Science Chapter 1 F2

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Chapter 1 The world through our senses
yschow@smkbpj(a) 1

1.1 Sensory Organs and Their Functions • A sensory organ is an organ that enables the body to respond to stimuli. • A stimulus is a change in the surroundings that can be detected by the sensory organs. • The five sensory organs are eye, ear, nose, tongue and skin.



• The ability of the sensory organs to detect stimuli is called senses. Sensory organ Eye Ear Nose Tongue Skin Sense Sight Hearing Smell Taste Touch Stimuli Light Sound Chemical in air Chemical in foods Pressure, heat, cold, touch



Pathway from stimulus to response.
Stimulus Response

Receptor in sensory organ

Effectors (muscle)

Sensory nerves


Motor nerves

1.2 Sense of Touch



1.2 Sense of Touch
• The skin is a sensory organ which responds to the sense of touch. • There are five types of receptors in the skin. Receptor Cold receptors Heat receptors Pain receptors Pressure receptors Touch receptors yschow@smkbpj(a)

Stimuli (sensitive to) Cold substances Heat Pain Large pressure Small pressure (touch ) 6

• The sensitivity of the skin depends on (a) the thickness of epidermis (b) the number of receptors present • Fingertip and neck are more sensitive . • Elbow , knee and back side are not so sensitive to touch.

Braille letters are specifically designed symbols for stimulating the fingertips, which allows blind people to read yschow@smkbpj(a) 7

1.3 Sense of Smell



1.3 Sense of Smell
• Mucus is produced to help keep the receptors moist. • When you inhale, the chemicals from the food enter the nasal cavity and dissolve in the mucus. • The smell receptors are stimulated. • The impulses from the smell receptors are sent to the brain by the olfactory nerves for interpretation. yschow@smkbpj(a) 9

• When we catch a cold, too much mucus is produced and this makes the receptors less sensitive.


Cold = flu (selsema 感冒 感冒)


1.4 Sense of Taste
yschow@smkbpj(a) 11

1.4 Sense of Taste • The tongue contains many taste buds. The taste buds are the taste receptors. • There are four types of taste receptors. • These receptors are sensitive to sweet, salty, sweet salty sour and bitter tastes. yschow@smkbpj(a) 12

How do you taste?
• The taste buds are able to detect the taste of the food when the food is dissolved in saliva. • The taste receptors will be stimulated and impulses are produced. • The impulses are sent to the brain for interpretation. yschow@smkbpj(a) 13

1.5 Sense of Hearing



1.5 Sense of Hearing • The ear is a sensory organ for hearing and balancing. • The car can be divided into three sections: i) outer ear, ii) middle ear and iii) inner ear. yschow@smkbpj(a) 15

How do we hear ?
1. The pinna collects sound waves and directs them into the auditory canal and to the eardrum. 2. The eardrum begins to vibrate and the vibrations are transferred to the ossicles



3. The ossicles magnify the vibrations and pass them to the oval window. 4. The oval window transmits the vibrations to the cochlea.



5. The cochlea converts the vibrations into impulses. 6. The impulses are sent by the auditory nerves to the brain for interpretation.



1.6 Sense of Sight



How do we see ?
• When you look at an object, the light rays from the object enter the eye. • The light rays are refracted by cornea, aqueous humour, lens and vitreous humour. • An image is formed on the retina. Impulses are produced and are sent to the brain by the optic nerves..



1.7 Light and Sight
• Light is a form of energy and light travels in straight lines. This causes the formation of eclipses. • Light cannot travel through opaque objects. Thus, shadows are...
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