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senses and coordination

Grade 8 Biology
Senses and coordination
 The senses
 The nervous system
 Neuron
*please note that you should refer to your text book for information.
The senses
Specialized neurons (nerve cells) in your body called sensory receptors enable you to taste, smell, hear, see, and
touch, and to detect motion and temperature.
Receptors are structures that detect stimuli and the nervous system then responds accordingly.
A stimulus: any change in the internal or external environment that causes an organism to react (respond).
Smell and taste
The senses of taste and smell are stimulated by chemicals and often function together. Specialized receptors
located high in the nose respond to chemicals in the air and send the information to the brain.
Taste buds are areas of specialized chemical receptors on the tongue that detect the tastes of sweet, sour, salty,
and bitter.
Signals from these receptors work together to create a combined effect in the brain. Try eating while holding your
nose. You will find that your food loses much of its flavor.
What we call “flavor” is mostly smell of the food rather than taste. This is why you may not taste food while you
have a blocked nose.
Sight or vision
Grade 8 Biology
Light first enters the eye through a transparent, layer of cells called the cornea. The cornea helps to focus the light
through an opening called the pupil. The size of the pupil is regulated by muscles in the iris—the colored part of
the eye. Behind the iris is the lens, which inverts (turns upside down) the image and projects it onto the retina.
The image travels through the vitreous humor, which is a colorless, gelatin-like liquid between the lens and the
The retina contains numerous receptor cells called rods and cones. Rods are light-sensitive cells that are excited
by low levels of light. Cones function in bright light and provide information about color to the brain. These
receptors send nerve impulses to the brain via the neurons in the optic nerve. The brain then interprets the
specific combination of signals received from the retina and forms a visual image.
Hearing and balance
Vibrations called sound waves cause particles in the air to vibrate. Sound waves enter the auditory, or ear canal
and cause a membrane, called the eardrum, at the end of the ear canal to vibrate. These vibrations travel through
three bones in the middle ear—the malleus (also called the hammer), the incus (anvil), and stapes (stirrup). As the
stapes vibrates the fluid inside the cochlea to move like a wave against the hair cells. The hairs cells respond by
generating nerve impulses in the auditory nerve and transmitting them to the brain.
The inner ear also contains organs for balance, including three semicircular canals. Semicircular canals transmit
information about body position and balance to the brain. The brain then is able to determine your position and
whether your body is still or in motion.
Grade 8 Biology
(*you do not need to know the names of the receptors)
Many types of sensory receptors that respond to temperature, pressure, and pain are found in the epidermis and
dermis layers of the skin.
Distribution of receptors is not uniform in all areas of the body. The tips of the fingers have many receptors that
detect light touch. The soles of the feet have many receptors that respond to heavy pressure.
Pain receptors are very simple, consisting of free nerve endings that are found in all tissues of the body except the
The nervous system
The nervous system consists of two major divisions: the central nervous system (CNS) and the peripheral nervous
system (PNS). The brain and the spinal cord make up the central nervous system. The peripheral nervous system
consists of the sensory neurons and motor neurons that carry information to and from the CNS.
The central nervous system
The brain
Because the brain maintains homeostasis and is involved with almost all of the body’s activities, it is sometimes
called the control center of the body.
 The cerebrum is the largest part of the brain and is divided into two halves called hemispheres. The cerebrum
carries out thought processes involved with learning, memory, language, speech, voluntary body movements,
and sensory perception.
 The cerebellum controls balance, posture, and coordination, and is located at the back of the brain. The
cerebellum controls the smooth and coordinated movement of skeletal muscles and also is involved with
motor skills, such as playing the piano or riding a bike.
 The brain stem connects the brain to the spinal cord and is made up of two regions called the medulla
oblongata and the pons. The medulla oblongata relays signals between the brain and the spinal cord. It also
helps control breathing rate, heart rate, and blood pressure.
Grade 8 Biology
 The hypothalamus regulates body temperature, thirst, appetite, and water balance.
The spinal cord
The spinal cord is a nerve column that extends from the brain to the lower back. It is protected by the vertebrae
(the backbone). Spinal nerves extend from the spinal cord to parts of the body and connect them to the central
nervous system. Reflexes are processed in the spinal cord.
A neuron or neurone is a nerve cell. There are 3 main type you will study in grade 8.
Neurones are a particular type of cell that carry information messages or signals to and from the brain and the
rest of the body.
A nerve impulse is an electrical charge traveling the length of a neuron. An impulse results from a stimulus, such
as a touch or perhaps a loud bang that causes you to jump.
A small gap exists between the axon of one neuron and the dendrite of another neuron. This gap is called a
A motor neuron carries impulses from the CNS to the parts of the body that react (effectors)
Grade 8 Biology
A sensory neuron transmits impulses from the receptors to the CNS.
Activity 1
Identify 5 different sensory organs in a human and what each is able to detect.
Activity 2
Draw the structure of the eye
List out the structures and write down the function of each part.
(you may recall the knowledge from grade 6 and also refer to the text book)
Activity 3
Compare and write the differences between the following.
a. rods —cones
b. cochlea—semicircular canals
c. retina—taste buds
Activity 4
Questions from the textbook.
Page number
Question number
1 to 8
Activity 5
Make a poster each for the following topics (one A4 sheet size per poster only)
a. One of the sense organs of your choice
b. The brain
c. Neurons