Special Senses

The Eye (Vision)

Vision is perception of light emitted or reflected from objects in the

environmentStimulus- light waves

An adult eye is a sphere that measures 1 inch (2.5cm) in diameter.

Accessory structures include the extrinsic eye muscles Eyelids conjunctiva lacrimal apparatus.

Eyelids protect the eyes anteriorly which meets at the lateral

corners of the eye.

Palpebral fissure the space between the eyelids in an open eye.

Anatomy of the EyeExternal and Accessory Structures

Eyelashesprojects from the boarder of each eyelids.Tarsal glandsThese are modified sebaceous glands associated with

the eyelids. Produces oily secretion that lubricates the eye.

Ciliary glandsmodified sweat glands, lie between the eyelashes. Conjunctiva- lines the eyelids and covers part of the outer surface

of the eyeball. It secretes mucus, which helps to lubricate the eyeball and keep it moist.

Lacrimal apparatusconsists the lacrimal gland and a number of ducts

that drains the lacrimal secretion into the nasal cavity.

Lacrimal glands Located above the lateral end of each eye. They

continually release dilute salt solution (tears) onto the anterior surface of the eyeball through several small ducts.

The tears flush across the eyeball into the lacrimal canaliculi medially, then into the lacrimal sac, and finally into nasolacrimal duct, which empties into the nasal cavity.

Lacrimal secretion contains antibodies and lysozyme.

Lysozyme enzymes that can destroy bacteria. Cleanses and

protects the eye surface as it moistens and lubricates it.

Six extrinsic or external, eye muscles are attached to the outer surface of each eye. These muscles produce gross eye movements and make it possible for the eyes to follow a moving object.

Eyeball the eye itselfA hollow sphereComposed of 3 layersFilled with fluids called bumors that help

maintain shapeLensMain focusing apparatus of the eye.Supported upright within the eye cavity,

dividing it into two chambers.

Internal Structures

Fibrous Layer Consist of a protective sclera and a

transparent cornea. The central anterior portion is crystal clear. Sclera a thick, glistening white connective tissue,

seen anteriorly as the “white of the eye”. Cornea Known as the “window” where light enters. Well supplied by nerve endings.

Layers Forming the Wall of the Eyeball

Supplied pain fibers.Most expose part of the eye. Very

vulnerable to damage.Ability to repair itself extraordinarily.

The cornea is the only tissue in the body that can be transplanted from one person to another without worrying rejection. It has no blood vessels and its beyond the reach of the immune system.

Vascular LayerMiddle layer of the eyeball.3 Distinguishable regions:1. Choroid- most posterior. A blood-rich

nutritive tunic that contains a dark pigment. The pigment prevents light from scattering inside the eye.

2. Smooth muscle structuresa) Ciliary body- Where lens is attached by a

suspensory ligament called ciliary zonule. b) Iris- has rounded opening, the pupil,

through which light passes. Acts like the diaphragm of the camera.

Regulates the amount of light entering the eye so that one can see as clearly as possible in the available light.

Close Vision and Bright light the circular muscles contract, and the pupil constricts.

Distant vision and Dim light, the radial fibers contract to enlarge (dilate) the pupil, which allows more light to enter the eye.

Sensory Layer Innermost sensory layer of the eye. Contains the two layered retina.

Two Layers of RetinaI. Outer Pigmented Layer- Composed of pigmented

cells that absorbs light and prevent light from scattering inside the eye. These cells act as phagocytes that remove dead or damaged receptor cells and stores needed vitamin A for vision.

II. Transparent Inner Neural Layer- contains millions of receptor cells, the rods and cones which are called photoreceptors.

Photoreceptors- Responds to light. Electrical signals pass from the photoreceptors via two neuron chain bipolar cells and ganglion cells before leaving retina via the optic nerve as nerve impulses that are transmitted to the optic cortex.

THE PROCESS OF SEEING

1. Formation of retinal image

Processes involved:

a. refraction of light rays- due to cornea, aqueous humor, lens, vitreous humor

b. accommodation of lens

Accommodation of Lens

2. Constriction of pupil-directs light rays to retina

3. Convergence of eyes- eyeballs converge so that visual axes come together at the object viewed

• Neural apparatus includes the retina & optic nerve

• Retina forms as an outgrowth of the brain attached only at optic disc where optic nerve begins

•Detached retinablow to head or lack of sufficient vitreous bodyblurry areas in field of visionleads to blindness due to disruption of blood supply

Test for Blind Spot

Optic disk or blind spot is where optic nerve exits the posterior surface of the eyeball no receptor cells are found in optic disk. Blind spot can be seen using the above illustration in the right position, stare at X and red dot disappears. Visual filling is the brain filling in the green bar across the blind spot area

Retina

blind spot macula

Details of Retina

Ganglion

Amacrine

Bipolar neuron

Horizontal cells

photoreceptive cells

Choroid

Sclera

Internal Anatomy of the Eye

Night Blindness

Inability to see well at night or in poor light. It is not a disorder in itself, but rather a symptom of an underlying disorder or problem, especially untreated myopia (nearsightedness).

Night blindness may exist from birth, or be caused by injury or malnutrition (for example, a lack of vitamin A). It can be described as insufficient adaptation to darkness.

Disorders of the Eyes

CausesMyopiaGlaucoma

medications that work by constricting the pupil

CataractsRetinitis

pigmentosaVitamin A

deficiency

Treatment Depends upon its cause. Treatment may be as simple as getting a new eyeglass

prescription or switching glaucoma medications, or it may require surgery if the night blindness is caused by cataracts.

Cataracts Is the clouding of the lens of the eye, which impedes

the passage of light. Although most cases of cataract are related to the

aging process, occasionally children can be born with the condition, or a cataract may develop after eye injuries, inflammation, and some other eye diseases

Causes vision to become hazy and distorted.

Causeso Diabetes Mellitus o Frequent exposure to sunlighto Heavy smoking

Treatment Surgical removal of the lens and replacement

with a lens implant or special cataract glasses.

Glaucoma

a term describing a group of ocular disorders with multi-factorial etiology united by a clinically characteristic intraocular pressure-associated optic neuropathy.]This can permanently damage vision in the affected eye(s) and lead to blindness if left untreated. It is normally associated with increased fluid pressure in the eye .

Stills sight slowly and painlessly until damage is done.Tonometer- used to measure the intraocular pressure.

TreatmentEyedrops- increases the rate of aqueous

humor drainage. Laser or Surgical enlargement of the

drainage of channels can be used. Opthalmoscope- instrument that illuminates

the inferior of the eyeball allowing the retina, optic disc, and internal blood vessels at the fundus, or posterior wall of the eye to be viewed and examined. Certain pathological conditions, such as diabetes, arteriosclerosis, and degeneration of the optic nerve and retina can be detected by such examination.

Emmetropia- Literally means harmonious vision.

Myopia- Short vision. It occurs when the parallel rays from distant objects fail to reach the retina and instead focus in front of it. Distant objects appear blurry to myopic people. Results from an eyeball that is too long, a lens to strong, or a cornea that is too curved.

Treatment for Myopia:Requires concave corrective lenses that

diverge the light rays before they enter the eye, so that they converge farther back.

Hyperopia- Far vision. Occurs when parallel light rays from distant objects are focused behind the retina at least in the resting eye in which the lens is flat and the ciliary muscle is relaxed. Results from an eyeball that is too short or a lazy lens. See distant objects clearly because their ciliary muscles contract continuously to increase the light-bending power of the lens, which moves the focal point forward onto the retina. Near by objects are blurry. Hyperopic people are subject to eye strains as their endlessly contracting ciliary muscles tire from overwork.

Treatment for Hyperopia: Correction requires convex corrective lenses that

converge the light rays before they enter the eye.

AstigmatismUnequal curvatures in different parts of the cornea

or lens. Blurry images occur because points on the retina but as lines. Eyes that are myopic or hyperopic and astigmatic require a more complex correction.

TreatmentSpecial cylindrically ground lenses or contacts are

used to correct this problem.

Astigmatism