Download Topic guide 8.4: The nervous system

Unit 8: Pharmacological principles of drug actions
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The nervous system
The nervous system is a highly specialised body system that enables the
body to respond to changes in the environment. It enables communication
and coordination throughout the body, thus maintaining a constant internal
environment.
It is made of two parts, the central nervous system and the peripheral nervous
system.
On successful completion of this topic you will:
•• understand the transmission of nerve impulses, diseases that affect
transmission and their modification by drugs (LO3).
To achieve a Pass in this unit you need to show that you can:
•• explain key stages in the transmission of nerve impulses (3.1).
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Unit 8: Pharmacological principles of drug actions
1 Central nervous system
The central nervous system (CNS) is made up of the spinal cord and the brain.
The spinal cord is approximately 5 mm in diameter. It starts at the base of the
brain and it ends at the lumbar vertebrae. It is not only protected by the vertebrae
– there are membranes called spinal meninges and cerebrospinal fluid that also
provide protection. It extends to the peripheral nervous system (PNS) through
the gaps on either side of the vertebrae.
The brain is protected by the cranium and the cranial meninges. It receives many
impulses and controls involuntary movements such as heart rate and breathing
rate. The hypothalamus is the control centre of the brain and it controls the
autonomic nervous system, helping to regulate factors such as temperature and
blood glucose levels.
2 Peripheral nervous system
Key terms
Central nervous system (CNS):
Consists of the brain and spinal cord.
Peripheral nervous system (PNS):
The extensive network of nerves
that extend from the central nervous
system.
Hypothalamus: The control centre of
the brain.
Afferent: Carry nerve impulses from
receptors to the central nervous system.
Efferent: Carry nerve impulses from
central nervous system to effectors.
Receptor: The part of the body that
detects a change in the environment.
Effector: The muscle or organ which
produces an effect in response to a
stimulus.
Neurones: Cells that emit electrical
and chemical impulses.
The peripheral nervous system (PNS) is the extensive network of nerves that
extends from the central nervous system. Afferent nerves carry information from
receptors to the central nervous system and efferent nerves carry information to
effector organs from the central nervous system.
Somatic nervous system
The somatic nervous system is part of the peripheral nervous system made
of both afferent and efferent nerves. The somatic nervous system is involved
with voluntary control of body movements via skeletal muscles. It processes
information from receptors in skin, tendons, eyes, nose, ears, tongue and more
that gives us sensations of sight, smell, taste, sound and touch.
Autonomic nervous system
This is made up of only efferent nerves. It consists of the sympathetic and
parasympathetic systems opposing each other. For example, it controls heart rate
– the sympathetic system increases heart rate and the parasympathetic decreases
it. The sympathetic system can be seen to prepare the body for flight or action, for
instance, in stressful or frightening situations. It is the parasympathetic system that
will return the body back to normal.
3Neurones
Neurones are specialised cells that make up the nervous system, carrying
electrical impulses from one part of the body to another. Nerves are bundles of
neurones. Sensory nerves take information into the central nervous system and
motor nerves take information from the brain to effectors such as muscles.
A motor neurone consists of a cell body with extensions called dendrites and a
long axon insulated by a myelin sheath (see Figure 8.4.1).
8.4: The nervous system
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Unit 8: Pharmacological principles of drug actions
Figure 8.4.1: The structure of a
neurone showing the cell body,
dendrites and the axon.
Dendrites
Cell body
Dendrites
Axon
Axon
Synapses
A sensory neurone is structurally similar – it contains a smaller cell body, dendrites,
a dendron (carries impulse from receptor to cell body) and a short axon.
Resting potential
The resting potential is the state that each neurone is in when it is ready to
conduct an electrical impulse. During this time the fluid inside the axon is
negatively charged due to an unequal distribution of ions, so the axon is polarised.
Active transport and facilitated diffusion of sodium and potassium results in
an unequal exchange of positive ions leaving the cell, hence leaving the inside
negatively charged.
The action potential
Key terms
Resting potential: A nerve cell
resting and ready for action but
not active.
A receptor detects a change in the environment and generates an action
potential. A wave of depolarisation occurs along the axon: the permeability of the
axon is reversed and sodium ions suddenly flow into the axon, making it positive
in relation to the outside. The sodium and potassium channels are gated and can
limit or allow more molecules through when needed. Some of the gated channels
detect the change in the voltage and open to allow sodium ions to diffuse in.
Action potential: The change in
electrical potential of an impulse
along a nerve cell.
If the threshold value is reached then all the gates open for approximately
0.5 milliseconds. This allows more sodium ions to diffuse inside quickly, producing
a positive charge inside the axon.
Repolarisation: Change in
membrane potential that returns the
membrane potential to negative after
the depolarisation.
Repolarisation takes place when the membrane potential is reduced to zero –
the potassium channels open for 0.5 milliseconds so potassium ions diffuse out,
leaving the charge inside the axon negative again, allowing the neurone to return
to resting potential.
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Unit 8: Pharmacological principles of drug actions
Action potentials are repeatedly fired along the nerve fibres but there is a time
delay between each action potential so the channels can close. The time between
each action potential is called the refractory period.
The speed of the action potential depends on the diameter of the axon – the
larger the axon, the faster the action potential. If the neurone is myelinated the
impulse will travel faster because the Schwann cells that form around the axon
provide insulation. If there are many synapses involved, this slows down the action
potential.
In the propagation of an action potential along an axon the channels are shut
when the membrane potential is near the resting potential of the cell, but
they rapidly begin to open if the membrane potential increases. When the
channels open, they allow an inward flow of sodium ions, which changes the
electrochemical gradient, which in turn produces a further rise in the membrane
potential. This then causes more channels to open, producing a greater electric
current, and so on.
Key terms
Refractory period: The period
following the stimulation of a
nerve cell.
Synapses
Axons are not attached to one another; where one axon ends and another starts
there are small gaps. These gaps are called synapses (see Figure 8.4.2).
Synapse: The gap between two
neurones.
Axon
Figure 8.4.2: Diagram showing a
gap between two neurones where
neurotransmitters diffuse across
to transport the chemical message
to the next neurone, where the
electrical impulse will continue.
Synaptic vesicle
Synapse
Neurotransmitter
Receptor
8.4: The nervous system
Dendrites
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Unit 8: Pharmacological principles of drug actions
The synapse will not allow the action potential to jump across – instead
neurotransmitters transport the message across the synapse and initiate an action
potential in the next axon.
Synaptic vesicles carry neurotransmitter chemicals that diffuse easily across the
synaptic cleft. When the action potential reaches the synaptic bulb, calcium
channels open in the presynaptic membrane, and because of the concentration
difference calcium ions diffuse in.
As the concentration increases, it causes the synaptic vesicles to fuse with the
membrane releasing the neurotransmitter into the synapse. The neurotransmitter
will reach the next synaptic bulb or effector within a millisecond, where the
neurotransmitter binds to receptors on the surface. The neurotransmitter chemicals
are released and initiate the action potential that excites the next neurone.
There are four main groups of neurotransmitters, acetylcholine, amino acids,
monoamines and neuropeptides. Acetylcholine acts in the brain to change the
activity of other neurotransmitters, usually involved with memory and motivation.
Acetylcholinesterase breaks down acetylcholine into acetic acid and choline,
which can be reabsorbed through the presynaptic membrane where they join
back together to produce acetylcholine – this can then be used again. See the
Case study to see how drugs can affect this process.
Synapses allow information to be passed from one axon to another ensuring that
the electrical impulse travels in the right direction.
Checklist
In this topic you should now be familiar with the following ideas about the nervous system:
 the central nervous system consists of the brain and the spinal cord
 the peripheral nervous system is the network of nerves that extends out throughout the body
connecting extremities to the CNS
 neurones are specialised cells that carry electrical impulses as information
 bundles of neurones are called nerves
 sensory neurones sense the change in stimuli from a receptor cell and send the information to
the brain
Portfolio activity (3.1)
You can generate evidence for your
portfolio by doing the following:
•• draw a diagram of four axons –
include the synapses
•• illustrate the initial action
potential
•• illustrate the activity at the
synapse.
8.4: The nervous system
 the brain processes the information and sends out a response via the motor nerve to an
effector
 effectors are usually muscles or glands
 synapses are small gaps between neurones where neurotransmitters transmit the action
potential to excite the next neurone
 action potentials are created by a change in the charge across the membranes of the axon
 the speed of action potentials depends on thickness of axon, number of synapses and
whether there is a myelin sheath present.
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Unit 8: Pharmacological principles of drug actions
Case study
Further reading
Olesky, W. (2000) The Nervous System
Rosen Publishing Group.
The first nerve gas produced for military use was in 1936 – it was used in the war to directly affect
the nervous system. Nerve agents are similar to insecticides and they can be deadly if people are
exposed to them. This nerve gas contained organophosphates that blocked acetylcholinesterase.
Therefore, there was no way to stop the effect of acetylcholine and it built up in the synapses
because it was not reabsorbed. This could cause symptoms such as twitching, diarrhoea and sickness.
Acknowledgements
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All other images © Pearson Education
Every effort has been made to trace the copyright holders and we apologise in advance for any
unintentional omissions. We would be pleased to insert the appropriate acknowledgement in any
subsequent edition of this publication.
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