1. Higher Human Biology Unit 3 – Neurobiology and Immunology
Section 20 – Cells of the Nervous System and Neurotransmitters at Synapses

2. a) Cells of the Nervous System
We will be learning…
To describe the structure and function of neurons
To describe the structure and function of myelin sheath
To state that myelination continues from birth to adolescence
To be able to explain how certain diseases can destroy the myelin sheath causing a loss of co-ordination e.g. Multiple Sclerosis
To state that glial cells produce the myelin sheath and support neurons

3. What are 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.

4. 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

5. 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.
Nerve impulses always travel in the same direction:
Dendrites Cell body axon.
cell body
dendrites
axon

7. The Function of Parts of a Neuron
Control metabolism, contains ribosomes for the production of neurotransmitters

8. 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.

9. The cell bodies of sensory neurons are located within the vertebrae, which surround and protect the spinal cord. In motor neurons the cell body lies within the central grey matter of the spinal cord, surrounded by the outer white matter, which is composed of axons with their fatty myelin sheaths. The grey matter of the brain similarly consists mainly of cell bodies and dendrites, whilst the white matter consists of axons.

10. Structure and Function of the Myelin Sheath
The axons of neurons are surrounded in a layer of lipid (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 between the myelin sheaths are called nodes.
axon
myelin sheath

11. Myelination
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.
From Birth
To Adolescence

12. Why is Myelination Important
Myelination is the process of coating the axon of each neuron with a fatty coating called myelin, which protects the neuron and helps it conduct signals more efficiently.

13. Types of Neurones There are three main types of neuron:
Sensory Neurones
From sense organs to CNS
Inter Neurones
From sensory neurones to motor neurones.
Are in CNS
Motor Neurones
- From CNS to effectors

14. 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

15. Inter Neurone Connects sensory neurons to motor neurons.
Has many dendrites which form many complex, connections.
Very short axon

16. 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

18. Diseases Diseases such as multiple sclerosis and poliomyelitis
Cause the myelin sheath to become damaged or destroyed
Resulting in loss of muscular coordination

19. How is the Myelin Sheath damaged in Multiple Sclerosis
Multiple Sclerosis is a chronic, typically progressive disease involving damage to the sheaths of nerve cells in the brain and spinal cord, whose symptoms may include numbness, impairment of speech and of muscular coordination, blurred vision, and severe fatigue.

20. What are 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

21. 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

22. 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.

23. Questions Describe the structure and function of a neuron.
Describe the pathway of a nerve impulse through a neuron.
Describe the features and functions of sensory, motor and inter neurons.
Describe the structure and function of the myelin sheath.
Explain the relationship between myelination, co-ordination and development from birth.
Describe the function of the glial cells

24. Answers 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.

25. 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
Inter neuron Connects sensory neurons to motor neurons. Has many dendrites which form many complex, connections.
Motor neuron
Has short dendrites which connect to neurons in the CNS. Has a long axon carries nerve impulses to muscle connections

26. 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, co-ordination and development from birth.
Myelination is not complete at birth. Therefore co-ordination and development will improve as this is completed
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

27. b) Neurotransmitters at Synapses
We will be learning…
To be able to describe how a nerve impulse is transmitted from one neuron to another across the synaptic cleft
To explain the structure and function of vesicles, synaptic cleft and receptors.
To explain the requirement for the removal of neurotransmitters by enzymes or reuptake to prevent continuous stimulation of postsynaptic neurons.
To explain that receptors determine whether the signal is excitatory or inhibitory
To explain that synapses can filter out weak stimuli as a result from insufficient secretion of neurotransmitters.
To state that accumulation of weak stimuli can release enough neurotransmitter to trigger an impulse.

28. 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).

29. 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
Neurotransmitters
Postsynaptic Neuron
(dendrite)

30. 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.
The neurotransmitters are stored in vesicles to ensure that resources are not wasted
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 a specific receptor molecule on the dendrites of the next neuron; this transmits the impulse to the next neuron.

31. Vesicles and Receptors
direction of nerve impulse

32. 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)
If the number of excitatory signals exceeds the number of inhibitory signals, the neuron will 'fire', carrying an impulse to the next synapse, but this only occurs if a certain threshold is reached.

33. There is more than one type of receptor for each neurotransmitter
There is more than one type of receptor for each neurotransmitter. A given neurotransmitter can usually bind to and activate multiple different receptor proteins. Whether the effect of a certain neurotransmitter is excitatory or inhibitory at a given synapse depends on which of its receptor(s) are present on the postsynaptic (target) cell.
Acetylcholine
The neurotransmitter acetylcholine is excitatory at the neuromuscular junction in skeletal muscle, causing the muscle to contract. In contrast, it is inhibitory in the heart, where it slows heart rate. These opposite effects are possible because two different types of acetylcholine receptor proteins are found in the two locations.

34. Removal 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.

35. 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.

36. 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.

37. 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.
A very large quantity of neurotransmitter has no extra effect. Strong signals do not paralyse the nervous system with excessive contractions.

38. 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.

39. 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.

40. Questions Describe what is meant by a ‘neurotransmitter’
Describe how a nerve impulse is transmitted at the synapse (to include vesicles, synaptic cleft and receptors)
Describe how neurotransmitters are removed and explain why this is necessary.
What will receptors determine about the signal?
Describe how synapses can ‘filter out’ weak stimuli.
Describe the what is meant by ‘summation’

41. Answers 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.

42. 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 the post synaptic neuron
4.What will receptors determine about the signal?
Whether the signal is excitatory or inhibitory
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.

43. c) Neurotransmitter Effects on Mood and Behaviour
We will be learning…
To state that neurotransmitters can have an effect on mood and behaviour
To explain the function of endorphins
To describe that the production of endorphins increases in response to severe injury, prolonged and continuous exercise, stress and certain foods.
To describe the function of dopamine and the reward pathway that involves neurons with secrete or respond to dopamine
To state the conditions that activates the reward pathway

44. What is the difference between Mood and Behaviour?
Certain neurotransmitters are particularly associated with the control of mood and behaviour. Mood is a psychological state which is less immediately affected by events than emotion, and less permanent than personality or temperament. Behaviour is the response of an organism to internal and external stimuli.

45. Can Neurotransmitters affect Mood and Behaviour?
Dopamine causes feelings of pleasure and euphoria, and, consequently, any activity which induces dopamine release will tend to be repeated. It is therefore associated with beneficial behaviours, such as eating when hungry.
Endorphins are a group of at least twenty related chemicals which are produced from the pituitary and the hypothalamus in response to a variety of different stimuli, both physical and mental. They act like the opiate drugs after which they are named, relieving pain and creating a feeling of well-being. This effect is achieved because the increased levels of endorphins in turn stimulate the release of dopamine.

46. 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)

47. Action of Endorphins

48. 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

49. Other Ways to Increase Endorphin Production
sunlight
exercise
Listen to music
meditation
laugh

50. 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 involved 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 increased levels of dopamine.

51. d) Neurotransmitter-related disorders and their Treatment
We will be learning…
To be able to describe neurotransmitter-related disorders and their treatment.
To define the term agonist in drug treatment in neurotransmitter-related diseases
To define the term antagonist in drug treatment in neurotransmitter-related diseases
To describe the action of other drugs that act by inhibiting the enzymes that degrade neurotransmitters or by inhibiting reuptake of the neurotransmitter at the synapse causing an enhanced effect.

52. Neurotransmitter Related Disorders
Some Neurotransmitter –related diseases are well known such as Alzheimer’s and Parkinson’s.
They can be caused by:
an over- or under-production of the neurotransmitter,
Imbalance between the production of different neurotransmitters which operate together (e.g. one being inhibitory and the other excitatory).
Receptors being blocked so that the neurotransmitters cannot bind to them
Consequently, many of the drugs used to treat these conditions are similar to the neurotransmitters which are present in abnormal levels.

53. 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.
Drugs which are agonists therefore have a similar effect on the natural agonist, the neurotransmitter.

54. 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.
Antagonists may exert their effect by binding to the active site of the receptor in competition with the neurotransmitter, later separating from the active site. However, some antagonists bind permanently to the active site, preventing it ever subsequently receiving a neurotransmitter. Also, similarly to enzyme inhibitors, some antagonists bind to a part of the receptor other than the active site, distorting the shape of the active site and preventing neurotransmitters from binding.
Other drugs, known as inhibitors, inhibit the enzymes which degrade neurotransmitters or inhibit re-uptake.

55. Agonists & Antagonists

56. Neurotransmitter related disorders
Below are some examples of neurotransmitter related disorders:
Many drugs which treat neurotransmitter related disorders are similar to neurotransmitters.
Disorder
Cause
Treatment
Alzheimer’s disease
Loss of cells synthesising acetylcholine.
Cholinesterase inhibitors -  slows the rate at which acetylcholine is degraded
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
Depression
Low levels of serotonin
Norepinephrine re-uptake inhibitors and monoamine oxidase enzyme inhibitors

57. Questions State the function of endorphins.
Describe the effect of endorphins on the body (i.e. mood)
State the factors which result in an increase in endorphin production.
State the function of dopamine and its effect on the body.
Give examples of neurotransmitter related diseases.
Describe the action of agonists and antagonists.
Describe the action of inhibitor drugs.

58. Answers 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
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.

59. 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.

60. e) Mode of Action of Recreational Drugs
We will be learning…
To be able to describe the action of recreational drugs
To state that recreational drugs can act as agonists or antagonists
To state that recreational drugs can affect neurotransmission at synapses in the brain which can alter an individual’s mood, cognition, perception and behaviour
To describe how recreational drugs can affect neurotransmission in the reward pathway of the brain
To state that drug addiction is caused by repeated use of drugs that act as antagonists
To state that drug tolerance is caused by repeated use of drugs that act as agonists

61. Recreational drugs
Drugs which are referred to as recreational are introduced into the body either because they generate pleasurable sensations in of themselves, or because they enhance some other leisure experience. They are also taken in other circumstances, e.g. to help cope with pain or other conditions.
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.

62. 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. colours, sounds, sense of time
behaviour
person is able to stay awake for longer and talk about themselves endlessly

63. Drug Addiction and 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.

64. 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.

65. 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.

66. Questions What do recreational drugs mimic?
What do changes in neurochemistry caused by recreational drugs cause?
Describe the meanings of the terms ‘drug addiction’ and ‘drug tolerance’.
Describe the meaning of the term ‘sensitisation’ and explain how this leads to drug addiction.
Describe the meaning of the term ‘desensitisation’ and explain how this leads to drug tolerance.

67. Answers 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
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.

68. 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
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.

69. a) Cells of the Nervous System
Now I can…..
Describe the structure and function of neurons
Describe the structure and function of myelin sheath
State that myelination continues from birth to adolescence
Explain how certain diseases can destroy the myelin sheath causing a loss of co-ordination e.g. Multiple Sclerosis
State that glial cells produce the myelin sheath and support neurons

70. b) Neurotransmitters at Synapses
Now I can…..
Describe how a nerve impulse is transmitted from one neuron to another across the synaptic cleft
Explain the structure and function of vesicles, synaptic cleft and receptors.
Explain the requirement for the removal of neurotransmitters by enzymes or reuptake to prevent continuous stimulation of postsynaptic neurons.
Explain that receptors determine whether the signal is excitatory or inhibitory
Explain that synapses can filter out weak stimuli as a result from insufficient secretion of neurotransmitters.
State that accumulation of weak stimuli can release enough neurotransmitter to trigger an impulse.

71. c) Neurotransmitter Effects on Mood and Behaviour Now I can…..
State that neurotransmitters can have an effect on mood and behaviour
Explain the function of endorphins
Describe that the production of endorphins increases in response to severe injury, prolonged and continuous exercise, stress and certain foods.
Describe the function of dopamine and the reward pathway that involves neurons with secrete or respond to dopamine
State the conditions that activates the reward pathway

72. d) Neurotransmitter-related disorders and their Treatment
Now I can…..
To be able to describe neurotransmitter-related disorders and their treatment.
To define the term agonist in drug treatment in neurotransmitter-related diseases
To define the term antagonist in drug treatment in neurotransmitter-related diseases
To describe the action of other drugs that act by inhibiting the enzymes that degrade neurotransmitters or by inhibiting reuptake of the neurotransmitter at the synapse causing an enhanced effect.

73. e) Mode of Action of Recreational Drugs
Now I can…..
To be able to describe the action of recreational drugs
To state that recreational drugs can act as agonists or antagonists
To state that recreational drugs can affect neurotransmission at synapses in the brain which can alter an individual’s mood, cognition, perception and behaviour
To describe how recreational drugs can affect neurotransmission in the reward pathway of the brain
To state that drug addiction is caused by repeated use of drugs that act as antagonists
To state that drug tolerance is caused by repeated use of drugs that act as agonists

74. Questions
Compare and contrast sensory and motor neurons and describe events that occur at a synapse. (10 marks)
Sensory neurons pass messages from sense organs to the central nervous system whereas motor neurons transfer messages from the CNS to muscles and glands.
Both neurons consist of cell body, axon and dendrites.
The cell body is found part way along the axon of a sensory neuron, whereas the axon grows out from one side of the cell body in the motor neuron.
In each case, the axon is wrapped in a myelin sheath with nodes every few millimetres.
At a synapse, neurotransmitters cross from the pre-synaptic neuron to the post-synaptic neuron.
Neurotransmitters include acetylcholine and noradrenaline.
The type of receptor on the post-synaptic dendrite, to which the transmitter chemical binds, determines whether the next neuron is inhibited or excited.
Acetylcholine is immediately degraded by an enzyme.
Noradrenaline is reabsorbed by active uptake.
Synapses filter out single weak impulses, but can sum several weak impulses.

75. chemical neurotransmitter or a drug that mimics one
Word
Definition
Agonist
chemical neurotransmitter or a drug that mimics one
Antagonist
drug that inhibits transmission of nerve impulses
Axon
neural fibre that conducts impulses away from the cell body
Dendrite
neural fibre that conducts impulses towards the cell body
Desensitisation
decrease in number of sensitivity of synaptic receptors as a result of exposure to drugs
Dopamine
neurotransmitter involved in inducing the feeling of pleasure and other actions
Endorphin
neurotransmitter involved in reducing intensity of pain and other actions

76. signal that affects a synaptic receptor but is not passed on
Word
Definition
Inhibitory Signal
signal that affects a synaptic receptor but is not passed on
Inter Neuron
conducts impulses within the CNS, linking sensory and motor neurons
Motor Neuron
carries impulses from the CNS to muscles or glands
Excitatory Signal
signal that affects a receptor and which can be passed on
Glial Cell
cell in the nervous system that physically supports neurons and produces myelin sheaths
Myelin Sheath
layer of fatty insulation round a nerve fibre
Myelination
insulation of nerve fibres with myelin sheaths
Neuron
conducting nerve cell

77. process by which phagocytes engulf and destroy foreign material
Word
Definition
Neurotransmitter
chemical released into a synaptic cleft to transmit impulses to the next cell
Phagocytosis
process by which phagocytes engulf and destroy foreign material
Post-synaptic membrane
membrane of the neuron that contains receptors for neurotransmitters
Pre-synaptic Membrane
membrane of the neuron that releases neurotransmitters
Receptor
protein found in the post-synaptic membrane that binds neurotransmitter
Recreational Drug
substance that changes neurochemistry; many are illegal
Reward Pathway
neural pathway that produces pleasure

78. neuron that carries impulses into the CNS from a sense organ
Word
Definition
Sensitisation
increase in number or sensitivity of receptors as a result of exposure to drugs
Sensory Neuron
neuron that carries impulses into the CNS from a sense organ
Synapse
junction between neurons
Synaptic cleft
gap between neurons at a synapse
Synaptic vesicle
tiny vacuole containing neurotransmitter; found in pre-synaptic neurons

79. Suggested activities ?