The three main senses organs are the eyes, ears, and skin. The eyes contain light-sensitive receptors that detect light and transmit signals to the brain. These signals are interpreted as visual images. The ears contain receptors for both hearing and balance. They detect sound vibrations and head position. The skin is our largest sensory organ and contains receptors for touch, temperature, and pain that provide information about our external environment.

1. The Sensory Organs

2.  Our “senses” continually provide us with
information about our surroundings.
 Sense organs are complex organs like the eye or
specialized receptors in areas such as the nasal
mucosa or tongue.

3.  Conversion of a stimulus to a sensation:
• Stimuli (light, sound, temperature, etc. are changed
into an electrical signal or nerve impulse.
• The signal is then transmitted over a ? neuron to the ?
• The signal is interpreted and we become consciously
aware of a sensation.

4.  Your senses are means of detecting and interpreting
stimuli of you external environment.
 These are the sense of taste, sight, hearing, touch and
smell.
 A sensory organ is a specialised organ that allows for
the reception of a stimulus, the formation of an
impulse and it’s transmission to the brain for
interpretation.
 The main sense organs are your eyes, ears, olfactory
organs in your nose, taste buds on your tongue and
your skin.

5.  Contains receptors for vision and a refracting
system that focuses light rays on the receptors in
the retina.
 The eye sits in the orbit formed by the maxilla,
zygomatic, frontal, sphenoid and ethmoid bones.
Extrinsic muscles attach the surface of the
eyeball to bones.

10.  Eyelids – contain skeletal muscle that allow us to
close them and totally cover the exterior eyeball.
 Eyelashes – help to keep dust out of our eyes.
 Tears
 Cranial Nerves
• Optic – vision
• Oculomotor, abducens and trochlear – eye movment
 The eye contains 3 layers

11.  Structure of the eyeball
• Sclera – tough fibrous tissue.
 Front surface is the “white” of our eyes and the cornea. The cornea is
transparent, receives no blood supply and is nourished by the aqueous
humor.
 Sclera is covered by the conjunctiva in the front of the eyeball.
• Choroid - contains a dark pigment to prevent scattering of
light that enters the eyeball. Also contains blood vessels and
2 involuntary muscles.
 Iris –
 Ciliary body (muscle) –
• Lens – composed of transparent, elastic protein; no blood
supply, nourished by the aqueous humor.
• Retina – contains microscopic receptor cells called rods and
cones.
 Rods –
 Cones –
 Fovea –

12.  Layer of the eye
• Retina
 Ganglionic neurons carry impulses generated by the rods and
cones until they converge at the optic disc. From the optic disc
they form the optic nerve and pass through the wall of the
eyeball to the occipital nerve.
 Optic disc – also know as the “blind spot”, no rods or cones; exit
to the optic nerve.
 Occipital lobe of the cerebrum – visual interpretation.

14.  Structure - fluids of the eyeball – 2 types:
 Aqueous humor – watery fluid in front of the lens
(anterior cavity) nourishes the lens and cornea.
○ Continually formed by the capillaries in the ciliary body, flow
through the pupil and is reabsorbed in the canal of Schlemm.
○ If drainage is blocked, the internal pressure in the eye
increases and may damage the eye and lead to blindness =
glaucoma.

15.  Structure - fluids of the eyeball – 2 types:
• Vitreous humor – jelly-like fluid behind the lens
(posterior cavity). Literally holds the retina in place
and gives structure to the eyeball.

16.  Focusing Problems
• Presbyopia – “old sightedness” or “short arm
syndrome”. Ciliary bodies lose their elasticity and can
no longer change the shape of the lens to bring near
objects into focus.
• Myopia – nearsightedness, image focuses in front of
the retina rather than on it, eyeball is elongated.
Corrected by glasses, contacts or radial keratotomy
(Lasix).

17.  Focusing Problems
• Hyperopia – “farsightedness”, image focuses behind
the retina, produces a fuzzy image. Corrected by
lenses.
• Astigmatism – refraction error – fuzzy image, irregular
curvature of the cornea or lens, requires special lenses
to correct (Toric lenses) or contacts.

19.  Light enters the eye, the amount of which is
controlled by the iris and is bent (refracted)
slightly by the cornea.
 It is then focused by the lens (controlled by the
ciliary muscles) onto the retina.
 On the retina, rods and cones convert the light
into a nerve impulse, carrying the image to the
brain.
 The brain interprets the images from both our
eyes forming a composite image.

22. Short Sight (Myopia)
 A short sighted person can view close objects
clearly but distant objects are out of focus.
 Light rays are focussed short of the retina.
 Cause: eyeball is too long or the focussing
elements of the eye are too strong.
 Correction: use a concave (divergent) lens to
widen the angle over which the light rays have to
be refracted.

24. Long Sight (Hyperopia)
 Condition: can view distant objects clearly but
close objects are out of focus,
 The focal point is long of the retina i.e. is behind
retina.
 Cause: eyeball is too short or the focussing
elements of the eye are too weak.
 Correction: use a convex (convergent) lens to
reduce the angle over which the light rays have to
be refracted.

26.  The ear has two major functions:
 Hearing: detection of vibrations, their frequency
(pitch) and amplitude (loudness).
 Balance: detection of direction of motion,
acceleration and head position related to gravity.
 It is divided into three sections, the outer ear,
middle ear and inner ear.

27.  External ear –
 External Auditory canal – a curving tube about one inch
long; extends into the temporal bone and end at the
tympanic membrane (eardrum).

28.  Middle ear – tiny epithelium lined cavity which
is hollowed out of the temporal bone.
• Tympanic membrane – separates the external and
middle ear and vibrates when sound waves strike it.
• 3 tiny bones called ossicles (bones) transmit sound
waves.

29.  Middle Ear
• Bones
 Malleus –
 Incus –
 Stapes –

30.  Middle Ear
• Oval Window separates the middle ear from the inner
ear.
• Eustachian tube – connects the throat with the middle
ear; allows air to enter and leave the middle ear which
equalizes pressure. Why do throat and ear infections
occur together?

32.  Middle Ear - Hearing Sequence
• Sound waves cause the eardrum to vibrate, and this
movement is transmitted and amplified by the ear
ossicles.
• Movement of the stapes against the oval window
causes movement of fluid in the inner ear which
generates electrical impulses.

34.  Inner Ear – contains mechanoreceptors that are
activated by vibration and generate nerve
impulses that result in hearing and equilibrium.
The 3 spaces are called the bony labyrinth and
contain fluids called perilymph and endolymph.
• Vestibule – membranous sacs (utricle and saccule)
adjacent to the oval window and between the
semicircular canals. Contains receptors for
equilibrium.

35.  Inner Ear
• Cochlea – snail shell; contains the Organ of Corti
which holds the receptors for hearing (hair cells). As
the hairs bend (vibration) they generate an electrical
impulse.
• Semicircular Canals – contain the crista ampularis
which is a specialized receptor that generates a nerve
impulse when you move your head. Receptors for
equilibrium.

39.  The skin is the outer layer of vertebrate animals. Its
major functions are protection, temperature regulation
and to act as a sense organ.
 Two major layers of the skin: epidermis and dermis.
Epidermis
 The epidermis is the outer renewable layer of the skin.
 Malpighian Layer: This is the base layer, which is
constantly producing new cells by mitosis. The new cells
are pushed towards the surface. The dark pigment
melanin is produced here.
 Granular Layer: The protein keratin accumulates in the
cells giving them a granular appearance. The cells finally
die.
 Cornified Layer: This is the surface layer of dead
keratinised cells, which is constantly being eroded.

41. Protection
 Prevents excessive loss of water (the cornified layer
of the epidermis is waterproof).
 Prevents the entry of pathogens.
 ‘Sebum oil’ keeps the skin intact preventing it from
‘cracking’.
 Sebum from the sebaceous glands contain anti-
microbial chemicals.
 Melanin gives protection against the damaging UV
rays of sunlight.
 The dermis and adipose tissue protect against
mechanical injury.

42. Sense Organ
 The skin contains receptors for touch, pressure, pain,
temperature rise and temperature decrease. The skin
supplies information about a variety of external
environment conditions.
Vitamin D Production
 Made when ultraviolet light penetrates the skin
converting a chemical in the blood to vitamin D. As a
result vitamin D is often called the ‘sunshine
vitamin’.
Excretion
 The skin has about 2.5 million sweat glands.
 Sweat is a dilute solution of water, sodium chloride,
urea, ammonia, uric acid and lactic acid.

43. Energy Storage
 There is a layer of fat storage adipose tissue
below the dermis of the skin.
 Fat is also a poor conductor of heat and so the
skin acts as a heat insulator.
 This fat layer also acts as a shock absorber
protecting against mechanical damage.
The Skin is a Homeostatic Organ!

44.  The olfactory region is high up in the nasal cavity
about 5 cm2 in area with about 20,000 receptors.
 Stimulatory chemicals must be volatile and
soluble in water.
 There are 50 primary smell qualities but in
combination they can produce over 3,000
different odours.
 Prolonged exposure to a particular chemical
causes fatigue of that sensation.

45.  Olfactory receptors – chemical receptors responsible for
the sense of smell are located in the upper part of the
nasal cavity.
 Olfactory receptors are stimulated by chemicals dissolved
in the watery mucus that lines the nasal cavity. We detect
about 10,000 different scents.
 Olfactory receptors are easily fatigued – many odors are
not noticeable after a time.

48.  There are four primary taste qualities: bitter,
sour, salty, sweet.
 Most tastes are combinations of these.
 The taste receptors are collected in groups of
about ten in the taste buds.
 Taste buds are present on the tongue’s edges and
upper surface.
 Taste buds are also present on the soft palate and
on the back of the pharynx.
 Certain regions of the tongue are more sensate to
a particular primary tastes than others.

49.  Taste buds – chemical receptors that generate
nervous impulses resulting in the sense of taste.
There are about 10,000 microscopic taste buds
located on the papillae of the tongue.
 Gustatory cells –

50.  Taste Sensations
• Sweet, sour, bitter, salty, and Umami (=savory).
• Other flavors results from a combination of taste bud
stimulations and olfactory receptor stimulation. i.e.
our taste sensations include odors as well.