TASTE
The sense of taste (gustation) is based on the perceptions of sweet, salty, sour, and bitter flavors and their combinations. Taste receptors, located on the tongue, are called taste buds. Chemoreceptors in the taste buds detect chemicals in the solution in the mouth. Basic tastes are perceived best by taste buds in certain areas of the tongue (Fig. 21-6). Bitter tastes are sensed at the back, sour on both sides, salty on the sides and the tip, and sweet at the tip (apex). (This information may be helpful when administering certain medications.)
Most foods have a combination of taste sensations. In addition, many characteristic tastes are greatly influenced by the sense of smell. (For example, try tasting an onion while holding your nose; it usually does not taste like an onion.)
Two cranial nerves carry the sensation of taste to the brain: the facial nerve (cranial nerve VII) and the glossopharyngeal nerve (cranial nerve IX). The sense of taste is interpreted in the parietal and temporal lobes of the cerebral cortex.
SMELL
The nerve of smell or olfaction is the olfactory nerve (cranial nerve I). Odors are tiny molecules of chemicals in the air, which may be mixed together. These molecules rise in the upper nasal cavity to the sensory area (olfactory epithelial tissue), which is about the size of a postage stamp. There are different receptors for different odors, which interact in a “lock-and-key” fashion with the nerve cells. The millions of sensory receptors in the olfactory nerve receive and transmit these impulses. Interpretation occurs in the olfactory center of the temporal lobe.
Key Concept Humans can differentiate between 5,000 and 10,000 different odors. Women generally have a more sensitive sense of smell than men.
TOUCH/TACTILE SENSE
Tactile receptors or corpuscles are the receptors for the sense of touch, found mostly in the skin. They are constantly receiving nerve impulses, allowing you to feel pressure, vibration, pain, or the pleasures of softness and warmth. They also alert you to the dangers of too much heat or cold. A greater number of touch receptors are located in some areas, such as the fingertips and around the lips.The parietal lobe of the brain is responsible for interpreting tactile stimuli. Temperature, pain, pressure, and proprioception (position of the body in space) are types of internal tactile stimuli that are described below.
OTHER SENTATIONS
Temperature
Two types of temperature receptors exist: those for heat (heat receptors) and those for cold (cold receptors). Temperature receptors are widespread in the body. The ratio between heat and cold receptors varies in different locations. For example, there are 10 to 15 times more cold receptors than heat receptors in the skin. Both types of receptors are abundant in the lips and in the mucous membranes of the oral and anal regions. The temperature sense protects the body, for example by preventing burns or by causing sweating or shivering in response to hot or cold stimuli.
Pain
Pain is usually protective. It causes people to move away from a painful stimulus or to seek medical attention for an internal disorder. Different types of pain can occur, depending on where the pain is located. Pain that is located in an external location, such as the skin, is considered superficial pain. Pain that is located in an internal organ system is described as visceral pain. Pain receptors also respond to extremes in temperature, such as when an ice cube touches a decayed tooth.
Pain that originates in an internal organ and is perceived in another location is called referred pain. (This type of pain refers to the site where the organ was originally located during fetal development.) The actual pain location and the referred site could both experience the sensation of pain, because the same part of the spinal cord connects to both areas. Although the organs have migrated during fetal development, the nerves still perceive pain as coming from the original site. Referred pain is extremely helpful in diagnosing many medical conditions. For example, a person suffering a heart attack may feel referred pain in the left arm, shoulder, neck, or jaw area. A person suffering from a gallbladder attack might have referred pain in the right shoulder (scapula area).
Pressure
The brain evaluates mechanical forces that distort or displace tissue. Tactile receptors can detect fine or light touches. More tactile receptors are found in the hairless portions of the body, such as the eyelids, lips, and fingertips. Pressure receptors within deeper tissues, some joints, and some visceral organs permit the sensations of deep pressure.
Proprioception
Location or position of the body (proprioception) depends on proprioceptors and the semicircular canals of the inner ear. Proprioceptors are located in muscles, tendons, and joints. They relay information about the relationships of body parts to one another. The nerve fibers from these structures are located in the posterior portion of the spinal cord. The cerebellum’s interpretation of these relationships allows us to walk and safely perform other physical activities.
EFFECTS OF AGING ON THE SYSTEM
Table 21-3 summarizes the effects of aging on the sensory system.
Eye and Vision Changes
Presbyopia
The lens of the eye becomes less elastic with aging, causing a gradual loss in accommodation. This is called presbyopia and usually begins between ages 40 and 45. In addition, this causes a decrease in the size of the pupil, admitting less light. Most older people require additional light for reading and may have difficulty with quickly changing levels of light, such as when driving at night. The stiffened lens often is not able to accommodate sufficiently well to focus on close objects. Reading material must be held at arm’s length (farsightedness) for print not to seem blurry. (In some cases, a topic may need to be held so far away that letters are no longer identifiable.) A person can be provided with eyeglasses, bifocals or trifocals, which will correct the situation. Bifocals have two types of lenses; the top is formulated to permit a person to see items far away. The bottom section is created to facilitate near vision. Trifocals are formulated in the same manner, except that there are three sections of the lens, one for distance, one for mid-range, and one for near vision. Graduated or line-free lenses are available, making the transitions between the corrections less noticeable and enhancing wearability.
Other Visual Disorders
Loss of visual acuity may also occur if debris (waste material) builds up within the eye. This process may occur at any time, but is more common in older people because of extended exposure to the sun’s ultraviolet rays, as well as to dust and wind. Color changes can also occur in the sclera, causing yellowing or browning (more common in people with dark skin) or a bluish hue (owing to thinning of the sclera). The vitreous gel can liquefy, causing posterior vitreous detachment (PVD). This causes opacities within the eyeball which are seen as “floaters.” Color perception and depth perception may also be reduced with aging.
TABLE 21-3. Effects of Aging on the Sensory System
|
FACTOR |
RESULT |
NURSING IMPLICATIONS |
|
VisionlEye Changes Lens accommodation decreases. |
Presbyopia |
Advise a vision check for corrective lenses. |
|
Depth perception decreases. |
Difficulty judging the height of curbs and steps Falls common |
Encourage use of hand rails, canes, and walkers. Advise to avoid fast moves or turns. Make client aware of dangers. |
|
Peripheral vision decreases. |
Driving may be dangerous |
Encourage a defensive driving class. Avoid standing at client’s side. |
|
Ability to react to darkness and to bright light decreases; night vision decreases. |
Takes longer for eyes to adjust when entering a dark room or bright sunlight |
Advise use of a night light. Advise person to avoid night driving if possible. |
|
Color perception decreases. |
Difficulty discerning hues of blue, green, and violet |
Use yellow, red, and black for signs. |
|
A grayish white ring (arcus senilis) forms around the iris. |
May lower self-esteem and body image |
Enhance client’s self-esteem. |
|
Tear formation decreases. |
Dry, itchy eyes More susceptible to infections |
Advise about medication ("artificial tears”). Advise against rubbing eyes. |
|
Fluid circulation in eye decreases. |
Increased risk for glaucoma |
Encourage regular visual examinations, including intraocular pressure measurement. |
|
Hearing/Ear Changes Numerous functional and structural changes occur in ear components. |
Presbycusis Progressive hearing loss |
Discuss hearing aid evaluation. Person may benefit from “helper” dog or special telephone volume controls. |
|
Loss of perception of high pitch, sound location, tracking sounds, normal conversation |
Face person when talking to him or hen Speak clearly but not too loudly. |
|
|
Increased incidence of vertigo, dizziness, and tinnitus |
Advise against driving if hearing is compromised. Tell client to use hand rails and to avoid sudden movements. Vertigo may be relieved by specific exercises (Epley maneuver). |
|
|
Structural changes affect balance and equilibrium. |
Increased incidence of falls Dizziness on change of position |
Advise the use of hand rails. Advise person to change positions slowly. |
|
Increased buildup of cerumen. |
Hearing loss |
Suggest frequent ear examinations. Cerumen removal may be required. |
|
Taste Changes Taste sensation decreases. |
Decreased appetite |
Monitor nutritional status. |
|
May try to compensate by increasing salt and sugar intake, aggravating conditions such as hypertension and diabetes |
Teach proper nutrition. |
|
|
Risk for consuming spoiled foods |
Teach client to check expiration dates on containers. |
|
|
Smell Changes Smell perception decreases. |
May not smell smoke or poisonous substances (e.g., gas leak) Affects sense of taste |
Teach client to install smoke detectors and preventive safety measures. Teach safe dietary modifications. |
|
Tactile Changes Efficiency and the number of sensory nerve endings (all sensations affected) decrease. |
Stronger stimuli are needed for person to perceive sensations. |
Monitor client’s overall condition. |
|
Pain associated with some conditions may differ; |
Do not ignore complaints. |
After about age 60, a prominent gray-white ring often occurs on the edge of the cornea (a deposit of calcium and cholesterol salts). This is called arcus senilis and does not affect vision.
The slowing of all body secretions and reduction of mucous cells in the conjunctiva may affect the lacrimal glands and may lead to extraordinarily dry eyes. “Artificial tears,” a type of eye drops, help provide moisture and lubrication to the eyes.
Clouding of the lens may occur at any age. The cloudy or opaque lens that results is called a cataract. It most often occurs by later middle age. When visual changes interfere with daily living or quality of life, the cataract is surgically removed and an artificial lens is implanted.
Other eye disorders commonly associated with aging include macular degeneration, retinal detachment, and diabetic retinopathy. Weakening of eyelid muscles can result in the lower eyelid falling away from the eyeball, ectropion.
Ptosis, a paralytic drooping of the upper eyelid, may occur as a result of the normal aging process or because of continued eye strain or wearing of certain types of contact lenses over many years. Severe ptosis that interferes with vision can be surgically corrected.
Special Considerations :LIFESPAN
Macular Degeneration
With advancing age, the retina’s macula or central part may degenerate, causing a loss of central vision. This loss of central vision is called macular degeneration. If the cause of this condition is an abnormal growth of blood vessels in the eye, and this cause is found early, it can be treated with laser therapy. Also, in most cases of macular degeneration, peripheral vision is maintained.
Ear and Hearing Changes
Many older people experience a degenerative loss of hearing, called presbycusis, which often begins at about age 60. The most common cause of presbycusis is deterioration of the cochlear structures. This specific difficulty is most often a result of the loss of hair cells in the organ of Corti. Initially, the ability to hear the highest pitches is affected; later, the lower frequencies are also difficult to hear. This may also involve difficulty in hearing specific consonants, such as k, t, s, p, and ch, which involve higher tones.
Other causes of impaired hearing in older people include:
• An injury or illness earlier in life (diabetes, otitis media, Ménière’s syndrome)
• Fusing of the ossicles in the middle ear, impairing their ability to vibrate freely
• Bone formation around the oval window
• Decalcification of skull bone, impairing bone conduction of sound
• Decreased cochlear function (may be caused by illness or injury)
• A disorder in the nerve pathway that carries sound impulses to the brain
• A disorder in the area of the brain where the impulses are received, which interferes with interpretation of sound impulses
• Lifelong exposure to loud noises, damaging the eardrum or structures of the inner ear
• Excess cerumen (ear wax)
• Certain medications: quinine, aminoglycosides (i.e., gentamicin, streptomycin), aspirin, loop diuretics (e.g., furosemide [Lasix])
A hearing aid may help many older people with these and other hearing problems.
Changes in Other Senses
The senses of smell and taste gradually may become less keen as a person ages. These changes result from loss of nerve function, which interferes with the transmission of the sensations to the brain. Another factor relates to the taste buds. Connective tissue cells gradually replace the taste buds as the person ages. By age 75, estimates are that a person has approximately 40% of the taste buds that were functioning at age 30. With the loss of the sense of taste, the person may not feel as hungry and may need to be encouraged to eat even when not hungry. He or she may also use salt and spices excessively to compensate.
The vocal cords of the older person may simply “wear out,” making speech difficult or impossible. The person is able to whisper at first, but may lose that ability as the condition worsens. The voice may deepen because of thickening and loss of mobility of the vocal cords and hormonal changes.
A condition in which calcium sludge forms particles in one of the semicircular canals of the inner ears is called benign positional paroxysmal vertigo (BPPV, BPV). This causes dizziness and often can be relieved by the Epley maneuver (the person’s head is turned rapidly while he or she is recumbent). This breaks up the particles and redistributes them.
NCLEX Alert The effects of aging on the sensory system may be integrated into the NCLEX exam as they impact safety effective basic care, pain control or comfort, and teaching. The options could reflect nursing actions in a variety of clinical settings.
KEY POINTS
• The five senses are seeing, hearing, tasting, smelling, and touching.
• The eye is the organ of vision. It has many protective mechanisms. Light rays travel through several structures of the eye before focusing on the retina.
• The eyeball has three major layers of tissue: sclera and cornea, choroid layer, and retina.
• Several cranial nerves are involved in vision.
• Three sets of extraocular muscles control eye movements.
• The three parts of the ear are the external, the middle, and the inner ear.
• The external ear is responsible for protecting the internal structures of the ear from foreign substances and for catching and carrying sound waves to the middle ear. The middle ear is responsible for the transmission of sound. The inner ear is responsible for transmitting sound waves and information about body position to the brain.
• The semicircular canals of the inner ear are responsible for the sensation of one’s position in space. Many factors contribute to the achievement of body balance.
• Taste buds are responsible for the perceptions of sweet, salty, sour, and bitter.
• Receptors for the sense of smell are located in the upper nasal cavity (olfactory receptors).
• Receptors for the sense of touch are called tactile receptors; they are located throughout most areas of the body.
• Proprioceptors are located in muscles, tendons, and joints and contribute to the sense of balance.
• Referred pain is perceived in a place other than where it originates.
• The temporal lobe of the brain is responsible for interpreting sounds via the vestibulocochlear nerve in the inner ear. The parietal and temporal lobes are responsible for interpreting taste. The occipital lobe receives visual information via the optic nerve. The cerebellum is responsible for maintaining equilibrium and coordination.
