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Sensory organs are organs thatdetect stimuli.
Stimuli (singular: stimulus) arechanges that occur in thesurroundings. Eg: sound, chemical
substances and light intensities. The ability of an organism to detect
a stimulus is known as sense.
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Sense Sensoryorgan
Stimuli detected
Touch Skin Touch,pressure,pain,heat,cold
Taste Tongue Taste (chemicals in food)
Smell Nose Smell (chemicals in air)
Sight Eyes Light
Hearing Ears Sound
Five senses in humans
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When an organism receives a stimulus,it will react to it. How it react to a
stimulus is known as a response. In certain cases, responses prevent us
from harm which is important for our
survival.
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Stimulus Response
Ringing of school bell Walk towards the
classroomLight from camera
flashlightClosing of eyes
Heat from a boilingkettle Pulling away of handsfrom the kettle
Examples of responses
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Stimulus
Response
Receptorsin sensoryorgans
Nerves
NervesEffectors
Pathway from stimulus to
response in humans
Brain
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Our sensory organs have specialstructures called receptors. The nerves
are attached to them. When a stimulus is received by a sensory
organ, the receptors in the sensory organ
produce electrical messages known asnerve impulses, which sent along thenerves to the brain.
Then, the brain interprets the messagesand decides how the body should respond. Effectors are part which carry out the
response.
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Sensoryorgan
Brain
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When the girl touches the cat, thereceptors in her hand receives the
stimulus (touch). The receptorsproduce nerve impulses which sent
along to the brain and interpreted.The touch is felt as soft and warm.Then, the brain produces new
impulses and sent along the nerves ,cause the hand to caress the cat as aresponse.
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The skinThe skin is the sensory organ
that gives us the sense oftouch.
It can detect stimuli such astouch, pain, pressure, heat and
cold.
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Structure of human skin
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The skin consist of two main layers;epidermis and dermis. Below the skin
is a layer of fatty tissue. Epidermis: the outer layer of the skin,
made up of dead cells, waterproof,tough and act as a protective layer.
Dermis: inner layer of the skin,
consist of living cells, blood vessels,sweat glands and nerves.
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Receptors in human skin:
Touch
receptors
Sensitive to touch;many found in hairless
areas such as lips and fingertips
Painreceptors
Heatreceptors
Coldreceptors
Pressure
receptors
Sensitive to pain;lie in the epidermis,close to the surface
Sensitive to heat
Sensitive to cold
Sensitive to heavy pressure
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The sensitivity of the skin
Different parts of body have different
degrees of sensitivity. The sensitivity of the skins depends on:
a) how close together the receptors are;the larger the concentration ofreceptors, the more sensitive the skin
b) how deep the receptors are; thethicker the epidermis, the lesssensitive the skin is
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Examples:
- The lips and fingertips are verysensitive to touch due to the largenumber of touch receptors. They
have thin epidermis too.- The back of the body is less sensitive
as it has very few touch receptors.
- The knees and elbows are lesssensitive as they have a thick
epidermis.
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1. Which of the following sensory organs candetect p pressure, heat and pain?
A) Nose B) Skin C) Tongue D) Ears
2. Which of the following structures detectsstimulus from the surroundings?
A) Receptor B) Effector
C) Nerve D) Brain
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3. Which of the following areas is the mostsensitive to touch?
A) Elbow B) FingertipC) Back of body D) Palm of hand
4. The knee is less sensitive to touch becauseA) it is wrinkled
B) it is has no receptors
C) it has little blood vesselsD) it has a thick epidermis
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5. The lips are very sensitive to touchbecause
I. it has a thin epidermisII. it has a large number of receptors
III. it has many blood vessels
A) I and II only
B) I and III only
C) II and III onlyD) I, II and III
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6. The sequence below shows the pathway fromstimulus to response in humans.
J Nerves Brain K L
Which of the following represent J, K and L?
AB
C
