SENSORY ABILITIES

Chapter 42 SENSORY ABILITIES




KEY TERMS/CONCEPTS























CHAPTER FOCUS


The sensory abilities of taste, smell, touch, sight and hearing enable the client to receive stimuli that facilitate interaction and provide information about the environment. Perception of sensory stimuli has its origin in the five special sense organs, which are specially adapted for the reception of specific stimuli — tongue (taste), nose (smell), skin (touch), eyes (sight) and ears (hearing and maintenance of balance). Receptors in the sense organs pick up stimuli from the environment and transmit this information to the brain via pathways in the nervous system. In the brain the information is processed and interpreted. A person’s senses are essential for growth, development and survival. Sensory stimuli give meaning to events in the environment. Any alteration in the individual’s sensory function can affect their ability to function within the environment. Alterations in sensory function may lead to dysfunctions of sight, hearing, smell, taste, balance and coordination.


As the eyes and ears are the two major structures by which an individual receives information about the external environment, this chapter focuses mainly on the care of these organs. The eye is the means by which light, reflected from objects, travels to the retina so that an image is formed. Nerve endings in the retina transmit electrical impulses along the optic nerve to the brain for interpretation. The ear is the means by which soundwaves are collected and amplified. Nerve endings in the inner ear transmit electrical impulses along the auditory pathways to the brain for interpretation. The auditory system is also responsible for maintenance of balance.





COMPONENTS OF THE SENSORY EXPERIENCE



THE TONGUE (TASTEBUDS)


The sensory receptors for taste are the taste buds. They are widely scattered in the oral cavity, with most concentrated over the surface of the tongue (Figure 42.1) and a few found on the soft palate and inner surface of the cheeks. They are chemoreceptors that respond to substances present in food and generate nerve impulses that are transmitted to the brain for interpretation. The upper surface of the tongue is covered with small projections (papillae) some of which contain a taste bud. Each taste bud consists of sensory and supporting cells situated in the epithelium and opening into the surface through a small gustatory pore. Between the cells of the taste bud lie the ending of afferent nerve fibres derived from several cranial nerves. Taste buds on the anterior two-thirds of the tongue connect with fibres of the facial (seventh cranial) nerve. Taste buds on the posterior one-third of the tongue are associated with the fibres of the glossopharyngeal (ninth cranial) nerve, while pharyngeal taste buds send impulses to the brain via the vagus (tenth cranial) nerve.



These taste buds can differentiate among sweet, salty, sour and bitter stimuli. The tip of the tongue is most sensitive to sweet and salty substances, the edges of the tongue are most sensitive to sour substances, and the back of the tongue is most sensitive to bitter substances. Substances must be in solution (saliva) so that they can enter the opening in a taste bud, and stimulate the nerve ending. Molecules pass into solution on the surface of the tongue then combine with the surface membranes of the receptor cells. Transmitter substances are released, which evoke action potentials on the sensory nerve fibres. Fibres from the seventh, ninth and tenth cranial nerves carry the taste impulses via the brainstem to an area of the cerebral cortex where the taste is experienced.


The sense of taste is intricately linked with the sense of smell, and the sense of taste depends on stimulation of the olfactory receptors. Both senses have a protective function; for example, in detecting substances that may be harmful. Interruption to the transmission of taste stimuli to the brain may cause taste abnormalities. Taste abnormalities may result from trauma, infection, vitamin or mineral deficiencies, neurological or oral disorders and the effects of drugs. Because tastes are most accurately perceived in a fluid medium, mouth dryness may interfere with taste. Alterations in taste may include ageusia (a complete loss of taste), hypogeusia (a partial loss of taste), dysgeusia (a distorted sense of taste) and cacogeusia (an unpleasant taste).



THE NOSE (OLFACTORY RECEPTORS)


The specific receptors for the sense of smell are the olfactory receptors. They are chemoreceptors that respond to airborne chemicals and generate impulses that are transmitted to the brain for interpretation. The olfactory receptors are situated in the mucous membrane lining the upper part of the nose (Figure 42.2). When these receptors are stimulated by chemicals they transmit impulses along the olfactory nerve to the brain. These receptors adapt quickly when exposed to an unchanging stimulus, which means that the client can become accustomed to an odour when constantly exposed to it.



Permanent alterations in the sense usually result when the olfactory neuroepithelium or part of the olfactory nerve is destroyed. Permanent or temporary loss can occur from inhaling irritants such as acid fumes that paralyse nasal cilia. Conditions such as ageing, Parkinson’s disease and Alzheimer’s disease may alter the sense of smell. Alterations in smell include anosmia (a total loss of sense of smell), hyposmia (an impaired sense of smell) and parosmia (an abnormal sense of smell).


There is a close relationship between the sense of smell and the sense of taste, and therefore they are not always distinguishable. The sensations of smell and taste play an important part in stimulating the secretions of digestive juices.


The nurse plays an important role in promoting the sense of taste and stimulating the sense of smell when caring for a client with altered sensory function. Client education regarding good oral hygiene practices will enhance taste perception, and discussing with the client what foods are the most taste appealing. If taste perception is improved, food intake and appetite will also improve. Taste perception is increased when foods are seasoned. The client’s sense of smell can be stimulated by pleasant aromas such as flowers, perfumes and food.






SENSORY ALTERATIONS


People become accustomed to certain sensory stimuli and, when these change markedly, the individual may experience discomfort. Factors that contribute to alterations in behaviour are as follows.






Factors affecting sensory function


A range of factors affect the amount and quality of sensory stimulation, including a person’s developmental stage, culture, level of stress, medications, illness and lifestyle. These are outlined in Clinical Interest Box 42.3.



CLINICAL INTEREST BOX 42.3 Factors that influence sensory function











ASSESSING SENSORY FUNCTION


Nursing assessment of sensory-perceptual functioning includes:








CLINICAL INTEREST BOX 42.4 Interview to assess a client’s sensory-perceptual functioning









THE EYE



STRUCTURE OF THE EYE


The eye is a spherical structure (Figure 42.3) about 2.5 cm in diameter, consisting of three principal layers.







Extraocular structure — accessory structures


Extraocular, or accessory, structures of the eye are portions of the eye outside the eyeball. These structures consist of the eyebrows, eyelids, eyelashes and lacrimal apparatus. The eyeball is anchored into position by several structures, including the extraocular muscles, the conjunctiva and the eyelids. The eyeball is surrounded and protected by a bony orbit, the eyebrow ridge and some fatty tissue. The eyeball is lubricated by the lacrimal glands.


The extraocular muscles bring about rotational movements of the eyeball. The muscles arise from the orbit and consist of four rectus muscles, which are attached to the sclera, and two oblique muscles. The oblique muscles are arranged so that, for part of their length, they lie around the circumference of the eyeball. The eyebrows and eyelashes are short coarse hairs that shade the eyes and protect the eyes from dust and sweat. The eyelids consist of connective tissue covered by skin, and lined with mucous membrane. The lining is reflected over the eyeballs, and is called the conjunctiva. The eyelids protect the eye from foreign bodies and excessive light as well as distributing tears by blinking.


The lacrimal apparatus (Figure 42.4) consists of the lacrimal gland, which is situated over the eye at the upper outer corner and secretes tears, which constantly wash over the conjunctiva. Tears leave the gland via several small ducts, and pass over the front of the eye eventually draining into the lacrimal sac, which is the expanded end of the nasolacrimal duct. Tears are secreted on to the anterior surface of the eyeball and are spread over it by the blinking movements of the eyelids. An antimicrobial substance in tears protects the eyes against microorganisms. Excess tears drain down the nasal cavity. The lacrimal glands are stimulated in response to chemical and mechanical irritants, thus producing tears to wash away irritants. Tears may also be produced as a result of emotion such as sadness, or happiness.




The refractive media


The refractive media are the transparent parts of the eye, and have the ability to bend light rays at the surfaces of two transparent media. The refractive media are the cornea, the lens, the aqueous humour and the vitreous humour. The cornea functions as a refracting and protective layer through which light rays pass en route to the brain.


The lens is a transparent, biconvex encapsulated structure suspended from the ciliary body posterior to the iris. The lens is an elastic structure and this allows its shape to change when the eye is focused. The function of the lens is to refract (bend) light rays and to focus them on the retina.


Aqueous humour is a clear watery fluid that fills the cavities around the lens. These cavities are called the anterior and posterior chambers. Aqueous humour is derived from the plasma in the capillaries of the ciliary body, and passes into the posterior chamber. It then passes forwards through the pupil to the anterior chamber, where it is absorbed into the ciliary veins. The rate of secretion and reabsorption is balanced so that intraocular pressure is regulated. Aqueous humour serves as a refractory medium and provides nutrients to the lens and cornea.


Vitreous humour is a clear jelly-like substance that fills the intraocular space from the posterior lens to the retina. Because it does not regenerate, any significant loss of vitreous humour; for example, as a result of injury to the eye, may distort other ocular structures. Vitreous humour helps maintain the shape of the eyeball, helps keep the retina in position, and helps with the refraction of light rays.



THE PHYSIOLOGY OF SIGHT


Light travels from its source to various objects, where it undergoes reflection. This reflected light can then travel towards the eye, where it passes through the cornea, aqueous humour, the lens and the vitreous humour, before forming an image on the retina. Nerve endings in the retina transmit electrical impulses along the optic nerve to the brain. A coordinated process of refraction, accommodation, regulation of pupil size and convergence makes normal binocular vision possible.







DISORDERS OF THE EYE



FACTORS AFFECTING SENSORY FUNCTION


Normal function of the eye can be altered by a variety of pathophysiological factors, including those that are congenital, degenerative, infectious, neoplastic or traumatic. The effects may be mild and temporary, severe or permanent. Disorders of vision include alterations in visual acuity, ocular movement, accommodation, refraction and colour vision.








MAJOR MANIFESTATIONS OF EYE DISORDERS


The major manifestations of disorders that affect the eye are changes in vision, pain or discomfort, excessive production of tears or dryness, and discharge.







SPECIFIC DISORDERS OF THE EYE




Feb 12, 2017 | Posted by in NURSING | Comments Off on SENSORY ABILITIES
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