The senses are divided according to whether they are widely distributed or localized in special sense organs. The receptors for the general senses are found throughout the body. Many are located in the skin. These senses include the following.

• Pain. These receptors are found in the skin and also in muscles, joints, and internal organs.
• Touch, the tactile sense, located in the skin. Sensitivity to touch depends on the concentration of these receptors in different areas—high on the fingers, lips, and tongue, for example, but low at the back of the neck or back of the hand.
• Pressure, or deep touch, located beneath the skin and in deeper tissues.
• Temperature. Receptors for heat and cold are located in the skin and also in the hypothalamus, which regulates body temperature.
• Proprioception, the awareness of body position. Receptors in muscles, tendons, and joints help to judge body position and coordinate muscle activity. They also help to maintain muscle tone.

Receptors for general senses in the skin

Synapses for these pathways are in the spinal cord.

The special senses are localized within complex sense organs in the head. These include the following.

• Gustation (taste) is located in receptors in taste buds on the tongue. These receptors basically detect only sweet, sour, bitter, salty, and umami (oo-MOM-e), a savory flavor triggered by certain amino acids and found in proteins and the flavor enhancer MSG. Researchers have also identified receptors for alkali (bases) and metallic tastes. The senses of smell and taste are chemical senses, that is, they respond to chemicals in solution.
• Olfaction (smell) is located in receptors in the nose. Many more chemicals can be discriminated by smell than by taste. Both senses are important in stimulating appetite and warning of harmful substances.
• Hearing receptors are located in the ear. These receptors respond to movement created by sound waves as they travel through the ear.
• Equilibrium receptors are also located in the ear. These receptors are activated by changes in the position of cells in the inner ear as we move.
• Vision receptors are light-sensitive and located deep within the eye, protected by surrounding bone and other support structures. The coordinated actions of external and internal eye muscles help in the formation of a clear image.

Suffixes pertaining to the senses are listed in the table below. The remainder of this module concentrates on hearing and vision, the senses that have received the most clinical attention.

Suffixes Pertaining to the Senses

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Key Terms: Senses

The terms listed below are emphasized in this module. Knowing them will help you organize and prioritize your learning.

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Fill in blanks. Define the following words.

The ear has the receptors for both hearing and equilibrium. For study purposes, it may be divided into three parts: the outer, middle, and inner ear.

Parts of the ear

The outer ear consists of the projecting pinna (auricle) and the external auditory canal (meatus). This canal ends at the tympanic membrane, or eardrum, which transmits sound waves to the middle ear. Glands in the external canal produce a waxy material, cerumen, which protects the ear and helps to prevent infection.

Spanning the middle ear cavity are three ossicles (small bones), each named for its shape: the malleus (hammer), incus (anvil), and stapes (stirrup). Sound waves traveling over the ossicles are transmitted from the footplate of the stapes to the inner ear. The auditory tube, also called the eustachian tube, connects the middle ear with the pharynx and serves to equalize pressure between the outer ear and the middle ear.

The inner ear, because of its complex shape, is described as a labyrinth, which means “maze”. It consists of an outer bony framework containing a similarly shaped membranous channel. The entire labyrinth is filled with fluid.

The inner ear

The outer bony labyrinth contains the membranous labyrinth. Receptors for equilibrium are in the vestibule and the semicircular canals. The cochlea contains the hearing receptor, the spiral organ. Sound waves enter the cochlea through the oval window, travel through the cochlea, and exit through the round window. The inner ear transmits impulses to the brain in the vestibulocochlear nerve (eighth cranial nerve).

The cochlea, shaped like a snail’s shell, has the specialized spiral organ (organ of Corti), which is concerned with hearing. Cells in this receptor organ respond to sound waves traveling through the cochlea’s fluid-filled ducts. Sound waves enter the cochlea from the base of the stapes through an opening, the oval window, and leave through another opening, the round window.

The sense of equilibrium is localized in the vestibular apparatus. This structure consists of the chamber-like vestibule and three projecting semicircular canals. Special cells within the vestibular apparatus respond to movement. (The senses of vision and proprioception are also important in maintaining balance.)

Nerve impulses are transmitted from the ear to the brain by way of the vestibulocochlear nerve, the eighth cranial nerve, also called the acoustic or auditory nerve. The cochlear branch of this nerve transmits impulses for hearing from the cochlea; the vestibular branch transmits impulses concerned with equilibrium from the vestibular apparatus.

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Outer ear

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Pinna

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External auditory canal

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Tympanic membrane

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Middle ear

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Malleus

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Incus

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Stapes

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Inner ear

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Semicircular canals

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Vestibule

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Cochlea

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Vestibular nerve

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Cochlear nerve

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Vestibulocochlear nerve (VIII)

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Auditory tube

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Pharynx

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Cerumen

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Temporal bone

Key Terms: Normal Structure and Function of the Ear

The terms listed below are emphasized in this module. Knowing them will help you organize and prioritize your learning.

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Hearing Loss

Hearing impairment may result from disease, injury, or developmental problems that affect the ear itself or any nervous pathways concerned with the sense of hearing.

Sensorineural hearing loss results from damage to the inner ear, the eighth cranial nerve, or central auditory pathways. Heredity, toxins, exposure to loud noises, and the aging process are possible causes for this type of hearing loss. It may range from inability to hear certain sound frequencies to a complete loss of hearing (deafness). People with extreme hearing loss that originates in the inner ear may benefit from a cochlear implant. This prosthesis stimulates the cochlear nerve directly, bypassing the receptor cells of the inner ear, and may allow the recipient to hear medium to loud sounds.

Conductive hearing loss results from blockage in sound transmission to the inner ear. Causes include obstruction, severe infection, or fixation of the middle ear ossicles. Often, physicians can successfully treat the conditions that cause conductive hearing loss.

Otitis

Otitis is any inflammation of the ear. Otitis media refers to an infection that leads to fluid accumulation in the middle ear cavity. One cause is malfunction or obstruction of the auditory tube, as by allergy, enlarged adenoids, injury, or congenital abnormalities. Another cause is infection that spreads to the middle ear, most commonly from the upper respiratory tract. Continued infection may lead to accumulation of pus and perforation of the eardrum. Otitis media usually affects children under 5 years of age and may result in hearing loss. If not treated with antibiotics, the infection may spread to other regions of the ear and head. An incision, a myringotomy, and placement of a tube in the tympanic membrane helps to ventilate and drain the middle ear cavity in cases of otitis media.

Otitis externa is inflammation of the external auditory canal caused by repeated fungal or bacterial infections. It is most common among those living in hot climates and among swimmers, leading to the alternative name, “swimmer’s ear.”

Otosclerosis

In otosclerosis, the bony structure of the inner ear deteriorates and then reforms into spongy bone tissue that may eventually harden. Most commonly, the stapes becomes fixed against the inner ear and is unable to vibrate, resulting in conductive hearing loss. The cause of otosclerosis is unknown, but some cases are hereditary. Surgeons usually can remove the damaged bone. In a stapedectomy, the stapes is removed, and a prosthetic bone is inserted.

Ménière disease

Ménière disease is a disorder that affects the inner ear. It seems to involve production and circulation of the fluid that fills the inner ear, but the cause is unknown. The symptoms include vertigo (dizziness), hearing loss, tinnitus (ringing in the ears), and a feeling of pressure in the ear. The course of the disease is uneven, and symptoms may become less severe with time. Ménière disease is treated with drugs to control nausea and dizziness, such as those used to treat motion sickness. In severe cases, the inner ear or part of the eighth cranial nerve may be surgically destroyed.

Acoustic neuroma

An acoustic neuroma (also called schwannoma or neurilemmoma) is a tumor that arises from the neurilemma (sheath) of the eighth cranial nerve. As the tumor enlarges, it presses on surrounding nerves and interferes with blood supply. This leads to tinnitus, dizziness, and progressive hearing loss. Other symptoms develop as the tumor presses on the brainstem and other cranial nerves. Usually, it is necessary to remove the tumor surgically.

Key Terms: Disorders and Treatment of the Ear

The terms listed below are emphasized in this module. Knowing them will help you organize and prioritize your learning.

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Key Terms: More About Hearing, Conditions and Treatments

The terms listed below increase your knowledge of this topic.

The Rinne test

This test assesses both bone and air conduction of sound. A. Test of bone conduction through the mastoid process behind the ear. B. Test of air conduction.

The Weber test

This test assesses bone conduction of sound.

Abbreviations

The abbreviations listed below are emphasized in this module.

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The eye is protected by its position within a bony socket or orbit. It is also protected by the eyelids, or palpebrae; eyebrows; and eyelashes. The lacrimal (tear) glands constantly bathe and cleanse the eyes with a lubricating fluid that drains into the nose. The protective conjunctiva is a thin membrane that lines the eyelids and covers the anterior portion of the eye. This membrane folds back to form a narrow space between the eyeball and the eyelids. Medications, such as eye drops and eye ointments, can be instilled into this conjunctival sac.

The eye’s protective structures

The lacrimal gland produces tears that flow across the eye and drain into the lacrimal canals.

The wall of the eye is composed of three layers. Named from outermost to innermost, they are as follows.

The sclera, commonly called the white of the eye, is the tough surface protective layer. The sclera extends over the eye’s anterior portion as the transparent cornea.

The uvea is the middle layer, which consists of the:

• Choroid, a vascular and pigmented layer located in the posterior portion of the eyeball. The choroid provides nourishment for the retina.
• Ciliary body, which contains a muscle that controls the shape of the lens to allow for near and far vision, a process known as accommodation. The lens must become more rounded for viewing close objects.

Accommodation for near vision

When viewing a close object, the lens must become more rounded to focus light rays on the retina.

• Iris, a muscular ring that controls the size of the pupil, thus regulating the amount of light that enters the eye. The genetically controlled pigments of the iris determine eye color.

Function of the iris

In bright light, muscles in the iris constrict the pupil, limiting the light that enters the eye. In dim light, the iris dilates the pupil to allow more light to enter the eye.

The retina is the innermost layer and the actual visual receptor. It consists of two types of specialized cells that respond to light.

• The rods function in dim light, provide low visual acuity (sharpness), and do not respond to color.
• The cones are active in bright light, have high visual acuity, and respond to color.

The eye

The three layers of the eyeball are shown along with other structures involved in vision.

Proper vision requires the refraction (bending) of light rays as they pass through the eye to focus on a specific point on the retina. The impulses generated within the rods and cones are transmitted to the brain by way of the optic nerve (second cranial nerve). Where the optic nerve connects to the retina, there are no rods or cones. This point, at which there is no visual perception, is called the optic disk, or blind spot. The fovea is a tiny depression in the retina near the optic nerve that has a high concentration of cones and is the point of greatest visual acuity. The fovea is surrounded by a yellowish spot called the macula.

The fundus (back) of the eye as seen through an ophthalmoscope

The optic disk (blind spot) is shown as well as the fovea, the point of sharpest vision, in the retina.

The eyeball is filled with a jelly-like vitreous body, which helps maintain the shape of the eye and also refracts light. The aqueous humor is the fluid that fills the eye anterior to the lens, maintaining the cornea’s shape and refracting light. This fluid is constantly produced and drained from the eye.

Six muscles attached to the outside of each eye coordinate eye movements to achieve convergence, that is, coordinated movement of the eyes so that they both are fixed on the same point.

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Refer to the image below to complete the matching activity.

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Retina

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Choroid

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Sclera

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Ciliary body

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Suspensory ligaments

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Cornea

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Iris

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Pupil

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Lens

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Aqueous humor

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Vitreous body

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Fovea

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Optic disc

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Blood vessels

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Optic nerve

Key Terms: Normal Structure and Function of the Eye

The terms listed below are emphasized in this module. Knowing them will help you organize and prioritize your learning.

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Roots for External Eye Structures

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Roots for the Eye and Vision

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Suffixes for the Eye and Vision^*

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Errors of Refraction

If the eyeball is too long, images will form in front of the retina. To focus clearly, one must bring an object closer to the eye. This condition of nearsightedness is technically called myopia.

The opposite condition is hyperopia, or farsightedness, in which the eyeball is too short and images form behind the retina. One must move an object away from the eye for clear focus. The same effect is produced by presbyopia, which accompanies aging. The lens loses elasticity and can no longer accommodate for near vision, so a person gradually becomes farsighted.

The term astigmatism describes an irregularity in the curve of the cornea or lens that distorts light entering the eye and blurs vision.

Errors of refraction

A. Myopia (nearsightedness). B. Hyperopia (farsightedness). A concave (inwardly curved) lens corrects for myopia; a convex (outwardly curved) lens corrects for hyperopia.

Clinical Perspectives. Eye Surgery: A Glimpse of the Cutting Edge

Cataracts, glaucoma, and refractive errors are common eye disorders. In the past, cataract and glaucoma treatments concentrated on managing the diseases. Refractive errors were corrected using eyeglasses and, more recently, contact lenses. Today, using laser and microsurgical techniques, ophthalmologists can remove cataracts, reduce glaucoma, and allow people with refractive errors to put their eyeglasses and contacts away. These cutting-edge procedures include:

• LASIK (laser in situ keratomileusis) to correct refractive errors. During this procedure, a surgeon uses a laser to reshape the cornea so that it refracts light directly onto the retina, rather than in front of or behind it. A microkeratome (surgical knife) is used to cut a flap in the cornea’s outer layer. A computer-controlled laser sculpts the middle layer of the cornea and then the flap is replaced. The procedure takes only a few minutes, and patients recover their vision quickly and usually with little postoperative pain.
• Phacoemulsification to remove cataracts. During this procedure, a surgeon makes a very small incision (~3 mm long) through the sclera near the cornea’s outer edge. An ultrasonic probe is inserted through this opening and into the center of the lens. The probe uses sound waves to emulsify the lens’s central core, which is then suctioned out. An artificial lens is then permanently implanted in the lens capsule. The procedure is typically painless, although the patient may feel some discomfort for one to two days afterward.
• Laser trabeculoplasty to treat glaucoma. This procedure uses a laser to help drain fluid from the eye and lower intraocular pressure. The laser is aimed at drainage canals located between the cornea and iris and makes several burns that are believed to open the canals and allow better fluid drainage. The procedure is typically painless and takes only a few minutes.

Infection

Several microorganisms can cause conjunctivitis (inflammation of the conjunctiva). This is a highly infectious disease commonly called “pink eye.”

The bacterium Chlamydia trachomatis causes trachoma, inflammation of the cornea and conjunctiva that results in scarring. This disease is rare in the United States and other industrialized countries but is a common cause of blindness in underdeveloped countries, although it is easily cured with sulfa drugs and antibiotics.

Gonorrhea is the usual cause of an acute conjunctivitis in newborns called ophthalmia neonatorum. An antibiotic ointment is routinely used to prevent such eye infections in newborns.

Disorders of the Retina

In cases of retinal detachment, the retina separates from the underlying choroid layer of the eye as a result of trauma or an accumulation of fluid or tissue between the layers. This disorder may develop slowly or may occur suddenly. If it is left untreated, complete detachment can occur, resulting in blindness. Treatment includes use of an electric current or weak laser beam to create pinpoint scars that reattach the retina. Posterior detachment of the vitreous body, as may occur in middle age and beyond, can also pull the retina away. Danger signs include light flashes with eye movement, floater showers, or the appearance of a “black curtain” over part of the visual field. If any of these symptoms appear, a person should consult an ophthalmologist immediately.

Retinal detachment

Degeneration of the macula, the point of sharpest vision, is a common cause of visual problems in the elderly. When associated with aging, this deterioration is described as age-related macular degeneration (AMD). In nonexudative (“dry”) macular degeneration, material accumulates on the retina. Vitamins C and E, beta carotene, and zinc supplements may delay this process. In neovascular (“wet”) AMD, abnormal blood vessels grow under the retina, causing it to detach. Laser surgery may stop the growth of these vessels and delay vision loss. More recently, ophthalmologists have had success in delaying the progress of wet AMD with regular intraocular injections of a drug (e.g., Lucentis) that inhibits blood vessel formation. Macular degeneration typically affects central vision but not peripheral vision. See the images below, comparing normal vision to the effects of blurred central vision in blocks a and b of the image. Other causes of macular degeneration are drug toxicity and hereditary diseases.

Circulatory problems associated with diabetes mellitus eventually cause changes in the retina referred to as diabetic retinopathy. In addition to vascular damage, there is a yellowish, waxy exudate high in lipoproteins. With time, new blood vessels form and penetrate the vitreous humor, causing hemorrhage, detachment of the retina, and blindness. The visual effects of diabetic retinopathy can be seen in box c of the image.

Cataract

A cataract is an opacity (cloudiness) of the lens that blurs vision (see block d of image). Causes of cataract include disease, injury, chemicals, and exposure to physical forces, especially the ultraviolet radiation in sunlight. The cataracts that frequently appear with age may result from exposure to environmental factors in combination with degeneration attributable to aging.

To prevent blindness, an ophthalmologist must remove the cloudy lens surgically. Commonly, the lens’s anterior capsule is removed along with the cataract, leaving the posterior capsule in place. In phacoemulsification, the lens is fragmented with high-frequency ultrasound and extracted through a small incision. After cataract removal, an artificial intraocular lens (IOL) is usually implanted to compensate for the missing lens. The original type of implant provides vision only within a fixed distance; newer implants are designed to allow for near and far accommodation. Alternatively, a person can wear a contact lens or special glasses.

Cataract extraction surgeries

A. Cross-section of normal eye anatomy. B. Extracapsular lens extraction involves removing the lens but leaving the posterior capsule intact to receive a synthetic intraocular lens. C. Intracapsular lens extraction involves removing the lens and lens capsule and implanting a synthetic intraocular lens in the anterior chamber.

Glaucoma

Glaucoma is an abnormal increase in pressure within the eyeball. It occurs when more aqueous humor is produced than can be drained away from the eye. There is pressure on blood vessels in the eye and on the optic nerve, leading to blindness. There are many causes of glaucoma, and screening for this disorder should be a part of every routine eye examination. Fetal infection with rubella (German measles) early in pregnancy can cause glaucoma, as well as cataracts and hearing impairment. Glaucoma is usually treated with medication to reduce pressure in the eye and occasionally is treated with surgery.

Key Terms: Disorders of the Eye

The terms listed below are emphasized in this module. Knowing them will help you organize and prioritize your learning.

Practice Activity

Key Terms: More About Vision, Conditions and Treatments

The terms listed below expand on the key terms to increase your knowledge of this module topic.

Practice Activity

Abbreviations: The Eye

The abbreviations listed below are emphasized in this module.

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