If the pterygium is large enough to threaten sight, is growing or is unsightly, it can be removed surgically

Retinitis pigmentosa

• Inherited, progressive retinal sclerosis characterized by pigment deposits in the retina and by retinal atrophy

In the progression of symptoms for RP, night blindness generally precedes tunnel vision by years or even decades. Many people with RP do not become legally blind until their 40s or 50s and retain some sight all their life. Others go completely blind from RP, in some cases as early as childhood. Progression of RP is different in each case.RP is a group of inherited disorders in which abnormalities of the photoreceptors (rods and cones) or the retinal pigment epithelium (RPE) of the retina lead to progressive visual loss. Affected individuals first experience defective dark adaptation or nyctalopia (night blindness), followed by constriction of the peripheral visual field and, eventually, loss of central vision late in the course of the disease.

Retinoblastoma

• Inherited, highly malignant tumor arising from immature retinal cells

Retinoblastoma is a rare cancer of the retina (the innermost layer of the eye, located at the back of the eye, that receives light and images necessary for vision). About 300 children will be diagnosed with retinoblastoma this year. It accounts for 3 percent of childhood cancers. Treatments include surgery, radiation, chemotherapy, laser therapy, phototherapy, thermal therapy, and cryotherapy.

Tinnitus

• Ringing or buzzing noise in the ears.

Ringing, buzzing, whistling, or roaring noises in the ear). These noises may come and go or may always be present. The noises may get louder just before a vertigo attack.

Trachoma

• Bacterial disease of the eye that causes conjunctivitis, which may lead to blindness

Trachoma, an infection of the eye caused by Chlamydia trachomatis, ranks worldwide as the most common preventable cause of blindness and the second most common cause of blindness after cataract. It has been estimated to cause 15% of the world's blindness.1,20 The disease is endemic in 48 countries in Latin America, Africa, the Middle East, Asia, and Australasia [see Fig. 1], and is most prevalent in poor, rural communities with lower standards of hygiene and sanitation.2 The WHO currently estimates that 6 million people are blind due to trachoma, and that an additional 146 million people have active forms of the disease.

Tympanoplasty

• Surgical reconstruction of the middle ear bones and the establishment of continuity from the eardrum to the oval window.

Uveitis

• Inflammation of the uvea, the region of the eye that includes the iris, ciliary body, and choroid coat.

There are different types of uveitis, depending on which part of the eye is affected: When the uvea is inflamed near the front of the iris, it is called iritis. If the uvea is inflamed in the middle of the eye, it is called cyclitis. Cyclitis affects the muscle that focuses the lens.An inflammation in the back of the eye is called choroiditis. Eye drops, especially steroids and pupil dilators, can reduce inflammation and pain. For more severe inflammation, oral medication or injections may be necessary. Uveitis can have these complications: Glaucoma (increases pressure in the eye); Cataract (clouding of the eye's natural lens); Neovascularization (growth of new, abnormal blood vessels).

Vertigo

• Sensation of dizziness

Lab Review

• Visual Acuity• Astigmatism • Accomodation• Blind Spot• Photopupillary Reflex• Accommodation Pupillary Reflex• Convergence Reflex

*clinical connection p276