2. Objectives
To provide students with a general overview of the
sense organs in the human body.
3. Sensory System
• A major characteristic of living organisms is the
ability to respond to stimuli.
• The human sensory system is very advanced and is
able to detect and process thousands of incoming
messages simultaneously.
• The core structure that enables us to detect stimuli is
the sensory receptor.
5. Sensory Receptors
A sensory receptor is a specialised structure which
has the ability to detect a particular stimulus.
Receptors vary in complexity:
Simple nerve endings (dendrites)
Complex sense organs (e.g. eyes )
Nerve endings combined with other tissues such as
connective, epithelial and muscular tissues.
6. Sensory Receptors
There are several methods for classifying sensory
receptors e.g.
1. Based on location/ origin of stimulus
2. Based on type of stimuli detected
7. Classification by location/ origin of stimulus
Three types
1. Exteroceptors:
Detect stimuli external to the body (outside/surface of body)
Located at or near the surface of the skin
Include receptors for tactile sensations, such as touch, pain,
and temperature, as well as those for vision, hearing, smell,
and taste.
2. Interoceptors (visceroceptors)
Detect stimuli occurring in the body from visceral organs
and blood vessels.
Produce sensations such as visceral pain, nausea and
stretch.
8. Classification by location/ origin of stimulus
3. Proprioceptors
Respond to stimuli occurring in skeletal muscles, tendons,
ligaments, and joints.
Sense the position and movement of the body or its parts.
9. Classification by type of stimulus detected
1. Mechanoreceptors
Respond to physical force such as pressure, vibrations, touch,
stretch and tension.
E.g. pressure receptors in the skin
2. Thermoreceptors
Respond to temperature changes (heat or cold).
E.g. thermoreceptors in the skin
3. Chemoreceptors
Respond to chemicals such as odours, tastes and changes in
internal body chemistry (e.g. levels of O2, CO2, or H+ in the blood)
10. Classification by type of stimulus detected
4. Nociceptors (Pain receptors)
Respond to tissue damage, ischemia (inadequate blood
supply) or excessive stimulation by agents such as heat and
chemicals.
Found in skin and in internal organs
5. Photoreceptors
Stimulated by light
Limited to the eye
12. Senses of taste & smell
Chemical Senses
Have chemoreceptors that are sensitive to chemicals
Stimuli are chemical molecules in food and air we
inhale
13. TASTE (Gustation)
The sense of taste (gustatory sense) occurs in the
taste buds.
The sensory receptors for taste are located in the taste buds.
Taste is basically a sensation that results from the action of
chemicals on the taste buds.
Taste buds are primarily located embedded in the
epithelium on the tongue
Reside in papillae – bumps on the tongue that give it a rough
texture.
14. TASTE
There are five primary types of taste sensations:
1. Sweet
2. Sour
3. Salty
4. Bitter
5. Umami
15. Structure of taste bud
Lemon-shaped
Composed of an aggregate of cells (3 types)
Taste (gustatory) cells – elongated
Supporting cells
Basal cells
Taste cells have microvilli which project into a pit
(taste pore) on the epithelial surface of the tongue.
Microvilli are the receptor surfaces for taste molecules.
When taste molecules bind to receptor proteins in microvilli,
nerve impulses go to the brain (which interprets them as tastes)
18. SMELL
Sensory receptors are olfactory cells
Olfactory cells are located within the olfactory
epithelium in the roof of the nasal cavity.
Olfactory cells are modified neurons.
Each cell ends in a tuft of about five olfactory cilia.
Olfactory cilia bear receptors for odour molecules.
When odour molecules bind to the receptor proteins,
nerve impulses are sent to the brain for
interpretation.
22. VISION
Vision is described as the perception of objects in the
surroundings of an individual by means of the light
they emit or reflect.
To see an object, light has to travel from that object to you.
Human can only perceive radiations in the range 400-750nm
(visible light) in the EMS.
The eyes have photoreceptors for sight.
Eyes are located in the orbits of the skull and has
accessory structures located in and around it.
23. Accessory Structures of the Eye
Eyebrows:
Short, thick hairs located
above the eye along the
supraorbital ridge.
Mainly useful in enhancing
facial expressions
Shade eye from sun &
prevent perspirations from
falling into the eye.
Eyelashes:
Trap debris and keep them
from entering the eye
Eyelids (palpebrae):
Layer of skin
Blinking keeps eye
lubricated and free of
debris
Controlled by 2 muscles
Orbicularis oculi – closes
Levator palpebrae
superioris - raises
24. Accessory Structures of the Eye
Lacrimal Apparatus:
Consists of the lacrimal
gland and lacrimal sac.
The lacrimal gland –
Lies in the orbit above the
eye
Produces tears that flow over
the eye when the eyelids are
blinked
Lacrimal sac has a duct
which carries tears to an
opening of the nasal cavity.
25. Accessory Structures of the Eye
Extrinsic Muscles:
3 pairs of antagonistic
muscles
Hold eye in place in each
orbit
Also move the eye
Origin of these muscles
found in the bone in the
orbit and insertions
through tendons to outer
layer of the eyeball.
26.
27. THE EYEBALL
The eyeball describes the
major structural unit of
the eye.
Shaped like an elongated
hollow sphere.
Diameter of about 2.5cm.
The wall of the eyeball
consists of 3
tunics/layers:
1. Sclera
2. Choroid
3. Retina
28. SCLERA
Tough outermost
connective tissue layer
Avascular
Visible forward portion is
the white of the eye
The forward 1/6 portion
of this layer is different
from the rest – Cornea
The cornea is made up
transparent collagen
fibres.
Serves as window which
allows light to enter eye
Functions of sclera:
Maintains the shape of the
eyeball
Provides surface for
attachment of eye muscles.
29. CHOROID
Middle, thin darkly
pigmented layer
Vascular
Iris is the ‘donut-
shaped’ part of choroid
located at the front of the
eyeball
It regulates amount of
light entering the eye by
controlling the size of its
circular opening – Pupil.
Colour of eye ‘determined’
by colour of Iris.
30. CHOROID
Behind the iris is a circular
thicken portion called ciliary
body.
Ciliary body contains the
ciliary muscle.
Ciliary muscle controls shape
of lens
Lens attached to ciliary body
by suspensory ligaments.
Lens divides eye into anterior
& posterior compartments.
Anterior compartment filled
with aqueous humour (clear,
watery fluid).
31. RETINA
Innermost layer –
nervous tissue covered by
epithelium
The layer which contains
the photoreceptors for
light
2 types of
photoreceptors:
Rods
Cones
Rods
Very sensitive to light and
more numerous than cones.
Provide vision in dim light.
Cannot detect colour
Cones
Respond to bright light and
sensitive to different
wavelengths of light.
Thus can detect colour.
Transmit sharp images.
32. RETINA
Fovea centralis – special region of retina
Contains highest concentration of cones
Site on retina that provides the highest visual acuity
(sharpness)
Sensory fibres from the retina form the optic nerve
Optic nerve transmits nerve impulses to the brain
Site of optic nerve on retina is also the blind spot.
Contains no cones nor rods thus vision not possible in that area
33.
34.
35. HEARING
Hearing is dependent on the ear
The ear is divided into three parts:
outer, middle, and inner.
37. Conti..
The outer ear consists of the pinna and the auditory
canal, which direct sound waves to the middle ear.
The middle ear begins with the tympanic membrane
and contains the ossicles (malleus, incus, and
stapes).
The malleus is attached to the tympanic membrane,
and the stapes is attached to the oval window, which
is covered by a membrane.
38. Conti..
The inner ear contains the cochlea and the semi-
circular canals, plus the utricle and the saccule.
Hearing begins when the outer ear receives and the
middle ear amplifies the sound waves that then
strike the oval window membrane.
Its vibrations set up pressure waves across the
cochlear canal, which contains the spiral organ,
consisting of hair cells whose stereocilia are
embedded within the tectorial membrane.
40. When the basilar membrane vibrates, the stereocilia
of the hair cells bend. Nerve impulses begin in the
cochlear nerve and are carried to the brain.
