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.

Figure 42.1 The location and structure of the taste buds in the tongue

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.

Figure 42.2 The location and structure of the olfactory receptors in the nose

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.

THE SKIN AS AN ORGAN OF SENSATION

The skin is an important sensory organ that allows us to receive information about our external environment. Distributed widely in the dermis of the skin are sensory receptors called cutaneous receptors, which are stimulated by heat, cold and touch, including pressure. Sensory neurons carry impulses from these receptors to the brain where the sensation is interpreted. The nurse can help stimulate altered sensory function by discussing with the client the introduction of touch therapy. This may include such things as touching arms, back rub and brushing of hair.

THE EYES

Sight, or vision, is a special sense that allows us to interact and experience our environment. The eyes allow us to see by providing a pathway for visual stimuli to reach the brain. Vision is the result of light rays being received by the eyes and transmitted by the optic nerve to the brain, where they are interpreted. The eyes are located within the orbits of the skull, one on either side of the nose. The ciliary artery and cental retinal artery, which are branches of the ophthalmic artery, supply arterial blood to the eye.

THE EARS

The ear is specially adapted as the organ of hearing but it is also concerned with sense of position, balance and equilibrium. Hearing is a complex mechanism in which the ears receive soundwaves and convert them into nerve impulses. The nerve impulses are transmitted by the acoustic (eighth cranial) nerve to the brain, where they are interpreted. The externally visible portion of each ear is located on the lateral surface of the head on each side. The remaining parts of each ear are embedded in the bone of the skull beneath.

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.

Age

• Infants are unable to discriminate sensory stimuli. Nerve pathways are immature

• Visual changes during adulthood include presbyopia (inability to focus on near objects) and the need for glasses for reading (usually occurring from age 40–50)

• Hearing changes, which begin at age 30, include decreased hearing acuity, speech intelligibility, pitch discrimination and hearing threshold. Tinnitus often accompanies a hearing loss as a side effect of drugs. Older adults hear low-pitched sounds the best but have difficulty hearing conversation over background noise

• Older adults have reduced visual fields, increased glare sensitivity, impaired night vision, reduced accommodation and depth perception, and reduced colour discrimination

• Older adults have difficulty discriminating the consonants (f, s, th, ch). Speech sounds are garbled and there is a delayed reception and reaction to speech

• Gustatory and olfactory changes include a decrease in the number of taste buds in later years and reduction of olfactory nerve fibres by age 50. Reduced taste discrimination and reduced sensitivity to odours are common

• Proprioceptive changes after age 60 include increased difficulty with balance, spatial orientation and coordination

• Older adults experience tactile changes, including declining sensitivity to pain, pressure and temperature

Medications

• Some antibiotics (e.g. streptomycin, gentamicin) are ototoxic and can permanently damage the auditory nerve; chloramphenicol can irritate the optic nerve. Narcotic analgesics, sedatives and antidepressant medications can alter the perception of stimuli

Environment

• Excessive environmental stimuli (e.g. equipment noise and staff conversation in an intensive care unit [ICU]) can result in sensory overload, marked by confusion, disorientation, and the inability to make decisions. Restricted environmental stimulation (e.g. with protective isolation) can lead to sensory deprivation. Poor-quality environmental stimuli (e.g. reduced lighting, narrow walkways, background noise) can worsen sensory impairment

Comfort level

• Pain and fatigue alter the way a person perceives and reacts to stimuli

Pre-existing illness

• Peripheral vascular disease can cause reduced sensation in the extremities and impaired cognition. Chronic diabetes mellitus can lead to reduced vision, blindness or peripheral neuropathy. Strokes often produce loss of speech. Some neurological disorders impair motor function and sensory reception

Smoking

• Chronic tobacco use can cause the taste buds to atrophy, lessening the perception of flavours

Noise levels

• Constant exposure to high noise levels (e.g. on a construction job site) can cause hearing loss

Endotracheal intubation

• Temporary loss of speech results from insertion of an endotracheal tube through the mouth or nose into the trachea

(adapted from Potter & Perry 2008)

ASSESSING SENSORY FUNCTION

Nursing assessment of sensory-perceptual functioning includes:

• Nursing history: Clinical Interest Box 42.4 gives some examples of interview questions to elicit data about the client’s sensory-perceptual functioning

• Mental status, including level of consciousness, orientation, memory and attention span

• Physical examination — the client’s specific visual and hearing abilities; perception of heat, cold, light touch and pain in the limbs; and awareness of the position of body parts

• The client’s environment — assess for quantity, quality and type of stimuli

• Social support network — the degree of isolation a person feels is significantly influenced by the quality and quantity of support from family and friends (Kozier et al 2000).

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

Gustatory

• Have you experienced any changes in taste (e.g. difficulty in differentiating sweet, sour, salty and bitter tastes)?

• Do you enjoy the taste of foods as you did previously?

Kinaesthetic

• Have you noticed any difficulty in perceiving the position of parts of your body?

(Kozier et al 2000: 892)

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.

Figure 42.3 The eye

The outer layer

The outer fibrous layer consists of the sclera and the cornea, which is a transparent structure continuous with the sclera. About five-sixths of the outer surface is made up of a tough white opaque fibrous layer (sclera), while the cornea comprises the anterior one-sixth. Both the sclera and the cornea consist of layers of collagen fibres. In the cornea, the fibres are regular in size and arrangement, which accounts for its transparency. The cornea functions as a refracting and protective layer through which light rays pass en route to the brain.

The middle layer

The middle pigmented vascular layer is composed of three structures: the choroid, the iris and the ciliary body. The choroid is a layer of tissue that lies between the sclera and the retina. It is composed largely of blood vessels, and contains highly pigmented cells that absorb light and prevent it from being reflected within the eyeball.

The iris is a pigmented circular membrane between the cornea and the lens, and gives the eye its colour. Sphincter and dilator muscles within the iris regulate the central aperture (the pupil). By either dilation or constriction of the pupil, the amount of light entering the eye is regulated. The pupil appears black because light rays entering the eye are absorbed by the choroid and are not reflected.

The ciliary body is a thickened area of the choroid, lying anterior to the choroid extending to the root of the iris. A circular array of fibres stretches from the ciliary body to the lens, to hold it in place. The ciliary body controls focusing of the lens and contains glands that secrete aqueous humour.

The innermost layer

The innermost layer is a thin structure called the retina. The retina lines the inner wall of the posterior portion of the eyeball and is comprised of a number of layers. The two main layers are the pigmented layer (the outermost layer of the retina, lying next to the choroid, and whose cells contain melanin) and the rod and cone layer, which lies next to the pigmented layer and is highly sensitive to light.

Rods and cones (specialised nerve endings) are distributed as a tightly packed mass throughout the retina, except at a point where the ganglionic fibres converge to form the optic nerve. This area, which is about 1.5 mm in diameter, is termed the optic disc. Since it possesses no photosensitive cells it is also known as the ‘blind spot’. The prime function of rods and cones is to absorb light. Rods can be stimulated by dim light and allow perception of shapes and movement in dim light. Rods also provide far peripheral vision. Cones are specialised for fine visual discrimination and colour perception. There are three types of cones: one type responds mostly to blue light, another to red light, and the third type responds to green light.

In the centre of the retina is an oval yellowish area, the macula lutea. In the centre of the macula lutea is a small depression, the fovea centralis. Because the photosensitive cells are more exposed to light here than over the rest of the retina, visual acuity is at its highest.

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.