This document provides an overview of the anatomy and physiology of the special senses, including the ear, eye, taste, smell, and touch. It describes the structure and function of the external, middle, and inner ear. It explains how sound is transmitted through the ear and the physiology of hearing and balance. It also details the anatomy of the eye, including its layers, chambers, muscles, and optical components. It discusses image formation through refraction, accommodation, and pupil constriction.

1. SPECIAL
SENSES
Anatomy &
Physiology-II Unit-V
Shabnam
Faculty INS-
KMU
Acknowledgement: Muhammad
Iqbal Lecturer INS-KMU

2. Objective
s
At the end of this unit, the learners will be
able to:
 Define key terms
 Describe the structure and function of Ear
 Discuss the transmission of sound through
the Ear.
 Describe the physiology of hearing and
Balance.

3. Classification of
Receptors
 Chemoreceptors: respond to chemicals including
odor, taste, and body fluid composition.
 Thermorecptors: respond to heat and cold.
 Nociceptors: are pain receptors which respond to
tissue damage resulting from trauma, ischemia,
etc.
 Mechanoreceptors: respond to physical
deformation caused by vibration, touch, pressure,
stretch, or tension.
 Photoreceptors: respond to light like eyes.
 Proprioceptors: sense the position and movements
of the body or its parts. They are found in muscles,
tendons, and joint capsules.

4. Anatomy of the
Ear
The ear has three regions:
1. External ear
2. Middle ear
3. Inner ear
The first two are concerned only with transmitting
the sound to the inner ear.

5. 1. External Ear
 The external ear is a funnel shaped tube called
auditory canal that conducts air vibrations to the
ear drum. The outer part of ear is pinna or auricle.
 The external auditory canal contains a few hairs and
specialized sweat glands called ceruminous glands
that secrete earwax or cerumen. The hair and
ceriman help prevent the entry of dust and microbes
from entering the ear.
2. Middle ear
 It is located in the tympanic cavity of the temporal
bone.
 It begins with the ear drum/tympanic membrane.

6.  Posterior the tympanic cavity is continuous with the
mastoid air cells in the mastoid process. It is filled with
air that enters by way of the auditory (Eustachian)
tube, a passageway to the nasopharnynx.
 The tympanic cavity, 2 to 3 mm wide between outer
and inner ear, contains the three smallest bones
and two smallest muscles of the body.
 The bones, called the auditory ossicles, connect the
eardrum to the inner ear. Progressing inward, the first
bone is malleus, the second is incus, and the third is
stapes.
 The two muscles are stapedius, connecting stapes
to the posterior wall of the cavity, and tensor
tympani which connects malleus.

7. 3. Internal Ear
The internal ear is also called labyrinth because of its
series of canals.
It consists of two main divisions:
 Bony labyrinth.
 Membranous labyrinth
The bony labyrinth, a series of cavities and
tubes in the temporal bone, divides into three
parts:
1. Three semicircular canals
2. The vestibule, both of them contain
receptors for equilibrium,
3. The choclea (=snail), which contains
receptors for hearing

8.  The bony labyrinth is lined with periosteum and
contains perilymph, a fluid between the bony
and the membranous labyrinth.
 The membranous labyrinth, a series of sacs
inside the bony labyrinth, is lined by epithelium
and contains endolymph.
 The vestibule is the oval central portion or
chamber of the bony labyrinth.
 Projecting superiorly and posterior from the
vestibule are the three bony semicircular
canals.

9. Inner Ear
cont…
Cochlea (=snail):
 The cochlea has three fluid-filled chambers
separated by membranes. The superior one is
called scala (= stair) vestibule, the middle one is
scala media or cochlear duct, and the inferior one
is scala tympani.
 Within the scala media is the organ of corti which is a
thick epithelium with associated structures. It converts
vibrations into nerve impulses.
 The epithelium in the corti is composed of hair cells
and supporting cells.

11. Ea
r

12. Organ of
Corti

13. Physiology of
Hearing
The following events are involved in the hearing:
 The auricle directs sound waves into the auditory
canal.
 The sound waves strike the eardrum and get it
vibrated.
 The central area of the eardrum is connected with the
malleus which starts to vibrate. The vibration is
transmitted to the incus and then to the stapes.
 As the stapes moves back and forth, it
vibrates the membrane of the oval window.

14.  The movement of the oval window vibrates the
fluid in the perilymph of the cochlea and the
endolymph inside the cochlear duct.
 The endolymph vibrates the hair cells in the
organ of corti which stimulates the
vestibulocochlear (VIII) nerve to transmit nerve
impulse to medulla

15. Physiology of
Equilibrium
 The position of the semicircular canals is vital in
maintaining the equilibrium (both static and
dynamic) particularly of the head.
 In the ampulla, the dilated portion of each canal,
contains a group of hair cells.
 By moving head, the hair cells are bent by the
endolymph in the ampulla.
 Bending of the hair bundles produce receptor
potentials.
 The receptor potential leads to nerve impulses that
pass along the vestibular branch of the
vestibulocochlear (VIII) nerve.
 Both cerebellum and cerebrum give feedback

16. Ey
e

17. Objective
s
At the end of this session, the students will be
able to:
1. Define Key Terms
2. Describe eye in terms of:
—Structure related to vision
—Accessory structures
3. Discuss image formation by describing
refraction, accommodation and constriction
of the pupil.
4. Explain the process of dark adaptation and
light adaptation.

18. Accessory structures of the
Eye
 Eyelids: The upper and lower eyelids or palpebrae
protect the eyes.
 Palpebral fissure is the space between the eyelids.
 Conjunctiva is a thin mucous membrane of
stratified columnar epithelium with goblet cells. The
one that lines the inner aspect of the palpebra is
called Palpebral conjunctiva, and the other which
covers the sclera but not the cornea is known as
bulbar conjunctiva.
 Eyelashes and eyebrows: both of them protect
the eyeballs from the foreign objects and sweat.
Sebaceous glands at the base of the hair follicles
of the eyelashes, called the sebaceous cilliary
glands, release sebum. Infection of these glands

19. Accessory eye structures
cont….
 The lacrimal apparatus: is a group of structures that
produce and drains lacrimal fluid or tears. The lacrimal
glands secrete tear which drains into the excretory
lacrimal ducts that empty it onto the surface of the
conjunctiva of the upper lid. After passing the anterior
surface of the eyeball it enters into an opening called
lacrimal punctum, to lacrimal canals, to lacrimal
sac, to nasolacrimal duct. This duct carries the tear
into the nasal cavity just inferior to the inferior nasal
concha.
 The infection of the lacrimal sac is called
dacryocystitis (dacryo=lacrimal sac) which results
in the blockage of the nasolacrimal duct.
 Apart from some mucus, the tears contain

20. Lacrimal
Apparatus

21. Extrinsic Eye
Muscles
Six extrinsic eye muscles move the eyes:
 Superior rectus
 Inferior rectus
 Lateral rectus
 Medial rectus
 Superior oblique
 Inferior oblique.
 They control the movement of each eye and
hold the eyes in the orbits.
 They are supplied by cranial nerves III, IV, VI.

22. Extrinsic Eye
Muscles
Fig
16.6

23. Anatomy of the
Eyeball
The adult eyeball measures about 2.5 cm (1
in.) in diameter
The eyeball consists of three layers:
1. Tunica fibrosa or Fibrous tunic (Sclera)
2. Tunica vasculosa or Vascular tunic
(Choroid)
3. Tunica interna (Retina)

24. I. Tunica Fibrosa
– It is tough outermost eyeball layer, divided into
two regions: the sclera and cornea
– Sclera—(scler=hard)—white of the eye; Covers
most of the eye surface, composed of collagen &
elastic fibers; optic nerve exits from sclera at the
back of eye.
– Cornea – is the anterior transparent region of
modified sclera that admits light into the eye.
– At the junction of sclera and cornea is an opening
called scleral venous sinus or canal of
schlemm. A fluid called aqueous humor drains
into this sinus.

25. II. Tunica Vasculosa
also called uvea (=grape) is the middle layer,
consists of the choroid, ciliary body, & iris
a. Choroid—thin, dark vascular layer behind the
retina. Choroid also contains melanocytes that
produce the
pigment melanin which absorbs stray light to
prevent reflection and scattering of light within
the eyeball.
Albinos lack melanin and thus need to wear
sunglasses to minimize the perception of bright
glare.

26. b. Ciliary body—A thick extension of the
choroid that forms a muscular ring around the
lens.
It supports the iris and lens.
The ciliary body consists of ciliary muscle and
ciliary process
The ciliary muscle is a circular band of smooth
muscle. Its contraction and relaxation changes
the tightness of the suspensory ligaments
(zonular fibers), which alter the shape of the
lens, adapting it for near and far vision.
It is controlled by CN III and parasympathetic
nerves

27.  The ciliary processes are the extensions of the
ciliary body.
 Epithelial cells together with blood capillaries in the
ciliary processes secrete aqueous humor.
 Extending from the ciliary process are
suspensory ligaments attached to the lens.

28. C. Iris -the colored portion of the eyeball with a
central aperture (pupil), located between the
cornea and lens and is attached at its outer
margin to the ciliary processes.
 It contains melanocytes and circular and radial
smooth muscle fibers.
 The amount of melanin in the iris determines the
eye color.
 The eyes appear brown to black when melanin
concentration is high, green when melanin
concentration is moderate, and blue when
melanin concentration is low.

29. Secretion and Drainage of Aqueous
Humour
 Anterior chamber and posterior chamber
 Secreted by ciliary processes (blood vessels and
epithelial cells) in posterior chamber at the rate of 2
to 3 L/min.
 Drained by canal of schlemm in the anterior
chamber.
 Intra ocular pressure (IOP) is 10—20 mmHg
(average 15) measured by tonometer.

30. Retina (Tunica Interna)
 The third and inner layer of the eyeball is retina,
which lines the posterior three-quarters of the
eyeball.
 It is the beginning of the visual pathway.
 The surface of the retina is the only place in the
body where blood vessels and optic nerve can de
directly seen through the pupil with the help of
opthalmoscope.
 The optic nerve leaves the eyeball at the site known
as the
optic disc.
 The retina consists of a pigmented layer and a neural
layer.

34. The Optical Components
 The optical components of the eye are transparent
elements that admit light rays, refract (bend) them,
and focus images on the retina. They include the
cornea, aqueous humor, lens, and vitreous body.
 The aqueous humor is a serous fluid secreted by
the ciliary body into the posterior chamber, a
space between the lens and iris. It flows through the
pupil into the anterior chamber between the iris
and cornea. From here it is reabsorbed by a ring like
vessel called the scleral
venous sinus or canal of schlemm.

35.  The lens is suspended behind the pupil by a ring of
fibers called the suspensory ligament.
 It is about 9 mm in diameter and 3.6 mm thick at
the middle.
 The vitreous body (vitre=glassy) or vitreous
humor is a transparent jelly that fills the large space
behind the lens.

36. The neural components
 The neural components are the retina and optic nerve.
 Retina is a thin transparent membrane attached at
only two points—the optic disc, where the optic
nerve leaves the rear (fundus) of the eye, and the
ora serrata (the junction b/w retina and cilliary
body).
 The retina is pressed smoothly against the rear of the
eyeball by the pressure of the vitreous body. It can
become detached (buckle away) from the wall of the
eyeball in some circumstances. It causes blurred
vision. It leads to blindness if the retina remains
detached for longer time from the choroid, on which it
depends for oxygen, nutrition, and waste removal.

37. • Directly posterior to the center of the lens, on the
visual axis of the eye, is a patch of cells called the
macula lutea (macula=spot; lutea=yellow) about 3
mm in diameter. In the center of the macula is a tiny
pit (small depression) called fovea centralis, which
contains only cones. The fovea centralis is the area of
highest visual acuity or resolution (sharpness of
vision).
• About 3 mm medial to the macula lutea is the optic
disc. Nerve fibers from all regions of the retina
converge on this point and exit the eye to form the
optic nerve.
• The optic disc contains no receptor cells, so it
produces a blind spot in the visual field of each
eye.

39. Photoreceptor
Cells
 Photoreceptor cells are the cells that absorb light
and generate a chemical or electrical signal.
There are three kinds of photoreceptors in the
retina: rods, cones, and ganglion cells. These are
special nerve cells.
 Rods contain a protein pigment called rhodopsin.
 Rod cells are responsible for night (scotopic)
vision
(scot=dark; op=vision).
 They can’t distinguish colors from each other
because they have only one kind of rhodopsin
which absorbs the light with wave length of 500
nm

40.  Cone cells are responsible for day (photopic)
vision as well as color vision.
 The cone cells have visual pigment as photopsin.
 There are three kinds of cones that absorb
different kind of wave length lights.
 Ganglion cells are the largest neurons of the
retina. Their axons form the optic nerve. Some of
the ganglion cells absorb light and transmit
signals to brain stem nuclei that control pupillary
diameter.

41. Rod and Cone
cell
Rod cell

42. Image
Formation
• The eye is just like a camera.
• Its optical elements focus an image of an object
on a light-sensitive ‘film’ called retina.
• Image formation involves three processes:
• Refraction—bending of light by the lens and
cornea.
• Accommodation—change in shape of the lens.
• Constriction—narrowing of the pupil.

43. Refraction of Light Rays
 By entering the light rays into the eyes, they
are refracted by cornea (75%) and lens
(25%) of the eyes so that the light rays may
come into exact focus on the retina.
 Images focused on the retina are real
and inverted.

44. Accommodation
 Accommodation of the eye refers to the act of
physiologically adjusting lens to alter (increase)
its refractive power the closer objects and
(decrease) for distant objects.
 The lenses of the eyes are convex and so
converge the light rays.
 The near point of vision is the minimum distance
from the eye that an object can be clearly focused
with maximum accommodation. This distance is
about 10 cm (4 in.) in a young adult.

45. Accommodatio
n

46. Constriction of the Pupil
 The diameter of the hole (pupil) is narrowed by
the contraction the circular muscles of the iris.
 This autonomic reflex occurs simultaneously with
accommodation and prevents light rays from
entering the eye through the periphery of the lens.
 Light entering the periphery cannot be refracted
and focused on the retina and would result in
blurred vision.
 The pupil also constricts in bright light.

47. Pupillary Diameter
control
The diameter of the pupil is controlled by two
sets of contractile elements in the iris:
1. The pupillary constrictor consists of smooth muscle
cells that encircle the pupil. When stimulated by the
parasympathetic nervous system, it narrows the pupil
and admits less light to the eyes.
2. The pupillary dilator consists of contractile epithelial
cells called myoepithelial cells. When stimulated by
the sympathetic nervous system, these cells
contract, widen the pupil, and admit more light to
the eyes.

48. Light and Dark
Adaptation
• Light adaptation occurs when you go from the dark
into bright light. Going from dark into bright light it is
difficult to open eyes properly because of a harsh
glare which over stimulates the retinas.
• The pupils quickly constrict to reduce the intensity
of stimulation. The rods bleach quickly in bright
light, and cones take over. It takes 5 to 10
minutes to restore the normal vision in the light.

49. • Dark adaptation occurs when you go from the
bright light into dark. The rod pigment is
bleached in the light. In the absence of light,
rhodopsin regenerates within 20 minutes.
• Dilation of the pupils also helps by admitting
more light to the eyes.

50. Refraction
abnormalities
 For focusing clear image on the retina, the normal
eye, known as emmetropic eye, can sufficiently
refract light rays from an object 6 m (20 ft) away.
Improper Refraction results in:
 Myopia or nearsightedness, when the lens
becomes thicker and the image converges in
front of the retina which allows the myopic
individuals to see closer objects clearly, but
not distant objects.

51. Refraction abnormalities
cont….
 Hyperopia or hypermetropia or farsightedness in
which the lens becomes thinner and the image
converges behind
the retina. Hypertropic individuals can see distant
object clearly, but not closer ones.
 Presbyopia (presby =old + opia = vision) is another
age related vision problem in which the lens loses its
elasticity and ability to focus on the nearby objects.
Therefore the older people cannot read the nearby
print.

52. Pathological
Terms
 Strabismus is a condition in which the eyes do not
properly align with each other when looking at an
object.
Types:
 Esotropia-The term derives from Greek, where ‘eso-‘
means ‘inward,’ and ‘trope’ means ‘turn it is a form of
strabismus in which one or both eyes turns inward.
The condition can be constantly present, or occur
intermittently, and can give the affected individual a
"cross-eyed" appearance.
 Exotropia- is a form of strabismus (eye misalignment)
in which one or both of the eyes turn outward. It is the
opposite of crossed eyes, or esotropia. Exotropia may
occur from time to time (intermittent exotropia) or may

53.  Hypertropia:A
hypertrop ia is a form
of vertical strabismus
where one eye is
deviated upwards in
comparison to the
fellow eye.

54.  Glaucoma- Glaucoma is a group of eye diseases
which result in damage to the optic nerve and
cause vision loss
 Cataracts- is the clouding of the eye's natural lens. It
is the most common cause of vision loss in people
over age 40 and is also the principal cause of
blindness in the world.
 Myosis-is a medical term referring to contraction of
the
eye’s pupil in response to bright light.
 Mydriasis- is the medical term for an unusual
dilation or widening of the pupils
 Pterygium- is a vascular pink tissue that can grow
from the conjunctiva onto the cornea. If it grows into
the line of vision (over the pupillary aperture), it can

55. Pterygiu
m
Catarac
ts
•
Mydria
si
Mydriasis

56. Any
Question?