Health and Physical Education
Unit: The Human Body
The Nervous System
 Importance
 Functions
 Structure and Workings
The Nervous system can be considered the most important system in the human body because it controls
everything that we do. It gathers information from the five senses, analyses it and initiates appropriate
responses. These responses can help you to deal with changes and safeguard your well-being.
Functions
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Controls all the functions of the human body
Collects information from inside and outside of the body
Relays messages to the brain and spinal cord
Processes the information to determine the best response
Sends messages to the muscles, glands and organs so that they can respond accordingly.
Structure and workings
The Nervous system is made up of the brain, the spinal cord and the nerves.
The brain is protected by the skull or cranium, weighs about 1.4.kgs and has two main parts:
 The cerebrum is the largest part of the brain and controls thought, speech and movement. The left
hemisphere of the brain controls the right hand side of the body and influences logical thought and
reasoning. The right hemisphere of the brain controls the left hand side of the body and influences
imagination and creativity.
 The cerebellum or “little brain” is located at the back and underneath the cerebrum. It is responsible
for maintaining balance and coordinating body movements.
The spinal cord emerges from the base of the brain, passing through a hole about 1cm wide in the base of the
skull. It extends all the way down the spine through gaps in the centre of the vertebrae and is made up of
spinal nerves which are able to transmit messages to and from the brain.
Nerve cells or neurons carry information to and from the spinal cord and brain thus allowing communication
between different parts of the body. Neuron fibres are very long, some of which exceeds 60 cms in length.
Some nerves relay information to the brain so that you can see, hear, smell, taste and touch while others
control your body movements. A neuron consists of 3 main parts:
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The cell body which contains the nucleus
Dendrites which are projections from the cell body and which receive messages in the form of nerve
impulses
The axon which is a long fibre from the cell body that ends at the axon terminals and transmit the
impulse away from the nucleus.
How do neurons relay messages?
Messages are in the form of electronic signals called impulses. A neuron relays impulses in only one
direction. Neurons are not actually connected to each other but are separated by a microscopic gap called
the synaptic gap. Messages are relayed from one neuron to another at the synapse, where the axon
terminals of one neuron are very close to the dendrites of another. Impulses are able to cross the gap via
the release of a chemical substance called acetylcholine which allows an impulse to travel through the
nervous system at great speed.
3
2
1
SYNAPSE
4
5
ACETYLCHOLINE
Direction of impulse
1
::
2
Electrical message travels along axon
3
Chemical messenger (acetylcholine) carries message across synapse
4
Chemical message changed back to electrical message
5
Electrical message continues through the next neuron
Chemical message released at the axon terminal
Types of neurons
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Sensory neurons: These receive messages from the receptor organs (muscles, skin and sensory organs)
and transmit impulses to the brain where the information is processed.
Interneurons: connect sensory and motor neurons and are only found in the Central Nervous System
Motor neurons: These transmit impulses from the brain to the effector organs (muscles, glands) where
the instructions are carried out for e.g. muscles contract and glands secrete hormones.
Interneuron
Sensory neuron
Receptor organs
Brain
Spinal Cord
(muscles, skin and sensory organs)
Motor neuron
Effector organs
(muscles, glands)
Divisions of the Nervous System
Nervous System
Central Nervous System
Peripheral Nervous System
Sensory Nervous System
Voluntary Nervous System
Motor Nervous System
Involuntary Nervous System
The Nervous System is divided into the Central Nervous System and the Peripheral Nervous System.
The Central Nervous System: This is the control centre of the body which consists of the brain and the spinal
cord. The spinal cord relays messages from the body to the brain. The brain analyses and interprets the
messages. It then responds by sending messages through the spinal cord to the rest of the body.
The Peripheral Nervous System: This consists of all of the nerves in the body that are not found in the Central
Nervous System. Its main function is to relay messages to and from the Central Nervous System. There are two
main groups of peripheral nerves:
 The cranial nerves: The 12 pairs of cranial nerves carry messages about pain, touch, temperature,
pressure, smell, vision and hearing to the brain. They also transmit messages from the brain to move
voluntary and involuntary muscles to make organs such as the heart and lungs work and also to
activate glands.
 The spinal nerves: the 31 pairs of spinal nerves branching out from the spinal cord carry messages to
and from many areas of the body including skeletal muscles.
The Peripheral Nervous System can be divided into the:
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Sensory Nervous System: comprises of neurons that relay messages from the sensory organs (eyes,
ears, nose, tongue, skin) to the Central Nervous System
Motor Nervous System: Motor neurons relay messages from the Central Nervous System to the
muscles or glands. This system is divided into the:
 Voluntary Nervous System: This regulates activities that are under our conscious control such
as a skeletal muscle contraction
 Involuntary Nervous System: controls the functions of the body that are automatic or not
under our conscious control for e.g. the workings of organs such as the heart, stomach, liver
and kidneys.
Reflexes
These are involuntary responses to stimuli. In a reflex action or response, a nerve impulse is sent from the
receptor organ( through the sensory neuron) to the spinal cord. Instead of travelling up to the brain, an
interneuron in the spinal cord passes the impulse to the appropriate effector organ to take action (through the
motor neuron). Examples of reflexes are the knee jerk reflex and the withdrawal reflex.
The withdrawal reflex
1. A sensory receptor in the hand detects a hot object
2. The sensory neuron carries the information in the form of a nerve impulse to the spinal cord
3. An interneuron in the spinal cord passes the impulse to a motor neuron
4. The motor neuron carries a nerve impulse to a muscle
5. The muscle contracts and the hand pulls away from the hot object.
Sensory receptor in hand
1
Sensory Neuron
Spinal Cord
Motor Neuron
Muscle
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3
4
5