Chapter 8: The special sense

Question 1

Hearing and the ear

Answer 1

The ear is the organ of hearing and is also involved in balance. It is supplied by the 8th cranial nerve, i.e., the cochlear part of the vestibulocochlear nerve, which is stimulated by vibrations caused by sound waves.

Question 2

The structure

Answer 2

The ear is divided into three distinct parts: the outer ear, middle ear (tympanic cavity) and inner ear.
The outer ear collects the sound waves and directs them to the middle ear, which in turn transfers them to the inner ear, where they are converted into nerve impulses and transmitted to the hearing area in the cerebral cortex.

Question 3

Outer ear

Answer 3

The outer ear consists of the auricle (pinna) and the external acoustic meatus (auditory canal).

Question 4

The auricle (pinna)

Answer 4

The medical term for the outer ear is the auricle or pinna. The outer ear is made up of cartilage and skin. There are three different parts to the outer ear; the tragus, helix, and lobule. EAR CANAL. The ear canal starts at the outer ear and ends at the eardrum.
The auricle, because of its shape, collects and concentrates the waves and directs them along the auditory canal causing the tympanic membrane to vibrate.

Question 5

External acoustic meatus (auditory canal)

Answer 5

The external acoustic meatus is a bony canal for the ear canal, a tube running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about 2.5 centimeters in length and 0.7 centimeters in diameter.

Question 6

Middle ear (tympanic cavity)

Answer 6

The tympanic cavity is a small cavity surrounding the bones of the middle ear. Within it sit the ossicles, three small bones that transmit vibrations used in the detection of sound.

Question 7

Auditory ossicles

Answer 7

These are three very small bones only a few millimeters in size that extends across the middle ear from the tympanic membrane to the oval window. They form a series of movable joints with each other and with the medial wall of the cavity at the oval window. The ossicles are held in place by fine ligaments and are named according to their shapes.

Question 8

The malleus

Answer 8

The is the lateral hammer-shaped bone. The handle is in contact with the tympanic membrane and the head forms a movable joint with the incus.

Question 9

The incus

Answer 9

This is the middle anvil-shaped bone. Its body articulates with the malleus, the long process with the stapes, and it is stabilized by the short process, fixed by fibrous tissue to the posterior wall of the tympanic cavity.

Question 10

The stapes

Answer 10

This is the medial stirrup-shaped bone. Its head articulates with the incus and its footplate fits into the oval window.

Question 11

Inner ear

Answer 11

The inner ear or labyrinth (meaning ‘maze’) contains the organs of hearing and balance. It is described in two parts, the bony labyrinth, and the membranous labyrinth, and is divided into three main regions:
-the vestibule, containing the utricle and saccule
-three semicircular canals
-the cochlea.
The inner ear is formed from a network of channels and cavities in the temporal bone (the bony labyrinth). Within the bony labyrinth, like a tube within a tube, is the membranous labyrinth, a network of fluid-filled membranes that lines and fills the bony labyrinth.

Question 12

The bony labyrinth

Answer 12

This is lined with periosteum. Within the bony labyrinth, the membranous labyrinth is suspended in a watery fluid called perilymph.

Question 13

The vestibule

Answer 13

This is the expanded part nearest the middle ear. The oval and round windows are in their lateral wall. It contains two membranous sacs, the utricle, and the saccule, which are important in balance.

Question 14

The semicircular canals

Answer 14

These are three tubes arranged so that one is situated in each of the three planes of space. They are continuous with the vestibule and are also important in balance.

Question 15

The cochlea

Answer 15

• The cochlea is a hollow, spiral-shaped bone found in the inner ear that plays a key role in the sense of hearing and participates in the process of auditory transduction. Sound waves are transduced into electrical impulses that can be interpreted by the brain as individual frequencies of sound

Question 16

Physiology of hearing

Answer 16

Sound waves have the properties of pitch and volume or intensity. Pitch is determined by the frequency of the sound waves and is measured in Hertz (Hz). Sounds of different frequencies stimulate the basilar membrane at different places along its length, allowing discrimination of pitch.
The volume depends on the magnitude of the sound waves and is measured in decibels (dB). The greater the amplitude of the wave created in the endolymph, the greater is the stimulation of the auditory receptors in the hair cells in the spiral organ, enabling perception of volume. Long-term exposure to excessive noise causes hearing loss because it damages the sensitive hair cells of the spiral organ.

Question 17

The semicircular canals and vestibule

Answer 17

Semicircular canals are three tiny, fluid-filled tubes in your inner ear that help you keep your balance. When your head moves around, the liquid inside the semicircular canals sloshes around and moves the tiny hairs that line each canal.
The vestibule is a small space or cavity at the beginning of a canal.

Question 18

Physiology of balance

Answer 18

The semicircular canals and the vestibule (utricle and saccule) are concerned with balance or equilibrium.
The arrangement of the three semicircular canals, one in each plane, not only allows perception of the position of the head in space but also the direction and rate of any movement. Any change of position of the head causes movement in the perilymph and endolymph, which bends the hair cells and stimulates the sensory receptors in the utricle, saccule, and ampullae.

Question 19

Sight and the eye

Answer 19

The eye is the organ of sight. It is situated in the orbital cavity and supplied by the optic nerve (2nd cranial nerve).
The space between the eye and the orbital cavity is occupied by adipose tissue. The bony walls of the orbit and the fat protect the eye from injury.
Structurally the two eyes are separate but, unlike the ears, some of their activities are coordinated so that they normally function as a pair. It is possible to see with only one eye (monocular vision), but three-dimensional vision is impaired when only one eye is used, especially in relation to the judgement of speed and distance.

Question 20

The structure of the eye

Answer 20

There are three layers of tissue in the walls of the eye:
-the outer fibrous layer: sclera and cornea
-the middle vascular layer or uveal tract: consisting of the choroid, ciliary body and iris
-the inner nervous tissue layer: the retina.
Structures inside the eyeball include the lens, aqueous fluid and vitreous body

Question 21

Sclera and cornea

Answer 21

The sclera, or white of the eye, forms the outermost layer of the posterior and lateral aspects of the eyeball and is continuous anteriorly with the cornea. It consists of a firm fibrous membrane that maintains the shape of the eye and gives attachment to the extrinsic muscles of the eye
Anteriorly the sclera continues as a clear transparent epithelial membrane, the cornea. Light rays pass through the cornea to reach the retina. The cornea is convex anteriorly and is involved in refracting (bending) light rays to focus them on the retina.

Question 22

Choroid

Answer 22

The choroid lines the posterior five-sixths of the inner surface of the sclera. It is very rich in blood vessels and is deep chocolate brown in color. Light enters the eye through the pupil, stimulates the sensory receptors in the retina and is then absorbed by the choroid.

Question 23

Ciliary body

Answer 23

The part of the eye that connects the iris to the choroid. It consists of the ciliary muscle (which alters the curvature of the lens), a series of radial ciliary processes (from which the lens is suspended by ligaments), and the ciliary ring (which adjoins the choroid).

Question 24

Iris

Answer 24

The iris is the colored part of the eye that controls the amount of light that enters the eye. It is the most visible part of the eye.

Question 25

Lens

Answer 25

The lens is a nearly transparent biconvex structure suspended behind the iris of the eye, the sole function of which is to focus light rays onto the retina.

Question 26

Retina

Answer 26

A layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses that pass via the optic nerve to the brain, where a visual image is formed.

Question 27

Blood supply to the eye

Answer 27

The eye is supplied with arterial blood by the ciliary arteries and the central retinal artery. These are branches of the ophthalmic artery, a branch of the internal carotid artery.
Venous drainage is done by several veins, including the central retinal vein, which eventually empties into a deep venous sinus.
The central retinal artery and vein are encased in the optic nerve, which enters the eye at the optic disc.

Question 28

Interior of the eye

Answer 28

The interior chamber of your eyeball is filled with a jelly-like tissue called the vitreous humor. After passing through your lens, light must travel through this humor before striking the sensitive layer of cells called the retina.

Question 29

Optic nerves (second cranial nerves)

Answer 29

The fibers of the optic nerve originate in the retina, and they converge to form the optic nerve about 0.5 cm to the nasal side of the macula lutea at the optic disc. The nerve pierces the choroid and sclera to pass backwards and medially through the orbital cavity. It then passes through the optic foramen of the sphenoid bone, backwards and medially to meet the nerve from the other eye at the optic chiasma.

Question 30

Optic chiasma

Answer 30

This is situated immediately in front of and above the pituitary gland, which is in the hypophyseal fossa of the sphenoid bone. In the optic chiasma the nerve fibers of the optic nerve from the nasal side of each retina cross over to the opposite side. The fibers from the temporal side do not cross but continue backward on the same side. This crossing-over provides both cerebral hemispheres with sensory input from each eye.

Question 31

Optic tracts

Answer 31

The pathway between the optic chiasma and the brain.

Question 32

Physiology of sight

Answer 32

Light is reflected into the eyes by objects within the field of vision. White light is a combination of all the colors of the visual spectrum (rainbow), i.e. red, orange, yellow, green, blue, indigo and violet. This is demonstrated by passing white light through a glass prism that bends the rays of the different colors to a greater or lesser extent, depending on their wavelengths. Red light has the longest wavelength and violet the shortest.
This range of color in the spectrum of visible light. In a rainbow, white light from the sun is broken up by raindrops, which act as prisms and reflectors.

Question 33

The electromagnetic spectrum

Answer 33

The range of wavelengths or frequencies over which electromagnetic radiation extends

Question 34

Refraction of the lights rays

Answer 34

Refraction is the bending of light (it also happens with sound, water and other waves) as it passes from one transparent substance into another. This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms and rainbows

Question 35

Focusing on an image of the retina

Answer 35

Light rays reflected from an object are bent (refracted) by the lens when they enter the eye in the same way as shown in, although the image on the retina is upside down. The brain adapts to this early in life so that objects are perceived ‘the right way up.
Abnormal refraction within the eye is corrected using biconvex or biconcave lenses

Question 36

Size of pupils

Answer 36

Pupil size contributes to clear vision by controlling the amount of light entering the eye. In bright light, the pupils are constricted. In the dim light, they are dilated.
The iris consists of one layer of circular and one of radiating smooth muscle fibers. Contraction of the circular fibers constricts the pupil, and contraction of the radiating fibers dilates it. The size of the pupil is controlled by the autonomic nervous system; sympathetic stimulation dilates the pupils and parasympathetic stimulation constricts them.

Question 37

Near vision

Answer 37

In order to focus on near objects, i.e., within about 6 meters, accommodation is required, and the eye must make the following adjustments:

• constriction of the pupils
• convergence
• changing the refractory power of the lens.

Question 38

Constriction of the pupils

Answer 38

This assists accommodation by reducing the width of the beam of light entering the eye so that it passes through the central curved part of the lens.

Question 39

Convergence (Movement of the eyeballs)

Answer 39

Light rays from nearby objects enter the two eyes at different angles and for clear vision they must stimulate corresponding areas of the two retinae. Extrinsic muscles move the eyes and to obtain a clear image they rotate the eyes so that they converge on the object viewed.
This coordinated muscle activity is under autonomic control. When there is voluntary movement of the eyes, both eyes move, and convergence is maintained. The nearer an object is to the eyes the greater the eye rotation needed to achieve convergence.

Question 40

Changing the refractory power of the lens

Answer 40

Changes in the thickness of the lens are made to focus light on the retina. The amount of adjustment depends on the distance of the object from the eyes, i.e. the lens is thicker for near vision and at its thinnest when focusing on objects more than 6 meters away. Looking at near objects ‘tires’ the eyes more quickly, owing to the continuous use of the ciliary muscle. The lens loses its elasticity and stiffens with age, a condition known as presbyopia.

Question 41

Distant vision

Answer 41

Objects more than 6 meters away from the eyes are focused on the retina without adjustment of the lens or convergence of the eyes.

Question 42

Functions of the retina

Answer 42

The retina is the light-sensitive (photosensitive) part of the eye. The light-sensitive nerve cells are the rods and cones and their distribution in the retina is shown in. Light rays cause chemical changes in light-sensitive pigments in these cells, and they generate nerve impulses that are conducted to the occipital lobes of the cerebrum via the optic nerves
The rods are much lighter sensitive than the cones, so they are used when light levels are low. Stimulation of rods leads to monochromic (black and white) vision.
The cones are sensitive to light and color; bright light is required to activate them and give sharp, clear color vision. The different wavelengths of visible light light-sensitive pigments in the cones, resulting in the perception of different colors.

Question 43

Color blindness

Answer 43

This is a common condition that affects more men than women. Although affected individuals see colors, they cannot always differentiate between them as the light-sensitive pigments (to red, green or blue) in cones are abnormal. There are different forms but the most common is red, green color blindness which is transmitted a by sex-linked recessive gene where greens, oranges, pale reds and browns all appear to be the same color and can only be distinguished by their intensity.

Question 44

Dark adaptation

Answer 44

When exposed to bright light, the rhodopsin within the sensitive rods is completely degraded. This does not affect vision in good light when there is enough light to activate the cones. However, moving into a darkened area where the light intensity is insufficient to stimulate the cones causes temporary visual impairment whilst the rhodopsin is being regenerated within the rods, ‘dark adaptation. When regeneration of rhodopsin has occurred, normal sight returns.

Question 45

Binocular vision

Answer 45

Binocular or stereoscopic vision enables three-dimensional views although each eye ‘sees’ a scene from a slightly different angle.

Question 46

Extraocular muscles of the eye

Answer 46

These include the muscles of the eyelids and those that move the eyeballs. The eyeball is moved by six extrinsic muscles, attached at one end to the eyeball and at the other to the walls of the orbital cavity. There are four straight (rectus) muscles and two oblique muscles
Moving the eyes to look in a particular direction is under voluntary control, but coordination of movement, needed for convergence and accommodation to near or distant vision, is under autonomic (involuntary) control.

Question 47

Nerve supply to the muscles of the eye

Answer 47

The oculomotor nerves supply the intrinsic eye muscles of the iris and ciliary body.
Medial rectus- rotates eyeball inwards
Lateral rectus- rotates eyeball outwards
Superior rectus- rotates eyeball upwards
Inferior rectus-rotates eyeball downwards
Superior oblique- rotates eyeball downwards and outwards
Inferior oblique- rotates eyeball upwards and outwards

Question 48

Accessory organs of the eye

Answer 48

The eye is a delicate organ which is protected by several structures:

• eyebrows
• eyelids and eyelashes
• lacrimal apparatus.

Question 49

Eyebrows

Answer 49

These are two arched ridges of the supraorbital margins of the frontal bone. Numerous hairs (eyebrows) project obliquely from the surface of the skin. They protect the eyeball from sweat, dust and other foreign bodies.

Question 50

Eyelids (palpebrae)

Answer 50

The eyelids are two movable folds of tissue situated above and below the front of each eye. On their free edges are short, curved hairs, the eyelashes. The layers of tissue forming the eyelids are:

• a thin covering of skin
• a thin sheet of subcutaneous connective (loose areolar) tissue
• two muscles – the orbicularis oculi and levator palpebrae superioris
• a thin sheet of dense connective tissue, the tarsal plate, larger in the upper than the lower eyelid, which supports the other structures
• a membranous lining, the conjunctiva.

Question 51

Conjunctiva

Answer 51

The mucous membrane that covers the front of the eye and lines the inside of the eyelids.

Question 52

Eyelids margins

Answer 52

Along the edges of the lids are numerous sebaceous glands, some with ducts opening into the hair follicles of the eyelashes and some onto the eyelid margins between the hairs. Tarsal glands are modified sebaceous glands embedded in the tarsal plates with ducts that open onto the inside of the free margins of the eyelids. They secrete an oily material, spread over the conjunctiva by blinking, which delays the evaporation of tears.

Question 53

Functions of the eyelid and eyelashes

Answer 53

The eyelids and eyelashes protect the eye from injury:
-reflex closure of the lids occurs when the conjunctiva or eyelashes are touched when an object comes close to the eye, or when a bright light shines into the eye this is called the corneal reflex
-blinking at about 3- to 7-second intervals spreads tears and oily secretions over the cornea, preventing drying.
When the orbicularis oculi contract, the eyes close. When the levator palpebrae contract, the eyelids open.

Question 54

Lacrimal apparatus

Answer 54

For each eye this consists of the structures that secrete tears and drain them from the front of the eyeball:
-1 lacrimal gland and its ducts
-2 lacrimal canaliculi
-1 lacrimal sac
-1 nasolacrimal duct
The lacrimal glands are exocrine glands situated in recesses in the frontal bones on the lateral aspect of each eye just behind the supraorbital margin. Each gland is approximately the size and shape of an almond and is composed of secretory epithelial cells. The glands secrete tears composed of water, mineral salts, antibodies and lysozyme, a bactericidal enzyme.

Question 55

Functions

Answer 55

The fluid that fills the conjunctival sac is a mixture of tears and the oily secretion of tarsal glands, which is spread over the cornea by blinking. The functions of this fluid include:

• provision of oxygen and nutrients to the avascular corneal conjunctiva and drainage of wastes
• washing away irritating materials, e.g., dust, grit
• the bactericidal enzyme lysozyme prevents microbial infection
• its oiliness delays evaporation and prevents friction or drying of the conjunctiva.

Question 56

Sense of smell

Answer 56

The sense of smell, or olfaction, originates in the nasal cavity, which also acts as a passageway for respiration.

Question 57

Olfactory nerves (first cranial nerves)

Answer 57

These are the sensory nerves of smell. They originate as chemoreceptors (specialized olfactory nerve endings) in the mucous membrane of the roof of the nasal cavity above the superior nasal conchae. On each side of the nasal septum, nerve fibers pass through the cribriform plate of the ethmoid bone to the olfactory bulb where interconnections and synapses occur. From the bulb, bundles of nerve fibers from the olfactory tract, which passes back to the olfactory area in the temporal lobe of the cerebral cortex in each hemisphere where the impulses are interpreted, and odor perceived.

Question 58

Physiology of smell

Answer 58

The human sense of smell is less acute than in other animals. Many animals secrete odorous chemicals called pheromones, which play an important part in chemical communication in, for example, territorial behavior, mating and the bonding of mothers and their newborn. The role of pheromones in human communication is unknown.
All odorous materials give off volatile molecules, which are carried into the nose with inhaled air and even very low concentrations, when dissolved in mucus, stimulate the olfactory chemoreceptors.

Question 59

Adaptation

Answer 59

When an individual is continuously exposed to an odor, perception of the odor decreases and ceases within a few minutes. This loss of perception affects only that specific odor.

Question 60

Sense of taste

Answer 60

The sense of taste, or gustation, is closely linked to the sense of smell and, like the smell, also involves the stimulation of chemoreceptors by dissolved chemicals.
Taste buds contain chemoreceptors (sensory receptors) that are found in the papillae of the tongue and are widely distributed in the epithelia of the tongue. They consist of small sensory nerve endings of the glossopharyngeal, facial, and vagus nerves (cranial nerves VII, IX, and X).

Question 61

physiology of taste

Answer 61

It is thought that all taste buds are stimulated by all ‘tastes. Taste is impaired when the mouth is dry because substances can only be ‘tasted’ when in solution.
The sense of taste is closely linked to the sense of smell. For example, when one has a cold, it is common for food to taste bland and unappealing. In addition, taste triggers salivation and the secretion of gastric juice. The sense of taste also has a protective function, e.g., when foul-tasting food is eaten, reflex gagging or vomiting may be induced.

Question 62

Presbycusis

Answer 62

This form of hearing impairment accompanies the ageing process and is therefore common in older adults. Degenerative changes in the sensory cells of the spiral organ result in sensorineural hearing loss. Perception of high-frequency sound is impaired first and later low-frequency sound may also be affected.

Question 63

Presbyopia

Answer 63

Age-related changes in the lens lead to loss of accommodation as the lens loses its elasticity and becomes firmer. This prevents focusing of light on the retina, giving blurred vision. Correction is achieved using glasses with convex lenses for near vision, e.g., reading

Question 64

Cataracts

Answer 64

Cataracts arise when there is an opacity of the lens. Weak light rays cannot easily pass through a less transparent or cloudy lens and are the reason why many older adults use brighter light for reading and may also have trouble with night vision. It is most commonly age-related occurring as a result of exposure to predisposing factors which include UV light, X-rays, and cigarette smoke.

Question 65

Disorders of the ear (hearing loss)

Answer 65

Hearing impairment can be classified into two main categories: conductive and sensorineural. Hearing impairment can also be mixed when there is a combination of conductive and sensorineural hearing loss in one ear.

Question 66

Ear infections (external otitis)

Answer 66

Infection by Staphylococcus aureus is the usual cause of localized inflammation (boils) in the auditory canal. More generalized inflammation may be caused by prolonged exposure to bacteria or fungi or by an allergic reaction to, e.g., dandruff, soaps, hair sprays, hair dyes.

Question 67

Stye

Answer 67

Also known as a hordeolum, this is an acute and painful bacterial infection of sebaceous or tarsal glands of the eyelid margin. The most common cause is Staphylococcus aureus. A ‘crop’ of styes may occur due to localized spread to adjacent glands. Infection of tarsal glands may block their ducts, leading to cyst formation (chalazion), which may damage the cornea.

Question 68

Blepharitis

Answer 68

This is a chronic inflammation of the eyelid margins, usually caused by bacterial infection or allergy, e.g., staphylococcal infection or seborrhea (excessive sebaceous gland secretion). If ulceration occurs, healing by fibrosis may distort the eyelid margins, preventing complete closure of the eye. This may lead to drying of the eye, conjunctivitis, and possibly corneal ulceration.

Question 69

Conjunctivitis

Answer 69

Inflammation of the conjunctiva may be caused by irritants, such as smoke, dust, wind, cold or dry air, microbes, or antigens and may be acute or chronic. Corneal ulceration is a rare complication.

Question 70

Glaucoma

Answer 70

A glaucoma is a group of eye conditions that damage the optic nerve, the health of which is vital for good vision. This damage is often caused by abnormally high pressure in your eye. Glaucoma is one of the leading causes of blindness for people over the age of 60.

Question 71

Cataract

Answer 71

A cataract is a cloudy area in the lens of the eye that leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry or double vision, halos around light, trouble with bright lights, and trouble seeing at night.