Tuesday, 25 October 2011


1. The eyes are the sensory organs of sight. They are sensitive to light.

2. The eyelids and eyelashes keep dust and dirt out of the eyes.

3. The eye brows help to prevent sweat from running into the eyes.

4. The wall of eyeball is made up of three layers.
(a) Sclere – tough, outer layer
(b) Choroid – black, middle layer
(c) Retina – sensitive to light , innermost layer
The structure of the human eye and the function of the different parts Part
White, fibrous, outer layer that is opaque and tough
Protects and maintains the shape of the eyeball
Middle layer that contains black pigment and many blood vessels
- The blood vessels supply nutrients and oxygen to the eye.
- The black pigment absorbs light and therefore prevents reflection of light inside the eye which would make an image less sharp.
The innermost layer of the eyeball. It contains many photoreceptors known as cones and rods
- Detect light and produces nerve impulses
- The cones detect colours in bright light, enabling us to have colour vision
-The rods enables us to see in dim light . They detect shades of grey in dim light.
The curved and transparent layer of the sclera at the front of the eye.
Its curved surface helps to refract (bend) light onto the retina.
The thin , transparent layer of membrane in front of the cornea
Protects the cornea
The coloured part of the eye that is continous with the choroids. It is made up of muscles
Controls the size of the pupil and thus the amount of light entering the eye.
The hole in the centre of the iris
Controls the amount of light entering the eye.
A transparent bioconvex and leastic disc.
-It refract and focuses an image onto the retina.
-The thickness of the lens is changed to focus near and distant objects.
Ciliary body
Make up of strong muscle
Contract and relaxes to change the thickness of the lens
Suspensory ligaments
Strong fibres that connect the lens to the ciliary body
Hold the lens in its position
Aqueous humour
A watery transparent liquid that fills the space between the cornea and the lens
- Helps to focus the image onto the retina
-Helps to maintain the shape of the eyeball
-The cornea , the conjunctiva and the lens obtain food and oxygen from the blood vessels in the choroids layer by diffusion through this liquid.
       5. How wee see

           i) When we look at an object, light rays from the object enter each eye through the pupil. The intensity  
              of  the light influences the size of the pupil.
          a) When it is very bright , the size of the pupil decreases to let in less light.
          b) When it is dim, the size of the pupil increases to let in more light.

           ii. As the light rays pass through the cornea, the aqueous humour , the lens and vitreous humour, the   
              rays are refracted to form an image on the retina.

         iii. The image is upside down and smaller than the actual object.

         iv. The brain interprets the impulses and we can see the object the right way up. The                      
              interpretation also allows us to know the shape, the colour , the size and the distance of the object.

          v. Eyes have the ability to focus on objects a different distance.


The human ear has three main parts – the outer ear, the middle ear and the inner ear.

Outer ear
a) Pinna (ear flap)
b) Ear cannal
Shape like funnel. Made of cartilage and skin.
A 2.5 cm long, narrow tube lined with hair.
Collects and directs sound waves into the ear canal
Directs sound waves to the eardrum
Middle ear
a) Eardrum
b) Ossicles
c) Oval window
d) Eustachian tube
A thin , stretched membrane at the end of the ear canal
Three small bones
A thin , small membrane at the end of the ossicles
A narrow tube that connects the middle ear to the back of the throat.
Vibrates when sound waves hit it
Amplify vibrations and transfer them from the eardrum to the oval window.
Transfers vibrations from the ossicles to the cochlea
Equalises the air pressure on both sides of the eardrum, thus preventing itu from damage.
Inner ear
a) Cochlea
b) Auditory nerve
c) Semicircular canals
A called tube filled with fluid and lined with tiny hair-like sensory cells.
Nerve fibres that connect the ear to the brain
Three semicircular cannals situated at right angles to each other. Contain fluid and sensory cells.
Detects vibrations and converts them into nerve impulses
Carries vibrations and converts them into nerve impulses
Detect the position and movement of the head to help us keep our balance (Note: They are not involved in the hearing mechanism.

 How sounds are detected by the ear :

a)    The pinna collects sound waves and directs them along the ear canal to the eardrum.

b)   When the sound waves hit the eardrum, it vibrates at the same frequency as the sound waves. The vibrations are then transferred to the ossicles.

c)    The ossicles amplify for the vibrations about 20 times before transferring them to the oval window.

d)    The vibration of the oval windows causes the fluid in the cochlea to move in the form of waves . The   
       movement of the fluid stimulates the sensory cells or receptors in the cochlea. Nerve impulses are produced.

e)    The auditory nerve carries the impulses to the brain.

f)    The brain interprets the impulses as sounds.


1. The tongue is the sensory organ related to the sense of taste.

2. It is sensitive to four basic tastes – sweet, salty, sour and bitter.

3. The surface of the tongue is covered with little bumps. These bumps contains tiny structures called taste 
    buds. Each taste bud contains many taste receptors which are sensitive to chemicals in food.
4. There are four types of taste buds , one for each basic taste. Different areas of the tongue are sensitive to
    different tastes. Refer to the diagram;
5. How taste detected ;
a) When we chew our food, the chemicals of the food dissolve in the salive.
b) The dissolved chemicals stimulate the taste receptors in the taste buds to
    produce nerve impulses.
c) Messages are sent to the brain to be interpreted. This enables us to identify
    the taste of the food.

6. The nasal cavity and the mouth cavity are connected. This allows the taste and
    smell of food in the mouth to be detected at the same time.


1. Smells are actually chemicals present in the air.

2. The nose can detect these chemicals.

3. The smell receptors or sensory cells are located at the roof of the nasal cavity. This position allows the 
    smell receptor to have the maximum exposure to the air current.

4. How smells are detected;
a) When air is breathed in, the air current passes through the nasal cavity. The
    chemicals present in the air dissolve in the mucous lining and stimulate the
    smell receptor.
b) These receptors send out nerve impulses to the brain. The brain interprets
    the message and identify the smells.


1. The skin is a sensory organ that gives us our sense of touch. It can detect stimuli such
as touch, pain, pressure, heat and cold.

2. The skin has five different receptor;
2.1 The touch receptor – sensitive to slight pressure
2.2 The pain receptor – sensitive to pain
2.3 The heat receptor – sensitive to heat
2.4 The cold receptor – sensitive to cold
2.5 The pressure receptor – sensitive to heavy pressure

3. The sensitivity of skin depend on;
a) How close together the receptor are
    Example : the lips and fingertips are very sensitive to touch due to the large
    concentration of touch receptor.
b) How deep the receptor are
    Example : The receptor with thick epidermis are less sensitive. The epidermis on the neck is thinner than    
    that on the knee. So neck more sensitive than the skin at the knee.
c) The sense of touch is very important to the blind because it helps them to identify
    object and feel their way around and to reads book in Braille .



Sunday, 16 October 2011




Sensory organ, senses and stimuli

Stimuli – changes that occur in the surroundings.
Example : sound, chemical sunstances and light intensity

Sensory organs – Organs that detect stimuli
Each sensory organ is sensitive to only certain types of stimuli. For example, the eyes are only sensitive to light but not to sound.

Sense – The ability of an organism to detect a stimulus

Human beings have five sensory organs
·         Senses of smell
·         Senses of touch
·         Senses of taste
·         Senses of hearing
·         Senses of sight

 Relation between sensory organ, stimuli, and senses in human beings

Relation between sensory organ, stimuli, and senses in human beings Sensory organ
Stimulus /stimuli detected
Sense of light
Sense of hearing
Sense of smell
Touch, pressure, pain, heat and cold
Sense of touch
Taste (chemicals in Food)
Sense of taste