Nervous System
Access Human Biology.
Clare Hargreaves-Norris
Introduction
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The nervous system is the main communication
system for the body and is responsible for
receiving and interpreting information between
the various parts of the body and the brain. The
three main sections of the nervous system are
called:
The central nervous system (CNS).
The peripheral nervous system (PNS).
The autonomic nervous system (ANS).
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Central Nervous System
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Structures of the central nervous
system
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The central nervous system comprises of
the:
Brain
Spinal cord
Cerebro-spinal fluid
Meninges
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Spinal cord
The spinal cord is a continuation of the medulla
oblongata and extends downwards through the
vertebral column, finishing on a level with the lumbar
vertebrae. It is protected by the vertebral column,
3 meninges and the cerebro-spinal fluid.
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The functions of the spinal cord are:
It conveys sensory impulses from one area of the
spine to another and from the skin and muscles of
the trunk and limbs to the brain.
It allows and co-ordinates spinal reflex actions,
which are a rapid response to a stimulus without any
conscious thought of the brain. This action protects
the body from danger before any harm is caused.
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The brain
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The brain is the most important part of the
nervous system.
It is the main communication centre.
The brain is protected by:
-The cranial bones of the skull.
-The cranial meninges (Three membranes that
cushion the brain and contain blood vessels).
-The subarachnoid space (Contains the cerebrospinal fluid, provides further cushioning and supplies
nutrients.
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Responsibilities of the brain
The brain is responsible for:
 Storing messages that it receives from all different parts
of the body via the sensory nerve endings.
 Transmitting messages via the motor nerves to all different
parts of the body to stimulate the required response.
 Co-ordinating the body’s movements.
 Controlling feeding.
 Controlling sleeping patterns,
 Controlling temperature regulation.
 Controlling the salt/water balance of the body.
 Storing information in the memory.
 Emotional and intellectual processes.
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Areas of the brain - cerebrum
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The cerebrum is the
largest region of the brain.
It is spilt into two
hemispheres
It has a surface called
grey matter.
It enables vision, touch,
taste, smell and hearing.
It also controls muscular
movements, emotions,
thought processes and
personality traits.
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Areas of the brain - diencephalo
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The diencephalon
comprises of the thalamus &
hypothalamus and makes up
part of the forebrain.
The thalamus relays sensory
impulses to the cerebrum.
The hypothalamus controls the
autonomic nervous systems.
The hypothalamus regulates:
- heart rate.
- body temperature.
- salt/water content,
- appetite.
- sleeping patterns.
Clare Hargreaves-Norris
Areas of the brain - cerebellum
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The cerebellum is
located in the
posterior aspect of
the brain.
It maintains muscle
tone, posture and
controls motor skills.
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Areas of the brain – brain stem
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The Pons and Medulla
oblongata are known as
the brain stem.
This is a continuation of
the spinal cord.
It controls what you have
no conscious control over
such as:
- blood pressure
- body temperature.
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Nerves
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Neurones, more commonly known as nerves, are made
from a collection of nerve fibres and all emerge
from the CNS. There are 3 types of nerves:
Motor (efferent) nerves - conveys impulses from
the brain, through the spinal cord to the muscles,
glands and smooth muscular tissue.
Sensory (afferent) nerves - conveys impulses from
the sensory nerve endings situated in various organs,
skin etc. to the brain & spinal cord.
Mixed nerves - are a combination of motor and
sensory nerve endings.
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Structure of a neurone
Each neurone has:
 A cell body – containing the
nucleus, mitochondria & other
organelles.
 Dendrites – several short
projections that receive
information from other cells
and transmit the message to
the cell body.
 Axon – one long projection that
conducts messages away from
the cell body. Axons have a
myelin sheath, which insulates
them to prevent against loss of
electrical impulses and
therefore increase the speed
at which the impulse is
conducted.
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Diagram of a neurone
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Transmission of nerve impulses
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Messages pass along the nerve fibres as electrical impulses,
which are the result of changes that are present inside and
outside the nerve membrane.
When an impulse reaches the end of the nerve fibre, a
chemical is released called a neurotransmitter substance.
The chemical passes across a tiny gap called a synapse and
generates an electrical impulse in the next neurone.
When the neurone reaches a muscle fibre, it is referred to as
a motor end plate. A similar reaction occurs at the motor end
plate, however a chemical transmitter substance is released
from the synaptic vesicle this time.
In simple terms, messages cross the synapse by electrical
impulses.
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Synapse at nerve muscle junction
known as motor end plate
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Passage of a nerve impulse
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A sensory receptor in the skin picks up a message from the outside world,
for example heat, cold, touch, pain or pressure.
The message, in the form of an electrical impulse, conveys from the
dendrites to the cell body and then along the axon.
The electrical impulse travels along the axon until it reaches the next
neuron, whereby a neurotransmitter substance is released to allow the
impulse to pass the small gap called a synapse.
Messages cross the synapse by an electrical impulse.
The impulse crosses the gap and onto the dendrite of the adjacent sensory
neuron, where it continues on its journey until it eventually reaches the
brain.
The brain perceives the sensation and sends a message back via motor
nerves; until the message reaches a neuro-muscular junction known as a
motor end plate.
The synaptic vesicle releases a chemical transmitter substance, allowing the
impulse to reach the muscle fibres.
The impulse tells the muscle fibres to contract or relax, thereby bringing
about a response.
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Reflex action
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Sometimes the impulse does not pass to the
brain.
It is dealt with by the spinal cord.
This is an involuntary response to a stimulus whereby
a very fast response is required.
This is referred to as a reflex action.
The message will be sent on to the brain after the
event to make you aware of what has occurred.
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Can you think of examples of reflex
actions?
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Someone blowing in your eye.
Burning your hand on something hot.
Patella knee jerk test.
Pricking your hand on a needle.
Choking.
Vomiting.
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Motor point
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The motor point is the position where
the motor nerve inserts into the muscle
belly at its most superficial point.
Therefore, it is the most accessible way to
stimulate the motor nerve in order to
produce a muscle contraction.
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Peripheral Nervous System
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Peripheral nervous system
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The peripheral nervous system is
composed of the parts of the nervous
system outside of the central nervous
system and consists of:
31 pairs of spinal nerves
12 pairs of cranial nerves
The autonomic part of the nervous system
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The cranial nerves
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There are 12 pairs of cranial nerves that
originate from the brain.
Most are confined to the head and neck, but
the tenth nerve has branches in the trunk.
Some of these nerves are mixed, while
others are either motor or sensory.
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The spinal nerves
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There are 31 pairs of spinal nerves, which
emerge from in-between the vertebrae of the
spinal column.
All spinal nerves are mixed nerves and they have
two points of attachment to the spinal cord and
these are referred to as roots.
The dorsal root carries sensory axons whose
impulses are passing inward.
The ventral root carries motor axons whose
impulses are passing outwards.
The names of the 31 pairs of spinal nerves depend
on the region of the spine they emerge from:
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Spinal Nerves (31 Pairs).
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Eight pairs of cervical nerves
Twelve pairs of thoracic nerves
Five pairs of lumbar nerves
Five pairs of sacral nerves
One pair of coccygeal nerves
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Nerve plexuses
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Each spinal nerve divides into branches, with the posterior
branches supplying the muscles and skin of the back. The
majority of the anterior branches form plexuses (networks)
on either side of the body:
Cervical plexuses cover the neck, head and upper shoulder
region.
Brachial plexuses supply the skin and muscles of the arms,
shoulders and upper chest.
Lumbar plexuses supply the abdomen and part of the leg; the
femoral nerves are the largest nerves of the lumbar
plexuses, and these supply the front of the thigh.
Sacral plexuses supply the buttocks and some of the leg; the
sciatic nerves from these plexuses are the largest nerves in
the body, and supply the muscles of the legs and feet.
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Autonomic Nervous System
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The autonomic nervous system
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This is the involuntary part of the peripheral nervous
system.
It controls the body activities that are not under conscious
control.
There are two parts to this system, which work
antagonistically, meaning that they produce opposite effects.
The parasympathetic system is concerned with energy
conversion.
The sympathetic system is concerned with increasing the
body’s use of energy.
Together they maintain homeostasis, as one system
counteracts the effects of the other.
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Sympathetic system
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The sympathetic system is
mainly active at times of stress.
It is concerned with increasing
the body’s use of energy.
It is comprised of interlinked
nerves called plexuses.
For example the system raises
the heart rate, increases the
breathing rate, and slows down
the digestive system to prepare
the body for fight or flight.
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Parasympathetic system
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The parasympathetic
system is mainly active at
times of peace.
It is concerned with
energy conservation.
For example the heart
rate is normalised, the
digestive functions are
maintained and the blood
supply to the muscles is
reduced.
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At times of stress
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The sympathetic impulses become stronger.
The heart beats faster, blood vessels dilate, hairs stand on
end.
The sweat glands produce more sweat and blood pressure
rises due to the constriction of small arterioles in the skin.
Adrenaline is produced and the metabolic rate is increased.
When the stressful situation passes the parasympathetic
nerves take over and help the function of the organs return
to normal and prepare the body for rest.
Clare Hargreaves-Norris