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Level 3 Pharmaceutical
Science
MODULE 6 PART 4
THE NERVOUS SYSTEM
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Level 3 Pharmaceutical Science
Module 6 Part 4 – The Nervous System
Contents
Chapter 1 - The Organisation and Function of the Nervous System .................................... 2
1.1 Nerve Cells ................................................................................................................. 4
1.2 A Synapse ................................................................................................................... 6
Chapter 2 - The Central Nervous System............................................................................ 9
2.1 Spinal Cord ................................................................................................................. 9
2.2 The Reflex Arc ............................................................................................................ 9
2.3 The Brain .................................................................................................................. 11
Chapter 3 - The Peripheral Nervous System ..................................................................... 13
3.1 The Somatic System ................................................................................................. 13
3.2 The Autonomic System ............................................................................................. 13
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Level 3 Pharmaceutical Science
Module 6 Part 4 – The Nervous System
Chapter 1 - The Organisation and Function of the Nervous System
By the end of this chapter you will be able to:
 Understand how the different parts of the nervous system are related to each other
 Describe the key features of the different types of neurons and explain their function
 Explain how a nervous impulse travels from one neuron to the next
This system, along with the endocrine system (the glands) organises the reactions of the
body to internal and external changes. It is composed of the brain, the spinal cord and the
nerves. They act together, communicating and carrying information to the brain, and
instructions from it.
There are two divisions of the nervous system:
 Brain and spinal cord forming the central nervous system
 Peripheral nervous system which connects the CNS to limbs and organs
Nerves carry electric impulses from the central nervous system (CNS) to all parts of the
body. They can make organs work and make glands secrete enzymes or hormones. Glands
and muscles are called effectors because they effect a change when they receive a message.
Nerves also carry messages back to the CNS from other parts of the body. They collect
impulses from all the sensory organs - the five senses: sight, sound, smell, taste and touch.
Nerve impulses from the sense organs to the CNS are called sensory. Those impulses from
the CNS to effectors are called motor impulses. Nerves which connect the body to the CNS
make up the peripheral nervous system.
The peripheral nervous system is divided into two systems: autonomic and voluntary.
A stimulus can therefore come from outside or inside the body. A change in blood glucose
level, oxygen or rate of breathing, as well as experiencing pain or heat can trigger a nervous
impulse.
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Module 6 Part 4 – The Nervous System
Overview of the Human Nervous System
Human Nervous System
Central Nervous System (CNS)
Composed of brain and spinal cord
Type of neuron – interneurons (relay)
Somatic Nervous System (voluntary)
Input from sense organs
Output to skeletal muscles
Sympathetic Nerves
Cause ‘fight or flight’ responses
Neurotransmitter noradrenaline
Peripheral Nervous System (PNS)
Connects CNS to limbs and organs
Types of neurons – sensory and motor
Autonomic Nervous System (involuntary)
Input from internal receptors
Output to smooth muscles and glands
Parasympathetic Nerves
Relaxing responses
Neurotransmitter acetylcholine
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Module 6 Part 4 – The Nervous System
1.1 Nerve Cells
The central nervous system and the peripheral nerves are made up of nerve cells called
neurons (or neurones – the spelling is interchangeable).
The motor neurons carry impulses from the CNS to muscles and glands.
The sensory neurons carry impulses from the sense organs to the CNS.
Each neuron has a cell body consisting of a nucleus surrounded by cytoplasm. Branching
fibres called dendrites make contact with other neurons. There are billions of them, all able
to make contact with hundreds of thousands of others. Depending on the number of
dendrites, a neuron could be classed as unipolar, bipolar or multipolar.
A long thread of cytoplasm surrounded by a sheath runs from the cell body. This can be very
short - just a few millimetres, or it can be very long - as much as a metre. The filament or
thread is called a nerve fibre or axon. The cell bodies are located in the brain or in the spinal
cord (or in ganglia outside of the CNS which we will discuss later) and the fibres run in the
nerves. So, a nerve actually consists of hundreds of fibres bundled together. Along the
length of the spinal cord are a number of junctions where messages are sorted or relayed to
the brain.
1.1.1 Motor Neuron
This relays messages from the brain or spinal cord to the muscles and organs
Schwann cell
nucleus
Axon
Myelin
sheath
Nodes of
Ranvier
Signal
Dendrites
Motor end plates
on muscle fibres
Cell body
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Module 6 Part 4 – The Nervous System
1.1.2 Sensory Neuron
This relays messages from sensory organs to the CNS
Nodes of
Ranvier
Myelin
sheath
Dendrite
Axon
Receptors
in skin
Signal
Signal
Cell body
Sensory Neuron
1.1.3 Interneuron (relay neuron)

Relays messages from sensory neuron to motor neuron

Make up the brain and spinal cord
Dendrites
Interneuron
Cell body
Signal
Axon
Synaptic
endings
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Synaptic
endings
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Module 6 Part 4 – The Nervous System
1.2 A Synapse
Impulses need to move from one neuron to another. The journey of an impulse from the
finger crosses 3 junctions before reaching the brain. These junctions are called synapses. A
synapse is a gap between two neurons. In an electrical circuit you might expect a gap to
result in a failure of the circuit. When the Christmas lights are switched on, if one bulb has
blown, sometimes the whole lot won't work.
However, there is a method of sending the message across the gap or synapse. At a synapse
one end of the fibre is only a short distance away from the dendrite of another. When the
impulse arrives, a tiny amount of a chemical substance called a neurotransmitter is released.
This must travel across the gap and be accepted by the dendrite. It can only do this by a lock
and key mechanism. It must be exactly the right shape to fit into the receptor or lock of the
dendrite. This sets off the impulse in the next neuron. Sometimes you need several
impulses to arrive before the neurotransmitter is released or ‘fired’.
Synapses are also located between a neuron and a target cell – in the autonomic nervous
system (involuntary) this is referred to as a neuro-effector junction, for example, the
synapse where the nervous system meets the cells of the diaphragm below the lungs. In the
somatic nervous system (voluntary) this is called the neuro-muscular junction, for example,
the synapse where the nervous system meets the muscle cells in your leg.
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Module 6 Part 4 – The Nervous System
When an electrical signal arrives at the end of a nerve fibre it triggers the release of the
neurotransmitter which then transmits the signal by chemical means to the next cell
At synapse or gap the neurotransmitter
crosses into the receptor of the next cell
Cell body of
1st neuron
Cell body of
2nd neuron
cell body of
3rd neuron
Electrical impulse travels along axon
The two most important neurotransmitters are called acetylcholine and noradrenaline.
Different neurotransmitters have different effects and we will need to look at these later,
but for now remember that each neuron only releases one transmitter.
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Module 6 Part 4 – The Nervous System
Take a Break!
Electrifying stuff! It can get quite confusing understanding
how the different parts of the nervous system link together. I
find it useful to refer back to the ‘Overview of the Human
Nervous System’ chart in this chapter if you ever find
yourself a bit lost!
Chapter 1 Summary



The human nervous system works to quickly to send messages from sensory organs and
receptors around the body to the brain, and then from the brain to muscles and organs.
It is divided into the central nervous system (CNS) and the peripheral nervous system
(PNS). The CNS receives messages from the PNS through sensory neurons; it interprets
them through an interneuron then passes the message back to the PNS, which then
sends a message to the relevant muscle or organ through motor neurons.
Messages are passed between neurons across a synapse which is a tiny gap between
neurons. When an impulse reaches the synapse a chemical called a neurotransmitter is
released. The neurotransmitter binds to a receptor in the next neuron and initiates
another nervous impulse. A synapse is also located between a neuron and a target cell –
in the autonomic nervous system (involuntary) this is referred to as a neuro-effector
junction. In the somatic nervous system (voluntary) this is called the neuro-muscular
junction.
Chapter 1 Quiz - Test Yourself
1. What is a synapse?
2. Name the two most common neurotransmitters.
3. What is an effector?
4. What type of nerves would carry an impulse from your ear to your brain?
5. What is the long filament or thread of a nerve cell called?
5. Axon
4. Sensory nerves
3. A target gland or muscle that effects a change when it receives a
message
2. Acetylcholine and noradrenaline
1. A tiny gap between two neurons. An impulse is passed across here
through the release of a neurotransmitter
Chapter 1 Quiz answers
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Module 6 Part 4 – The Nervous System
Chapter 2 - The Central Nervous System
By the end of this chapter you will be able to:



Describe how the spinal cord is involved in reflex actions
Explain what a reflex action is
Understand that there are different areas of the brain which are involved in different
functions. Some are voluntary and some are involuntary.
2.1 Spinal Cord
The spinal cord forms the link between the brain and the body. Most nerves pass through
the spinal cord on the way to the brain. The spinal cord has grey matter on the inside
shaped like an H and white matter surrounding it.
The spinal cord has two functions:
 Conduction of nerve impulses to and from the brain
 Control of spinal reflex actions.
The CNS receives messages from receptors. These messages have to go to the brain to be
interpreted, an action decided upon, and then a response sent as an order to the effector.
Sometimes this takes too long. If the body wants to bypass the brain it is called a spinal
reflex action.
Some nerves have structures called ganglia. In ganglia, the cell bodies and dendrites of
neurons are stored together while the axons project out to other parts of the body.
When the nerve leaves the brain or spinal cord it is called the pre-ganglionic nerve. After
the ganglion it is called a post-ganglionic nerve. This will then travel to the muscle or gland.
2.2 The Reflex Arc
A reflex action is an automatic response. Can you think of any reflex actions? List them here:
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Module 6 Part 4 – The Nervous System
You may have mentioned blinking or the knee-jerk.
In the knee-jerk one leg is crossed over another and the muscles are relaxed. If the tendon
below the kneecap of the upper leg is tapped sharply what happens?
A reflex arc makes the thigh muscle contract and the leg swings upwards. Hitting the tendon
stimulates a stretch receptor. A receptor is something that receives a message. The
receptor then sends a message via the sensory neurons. These impulses reach the CNS. In
the middle part of the spinal cord the sensory fibre passes the message to a motor neuron
across a synapse. This takes the message back to the effector, the thigh muscle.
Do the sensory impulses ever reach the brain? Yes, you know it is happening. However, the
reflex happens so quickly that the jerk is not caused by the brain. If you like, you could say it
was bypassed to get a quicker response. Like any boss, it finally found out what was
happening!
Look at the diagram below of a reflex arc for a knee jerk and touching a sharp object. Trace
the journey from the sensation to the action of the effector.
Sensory (pain)
receptor
Sensory
neuron
Sensory (stretch)
receptor
Spinal
cord
Sensory
neuron
Interneuron
Effector organ (bicep
muscle)
Motor
neuron
Motor
neuron
Effector organ (thigh
muscle)
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2.3 The Brain
The brain could be thought of as a swelling at the front end of the spinal cord. Certain areas
are greatly enlarged to deal with all the information arriving from the sensory fibres. I
wonder if the only people who really need to know about the brain are brain surgeons and
pathologists.
Cerebrum
Cerebellum
Brain stem
The following is a highly simplified representation of the regions of the brain.

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



The medulla (located in the brain stem) is concerned with involuntary processes such
as heart rate, temperature and breathing rate. It is therefore linked to the autonomic
nervous system.
The cerebellum controls posture, balance and co-ordination.
The mid brain deals with eye reflexes.
The cerebrum is thought to be the regions concerned with intelligence, memory,
reasoning and learnt skills. It is situated at the front of the head and is divided into
two halves called the right and left cerebral hemispheres which are connected
together. The cerebrum has areas which are involved in differing functions:
movement (the motor area), a sensory area receiving information from the skin,
speech areas, visual areas, auditory receiving sound, taste and smell, the frontal area
which makes decisions based on evidence from the others, and the memory area.
The outer layer of the cerebrum is called the cerebral cortex. It is composed of grey
matter, thousands of neurons. Most of the work is done here. It has a large surface
area because of the large number of folds.
The thalamus is situated below the cerebrum. It receives impulses from sense
organs. The thalamus transmits these messages to the cerebral cortex.
The hypothalamus is an important component of the autonomic nervous system as it
controls endocrine function (the release of hormones). It is situated in front of the
thalamus. It regulates thirst and hunger, it determines emotion, and it sends
messages to the pituitary gland.
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Take a Break!
Well there you have it, and here’s me thinking the knee jerk
was some type of 80’s dance move. Time to have a break
before tackling the end of chapter activities.
Chapter 2 Summary



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


The CNS is made up of the brain and spinal cord
Most of the nerves that pass to the brain run through the spinal cord.
Some reactions need to be instantaneous and do not have time to be processed by the
brain – these are called reflex actions.
There are interneurons within the spinal cord, and therefore messages can be loosely
interpreted here. During a reflex action and receptor will be stimulated and pass a nerve
impulse along sensory neurons to the spinal cord. Here the message is passed across an
interneuron to motor neurons which then send the message to an effector.
The brain is composed of several regions, all of which control certain processes.
The medulla conducts the autonomic nervous system – its output messages are sent via
sympathetic and parasympathetic nerves (you will learn about these later). The
hypothalamus is a component of the autonomic nervous system but sends its output
messages via hormones instead of nervous impulses.
The cerebellum and cerebrum deal with voluntary, thought-processed movement.
Chapter 2 Quiz - Test Yourself
1. What are the two functions of the spinal cord?
2 Which part of the CNS is involved in reflex actions?
3. What type of receptor is stimulated in the knee-jerk reflex?
4. In the knee-jerk reflex, what is the effector?
5. Which part of the brain conducts the autonomic nervous system through nerve
impulses only?
5. Medulla
4. Thigh muscle
3. Stretch receptor
2. Spinal cord
1. Conduction of nerve impulses to and from the brain and control of
spinal reflex actions
Chapter 2 Quiz answers
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Module 6 Part 4 – The Nervous System
Chapter 3 - The Peripheral Nervous System
By the end of this chapter you will be able to:

Describe the organisation of the peripheral nervous system

Explain some processes that are controlled by the somatic system

Describe the organisation of the autonomic system and explain some of the
processes that are controlled by it.

Explain how the sympathetic and parasympathetic nervous systems differ from each
other in terms of their effects and the neurotransmitters they use.
What does this do?
This connects the CNS with the rest of the body. The peripheral system can be divided into
two sections:
1. Voluntary or somatic
2. Involuntary or the autonomic system
3.1 The Somatic System
This carries nerve impulses from the sense organs to the CNS and then takes nerve impulses
from the CNS to the skeletal muscles (those attached to the bones).
Which nerves carry messages to the brain from other parts of the body?
These sensory nerve impulses are sorted in the spinal cord and are then sent on to the brain.
Messages from the brain are delivered to muscles by motor nerves. One motor nerve with
its branching fibres can control thousands of muscle fibres.
3.2 The Autonomic System
This deals with things such as digestion, respiration and circulation. It deals with processes
that are not under your conscious control.
The autonomic system is actually divided into two parts: the sympathetic and
parasympathetic.
Most organs are actually served by both systems. Generally, the effects of the
parasympathetic are very different from those of the sympathetic system. It is the balance
between these systems which keeps balance or homeostasis within the body. It is very
important to know which system produces which effect because drugs often work by
enhancing or blocking one system.
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Module 6 Part 4 – The Nervous System
3.2.1 The Sympathetic System
In very general terms the sympathetic system gets the body ready to fight or run. The
parasympathetic system is peaceful and calming. Let's look at some of the effects and I'll tell
you how I remember them. The sympathetic system is easier to remember if you think of a
character ....Sexy Sidney or Saucy Sue. Imagine this character has become quite excited. Let
us look at their bodily functions.
Sexy Sid
The Eyes: when you are attracted to someone your pupils dilate (get larger)
The Heart Rate: you might expect Sexy Sidney's heart rate to increase
Blood Vessels: these constrict leading to an increase in blood pressure
Breathing: he might require to do some very deep breathing so the airways widen and
breathing rate increases.
The Digestive System: not the time for a visit to the loo so this is slowed down
The Urinary System: as above
The Liver: energy required so glucose released into the system
The Skin: increased sweating in order to keep the body cool under stress, hairs stand to
attention
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Module 6 Part 4 – The Nervous System
3.2.2 The Parasympathetic System
I remember this as Placid Percy or Plump Penelope - not
interested in passion but dribbling at the sight of a
cream doughnut. You could also think parachute as the
parasympathetic system is all about slowing down and
relaxing actions.
The actions are the opposite of the sympathetic system so list them here:
Eye:
Circulation:
Respiratory:
Digestive:
Liver:
Excretory System:
Skin:
So, the parasympathetic system prevents the body systems from accelerating too much. It
acts as a damper.
So now you know what they do - but how do they manage it?
The neurotransmitters released are not the same.
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3.2.3 Neurotransmitters
In the autonomic nervous system, a neuro-effector junction is the junction between the
neuron and the target cell (muscle, gland or neuron); in the somatic nervous system, a
neuro-muscular junction is the junction between a neuron and a skeletal muscle.
In the sympathetic system a nerve leaves the CNS heading towards a gland or involuntary
muscle. It will have to pass a ganglion or two on the way. At the ganglion the transmitter is
acetylcholine, however at the junction between the nerve and the muscle of the organ or
gland (a neuro-effector junction) the neurotransmitter is noradrenaline.
In the parasympathetic system the transmitter at both the ganglion AND the neuro-effector
junction is acetylcholine.
The example below shows the sympathetic and parasympathetic nerves controlling the
heart rate, and the neurotransmitter that is released at each stage.
In the somatic nervous system it is acetylcholine which is released at the neuro-muscular
junction.
•
•
Drugs which behave like noradrenaline are called adrenergics
Drugs which behave like acetylcholine are called cholinergics
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Take a Break!
The different parts of the nervous system are constantly
interacting, and are so well co-ordinated that man can think,
feel, and act on many different levels, without serious
confusion, all at the same time. I hope your system has
served you well during this study session.
Chapter 3 Summary




The somatic system carries nerve impulses from the sense organs to the CNS and then
takes nerve impulses from the CNS to the skeletal muscles (those attached to the
bones). It is under voluntary control
Much of the human nervous system is concerned with routine, involuntary jobs, such as
homeostasis, digestion, posture, breathing, etc. This is the job of the autonomic
nervous system,
Its motor functions are split into two divisions, with anatomically distinct neurons. Most
body organs are innervated by two separate sets of motor neurons; one from the
sympathetic system and one from the parasympathetic system.
These neurons have opposite (or antagonistic) effects. In general the sympathetic
system stimulates the ‘fight or flight’ responses to threatening situations, while the
parasympathetic system relaxes the body. The details are listed in this table:
Organ
Sympathetic system
Parasympathetic system
Eye
Dilates pupil
Constricts pupil
Tear glands
No effect
Stimulates tear secretion
Salivary glands
Inhibits saliva production
Stimulates saliva production
Lungs
Dilates bronchi
Constricts bronchi
Heart
Speeds up heart rate
Slows down heart rate
Gut
Inhibits peristalsis
Stimulates peristalsis
Liver
Stimulates glucose production
Stimulates bile production
Bladder
Inhibits urination
Stimulates urination

At their neuro-effector junctions, parasympathetic nerves release the neurotransmitter
acetylcholine whilst sympathetic nerves release noradrenaline.
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Chapter 3 Quiz - Test Yourself
1. What does the autonomic nervous system do?
2. What does the somatic nervous system do?
3. How is the autonomic system divided?
4. What is the name of the neurotransmitter at the neuro-effector junction of the
sympathetic system?
5. What is the name of the neurotransmitter at the neuro-effector junction of the
parasympathetic system?
6. What effect does the sympathetic system have on heart rate?
7. What effect does the parasympathetic system have on digestion?
7. Stimulates digestion
6. Increases heart rate
5. Acetylcholine
4. Noradrenaline
3. Into the sympathetic and parasympathetic nerves
2. Control voluntary processes such as movement of the limbs
1. Control involuntary processes such as breathing rate, heart rate etc.
Chapter 3 Quiz answers
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