Transcript Figure 8.12

Special Senses
The Senses
 General senses:
-Cutaneous sensory organs
 Temperature (cold &heat)
 Pressure
 Fine touch
 Pain
-Proprioceptors of muscles and joints
 Special senses
 Sight
- Hearing
 Equilibrium
-Smell
-
Taste
The Eye and Vision
 The adult eye is a sphere with a diameter of
about 2.5 cm (1 inch).
 70 percent of all sensory receptors of the
body are in the eyes.
 Each eye has over a million nerve fibers
 Protection for the eye
 Most of the eye is enclosed in a bony
orbit(only 1/6 of the eye surface is
seen).
 A cushion of fat surrounds most of the
eye
Accessory structures of the eye
The accessory structures of the eye include:
 eyelids,
 conjunctiva,
 lacrimal apparatus,
 and extrinsic eye muscles.
I-Eyelids
Anteriorly, the eyes are protected by the mobile
eyelids or palpebrae (pal′pĕ-bre). The eyelids are
separated by the palpebral fissure (“eyelid slit”) and
meet at the medial and lateral angles of the eye—the
medial and lateral commissures (canthi),respectively.
 Projecting from the free margin of each eyelid are
the eyelashes. The follicles of the eyelash hairs are
richly innervated by nerve endings (hair follicle
receptors), and anything that touches the eyelashes
(even a puff of air) triggers reflex blinking.
Several types of glands are associated with the
eyelids:
 The tarsal glands (Meibomian glands; mi-bo′me-an) ,their
ducts open at the eyelid edge just posterior to the eyelashes.
These modified sebaceous glands produce an oily secretion
that lubricates the eyelid and the eye and prevents the
eyelids from sticking together.
 Associated with the eyelash follicles are a number of
smaller, more typical sebaceous glands, and modified sweat
glands called ciliary glands lie between the hair follicles
(cilium = eyelash).
 Infection of a tarsal gland results in an unsightly cyst called
a chalazion (kah-la′ze-on; “swelling”). Inflammation of any
of the smaller glands is called a sty.
Figure 8.1b
II-Conjunctiva
 Is a transparent mucous membrane. It lines the eyelids and
reflects (folds back) over the anterior surface of the eyeball.
The latter covers only the white of the eye, not the cornea.
The conjunctiva is very thin, and blood vessels are clearly
visible beneath it.
 The major function of the conjunctiva is to produce a
lubricating mucus that prevents the eyes from drying out.
 HOMEOSTATIC IMBALANCE Inflammation of the
conjunctiva, called conjunctivitis, results in reddened,
irritated eyes. Pinkeye, a conjunctival infection caused by
bacteria or viruses, is highly contagious.
III_Lacrimal apparatus Consists of :
1- the lacrimal gland :
It lies in the orbit above the lateral end of the eye
and is visible through the conjunctiva when the
lid is everted. It continually releases a dilute
saline solution called lacrimal secretion—or,
more commonly, tears—into the superior part
of the conjunctival sac through several small
excretory ducts. Blinking spreads the tears
downward and across the eyeball to the medial
commissure, where they enter:
Figure 8.1a
2-the paired lacrimal canaliculi via two tiny
openings called lacrimal puncta, visible as tiny red
dots on the medial margin of each eyelid. From the
lacrimal canaliculi, the tears drain into the:
3- lacrimal sac and then into:
4- the nasolacrimal duct, which empties into the
nasal cavity at the inferior nasal meatus.
Figure 8.1a
 Lacrimal fluid contains mucus, antibodies, and
lysozyme, an enzyme that destroys bacteria.
 it cleanses and protects the eye surface as it moistens
and lubricates it. When lacrimal secretion increases,
tears spill over the eyelids and fill the nasal cavities.”
This happens when the eyes are irritated and when
we are emotionally upset.
 HOMEOSTATIC IMBALANCE :Because
the nasal cavity
mucosa is continuous with that of the lacrimal duct
system, a cold or nasal inflammation often causes
the lacrimal mucosa to become inflamed and swell.
This constricts the ducts and prevents tears from
draining from the eye surface, causing “watery”eyes.
IV-Extrinsic Eye Muscles
The movement of each eyeball is controlled
by six straplike extrinsic eye muscles, which
originate from the bony orbit and insert into
the outer surface of the eyeball .
 These muscles allow the eyes to follow a
moving object, and help to maintain the shape
of the eyeball and hold it in the orbit.
Extrinsic Eye Muscles
 Muscles attach to the outer surface
of the eye
 Produce eye movements
Figure 8.2
HOMEOSTATIC IMBALANCE
Congenital weakness of the external eye muscles may cause
strabismus (strah-biz′mus; “cross-eyed”, squint), a condition in
which the affected eye rotates medially or laterally.
Internal Structure of the Eyeball
The eye itself, commonly called the eyeball, is a
slightly irregular hollow sphere .
-Its wall is composed of three layers (formerly called
tunics): the fibrous, vascular, and sensory layers.
-Its internal cavity is filled with fluids called
humors that help to maintain its shape.
-The lens, the adjustable focusing apparatus of the
eye, is supported vertically within the internal cavity,
dividing it into anterior and posterior segments, or
cavities.
 The wall is composed of three tunics:
 Fibrous layer – the outermost layer
(Sclera &Cornea)
 Vascular layer(Choroid)the
middle layer
 Sensory layer –
(Retina)the
 innermost layer
Figure 8.3a
 The Fibrous Layer The outermost coat of the eye,
and is composed of dense avascular connective
tissue. It has two obviously different regions: the
sclera and the cornea.
a-The sclera (skle′rah), forming the posterior portion
and the bulk of the fibrous layer, is glistening white
and opaque. Seen anteriorly as the “white of the
eye,” it protects and shapes the eyeball and provides
attachment for the extrinsic eye muscles. Posteriorly,
where the sclera is pierced by the optic nerve, it is
continuous with the dura mater of the brain.
b- the cornea The anterior one-sixth of the fibrous layer is
modified to form the transparent ,crystal-clear cornea . It
forms a window that lets light enter the eye, and is a major
part of the light-bending apparatus of the eye.
The cornea is well supplied with nerve endings, most of
which are pain receptors. When the cornea is touched,
blinking and increased tearing occur reflexively. Luckily,
its capacity for regeneration and repair is extraordinary.
Because it has no blood vessels, it is beyond the reach of the
immune system, so the cornea is the only tissue in the body
that can be transplanted from one person to another with
little or no possibility of rejection..
The Vascular Layer (Uvea),Choroid:
-Blood-rich nutritive layer
-Dark pigment prevents light from scattering
-Modified interiorly into two structures
 Cilliary body ,a thickened ring of tissue that encircles the
lens. The ciliary body consists chiefly of interlacing smooth
muscle bundles called ciliary muscles, which are important in
controlling lens shape.
 Iris ,the visible colored part of the eye, it lies between the
cornea and the lens and is continuous with the ciliary body
posteriorly. Its round central opening, the pupil, allows light to
enter the eye.
 The Inner Layer (Retina) : it is formed of twolayeres.
a- Its outer pigmented layer, a single-cell-thick
lining the choroid .
-These pigmented epithelial cells, like those of
the choroid, absorb light and prevent it from
scattering in the eye.
-They also act as phagocytes to remove dead or
damaged photoreceptor cells, and store vitamin
A needed by the photoreceptor cells.
b-The transparent inner neural layer extends
anteriorly to the posterior margin of the
ciliary body. Originating as an outpocketing
of the brain, the retina contains millions of
photoreceptors that transduce light energy.
-Although the pigmented and neural layers are
very close together, they are not fused.
- Only the neural layer of the retina plays a
direct role in vision.
 From posterior to anterior, the neural layer is
composed of three main types of neurons:
photoreceptors, bipolar cells, and ganglion cells.
Signals are produced in response to light and spread
from the photoreceptors to the bipolar neurons
and then to the innermost ganglion cells, where
action potentials are generated. The ganglion cell
axons leave the posterior aspect of the eye as the
thick optic nerve. The optic disc, is a weak spot in
the fundus (posterior wall) of the eye because it is
not reinforced by the sclera. The optic disc is also
called the blind spot because it lacks photoreceptors,
so light focused on it cannot be seen.
Neurons of the Retina
Figure 8.4
 The photoreceptors found in the neural retinas are of
two types: rods and cones.
-The more numerous rods are our dim-light and
peripheral vision receptors. They are far more
sensitive to light than cones are, but they do not
provide either sharp images or color vision. Most
are found towards the edges of the retina
-Cones, by contrast, operate in bright light and
provide high-acuity color vision. Densest in the
center of the retina. Lateral to the blind spot of
each eye, a minute (0.4 mm) pit called the fovea
centralis .It contains only cones; so it is the area of
greatest visual acuity.
Cone Sensitivity
 There are three
types of cones
blue,green, and
red.
 Different cones
are sensitive to
different
wavelengths.
 Color blindness is
the result of lack
of one cone type.
Figure 8.6
 HOMEOSTATIC IMBALANCE
Color blindness is due to a congenital lack of one or
more of the cone types.
-Inherited as an X-linked condition, it is far more
common in males than in females. As many as 8–
10% of males have some form of color blindness.
-The most common type is red-green color
blindness, resulting from a deficit or absolute absence
of either red or green cones. Red and green are seen
as the same color—either red or green, depending on
the cone type present.
-Many color-blind people are unaware of that.
Lens
 Lens is a biconvex, transparent, flexible structure
that can change shape to allow precise focusing of
light on the retina. Like the cornea, the lens is
avascular; blood vessels interfere with transparency.
 It is held in place
by suspensory
ligament.
Figure 8.3a
 HOMEOSTATIC IMBALANCE: A
cataract is a clouding
of the lens as if seen through frosted glass .Some
cataracts are congenital, but most result from agerelated hardening and thickening of the lens or are a
secondary consequence of diabetes mellitus.
- Heavy smoking and frequent exposure to intense
sunlight increase the risk for cataracts.
-whereas long-term dietary supplementation with
vitamin C may decrease the risk.
-Fortunately, the lens can be surgically removed and
an artificial lens implanted to save the patient’s sight
Internal Eye Fluids
 Aqueous humor
 Watery fluid found in the anterior chamber
(between the lens and cornea)
 Similar to blood plasma
 Helps maintain intraocular pressure
 Provides nutrients for the lens and cornea
 Reabsorbed into venous blood through the
canal of Schlemm which is located at the
junction of the cornea and sclera.
 Vitreous humor(vitreous body)
 Gel-like substance in the posterior
chamber (behind the lens).
 Keeps the eye from collapsing by
maintaining intraocular pressure.
 Lasts a lifetime and is not replaced.
 HOMEOSTATIC IMBALANCE
-If the drainage of aqueous humor is blocked,
pressure within the eye may increase to dangerous
levels and compress the retina and optic nerve—a
condition called glaucoma (glaw-ko′mah). The
eventual result is blindness.
-Unfortunately, many forms of glaucoma can not be
realized until the damage is done. Late signs include
seeing halos around lights and blurred vision. The
intraocular pressure is determined by Tonometer.
This exam should be done yearly after the age of 40.
Lens Accommodation
 Light must be
focused to a point
on the retina for
optimal vision
 The eye is set for
distance vision
(over 20 ft away)
 The lens must
change shape to
focus for closer
objects
Figure 8.9
Images Formed on the Retina
Figure 8.10
Visual Pathway
 Photoreceptors
of the retina
 Optic nerve
 Optic nerve
crosses at the
optic chiasma
Figure 8.11
Visual Pathway
 Optic tracts
 Thalamus (axons
form optic radiation)
 Visual cortex of the
occipital lobe
Figure 8.11
Eye Reflexes
 Internal muscles are controlled by the
autonomic nervous system.
 Bright light causes pupils to constrict
(photopupillary reflex) through action of
radial and circular muscles of iris.
 Viewing close objects causes pupils to
constrict ( accommodation pupillary
reflex)
 External muscles control eye movement to
follow objects.
 Viewing close objects causes the eyes to move
medially( convergence reflex).
 HOMEOSTATIC IMBALANCES
 Myopia (mi-o′pe-ah; “short vision”) occurs when
distant objects are focused not on, but in front of, the
retina .
 Hyperopia (hy′per-o″pe-ah; “far vision”), or
farsightedness, occurs when the parallel light rays
from distant objects are focused behind the retina.
 Astigmatism Unequal curvatures in different parts
of the cornea or lens lead to blurry images.
The Ear
 Houses two senses
 Hearing
 Equilibrium (balance)
 Receptors of the ear are called
mechanoreceptors, as they respond to
sound vibration(hearing receptors) and gross
movement of the head(balance organs)
 Different organs house receptors for each
sense.
Anatomy of the Ear
Figure 8.12
Anatomy of the Ear
External ear
 Involved in hearing only
 Structures of the external ear
-Ear Pinna (auricle)
- External auditory canal :Narrow
chamber in the temporal bone
-Lined with skin
-has Ceruminous (wax) glands .
-ends at the tympanic membrane
Figure 8.12
The Middle Ear or Tympanic Cavity
 Air-filled cavity within the temporal bone
 Only involved in the sense of hearing
 Two tubes are associated with the inner
ear
 The auditory canal which is closed by
the tympanic membrane
 The auditory tube connecting the
middle ear with the throat
 Allows for equalizing pressure (by yawning
or swallowing) on both sides of eardrum.
Figure 8.12
Bones of the Tympanic Cavity
 Three bones span the cavity
 Malleus (hammer)
 Incus (anvil)
 Stapes (stirrip)
 Vibrations from eardrum move the
malleus
 These bones transfer sound to the
inner ear
Figure 8.12
 HOMEOSTATIC IMBALANCE
Otitis media (me′de-ah), or middle ear inflammation, is a
fairly common result of a sore throat, especially in children,
whose auditory tubes are shorter and run more
horizontally.
Otitis media is the most frequent cause of hearing loss in
children.
In acute forms, the eardrum bulges and becomes inflamed
and red. Most cases of otitis media are treated with
antibiotics. When large amounts of fluid or pus accumulate
in the cavity, an emergency myringotomy (lancing of the
eardrum) may be required to relieve the pressure, and a tiny
tube implanted in the eardrum permits pus to drain into the
external ear. The tube falls out by itself within the year.
Inner (Inner )Ear
 Includes sense organs for hearing and
balance and Filled with perilymph
Figure 8.12
Inner Ear or Bony Labyrinth
 A maze of bony chambers within the
temporal bone called bony or osseous
labyrinth.
 It is divided to
 Cochlea
 Vestibule
 Semicircular
canals
Figure 8.12
Organs of Hearing
 Located within the cochlea is the
Organ of Corti which contains the
hearing Receptors or hair cells .
 Once stimulated, the hair cells
transmit impulses along the
Cochlear nerve which transmits
impulses to auditory cortex in the
temporal lobe.
Organs of Hearing
Figure 8.15
Mechanisms of Hearing
 Sound waves that reach the cochlea
through vibration of the ear drum,
ossicles , and oval window set the
cochlear fluid into motion that affects
the receptor cells in the organ of
Corti.
 An action potential starts in the
cochlear nerve.
N,B.,Continued stimulation can lead to
adaptation.
Mechanisms of Hearing
Figure 8.16a–b
Organs of Equilibrium
 Receptor cells are in two structures
-Vestibule(Static equilibrium)
-Semicircular canals(Dynamic equilibrium)
Figure 8.14a–b
Static Equilibrium
 Maculae – receptors in the vestibule
 Report on changes in the position of the
head in space with respect to the pull of
gravity when the body is not moving( static
equilibrium).They help us keep our head
erect and extremely important to divers
swimming in the dark depths enabling them
to tell which way is up.
 Send information via the vestibular nerve.
Function of Maculae
Figure 8.13a–b
Dynamic Equilibrium
 Receptors in the semicircular canals are
found in Crista ampullaris
 Respond to angular
or rotatory movement
of the head rather than
to straight-line movements.
 An impulse is sent via the
vestibular nerve to the
cerebellum.
Figure 8.14c
 Homeostatic Imbalances of Hearing
-Deafness
Any hearing loss is called deafness.Two types:
1 - Conduction deafness occurs when
something hampers sound conduction to the
fluids of the internal ear as impacted earwax or a
perforated (ruptured) eardrum. But the most
common causes of conduction deafness are
middle ear inflammations (otitis media) and
otosclerosis (o″to-sklĕ-ro′sis) of the ossicles.
Otosclerosis (“hardening of the ear”) occurs
when overgrowth of bony tissue fuses the
ossicles to one another.
2-Sensorineural deafness results from damage to
neural structures at any point from the cochlear
hair cells to the auditory cortical cells as in:
a- gradual loss of the hair cells throughout life.
b- hair cells can also be destroyed at an earlier age
by a single explosively loud noise or
c-prolonged exposure to high-intensity sounds
which tears off their cilia.
D- degeneration of the cochlear nerve, cerebral
infarcts, and tumors in the auditory cortex are
other causes.
-Tinnitus
Tinnitus (tĭ-ni′tus) is a ringing or clicking sound in
the ears in the absence of auditory stimuli.
It is more a symptom of pathology than a disease.
For example, tinnitus is:
- Is one of the first symptoms of cochlear nerve
degeneration.
- – symptom of inflammation of the middle or
internal ears and
- is a side effect of some medications, such as aspirin.
Chemical Senses –Taste and Smell
 Both senses use chemoreceptors which
are stimulated by chemicals in solution.
 Taste has at least four types of
receptors.
 Smell can differentiate a large range
of chemicals.
 Both senses complement each other
and respond to many of the same
stimuli.
Olfaction – The Sense of Smell
 Olfactory receptors ( in the roof of the nasal
cavity) are Neurons with olfactory hairs ,
(long cilia) that protrude from the nasal
epithelium and are continuously bathed by a
layer of mucus secreted by underlying
glands.
 Chemicals must be dissolved in mucus for
detection.
 Impulses are transmitted via the olfactory
nerve.
 Interpretation of smells is made in the
olfactory cortex.
Olfactory Epithelium
Figure 8.17
The Sense of Taste
 Taste buds house
the receptor organs
 Location of taste
buds:
 Most are on the
tongue
 Soft palate
 Cheeks
Figure 8.18a–b
The Tongue and Taste
 The tongue is covered with projections
called papillae .
 Taste buds are found on the sides of
papillae.
 Gustatory cells are epithelial cells that
have gustatory hairs (long microvilli)
 Hairs are stimulated by chemicals
dissolved in saliva. So they depolarise
and impulses are transmitted to the
brain.
Structure of Taste Buds
 Impulses are carried to the gustatory
cortex by three cranial nerves because
taste buds are found in different areas
 Facial nerve
 Glossopharyngeal nerve
 Vagus nerve
Anatomy of Taste Buds
Figure 8.18
Taste Sensations
 Sweet receptors: Sugars ,Saccharine, Some
amino acids
 Sour receptors :Acids
 Bitter receptors :Alkaloids
 Salty receptors : Metal ions
 Umami (u-mam′e; “delicious”), a new taste
discovered by the Japanese, is elicited by the amino acids
glutamate and aspartate .
 However, these differences are not absolute: Most taste
buds respond to two or more taste qualities, and many
substances produce a mixture of the basic taste
sensations.
 Taste maps that put sweet receptors
to the tip of the tongue, salty and
sour receptors to the sides, bitter
receptors to the back, and umami
receptors to the pharynx are
common in the literature. However,
researchers have known for years
that these mapped areas are
misrepresentations.
Developmental Aspects of the
Special Senses
 All special senses are formed early in
embryonic development.
 Eyes are outgrowths of the brain.
 All special senses are functional at birth.
 Vision
- As a rule, vision is the only special sense not fully functional at birth.
-Most babies are hyperopic. The newborn eye movements are
uncoordinated, and often only one eye at a time is used.
-The lacrimal glands are not completely developed until about two
weeks after birth, so babies are tearless for this period.
- By 5 months, infants can follow moving objects with their eyes.
- By the age of 5 years color vision is well developed.
- By school age, the initial hyperopia has usually been replaced by
emmetropia, and the eye reaches its adult size at 8–9 years of age.
- Emmetropia usually continues until presbyopia begins to set in around
age 40 owing to decreasing lens elasticity.
Hearing and Balance
 Newborn infants can hear, but early responses to sound are
mostly reflexive—for example, crying and clenching the
eyelids in response to a noise.
 By the fourth month, infants will turn to the voices of
family members.
 Except for ear inflammations, mostly due to infections, few
problems affect the ears during childhood and adult life.
 The ability to hear high-pitched sounds leaves us first. This
condition, called presbycusis (pres″bĭ-ku′sis), is a type of
sensorineural deafness. Although presbycusis is considered a
disability of old age, it is becoming much more common in
younger people as our world grows
noisier.
 Taste and Smell
Smell and taste are sharp, and infants relish food that
adults consider tasteless.
 There are few problems with the chemical senses
during childhood and young adulthood.
 Women generally have a more acute sense of smell
than men, and nonsmokers have a sharper sense of
smell than smokers.
 Beginning in the fourth decade of life, our ability to
taste and smell declines .