PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
CHAPTER 14
COORDINATION
AND
RESPONSE
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
How does an organism respond to a stimulus?
A stimulus (plural: stimuli) is any change in the environment that causes an organism to react.
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Plants grow towards the light
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Euglena swims toward the light
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Cockroaches move away from light
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What is the role of the nervous system in humans?
The nervous system regulates bodily functions and activities, and coordinates the body’s responses to stimuli.
It consists of the brain, spinal cord, spinal nerves and sense organs. The sense organs
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help the body to adjust rapidly to any changes in the environment;
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enable the various body parts to coordinate with one another quickly.
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What are the components of the human nervous system?
The human nervous system is made up of
• the central nervous system (CNS);
• the peripheral nervous system (PNS).
General arrangement of the brain, spinal cord and nerves in humans
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What are the components of the human nervous system?
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Sense organs receive stimuli, and are also called receptors. They transmit nerve impulses to the CNS.
Nerve impulses are carried or transmitted by nerves. Nerve impulses are transmitted within a fraction of a second.
How impulses are transmitted by nerves
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What are neurones?
Relation between a sensory neurone, a relay neurone and a motor neurone
(The arrows indicate the direction of travel of nerve impulses.)
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What is a synapse?
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Synapse is the tiny junction between two neurones.
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Nerve impulses cannot directly cross from one neurone to another neurone due to the synapse.
Nerve impulses are transmitted across a synapse by chemical released by the neurone.
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
How is an impulse transmitted across a synapse?
The swelling at the end of the sensory neurone is called the synaptic knob. The space between two neurones
is the synaptic gap.
Transmission of an impulse across a synapse
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
How does the nervous system process information?
Sensations
• When the receptors under the skin are stimulated, nerve impulses are produced and transmitted to the
brain. The brain interprets these impulses, and give us a sensation of the stimulus.
Voluntary actions
• A voluntary action is a deliberate action, which means it happens under conscious control.
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What is a reflex action?
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(a)
A reflex action is an immediate response to a specific stimulus without conscious control. Reflex actions are
rapid and automatic, and are examples of involuntary actions.
The brain and the spinal cord are reflex centres. Reflex actions are classified as follows:
Cranial reflexes (controlled by the
brain involuntarily)
(b)
Spinal reflexes (controlled by the
spinal cord)
Pupil reflex is an example of cranial reflex.
Knee jerk reaction is an example of spinal reflex.
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What happens when your hand touches a hot object?
Sequence of events in a reflex action
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What is a reflex arc?
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A reflex arc is the shortest pathway for nerve impulses to travel from receptor to effector in a reflex action.
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The nervous pathway in a reflex arc:
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
How is a reflex action different from a voluntary action?
Differences between a reflex action and a voluntary action
Reflex action
Voluntary action
It happens automatically, not under conscious control.
It happens under conscious control.
It is immediate or quick.
It is slower than a reflex action.
A specific stimulus is required.
A specific stimulus is not required.
Impulses are produced by the receptor.
Impulses are produced in the brain.
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
Let’s Practise 14.1 and 14.2
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Using an example, distinguish between a stimulus and a response.
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What are the components of the nervous system?
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State the names of three kinds of neurones.
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What are sense organs?
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Receptors or sense organs receive stimuli from the environment and inform your body of changes.
Sense organs and their stimuli
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What is the structure of the human eye?
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The eye is the sense organ responsible for sight.
Parts at the front of the eye
Rectus muscles of the eyeball
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What is the internal structure of the eye?
Vertical section of the eye
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What are photoreceptors?
Section through the retina
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What are photoreceptors?
Cones
Rods
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Cones enable us to see colours in bright light. There
are three different kinds of cone – red, blue and green.
Rods enable us to see in dim light, but only in
black and white. Rods are sensitive to dim light.
Problems with pigments in the cones cause colour blindness.
Left: normal vision; Right: Red-green colour blindness
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What is the pupil reflex?
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The right amount of light must enter the eye in order for a person to see clearly. The size of the pupil determines
how much light enters the eye.
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The pupil reflex is a reflex action.
The pupil reflex
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
How does the iris control the amount of light entering the eye?
Action of the iris
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
How does the iris control the amount of light entering the eye?
Reflex arc in the pupil reflex
The pathway of impulses in the pupil reflex:
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
Let’s Investigate 14A
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When the hand is suddenly removed from the eye, there is a change in light intensity that the eye is
exposed to.
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The pupil changes in size in response to the change in light intensity.
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
Let’s Practise 14.3
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Outline the processes that take place in the pupil reflex when you leave a brightly lit room and enter a dark
room. Name the receptor and the effector involved.
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
How is image formed on the retina of the eye?
Formation of an image on the retina
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What is focusing or accommodation?
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Focusing or accommodation is the adjustment of the lens of the eye, so that clear images of objects at
different distances are formed on the retina.
Focusing on a distant object (estimated at 7 m or more):
Focusing on a distant object
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
What is focusing or accommodation?
Focusing on a near object:
Focusing on a near object
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PowerPoint Slides for Cambridge IGCSETM
Biology Student's Book
Let’s Practise 14.4
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A person enters a cinema from a bright place. At first, he cannot even see the passageway. He waits
for some time before he can see. Explain what happens in his eyes that enables him to see in dim
light.
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