Copyright ©2012 Delmar, Cengage Learning. All rights reserved. Chapter 25 The Senses

Copyright ©2012 Delmar, Cengage Learning. All rights reserved.

Chapter 25

The Senses


The Senses of the Human Bodyand Corresponding Organs

• Sight: the eye• Hearing: the ear• Taste: the tongue• Smell: the nose• Touch: the skin

• Knowledge of the environment requires sensory organs, cranial nerves, and the brain to interpret stimuli.



The Structures of the Eye



• Sclera: tough outer covering – Maintains the shape of the eye

• Extraocular muscles (6)– Contract or relax to permit movement of the eyeball

• Choroid– Layer under the sclera– Contains the blood vessels that serve the eye tissues– This layer also has a nonreflective pigment that keeps

light from reflecting within the eye.



• Retina: focuses an image onto the surface• Iris: colored circular muscle that surrounds the

pupil; regulates the amount of light• Optic disk: blind spot • Pupil: central opening• Cornea: curved covering of the sclera that can

correct some unclear images



• Conjunctiva: the white of the eye• Vitreous humor: thick, jellylike substance that

maintains the shape of the eyeball• Aqueous humor: salty clear fluid; circulates

between the chambers



• Fovea centralis: posterior surface of the retina; the area of sharpest vision

• Rods: nerve cells that transmit the stimuli so we can see in black and white

• Cones: nerve cells that transmit the stimuli so we can see in color


The Effects of the Lens and Corneaon Focusing Images

• When the lens or cornea has an abnormal curvature, the image is improperly refracted.

• When the image is focused in front of the retina, the person is said to nearsighted.

• The medical term for nearsightedness is myopia.


The Effects of the Lens and Corneaon Focusing Images

• When the image is focused behind the retina, the person is said to be farsighted.

• The medical term for farsightedness is hyperopia.


Age-Related Macular Degeneration

• A disease that affects the most sensitive part of the retina, and alters the center of the visual field

• Results in a gradual loss of central vision, but side vision is maintained


Amblyopia

• Also called lazy eye• Most often affects children under age 5• One eye turns inward, causing blurred vision


Arcus Senilis

• Accompanies normal aging • Results in a thin greyish-white arc or circle not

quite at the edge of the cornea


Blepharitis

• Inflammation of the edges of the eyelids involving the hair follicle and glands

• Results in itching and burning, rubbing eyes, and constant blinking


Cataract

• Gradually developing opacity of the lens• Occurs most frequently in persons over the age of

70• Causes painless, gradual blurring and loss of vision


Conjunctivitis

• Caused by inflammation of the conjunctiva• Begins in one eye and spreads to the other;

causes redness and a “bloodshot” appearance• Usually caused by a bacterial infection


Corneal Abrasion

• A scratch or trauma to the cornea usually caused by a foreign body in the eye

• Vision can be affected if the location and extent of the injury are significant


Diabetic Retinopathy

• Usually results from juvenile or adult diabetes• Edema of the retina causes light to scatter• Capillary walls dilate, twist, and/or

hemorrhage• Causes glare, blurred vision, and reduced

visual acuity


• Characterized by excessive intraocular pressure that results in atrophy of the optic nerve; causes severe visual impairment and eventual blindness

Glaucoma


Diseases of the Eye• Closed-angle glaucoma

– Aqueous humor cannot drain because of a blockage and pressure rises slowly

• Acute closed-angle glaucoma– Obstruction of the drainage holes resulting from

automatically shallow chambers and a thickened iris that closes the passages

– There is a rapid onset of symptoms; this is considered an emergency.


Hordeolum and Iritis

• Hordeolum– Localized infection of the gland of the eyelid;

produces an abscess around an eyelash– Causes red, painful, and swollen eyes

• Iritis– Inflammation of the iris; causes moderate to

severe eye pain, sensitivity to light, and small, nonreactive pupils


Presbyopia and Ptosis• Presbyopia

– Inability of the lens to accommodate for near vision; usually occurs as a normal part of the aging process

• Ptosis– Drooping of the upper eyelid that results in the

eyes appearing to be only partially open– Could be the result of a congenital condition,

aging, excess fatty folds, or neurological factors


Retinal Detachment

• Results from separation of the retina from the choroid layer of the eyeball

• Causes the patient to see floating spots, flashes of light, and a gradual loss of vision


Strabismus

• One eye deviates with the gaze being abnormally inward or outward or higher or lower than the other eye

• This can results in:– Esotropia: abnormally inward gaze– Exotropia: abnormally outward gaze


The Structures of the Ear



• Pinna: external ear shell; also called the auricle; collects and directs sound down the external auditory canal

• External auditory canal: passageway from outside to the eardrum

• Tympanic membrane: eardrum; vibrates when sound waves hit it



• Vibrations from the tympanic membrane hit the three small bones of the middle ear.

• The malleus (hammer) strikes the incus (anvil), which moves the stapes (stirrups).

• This communicates the vibrations to the inner ear when the stapes pushes against the fluid in the vestibule of the inner ear.



• The middle ear is connected by the eustachian tube to the throat.

• The eustachian tube equalizes the air pressure in the middle ear with the outside air pressure.



• Vibrations from the middle ear continue through a snail-shaped organ called the cochlea.

• The cochlea contains a collection of specialized nerve cells called the organ of Corti.



• The organ of Corti transmits impulses to the auditory nerve, which passes them on to the auditory center of the temporal lobe of the cerebrum of the brain for interpretation.

• The inner ear also contains three semicircular canals that are responsible for maintaining balance.


The Balance Function of the Inner Ear

• Hairlike nerve cells within the three canals are embedded in a gelatin-like material that pushes against receptors when the head moves and transmits this change in position to the brain.


Auditory canal obstruction

• Anything in the ear canal that can occlude the opening


Hearing Loss

• Reduced ability to perceive sound at normal levels

• Hearing loss can be the result of obstruction, nerve damage from prolonged exposure to noise, or disease


Meniere’s Disease

• Disorder of the inner ear that usually affects only one ear; typically begins between ages 20 and 50

• Results in extreme dizziness (vertigo) and ringing in the ears (tinnitus).


Motion Sickness

• Loss of balance, perspiration, headache, nausea, and vomiting brought on by irregular motion

• Examples: riding in cars, boats, planes, or amusement park rides


Otitis

• Otitis externa– Infection of the external auditory canal that

causes pain and hearing loss• Otitis media

– Infection of the middle ear often associated with respiratory infections


Otosclerosis and Prebycusis

• Otosclerosis– Loss of movement of the stapes that results in

deafness called bone conduction deafness• Presbycusis

– Progressive hearing loss that usually results as a result of aging


Epistaxis and Nasal Polyps

• Epistaxis– Bleeding from the nose that can result from

trauma or a disease process• Nasal polyps

– Benign growths inside the nose that obstruct the airway


Rhinitis

• A reaction to airborne allergens that results in sneezing, watery eyes, and itching of the eyes and nose


The Structures of the Nose

• The sense of smell is caused by the olfactory organ in the top of the nasal cavity.

• Nerve fibers in the nose respond to stimuli and these nerve fibers run through tiny holes in the skull to the olfactory center in the brain.


The Structures of the Tongue

• The ability to taste flavors is located in the receptors of the taste buds on the tongue.

• These receptors are located at the tip, sides, and back of the tongue.


Taste Sensation and the Corresponding Areas of the Tongue

• Tip– Sweet

• Sides– Sour

• Back– Bitter

• Area between tip and sides– Salty


Candidiasis

• Fungal infection of the mouth and throat that results in white patches that appear on the tongue, mouth, or throat


Glossitis and Oral Cancer

• Glossitis– Inflammation of the tongue

• Oral cancer– Cancer of the tongue, mouth, or throat; often

linked to tobacco use


The Structures of the Skin

• Touch receptors are located in the fingertips.• Pain receptors are bare nerve endings in the

skin and other organs.• Skin receptors perceive heat and cold.


The Skin’s Contact Receptors

• The sense of touch involves pressure or traction that activates mechanoreceptors.

• Radiant energy, heat or cold, activates thermoreceptors.

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Copyright ©2012 Delmar, Cengage Learning. All rights reserved. Chapter 25 The Senses