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Unit 3 -Nerve Cells and Neural Pathways
CfE Higher Human Biology
19. Memory
Learning Intentions
I can identify the structures of a neurone including dendrites, cell body and
axon
I can identify sensory, motor and inter (relay) neurons
I can state that sensory neurons take impulses from sensory receptors to the
central nervous system
I can state that motor neurons take impulses from central nervous system to
an effector (e.g. muscle)
I can state that inter neurons transmit impulses between sensory and motor
neurons within the central nervous system
I can state that myelin sheath is composed of fatty material and surrounds
the axon
I can explain why myelination increases the speed of impulse transmission
I can state that myelination increases from birth to adolescence
I can state that glial cells support the function of neurons and produce myelin
sheath
I can state that glial cells also maintain a homeostatic environment around the
neurons and remove debris by phagocytosis
Neurons
• The nervous system is made up of a system of nerve
cells, known as neurons, which receive and transmit
electrical signals called nerve impulses.
• Glial cells support and maintain these neurons.
Function and Types of Neurone
• Neurones provide the
body with rapid
communication and
coordinated control.
• They conduct nerve
impulses from on one
part of the body to
another
• 3 types of neurones
– Sensory
– Inter(relay)
– motor
Inter
Neurons
Structure of neurons
• All neurons have the same basic structure, they are
composed of three key structures:
– Dendrites – nerve fibres that receives nerve impulses
towards a cell body
– a cell body – contains the nucleus and most of the
cytoplasm
– Axons – nerve fibres that carries nerve impulses away
from a cell body.
cell body
dendrites
axon
• Nerve impulses always travel in the same direction:
dendrites
cell body
axon.
The Function of Parts of a
Neuron
Control metabolism,
contains ribosomes for
the production of
neurotransmitters
Neurons
Structure of neurons
• Cell body - The cell body contains a nucleus and
cytoplasm. The cytoplasm contains organelles such as
mitochondria to provide energy for impulses and
ribosomes which synthesise proteins (e.g. enzymes) for
the synthesis of neurotransmitters.
• Dendrites – these fibres receive nerve impulses and
carry them towards the cell body
• Axon – this fibre carries nerve impulses away from the
cell body.
Neurons
Structure of neurons
• The axons of neurons are
surrounded in a layer of
fatty material known as
the myelin sheath. This
insulates the axon.
• The myelin sheath greatly
increases the speed of
transmission of a nerve
impulse from node to node.
• The small gaps are called
nodes.
axon
myelin
sheath
Myelination
Structure of neurons
• Myelination (the extent to which an axon is covered in
myelin) is not complete at birth.
• As a child ages, myelination increases and so does
nervous control. The responses of a two year old child
are therefore slower than those of an adult.
• Some diseases, such as Polio, Tay-Sachs and Multiple
Sclerosis (MS) can damage the myelin sheath and result
in loss of muscular co-ordination.
Neurones
Types of neurons
• There are three main types of neuron:
• SENSORY NEURONES
– From sense organs to CNS
• MOTOR NEURONES
– From CNS to effectors
• INTER NEURONES
– From sensory neurones to motor neurones.
– Are in CNS
Sensory Neurone
• Has dendrites in contact with sense organs.
• These dendrites merge to form a myelinated fibre which carries impulses to
the cell body.
• Has a short axon
• Forms connections with neurons in the CNS
DIRECTION OF IMPULSE
Inter Neurone
• Connects
sensory
neurons to
motor
neurons.
• Has many
dendrites
which form
many complex,
connections.
Motor Neurone
• Has short dendrites which connect to neurons in the CNS
• Has a long myelinated axon
• Axon carries nerve impulses to muscle connections.
DIRECTION OF IMPULSE
Diseases
• Diseases such as
multiple sclerosis
and poliomyelitis
• Cause the myelin
sheath to become
damaged or
destroyed
• Resulting in loss
of muscular
coordination
Glial cells
• Glial cells have a number of key
functions:
– physically support neurons
– produce the myelin sheath
– control the chemical
composition of the fluid
surrounding the neuron and so
maintain a homeostatic
environment.
– remove debris by phagocytosis
Glial cell
Glial Cells
• They provide physical
support to neurons
– (Oligodendrocytes) form
the myelin sheath around
axons.
– (Astrocytes) provide
nutrients to neurons,
maintain their
extracellular environment,
and provide structural
support (homeostatic
environment).
– (Microglia) scavenge
pathogens and dead cells
by phagocytosis
Blood Brain Barrier
The blood-brain barrier (BBB) is the close association
between projections from certain glial cells and the cells
forming the walls of capillary blood vessels.
This is the layer that lines the blood capillaries and is
made up of cells very closely packed together. This
separates the blood in the capillaries separate from the
extracellular fluid in the brain.
This prevents larger molecules and microorganisms moving
into the brain fluid from the bloodstream.
Some glial cells are phagocytic so they will remove foreign
material by phagocytosis.
Questions
1. Describe the structure and function of a neuron.
2. Describe the pathway of a nerve impulse through a
neuron.
3. Describe the features and functions of sensory,
motor and inter neurons.
4. Describe the structure and function of the myelin
sheath.
5. Explain the relationship between myelination, coordination and development from birth.
6. Describe the function of the glial cells
Answers
1. Describe the structure and function of a neuron.
All neurons have dendrites, cell body and axons.
Neurons transmit electrical signals called nerve
impulses.
2. Describe the pathway of a nerve impulse through a
neuron.
Impulse travels along a dendrite reaches the cell body
and then passes along an axon.
Answers (continued)
3. Describe the features and functions of sensory,
motor and inter neurons.
Sensory neuron
Has dendrites in contact with sense organs which merge to form a
fibre which carries impulses to the cell body. Has a short axon
Motor neuron
• Has short dendrites which connect to neurons in the CNS. Has a
long axon carries nerve impulses to muscle connections
Inter neuron Connects sensory neurons to motor neurons.Has many
dendrites which form many complex, connections.
Answers (continued)
4. Describe the structure and function of the myelin
sheath.
The myelin sheath is a layer of fatty material which
greatly increases the speed of transmission of a
nerve impulse.
5. Explain the relationship between myelination, coordination and development from birth.
Myelination is not complete at birth. Therefore coordination and dveopement will improve as this is
completed
Answers (continued)
6. Describe the function of the glial cells
Physically support neurons
Produce the myelin sheath
Control the chemical composition of the fluid
surrounding the neuron and so maintain a homeostatic
environment.
Remove debris by phagocytosis
Neurotransmitters at
Synapses
Learning Intentions
I can describe a synapse as an area of communication between axon of one neuron and
dendrite of another
I can explain that a neurotransmitter is a chemical that relays the message from the
pre synaptic neuron to the post-synaptic neuron
I can describe the chemical transmission at a synapse from vesicles containing
neurotransmitter crossing the synaptic cleft to receptors
I can state why it is important to remove neurotransmitters from the synapse
I can state that the type of receptor determines whether a signal is inhibitory or
excitatory
I can state that insufficient neurotransmitter at the synapse results in failure of
transmission of the impulse
I can state that a summation of a series of weak stimuli can trigger enough
neurotransmitter to fire an impulse.
I can define a converging neural pathway as several neurons sending an impulse to one
receiving neuron in order to increase the intensity of the impulse e.g. used to see in the
dark
I can define a diverging neural pathway as a motor neurone sending an impulse to
multiple neurons in order to provide fine motor control and hypothalamus co-ordinated
control of body temperature
I can define reverberating neural pathways as those which can repeat the original
impulse
Synaptic Cleft and Neurotransmitters
• The tiny area between the ending of an axon of one
neuron and the dendrite of another is known as a
synapse.
• The plasma membranes of each neuron are in very close
contact and are separated by a narrow space called a
synaptic cleft.
• Messages are passed across synaptic clefts by chemicals
called neurotransmitters.
• Two examples are acetylcholine and norepinephrine (also
known as noradrenaline).
Synaptic Cleft
• The neuron before the synaptic cleft is known as the
presynaptic neuron.
• The neuron after the synaptic cleft is known as the
postsynaptic neuron.
presynaptic neuron
(axon)
synaptic cleft
postsynaptic neuron
(dendrite)
neurotransmitters
Action of neurotransmitters
• When a nerve impulse passes through a neuron and
reaches the end of the axon (known as the axon
terminal), many vesicles containing neurotransmitters
are stimulated.
• These vesicles move to and fuse with the membrane at
surface of the axon terminal. The neurotransmitters
within the vesicles are then released (by exocytosis)
into the synaptic cleft.
• The neurotransmitter then diffuses across the cleft and
binds to receptor molecules on the dendrites of the next
neuron; this transmits the impulse to the next neuron.
Action of neurotransmitters
direction of nerve
impulse
Action of neurotransmitters
• http://www.youtube.com/watch?v=H_81gwAnjDU
• http://science.education.nih.gov/supplements/nih2/Addi
ction/activities/lesson2_neurotransmission.htm
Excitatory & inhibitory signals
• The type of receptor cells found on the postsynaptic
neuron will determine whether the signal is:
– excitatory (causes an increase in action e.g. cause
muscles to contract) or
– inhibitory (cause a decrease in action e.g. slow heart
rate)
Action of neurotransmitters
• Neurotransmitters must be rapidly removed as soon as
the impulse has been transmitted for the following
reasons:
– to prevent continuous stimulation of the postsynaptic
neuron
– so that the membrane is sensitive to the next
stimulus
– otherwise, the neurotransmitter would continue to
have an effect
– this allows a neurone to send many separate impulses
allowing a variety in the rate of impulse transmission.
Action of neurotransmitters
• Neurotransmitters can be removed from the synaptic
cleft by:
– enzyme degradation - this occurs with acetylcholine,
the products of which are absorbed and used to
synthesise new neurotransmitters
or
– re-uptake - this occurs with norepinephrine, which is
reabsorbed by presynaptic membrane.
Action of neurotransmitters
• The continual synthesis and removal of
neurotransmitters requires a very large amount of
energy.
• Neurones contain a large number of mitochondria to
provide ATP.
• This is why the brain is so easily damaged by oxygen
deprivation.
Weak stimuli
• A nerve impulse will only be transmitted across a
synaptic cleft if it causes the release of a sufficient
number of neurotransmitter molecules; this is known as
the threshold.
• Weak stimuli are known as sub-threshold stimuli and are
too weak to cause the transmission of a nerve impulse.
• When the stimulus is weak, the synapse acts as a gap
which the impulse cannot cross and the stimulus is
‘filtered out’ due to insufficient secretion of
neurotransmitters.
Summation
• A single weak stimulus will not trigger the release of
enough neurotransmitters to cause transmission of a
nerve impulse.
• However, a series of weak stimuli from many neurons can
bring about an impulse.
• The cumulative effect of a series of weak stimuli which
triggers an impulse is known as summation.
Summation
• If a weak stimulus passed along one axon this would not
trigger enough neurotransmitters to be released to
reach the threshold.
• When many axons release
their neurotransmitter at
the same time or in rapid
succession, this releases
enough chemical to fire a
response.
Questions
1. Describe what is meant by a ‘neurotransmitter’
2. Describe how a nerve impulse is transmitted at the
synapse (to include vesicles, synaptic cleft and
receptors)
3. Describe how neurotransmitters are removed and
explain why this is necessary.
4. What will receptors determine about the signal?
5. Describe how synapses can ‘filter out’ weak stimuli.
6. Describe the what is meant by ‘summation’
Answers
1.
Describe what is meant by a ‘neurotransmitter’
Neurotransmitters are chemicals which cause messages to pass
across synaptic clefts
•
2. Describe how a nerve impulse is transmitted at the synapse
(to include vesicles, synaptic cleft and receptors)
•
vesicles , at the axon terminal, are stimulated,
•
vesicles fuse with the membrane at surface of the axon
terminal. The neurotransmitters within the vesicles are then
released into the synaptic cleft.
•
The neurotransmitter then diffuses across the cleft and binds
to receptor molecules on the dendrites of the next neuron
•
this transmits the impulse to the next neuron.
Answers (continued)
3. Describe how neurotransmitters are removed and
explain why this is necessary.
Enzyme degradation and re-uptake
It is necessary to prevent continued stimulation of th
post synaptic neuron
4.What will receptors determine about the signal?
Whether the signal is excitatory or inhibitory
Answers (continued)
5. Describe how synapses can ‘filter out’ weak stimuli.
When the stimulus is weak, the synapse acts as a gap
which the impulse cannot cross and the stimulus is
‘filtered out’ due to insufficient secretion of
neurotransmitters.
6. Describe the what is meant by ‘summation’
The cumulative effect of a series of weak stimuli which
triggers an impulse is known as summation.
Neural pathways
Learning Intentions
I can define a converging neural pathway as several neurons sending an impulse to one
receiving neuron in order to increase the intensity of the impulse e.g. used to see in the
dark
I can define a diverging neural pathway as a motor neurone sending an impulse to
multiple neurons in order to provide fine motor controle.g. hypothalamus co-ordinated
control of body temperature
I can define reverberating neural pathways as those which can repeat the original
impulse
I can describe plasticity of response as the ability to suppress reflex actions and the
ability of the brain to bypass areas of brain damage and relearn skills
Complex neural pathways
• Neurons are connected to others in many different ways
in the CNS.
• This allows many complex interactions to occur between
neurons and so allows the nervous system to carry out
many complex functions.
• There are three main neural pathways:
– converging
– diverging
– reverberating
Converging neural pathways
• Converging neural pathways
have many neurons coming
together and feeding impulses
to one neuron.
• This allows for signals to be
brought together for a
combined or concentrated
effect (e.g. summation) at one
neuron.
• An example of this can be found with
the convergence of the neurons from
rod cells in the retina of the eye.
The direction of the impulse on the
diagram is very important.
Convergence of Neurons in Rods and Cones in Dim Light
Rods and cones are visual receptors found in the eye. They contain pigments that break down
in the presence of light. This breakdown releases a chemical trigger that sends impulses along
a pathway of neurones.
Cones – the pigment is not as sensitive so daylight is needed
Rods – this pigment is sensitive to very dim light so is rendered temporarily inactive in very
bright light.
As light intensity decreases, cones stop responding and rods take over. The nerve impulse
created from one rod is very weak so cant raise the postsynaptic membrane to threshold. The
convergent arrangement of several rods allows impulses to be transmitted simultaneously and
so release enough neurotransmitter.
To optic
nerve
Diverging neural pathways
• Diverging neural pathways have one
neuron branching out and feeding
impulses to many neurons.
• This allows for signals from a single
source to be sent to several destinations
and allows us to co-ordinate control (e.g.
when threading a needle. This is fine
motor control from the motor area of
the cerebrum).
• Similarly temperature in the
hypothalamus is found to diverge into
branches that lead to sweat glands, skin
arterioles and skeletal muscles.
The direction of the impulse on the
diagram is very important.
Reverberating neural pathways
• Reverberating neural pathways possess neurons later in
the pathway which form connections with neurons
earlier in the pathway.
• This allows for nerve impulses to be recycled and
repeatedly stimulate the circuit, these impulses will only
stop when they are no longer required. Complex
reverberating pathways in the brain are involved in the
control of rhythmic activities such as breathing.
The direction of the impulse on the
diagram is very important.
Plasticity of response
(P263)
• The brain can undergo change depending upon sensory
input in the synaptic network during our lifetime.
• The ability of brain cells to become altered and form
new neural pathways as a result of new environmental
experiences is known as plasticity of response.
• This allows new neural pathways to be formed during
early development when learning many new skills.
• Major plasticity of response can occur after brain
damage (e.g. stroke) and allows undamaged cells to form
new neural pathways to take on the functions of the
damaged area.
• Minor plasticity is used to suppress reflexes (e.g.
blinking or prevent the body dropping a hot object) or
suppress sensory responses (such as visual distractions).
Questions
1. Name the three main neural pathways.
2. Describe the features and functions of each of the
neural pathways.
3. Describe what is meant by ‘plasticity of response’
4. Give examples of when plasticity of response occurs
Answers
1.
Name the three main neural pathways.
Converging, diverging and reverberating
2.Describe the features and functions of each of the neural
pathways.
•
Converging -many neurons coming together and feeding impulses
to one neuron. This allows for signals to be brought together
for a combined effect at one neuron.
•
Diverging -one neuron branching out and feeding impulses to
many neurons. This allows for signals from a single source to be
sent to several destinations and allows us to co-ordinate control
•
Reverberating -neurons later in the pathway form connections
with neurons earlier in the pathway. This allows for nerve
impulses to be recycled and repeatedly stimulated
Answers(continued)
3.Describe what is meant by ‘plasticity of response’
The ability of brain cells to become altered and form
new neural pathways as a result of new environmental
experiences
4. Give examples of when plasticity of response occurs
• after brain damage (e.g. stroke)
• to suppress reflexes
• to suppress sensory responses
Neurotransmitters,
mood and behaviour
Learning Intentions Continued
I can state that endorphins are neurotransmitters that stimulate neurons involved in
reducing the intensity of pain
I can state that endorphins are also connected to feelings of euphoria, appetite
control and release of sex hormones
I can state that endorphin production increases in response to severe injury, prolonged
exercise, stress and certain foods e.g. chocolate
I can describe dopamine as a neurotransmitter that induces feelings of pleasure and
reinforces particular behaviours in the reward pathway
I can state that some disorders are caused by changes to neurotransmitter release
e.g. Alzheimer’s, Parkinson’s and some anxiety disorders
I can state that some drugs used to treat neurotransmitters are similar to
neurotransmitters e.g. agonists and antagonists
I can state that an agonist binds to and stimulates receptors mimicking the
neurotransmitter
I can state that antagonists bind to specific receptors blocking the action of the
neurotransmitter
I can state that other drugs inhibit the enzymes which breakdown neurotransmitters or
inhibit re-uptake at the synapse
Endorphins
• Endorphins are chemicals that function like
neurotransmitters. They act like natural painkillers by
combining with receptors at synapses and blocking the
transmission of pain signals. They are produced in the
hypothalamus.
• Endorphin production increases in response to:
– severe injury
– prolonged and continuous exercise
– physical & emotional stress
– certain foods
• (e.g. chocolate and chilli peppers)
Action of Endorphins
Endorphins
• Increased levels of endorphins can also bring about
other responses within the body, such as:
– euphoric feelings (intense happiness)
– regulation (modulation) of appetite
– release of sex hormones
Other Ways to Increase
Endorphin Production
sunlight
exercise
Listen to
music
laugh
meditation
Dopamine
• Dopamine is a neurotransmitter produced in several
areas of the brain which induces the feeling of pleasure.
They can also reduce anxiety and stress.
• Dopamine is also involve in reinforcing beneficial
survival-related behaviour (such as satisfying hunger by
eating, thirst or sexual need) by activating the reward
pathway.
• The reward pathway involves neurons which secrete or
respond to dopamine.
Agonists
• Agonists are chemicals that bind to and stimulate
specific receptors on the membrane of postsynaptic
neurons in a neural pathway.
• Agonists mimic the action of natural neurotransmitters
and so normal cell responses occur (i.e. nerve impulse is
transmitted) sometimes at an enhanced level.
Antagonists
• Antagonists are chemicals that bind to and block
specific receptors on postsynaptic neurons.
• Antagonists, by blocking the receptor sites, prevent the
normal neurotransmitter from acting.
• Antagonists can greatly reduce or even stop the normal
transmission of nerve impulses.
• Other drugs, known as inhibitors, inhibit the enzymes
which degrade neurotransmitters or inhibit re-uptake.
Agonists & Antagonists
Neurotransmitter related disorders
• Below are some examples of neurotransmitter related
disorders:
Disorder
Cause
Treatment
Alzheimer’s
disease
Loss of cells synthesising
acetylcholine.
Cholinesterase inhibitors
Parkinson’s
disease
Loss of dopamine
synthesising neurons.
Monamine oxidase inhibitors and
the potential use of adult stem cells
Schizophrenia
Overactive dopamine system
The use of dopamine antagonists
General anxiety
disorders
Imbalance in serotonin and
norepinephrin
The use of GABA agonists and
beta blockers
Low levels of serotonin
Norepinephrine re-uptake inhibitors
and monoamine oxidase enzyme
inhibitors
Depression
• Many drugs which treat neurotransmitter related
disorders are similar to neurotransmitters.
Questions
1. State the function of endorphins.
2. Describe the effect of endorphins on the body (i.e.
mood)
3. State the factors which result in an increase in
endorphin production.
4. State the function of dopamine and its effect on the
body.
5. Give examples of neurotransmitter related diseases.
6. Describe the action of agonists and antagonists.
7. Describe the action of inhibitor drugs.
Answers
1. State the function of endorphins.
Endorphins are neurotransmitters which act like natural
painkillers by stimulating neurons which are involved
in reducing the intensity of pain.
2. Describe the effect of endorphins on the body (i.e.
mood)
Euphoric feelings, regulation of appetite and the release
of sex hormones
Answers (continued)
3. State the factors which result in an increase in
endorphin production.
Severe injury,prolonged and continuous exercise,
physical & emotional stress, certain foods
4. State the function of dopamine and its effect on the
body.
It is a neurotransmitter which induces the feeling of
pleasure. Dopamine is also involve in reinforcing
beneficial behaviour by activating the reward
pathway.
Answers (continued)
5. Give examples of neurotransmitter related diseases.
Alzheimer’s disease, Parkinson‘s disease, Schizophrenia, General
anxiety disorders, Depression
6. Describe the action of agonists and antagonists.
Agonists are chemicals that bind to and stimulate specific
receptors on postsynaptic neurons.
Antagonists are chemicals that bind to and block specific receptors
on postsynaptic neurons
7. Describe the action of inhibitor drugs.
Inhibitors, inhibit the enzymes which degrade neurotransmitters
or inhibit re-uptake.
Mode of action of
recreational drugs
Learning Intentions Continued
I can state that recreational drugs can also mimic neurotransmitters
I can explain that as a consequence of taking recreational drugs
changes in neurochemistry alter mood, cognition, perception and
behaviour
I can state that many recreational drugs affect neurotransmission in
the reward circuit of the brain
I can state that recreational drugs can be either antagonistic or
agonistic
I can explain that antagonists lead to an increase in sensitivity and
number of receptors and as a consequence results in addiction =
sensitisation
I can explain that agonists lead to a decrease in sensitivity and
number of receptors and as a consequence results in drug tolerance =
desensitisation
Recreational drugs
• Many recreational drugs can mimic the action of
neurotransmitters and will affect the transmission of
nerve impulses in the reward circuit of the brain.
• Recreational drugs can stimulate the release of
neurotransmitters, act as agonists or antagonists and
inhibit their reuptake or enzyme degradation.
Recreational drugs
• Recreational drugs therefore alter a persons
neurochemistry and so can lead to changes in:
– mood
• e.g. happier/more confident/more aggressive
– cognition
• person becomes poorer at mental tasks such as problem solving and
decision making
– perception
• misinterpretation of environmental stimuli e.g. colurs, sounds, sense
of time
– behaviour
• person is able to stay awake for longer and talk about themselves
endlessly
Drug addiction/tolerance
• Drug addiction is a chronic disease. The sufferer will
compulsively seek out and use a drug regardless of the
consequences.
• The initial use of the drug is often voluntary but the
changes which occur after use soon override a persons
control.
• Drug tolerance occurs when a persons reaction to an
addictive drug decreases in intensity although the
concentration is the same. A large dose is then required
to bring about the original effect.
Sensitisation
• Sensitisation is an increase in the number and sensitivity
of neurotransmitter receptors.
• This occurs as a result of exposure to drugs which are
antagonists, which block receptors; the body then
responds by increasing the number of these receptors.
• Sensitisation leads to addiction.
Desensitisation
• Desensitisation is a decrease in the number and
sensitivity of neurotransmitter receptors.
• This occurs as a result of exposure to drugs which are
agonists, which stimulate receptors and cause feelings
of euphoria.
• The body responds to this overstimulation by decreasing
the number of these receptors and so a larger dose is
required to bring about the original effect.
• Desensitisation leads to drug tolerance.
Questions
1. What do recreational drugs mimic?
2. What do changes in neurochemistry caused by
recreational drugs cause?
3. Describe the meanings of the terms ‘drug addiction’
and ‘drug tolerance’.
4. Describe the meaning of the term ‘sensitisation’ and
explain how this leads to drug addiction.
5. Describe the meaning of the term ‘desensitisation’
and explain how this leads to drug tolerance.
Answers
1. What do recreational drugs mimic?
the effect of neurotransmitters and will affect the
reward circuit in the brain.
2.What do changes in neurochemistry caused by
recreational drugs cause?
It causes alterations in mood, cognition, perception and
behaviour
Answers (continued)
3. Describe the meanings of the terms ‘drug addiction’
and ‘drug tolerance’.
Drug addiction will compulsively seek out and use a drug
regardless of the consequences.
Drug tolerance occurs when a persons reaction to an
addictive drug decreases in intensity although the
concentration is the same.
Answers (continued)
4. Describe the meaning of the term ‘sensitisation’ and
explain how this leads to drug addiction.
Sensitisation is an increase in the number and
sensitivity of neurotransmitter receptors.
This occurs as a result of exposure to drugs which are
antagonists, which block receptors; the body then
responds by increasing the number of these
receptors which leads to drug addiction
Answers (continued)
5. Describe the meaning of the term ‘desensitisation’
and explain how this leads to drug tolerance.
This is a decrease in the number and sensitivity of
neurotransmitter receptors.
This occurs as a result of exposure to drugs which are
agonists, which stimulate receptors and cause
feelings of euphoria.
The body responds to this over stimulation by
decreasing the number of these receptors and so a
larger dose is required to bring about the original
effect. This leads to drug tolerance.