SCIENCE FORM 2

CHAPTER 1THE WORLD THROUGH OUR

SENSES

1.1 Sensory Organs

Sensory organs, stimuli and sensesa) Sensory organs : organs that detect

stimuli.

b) Stimuli : Any changes that occur in the surroundings which lead to respond.

c) Senses: detectors used to find out the stimuli.

sense sensory organs stimuli detected

touch skin touch, pressure, heat, cold, pain

taste tongue taste

smell nose smell

sight eyes light

hearing ears sound

d) HOTS Without sensory organs, we will not be able to know what is happening in our surroundings. So, we must take care of our sensory organs from injuries.

Responses to stimuli

• response: a reaction

• receptor: a structure that recognize a stimulus in the internal/ external environment of an organism.

• nerve impulses: electrical messages produced by receptor.

• effector: part of the body that carry out responses. Example: muscle and gland

Common pathway from the stimulus to response in humans

1.2 Sense of touch

The Skin - largest organ in the body.

The sensitivity of the skin

• Different parts have different degrees of sensitivity.

• Depends on:1.how close together the receptors are -

the higher the concentration of receptor, the more sensitive the skin is

2.how deep the receptor are - the thicker the epidermis, the deeper the receptors lie in the skin, the less sensitive the skin is.

Examples:

• The back of the body is less sensitive as it has very few touch reseptor.

• The knees and elbows are less sensitive as they have a thick epidermis.

• The lips and fingertips are very sensitive to touch due to the large number of touch receptor. They have thin epidermis too.

HOTS- APPLICATION

• Blind people use the high sensitivity of their fingertips to read in Braille. Raised dots are used to represent letters and numbers.

• Doctors give injections to their patients at the forearms or hips. One of the reasons for this is these areas are less sensitive to pain.

• The back of the hands has a lot of heat and cold receptors (temperature). Mother test the temperature of their babies' milk on the back of their hands.

1.3 Sense of smell

1.Two holes, called nostrils, open into a large hollow space called nasal cavity.

2.The nasal cavity is lined with mucus. Its roof has many smell receptors/ sensory cell.

3.Adaptation: The smell receptors are always covered

with mucus. Smells must dissolve in the mucus before they can be detected.

• Smell receptors are located at the roof of the nasal cavity. This position allows the receptors to have the maximum exposure to the air current.

Detecting smell.

• Smells are actually chemicals present in the air.

• Food, perfume .............air.

• When we breathe in, .........cavity.

• The ......cells.

• These ........smell.

• After smelling ......brain.

• Then we ....... smell it.

HOTS!!!!!

• When a person catches a cold, they cannot detect smell well. The nose produces too much mucus which prevents chemicals from stimulating the smell receptors.

• A dog's sense of smell is hundred times more sensitive than human. Therefore, dogs are being used to track criminals and to sniff out drugs or explosives.

1.4 Sense of Taste

• draw figure 1.8 pg 9

draw figure 1.9 pg 10

1.Tongue is sensitive to chemicals in food that give taste to food.

2.Can detect 4 tastes: _____, _____, _____ and _______.

3.The tongue is lined with taste buds which contain many taste receptor.

How tastes are detected?

• When we chew our ..................saliva.

• The dissolved.......................impulses.

• These are ................................tastes.

pg 10

The senses of taste and smell.

• Both senses are needed to detect the flavour of food/ drink accurately.

• The nasal cavity and mouth are connected. When we eat/ drink, chemicals from food move up to nasal cavity and stimulate the smell receptors in the nose as well.

• When we have cold, chemicals in the food cannot reach the smell receptors because excessive mucus or blocked nose. So, the food become less tasty.