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1. CHAPTER 1
STIMULI AND
RESPONSES

2. The Human Nervous System
1. The human nervous system is divided into:
a) The central nervous system (control centre) ; the
brain controls all activities of the body & the
spinal cord controls the involuntary actions like
knee jerks.
b) The peripheral nervous system transmits
impulses from the sensory organs through the
central nervous system to the muscles or glands
2. Voluntary actions are actions we aware of and
involuntary actions are actions which carry on
automatically.

3. Human Nervous System

7. The Brain

8. Voluntary and Involuntary action

11. Involuntary action

12. Reflex arc when hand is touch hot object

13. Stimuli and Responses in Human
◦ Humans face constant changes in
surroundings. These changes are called
stimuli.
◦ Examples of stimuli include light, sound and
chemical substances. Humans use their
sensory organs to detect stimuli.
◦ Humans have five sensory organs: eyes, ears,
nose, skin and tongue

14. Eye
Front view of eye
Eyelid
Pupil
Sclera
Iris

15. Structure of Human eye
Ciliary muscles
Eye Lens
Iris
Pupil
Cornea
Sclera Choroid
Retina
Yellow spot
Optic nerve
Blind spot
Vitreous humour
Aqueous
humour
Suspensory
ligament

16. Side view of eye

17. The functions of the different parts
of the eye
PART STRUCTURE FUNCTION
Sclera -White, tough and
fibrous coat
- Protect the eye
- Gives the eye fixed
shape
Choroid - Dark colour
- Has many blood
capillaries
- Absorbs light to
prevent reflection of
light inside the eye

18. PART STRUCTURE FUNCTION
Retina - Light-sensitive
layer of the eye
- Contains
photoreceptors
that detect light
- Place on which
images are formed
Yellow
spot
Shallow yellowish
depression in the
retina directly
opposite the centre
of the lens
- The area that is
most sensitive to
light
- The area where
images are normally
focused

19. PART STRUCTURE FUNCTION
Blind
spot
- Located immediately
over
the optic nerve
- Has no
photoreceptors
- The spot in the eye
where the optic nerve
enters the eye
- Not sensitive to light
Cornea - Transparent layer at
the front of the eye
- Curved shape
- Allow light to enter
the eye
- Directs light towards
the lens

20. PART STRUCTURE FUNCTION
Conjunctiva - Thin transparent
membrane covering
the exposed part of
the eye
- Protects the
cornea
Iris - Part of the choroid
layer which can be
seen from the front of
the eye as a disc-
shaped structure
- Consists of muscles
- Coloured, for
example, brown or
blue
- Controls the
size of the pupil
thus controlling
the amount of
light entering the
eye

21. PART STRUCTURE FUNCTION
Pupil - A small opening in
the centre of
the iris
- Enables light to
enter the eye
- Controls amount of
light entering the eye
Ciliary
muscles
- Consists of the
ciliary muscle
- Part of the choroid
layer
- Contracts and
relaxes to change the
thickness of the lens

22. PART STRUCTURE FUNCTION
Eye
Lens
- A transparent,
elastic and
biconvex disc
- Focuses light that
enters the eye onto
the retina to form an
image
- Focuses light from
near and distant
objects by changing
its thickness

23. PART STRUCTURE FUNCTION
Vitreous
humour
- Transparent jelly-
like substance
- Fills the space
behind the lens
- Helps maintain the
shape of the eyeball
- Helps focus images
on the retina
Suspensory
ligaments
- Consists of fibres
attaching the lens
to the ciliary body
- Holds the lens in
position

24. PART STRUCTURE FUNCTION
Aqueous
humour
- Watery,
transparent fluid
- Fills the space
between cornea
and lens
- Helps focus images
the retina
- Helps maintain the
shape of the eyeball
- Enables oxygen and
nutrients from the
choroid to diffuse to the
lens, cornea and
conjunctiva

25. PART STRUCTURE FUNCTION
Optic
nerve
- Nerve
connecting the
photoreceptors in
the retina to the
brain
- Carries impulses from
the retina to the brain

26. Eye
1. The eye is the sensory organ of sight that responds
to the light.
2. Sight mechanism:
a) Light rays from the object entering the eye
through the cornea, aqueous humour, pupil,
eyepiece and vitreous humour and focusing on
the retina.
b) The photoreceptors that are trigged and nerve
impulses that are formed are sent through the
optic nerve to the brain.

27. The retina has two types of photoreceptors:
rod cells and cone cells:
1. The rod cell in the retina is sensitive to light of
different intensity, such as at night. It is not
sensitive to colour and only a black and white
image I produced.
2. The cone cell in the retina is sensitive to light
of high intensity to detect colour.

29. How Human See
Light reflected from
an object enters the
eye through the
pupil
Lens bends the light
rays and focuses
them onto the retina
Inverted image
formed on the
retina
Photoreceptors
stimulated
Impulses formed and
sent through the
optic nerve to the
brain
Brain interprets the
image as upright

30. The Human Sight Mechanism
Cornea
Aqueous
humour
Pupil
Eye lens
Vitreous
humour
Retina
(Photoreceptor)
Optic nerve
Brain

32. Ear
1. The ear is the sensory organ for hearing.
2. Responds to sound stimuli.

33. The Human Ear
Outer
- Pinna
- ear canal
(filled with
air)
Middle
- eardrum
- ossicles
- oval window
- Eustachian tube.
(filled with air)
Inner
-Cochlea
- semicircular
canals
- auditory nerve
(contain fluid)

34. PART STRUCTURE FUNCTION
Outer ear
(a) Pinna
- Made of cartilage
and skin
- Collects and directs
sound waves into
the ear canal
(b) Ear
canal
-A narrow passage
- Walls near the
outside of the ear
covered with fine
hairs
-Leads to the ear
drum
- Directs sound
towards the ear
drum

35. PART STRUCTURE FUNCTION
Middle ear
(a) Ear
drum
- A thin membrane - Vibrates when
sound waves reach
it
(b)
Ossicles
- Three small bones
called the hammer-
bone, anvil-bone
and stirrup-bone.
- Amplifies and
transmits vibrations
of the ear drum to
the membrane
covering the oval
window

36. PART STRUCTURE FUNCTION
(c) Oval
window
- A small opening
covered by a
membrane
- Transmits vibrations
from middle ear to
inner ear
(d)
Eustachian
tube
- A narrow tube
- Connects the
middle ear with
the throat
- Balances air
pressure on both
sides of ear drum so
that the ear drum
can vibrate freely

37. PART STRUCTURE FUNCTION
Inner ear
(a)
Cochlea
- Coiled structure
- Filled with fluid
- Inner wall contain
receptors sensitive
to vibrations
- Changes sound
vibrations to nerve
impulses
(b)
Auditory
nerve
- Nerve fibres that
connect receptors in
the cochlea to
the brain
- Transmits impulse
from receptors in the
cochlea to the brain

38. PART STRUCTURE FUNCTION
(c)
Semicircular
canals
- Three semicircular
canals positioned at
right angles to each
other
- Contains sensory
cells and fluid
- Involved in
maintaining the
body balance
- Not involved in
hearing

39. Hearing Mechanism
1. The ear pinna receives and sends sound waves
through the auditory canal to the eardrum.
2. The vibration from the ear drum are amplified
by the ossicles and then sent to the cochlea
through the oval window.
3. The cochlea converts sound vibration to nerve
impulses and are transmitted through the
auditory nerve to the brain.

40. The pinna catches
sound waves and
directs the sound waves
to the ear
The ear drum vibrates
when the sound waves
strike it
Vibrations amplified
and transferred by the
ossicles to the
membrane of the oval
window
Vibrations of the
membrane of the oval
window causes the fluid
in the cochlea to move
in waves and stimulate
receptors that produce
nerve impulses
Nerve impulses carried by
the auditory nerve to the
brain
Impulses interpreted by the
brain as sound
The
membrane
of the oval
window
vibrates
The Hearing Mechanism
Figure 4. How we hear

42. Nose
1. The nose is the sensory organ for smell.
2. Cells sensitive to smell are found in the
epithelium located high in the nasal cavity.
3. Chemicals in the air dissolve in the mucus
layers that coat the sensory cell of smell
(olfactory cell) and then stimulate it to
produce nerve impulses.

43. Nose Structure

47. Chemicals in
the air
Air enters nasal
cavity through
nostrils
Chemicals
dissolve in the
mucus layer
Receptors
stimulated by the
chemicals
Receptors
(olfactory nerve)
send messages to
the brain
Brain interprets the
messages as a
specific smell
The detection of Smell

48. Tongue
1. The tongue is the sensory organ for taste.
2. Chemicals in food dissolve in saliva and are
absorbed into the taste buds through the
pores and stimulate the taste receptors in
them to produce nerve impulses.
3. There are five tastes: sweet, salty, sour , bitter
and umami
4. Tiny bumps found on the tongue called taste
buds contain the receptors that detect
different tastes.

49. 5. Different areas of the tongue detect different
tastes.
6. The saliva in the mouth has two functions:
◦ Dissolve substances so that they can be
detected by the taste receptors.
◦ Make chewed food easier to swallow.

52. Umami

54. Chemicals in
food
released by
chewing
Chemicals
dissolved by
saliva
Taste receptors
stimulated by
chemicals in saliva
Taste receptors
send messages to
the brain
Brain interprets
the messages as
a specific taste
Detected of Taste

55. Skin
1. The skin has five receptors that are
sensitive to heat, cold, pressure, touch and
pain stimuli respectively.
2. The thinner the epidermis or the more
receptors found on the skin, the more
sensitive is that part of the skin.

56. The Structure of Human Skin
Three layers:
Epidermis Dermis
Hypodermis
(fat layer)

58. Epidermis
- Outer layer of the skin divided into three layers.
- Outermost layer is made up of dead cells.
- Tough and water-resistant .
- It also protects the sensitive cells under it
- Prevents the entry of germs into the body.

59. Dermis
- Inner layer of skin.
- Blood vessels, glands and receptors are found
-The glands ~sweat glands and sebaceous glands.
- Receptors~ touch receptors, pain receptors,
heat receptors, cold receptors and
pressure receptors.
Hypodermis (fat layer)
- is a layer directly below the dermis and
serves to connect the skin to the fibrous tissue
of the bones and muscles.

61. Receptors in the Skin
1. Cold receptors
sensitive to cold.
2. Heat receptors
sensitive to heat.
3. Pain
receptors
- nearest to
the
epidermis.
- nerves
endings.
4. Touch receptors
- sensitive to slight
pressure
-found more
abundantly
in certain parts
(fingertips).
5. Pressure
receptors
- sensitive to
pressure.
- located the
furthest from
the
epidermis.
- stimulated
when any
object presses
hard against
the skin.

62. Sensitivity of the Skin at Different Parts
of the Body
1. The skin on different parts of the body has
different sensitivity.
2. The skin is more sensitive ~ fingertips, neck and
cheek.
3. These areas have more touch receptors or a
thinner epidermis.
4. Some areas of the body are less sensitive than
others.

63. 5. Example: the skin on the back, arms and legs have
fewer nerves endings.
6. The elbows, knees and soles of the feet are not
very sensitive because they have thicker
epidermis.

64. Limitation of the Sense of Sight
1. The limitation of the sense of sight is the ability
limit the eye to see the object.
2. The optical illusion occurs when our brain
cannot interpret accurately what is actually
seen by the eye.
3. The blind spot does not have any
photoreceptor and we cannot see an object if
its image is formed on it.

65. Optical Illusion

72. Eye Defects
In normal vision, light is focused accurately on
the retina to form an image on the retina. This
will produce a clear and sharp image on the
retina.
Defects of vision:
(a) short sightedness.
(b) long sightedness.
(c) astigmatism.

73. A short-sighted person
1. Can see nearby objects clearly but distant
objects appear blur due to the image of the
object which falls in front of the retina.
2. The eyeballs are too long and the eye lenses
are too thick. This is because the ciliary
muscles are too weak to make the eye lens
thinner.
3. Can be corrected by wearing concave lenses.

74. Vision
defect
Short sightedness Long sightedness
causes Eyeball is too long
or lens is too thick
Eyeball is too short
or lens is too thin
Symptom Can see near
objects clearly but
distant objects
appear blur
Near objects appear
blur but can see
distant objects
clearly

75. Vision
defect
Short sightedness Long sightedness
Method of
correction
Use a concave lens
to diverge (spread
out) light rays just
before they enter
the eyes so that
image if formed
further inside and
exactly on the
retina.
Use a convex lens to
converge light rays
(make them come
closer together) just
before they enter the
eyes so that image is
formed at a shorter
distance and exactly
on the retina.

76. Short sightedness Long sightedness
Light focus in front of retina Light focus behind the
retina

77. Short sightedness

78. Correction

79. Long sightedness

80. Correction

81. Astigmatism
1. Caused by irregular curvature of the cornea
or the lens.
2. The light rays from an object is split and
focused at different points in the eye.
3. Some light may be focused on the retina,
some light will be focused in front or behind
the retina.
4. Causes blurred vision for both near and
distant objects
5. Can be corrected with cylindrical lenses

84. Limitation of Sight and Hearing

85. Limitation of Sight
Limitation of sight Devices
Cannot see through
objects that are not
transparent
X-ray machine,
ultrasound scanning
device
Cannot see objects that
are very far away
Binoculars, telescope
Cannot see objects that
are very tiny
Magnifying glass,
microscope
Cannot see around
corners
Periscope

86. Ultrasound machine

87. Telescope

88. Microscope

89. X-ray machine

90. Limitation of Hearing
1. Defect of hearing occurs when the sense of
hearing of a person does not function
perfectly.
2. Defects of hearing are usually caused by
damage to the ear due to
 Infection by microorganisms
 Injury
 Aging process
 Continual loud sounds

91. Damage to the ear and ways to correct
them
Damage Ways to correct them
Ear canal clogged
by foreign object
Remove the foreign object
clogging the ear canal
Ruptured eardrum Repair the ruptured ear drum
using surgery
Damage ossicles Repair the damaged ossicles
using medicine or surgery
Damage cochlea Replace damaged cochlea with
cochlea implant
Auditory nerve No effective treatment yet

92. Stethoscope
Device to correct the hearing

93. Hearing aid

94. Microphone

95. Loud speaker

96. The Response in Plants
1. The response of plants to stimuli is called
tropism.
2. The tropic movements such as phototropism
are important to help the plants to obtain the
basic needs like light, water and minerals.

97. Stimuli and Responses in Plants
Responses of Plants to Stimuli
Plants respond by either positive tropism or
negative tropism.
Positive tropism- the growth of the part of the
plant towards the direction of the stimulus.
Negative tropism- the growth of the part of the
plant away from the direction of the stimulus.
Nastic movement – not dependent on the
direction of the stimulus.

98. Response Characteristic
Phototropism The movement or growth of
plants in response to light
• Shoots growth towards light
• Roots grow away from light
Geotropism The movement or growth of
plants in response to gravity
• Roots grow downwards in the
direction of gravity
• Shoots grow in the opposite
direction of gravity

99. Response Characteristic
Hydrotropism The movement or growth of
plants in response to water
• Roots grow downwards in the
direction of water
Thigmotropism • The movement of plants in
response to touch stimulus
• Example: bitter gourd have
tendrils that curl around
objects that touch them so
that they can grow toward the
sunlight

105. Thigmotropism
◦ Response towards touch or contact with a solid
object.
◦ The stems and tendrils of climbing plants grow
and twine around a support when they come into
contact with the support.
◦ Stems and tendrils- positive thigmotropism.

106. use twinning stem
Use tendril

107. Nastic Movement
The response of a plant part to a stimulus in
which the movement of the plant part is
independent of the direction of the stimulus.
Nastic movement takes place more quickly than
a tropism.
One type- seismonasty ~ Mimosa pudica and
Venus flytrap.

108. Leaves of Mimosa pudica close immediately when
they are touched. The leaves will open again after a
while. Thus, these leaves show seismonastic
response.
Venus flytrap- closes and trap insects that land on
it.
Photonastiy - nastic movement involved in growth.
Some flowers open in bright light and close in dim
light or vice versa.

112. Visions in Humans and Animals
1. Stereoscopic vision is the vision whereby both the
eye located in front of the head.
2. The production of three-dimensional images
enables humans and predators to estimate
distance accurately.
3. Monocular vision is the vision whereby both of
the eye are located at the side of the head.
4. The production of two-dimensional images
makes the preys difficult to estimate distance
accurately.
5. However, the field of monocular vision is wider.

113. Visions in Humans and Animals

115. Characteristics Monocular Vision Stereoscopic
Vision
Examples of
animals
Prey animals:
rabbit, deer and
rat
Human and
predator animals:
owl, tiger
Size of the field
of vision
At the side of the
head
In front of the
head
Size of the field
of vision
Wide Narrow
Overlap in fields
of vision
No overlap Has overlap
Type of vision Two dimensional
image
Three
dimensional
image

116. Characteristics Monocular Vision Stereoscopic
Vision
Estimation of
distance
Unable to
accurately
estimate
distances, size
and depth of
objects
Able to
accurately
estimate
distances, size
and depth of
objects
Effect on animal Wide field of
vision helps prey
animals to detect
enemies
Ability to
estimate distance
accurately helps
predator animals
to hunt

117. Sensory Organs Ensure the Survival
of Animals on Earth
Bats have poor eyesight but they have a
very sensitive sense of hearing

118. Grasshoppers have fine hairs all over their body
to detect air movements.

119. Leopards have very good stereoscopic vision to
help them hunt their prey.

120. The feelers on catfishes sense food

121. Chameleons can move their eyeballs
Independently to see two different
directions at the same time.

122. The earthworms’ body are covered with
chemoreceptors to taste and detect chemical
substances in the soil.