Chapter 4 - Neurones and Synapses

Question 1

What are the functions of the nervous system?

Answer 1

• To collect information about the internal and external environment. (Receptors e.g. eyes).
• To process and integrate the information, often in relation to previous experience. (Brain and spinal cord).
• To act upon the information, usually by coordinating the organisms activities. (Effectors e.g. muscles).

Question 2

Coordination in animals, as in plants, involves …

Answer 2

Hormones (called plant growth substances in plants)

Question 3

Coordination in animals, as in plants, involves hormones (called plant growth substances in plants). However, animals also have …

Answer 3

A nervous system

Question 4

Coordination in animals, as in plants, involves hormones (called plant growth substances in plants). However, animals also have a nervous system. The nervous system is based on …

Answer 4

A system of neurones (nerve cells) that transmit electrical nerve impulses throughout the body.

Question 5

Coordination in animals, as in plants, involves hormones (called plant growth substances in plants). However, animals also have a nervous system. The nervous system is based on a system of neurones (nerve cells) that transmit electrical nerve impulses throughout the body. Fine control and integration is provided through …

Answer 5

A system of synapses (junctions) between neurones that control the nerve pathways involved.

Question 6

What are nerves?

Answer 6

Nerves are bundles of neurones (nerve cells) grouped together

Question 7

Do not confuse neurones with nerves. Nerves are bundles of neurones (nerve cells) grouped together (analogous to how …

Answer 7

Individual electrical wires are grouped in electrical cabling).

Question 8

In general, how does nervous control compare with hormone action?

Answer 8

In general, nervous control is faster and more precise than hormone action.

Question 9

What does nervous control involve?

Answer 9

Nervous control usually involves receptors and effectors with an interlinking coordinator.

Question 10

Where in the body can receptors be found?

Answer 10

In the eye, ear and nose

Question 11

What makes receptors specific in nature?

Answer 11

Each type of receptor is sensitive to a particular type of stimulus.

Question 12

What is a stimulus?

Answer 12

Something we see, hear or smell.

Question 13

What are effectors in the body?

Answer 13

Effectors are parts of the body that produce the response to the stimulus.

Question 14

In mammals effectors are often in the form of …

Answer 14

Muscle

Question 15

What does CNS stand for?

Answer 15

Central nervous system

Question 16

Coordination of the nervous system in animals invariably involves …

Answer 16

The central nervous system (CNS)

Question 17

What is the central nervous system (CNS) comprised of …

Answer 17

The brain and spinal cord

Question 18

Coordination invariably involves the central nervous system (CNS) comprising the brain and the spinal cord. Consequently, many of the neurones in the body travel to …

Answer 18

The CNS from receptors

Question 19

Coordination invariably involves the central nervous system (CNS) comprising the brain and the spinal cord. Consequently, many of the neurones in the body travel to the CNS from receptors and from …

Answer 19

The CNS to effectors

Question 20

Name the three main types of neurone

Answer 20

Motor neurones
Sensory neurones
Connector (relay, association or intermediate) neurones

Question 21

What are motor neurones?

Answer 21

Carry impulses from the CNS (brain or spinal cord) to effectors (muscles and glands).

Question 22

What are sensory neurones?

Answer 22

Carry impulses from receptors to the CNS.

Question 23

What are connector neurones?

Answer 23

Connect neurones within the CNS

Question 24

Give the similarities and differences between the three main types of neurones

Answer 24

Similarities
• Each type of neurone has the same function - to conduct nerve impulses.

Differences
• The three types of neurone differ in location.
• Furthermore, they have different shapes and sizes.

Question 25

How do neurones in the body differ from diagrammatic representations found in textbooks

Answer 25

Neurones are much longer than they appear. Some neurones can have a thousand or more Schwann cells along the length of the axon.

Question 26

Draw a diagram of a motor neurone

Answer 26

Textbook page 56

Question 27

Draw a diagram of a sensory neurone

Answer 27

Textbook page 56

Question 28

Draw a diagram of a connector neurone

Answer 28

Textbook page 56

Question 29

In simple terms, a neurone can be said to consist of …

Answer 29

A cell body and an extended nerve fibre

Question 30

The cell body of a neurone is often referred as the …

Answer 30

Centron

Question 31

What does the cell body (centron) of a nerve cell (neurone) contain?

Answer 31

A nucleus, mitochondria and other organelles as well as Nissl’s granules (large groups of ribosomes).

Question 32

Terminology surrounding the nerve fibre depends on …

Answer 32

Whether the part involved carries impulses to the cell body or away from it.

Question 33

Terminology surrounding the nerve fibre depends on whether the part involved carries impulses to the cell body or away from it. If it transmits impulses away from the cell body, it is referred to as …

Answer 33

An axon

Question 34

Terminology surrounding the nerve fibre depends on whether the part involved carries impulses to the cell body or away from it. If it transmits impulses away from the cell body, it is referred to as an axon - in motor neurones …

Answer 34

The entire fibre is an axon.

Question 35

What is an axon?

Answer 35

A nerve fibre that transmits electrical nerve impulses away from the cell body (centron).

Question 36

What is a dendron?

Answer 36

A nerve fibre that transmits electrical nerve impulses to the cell body (centron).

Question 37

Terminology surrounding the nerve fibre depends on whether the part involved carries impulses to the cell body or away from it. If it transmits impulses away from the cell body, it is referred to as an axon - in motor neurones the entire fibre is an axon. However, if a part of the fibre is carrying impulses to the cell body (as in …

Answer 37

Sensory neurones).

Question 38

Terminology surrounding the nerve fibre depends on whether the part involved carries impulses to the cell body or away from it. If it transmits impulses away from the cell body, it is referred to as an axon - in motor neurones the entire fibre is an axon. However, if a part of the fibre is carrying impulses to the cell body (as in sensory neurones) it is called …

Answer 38

A dendron

Question 39

What is a dendrite?

Answer 39

Very small (and numerous) extensions that can conduct impulses into a dendron (for example, a sensory neurone) or into the cell body directly (for example, a motor neurone).

Question 40

Axons terminate in …

Answer 40

Synaptic bulbs (knobs).

Question 41

Comment on the length of nerve fibres

Answer 41

Nerve fibres can range in length from less than a millimetre (some connector neurones) to over a metre.

Question 42

In mammals, many nerve fibres (but not all) are …

Answer 42

Myelinated

Question 43

In mammals, many nerve fibres (but not all) are myelinated. What does this mean?

Answer 43

This means that their dendrons and axons are covered with an insulating myelin sheath.

Question 44

Describe the composition of the myelin sheath

Answer 44

The myelin sheath, rich in the lipid myelin, is formed from the greatly extended cell surface membrane of Schwann cells repeatedly being wrapped round the axon or dendron.

Question 45

Comment on the arrangement of Schwann cells along myelinated nerve fibres

Answer 45

• The Schwann cells (each about 1 mm in length) are arranged at intervals along the nerve fibre with small gaps between each cell called nodes of Ranvier.
• At these nodes the dendron or axon is exposed.

Question 46

What is the function of the myelin sheath?

Answer 46

The myelin sheath is both protective in function and also serves to speed up nervous conduction.

Question 47

Draw a diagram showing a longitudinal cross-section (LS) through a myelinated nerve cell axon

Answer 47

Textbook page 57, top diagram

Question 48

Draw a diagram showing a transverse cross-section (TS) through a myelinated nerve cell axon which intersects a Schwann cell nucleus

Answer 48

Textbook page 57 diagram A

Question 49

Draw a diagram showing a transverse cross-section (TS) through a myelinated nerve cell axon which does not intersect the Schwann cell nucleus

Answer 49

Textbook page 57 diagram B

Question 50

Draw a diagram showing a transverse cross-section (TS) through a myelinated nerve cell axon which intersects a node of Ranvier

Answer 50

Textbook page 57 diagram C

Question 51

What is the cell surface membrane of a neurone also referred to as?

Answer 51

Neurilemma

Question 52

What do all myelinated and non-myelinated neurones have around their axons?

Answer 52

A protective connective tissue sheath

Textbook page 57 bottom photo

Question 53

What are nerves?

Answer 53

Bundles of neurones protected within an outer protective layer.

Question 54

What type of neurones can nerves contain?

Answer 54

Nerves can contain sensory neurones only, motor neurones only, or can be mixed and contain both types.

Question 55

What makes neurones highly specialised cells?

Answer 55

Their ability to conduct electrical impulses.

Question 56

The nerve impulse - The resting potential

Being able to conduct electrical impulses, neurones are …

Answer 56

Highly specialised cells.

Question 57

Being able to conduct electrical impulses, neurones are highly specialised cells. They have a potential difference across their cell surface membrane called …

Answer 57

A resting potential

Question 58

Being able to conduct electrical impulses, neurones are highly specialised cells. They have a potential difference across their cell surface membrane called a resting potential, ie …

Answer 58

The neurones are polarised

Question 59

Being able to conduct electrical impulses, neurones are highly specialised cells. They have a potential difference across their cell surface membrane called a resting potential, ie the neurones are polarised as …

Answer 59

There is an electrochemical gradient across the membrane.

Question 60

What is the resting potential?

Answer 60

Neurones have a potential difference across their cell surface membranes called a resting potential, ie the neurones are polarised as there is an electrochemical gradient across the membrane.

Question 61

What causes the resting potential in neurones (nerve cells)?

Answer 61

• This potential difference is caused by there being an excess of positively charged ions (Na+) outside the membrane compared with inside.
• At rest, the outside of the neurone is positive relative to the inside (or the inside is negative relative to the outside) with a potential difference of around 70 mV (millivolts).

Question 62

How is the resting potential maintained in neurones (nerve cells)?

Answer 62

This differential can be maintained as the cell surface membrane is largely impermeable to the flow of sodium ions when not conducting an impulse (an important feature as if it was permeable, the Na+ positive ions would diffuse into the neurone down the concentration gradient).

Question 63

Draw a simple diagram showing the resting potential across the cell surface membrane of a nerve cell axon in transverse cross-section

Answer 63

Textbook page 58

Question 64

• The action potential

What happens when a neurone is stimulated?

Answer 64

When a neurone is stimulated the cell surface membrane becomes permeable to ions.

Question 65

• The action potential
  When a neurone is stimulated the cell surface membrane becomes permeable to ions. With an excess of positive ions outside the neurone relative to inside, they …

Answer 65

Diffuse into the neurone down the concentration gradient.

Question 66

• The action potential
  When a neurone is stimulated the cell surface membrane becomes permeable to ions. With an excess of positive ions outside the neurone relative to inside, they diffuse into the neurone down the concentration gradient. As the potential difference across the cell surface membrane decreases, a point is reached (…

Answer 66

The outside +55 mV relative to the inside)

Question 67

• The action potential
  When a neurone is stimulated the cell surface membrane becomes permeable to ions. With an excess of positive ions outside the neurone relative to inside, they diffuse into the neurone down the concentration gradient. As the potential difference across the cell surface membrane decreases, a point is reached (the outside +55 mV relative to the inside) where …

Answer 67

A number of gated ion channels open

Question 68

• The action potential
  When a neurone is stimulated the cell surface membrane becomes permeable to ions. With an excess of positive ions outside the neurone relative to inside, they diffuse into the neurone down the concentration gradient. As the potential difference across the cell surface membrane decreases, a point is reached (the outside +55 mV relative to the inside) where a number of gated ions channels open, rapidly increasing …

Answer 68

The rate of diffusion of ions

Question 69

• The action potential
  When a neurone is stimulated the cell surface membrane becomes permeable to ions. With an excess of positive ions outside the neurone relative to inside, they diffuse into the neurone down the concentration gradient. As the potential difference across the cell surface membrane decreases, a point is reached (the outside +55 mV relative to the inside) where a number of gated ions channels open, rapidly increasing the rate of diffusion of ions leading to …

Answer 69

Depolarisation of the neurone.

Question 70

Why do ions diffuse into a neurone once said neurone has been stimulated?

Answer 70

• When a neurone is stimulated the cell surface membrane becomes permeable to ions.
• With an excess of positive ions outside the neurone relative to inside, they diffuse into the neurone down the concentration gradient.

Question 71

• The action potential
  When a neurone is stimulated the cell surface membrane becomes permeable to ions. With an excess of positive ions outside the neurone relative to inside, they diffuse into the neurone down the concentration gradient.
  What happens as the potential difference across the cell surface membrane decreases?

Answer 71

As the potential difference across the cell surface membrane decreases, a point is reached (the outside +55 mV relative to the inside) where a number of gated ion channels open, rapidly increasing the rate of diffusion of ions leading to depolarisation of the neurone.

Question 72

• The action potential
  When a neurone is stimulated the cell surface membrane becomes permeable to ions. With an excess of positive ions outside the neurone relative to inside, they diffuse into the neurone down the concentration gradient. As the potential difference across the cell surface membrane decreases, a point is reached (the outside +55 mV relative to the inside) where a number of gated ions channels open, rapidly increasing the rate of diffusion of ions leading to depolarisation of the neurone.

As positive ions flood in, the inside becomes …

Answer 72

Positive relative to the outside

Question 73

• The action potential
  When a neurone is stimulated the cell surface membrane becomes permeable to ions. With an excess of positive ions outside the neurone relative to inside, they diffuse into the neurone down the concentration gradient. As the potential difference across the cell surface membrane decreases, a point is reached (the outside +55 mV relative to the inside) where a number of gated ions channels open, rapidly increasing the rate of diffusion of ions leading to depolarisation of the neurone.

As positive ions flood in, the inside becomes positive relative to the outside, reaching a potential difference of …

Answer 73

Around 40 mV

Question 74

When a neurone is stimulated the cell surface membrane becomes permeable to ions. With an excess of positive ions outside the neurone relative to inside, they diffuse into the neurone down the concentration gradient. As the potential difference across the cell surface membrane decreases, a point is reached (the outside +55 mV relative to the inside) where a number of gated ions channels open, rapidly increasing the rate of diffusion of ions leading to depolarisation of the neurone.

As positive ions flood in, the inside becomes positive relative to the outside, reaching a potential difference of around 40 mV. This depolarisation and reversal of potential difference in neurones is called …

Answer 74

An action potential

Question 75

• The action potential
  When a neurone is stimulated the cell surface membrane becomes permeable to ions. With an excess of positive ions outside the neurone relative to inside, they diffuse into the neurone down the concentration gradient. As the potential difference across the cell surface membrane decreases, a point is reached (the outside +55 mV relative to the inside) where a number of gated ions channels open, rapidly increasing the rate of diffusion of ions leading to depolarisation of the neurone.

As positive ions flood in, the inside becomes positive relative to the outside, reaching a potential difference of around 40 mV. This depolarisation and reversal of potential difference in neurones is called an action potential, a sequence of events that takes …

Answer 75

About 1 millisecond

Question 76

What is the action potential?

Answer 76

The depolarisation and reversal of potential difference in neurones.

Question 77

What is the potential difference across the cell surface membrane of a neurone at the peak of the action potential?

Answer 77

+40 mV relative to the outside

Question 78

At the peak of the action potential (inside of neurone +40 mV relative to outside) the …

Answer 78

Recovery phase starts

Question 79

At the peak of the action potential (inside of neurone +40 mV relative to outside) the recovery phase starts and …

Answer 79

The positive ions both diffuse and are pumped out of the neurone.

Question 80

At the peak of the action potential (inside of neurone +40 mV relative to outside) the recovery phase starts and the positive ions both diffuse and are pumped out of the neurone. This rapidly …

Answer 80

Restores the resting potential

Question 81

At the peak of the action potential (inside of neurone +40 mV relative to outside) the recovery phase starts and the positive ions both diffuse and are pumped out of the neurone. This rapidly restores the resting potential and the cell surface membrane becomes …

Answer 81

Largely impermeable again to Na+

Question 82

At the peak of the action potential (inside of neurone +40 mV relative to outside) the recovery phase starts and the positive ions both diffuse and are pumped out of the neurone. This rapidly restores the resting potential and the cell surface membrane becomes largely impermeable again to Na+. During this recovery phase (…

Answer 82

Refractory period)

Question 83

At the peak of the action potential (inside of neurone +40 mV relative to outside) the recovery phase starts and the positive ions both diffuse and are pumped out of the neurone. This rapidly restores the resting potential and the cell surface membrane becomes largely impermeable again to Na+. During this recovery phase (refractory period) a …

Answer 83

Further impulse cannot occur

Question 84

At the peak of the action potential (inside of neurone +40 mV relative to outside) the recovery phase starts and the positive ions both diffuse and are pumped out of the neurone. This rapidly restores the resting potential and the cell surface membrane becomes largely impermeable again to Na+. During this recovery phase (refractory period) a further impulse cannot occur as …

Answer 84

The gated ion channels are closed and the resting potential has not been fully restored.

Question 85

At the peak of the action potential (inside of neurone +40 mV relative to outside) the recovery phase starts and the positive ions both diffuse and are pumped out of the neurone. This rapidly restores the resting potential and the cell surface membrane becomes largely impermeable again to Na+. During this recovery phase (refractory period) a further impulse cannot occur as the gated ion channels are closed and the resting potential has not been fully restored. The entire electrochemical sequence of events associated with an action potential takes …

Answer 85

About 4 milliseconds

Question 86

What happens at the peak of the action potential in a neurone?

Answer 86

At the peak of the action potential (inside of neurone +40 mV) the recovery phase starts and the positive ions both diffuse and are pumped out of the neurone.

Question 87

What happens during the recovery phase of an action potential?

Answer 87

1. At the peak of the action potential (inside of neurone +40 mV) the recovery phase starts and the positive ions both diffuse and are pumped out of the neurone.
2. This rapidly restores the resting potential and the cell surface membrane becomes largely impermeable again to Na+.
3. During this recovery phase (refractory period) a further impulse cannot occur as the gated ion channels are closed and the resting potential has not been fully restored.
4. The entire electrochemical sequence of events associated with an action potential take about 4 milliseconds.

Question 88

Draw a graph showing the changes in the potential difference across the axon membrane of a neurone as an action potential occurs

Answer 88

Textbook page 58

Question 89

Page 58 of the textbook shows a graph displaying the changes in the potential difference across the axon membrane of a neurone as an action potential occurs. Explain what happens during period 1.

Answer 89

Period 1 = Resting potential with inside of axon -70 mV relative to outside (another way of saying outside of membrane is +70 mV relative to inside).

Question 90

Page 58 of the textbook shows a graph displaying the changes in the potential difference across the axon membrane of a neurone as an action potential occurs. Explain what happens during period 2.

Answer 90

Membrane becomes permeable and positive ions diffuse into the axon - membrane is starting to become depolarised.

Question 91

Page 58 of the textbook shows a graph displaying the changes in the potential difference across the axon membrane of a neurone as an action potential occurs. Explain what happens during period 3.

Answer 91

• At -55 mV (inside relative to the outside) gated channels open and positive ions flood in at an even more rapid rate.
• Rapid depolarisation of the membrane takes place and the inside becomes +40 mV relative to the outside - the action potential.

Question 92

Page 58 of the textbook shows a graph displaying the changes in the potential difference across the axon membrane of a neurone as an action potential occurs. Explain what happens during period 4.

Answer 92

• Positive ions diffuse out and are also pumped out of the axon.
• This stage is called the refractory period as the membrane cannot be depolarised again until the resting potential is restored.
• At the end of this stage there is a slight ‘overshoot’ as the inside of the axon membrane becomes slightly more negative (hyperpolarisation) than in the normal resting potential.

Question 93

Page 58 of the textbook shows a graph displaying the changes in the potential difference across the axon membrane of a neurone as an action potential occurs. Explain what happens during period 5.

Answer 93

The resting potential is restored and the axon can conduct another nerve impulse if stimulated.

Question 94

The sequence of events in the diagram on textbook page 58 representing the action potential describes the sequence …

Answer 94

As it occurs at one point on the axon over a very short period of time (a few milliseconds).

Question 95

What are the functions of the refractory period?

Answer 95

1. It ensures that the action potentials are propagated in one direction only. This is important as axons are physiologically capable of transmitting an impulse in either direction.
2. It also limits the number of action potentials that can be fired and ensures each action potential is a discrete entity.

Question 96

What is the threshold stimulus?

Answer 96

The threshold stimulus refers to the level of stimulus a neurone requires before an action potential is produced, for example, a small degree of polarisation in the cell surface membrane of the neurone can occur without resulting in an action potential but at a critical point (the threshold potential) an action potential will result.

Question 97

What is the all-or-nothing-law?

Answer 97

The all-or-nothing-law refers to the principle that once the threshold stimulus is reached, the action potential results, ie an action potential either occurs or it does not; different intensities of action potential do not occur - they are all the same.

Question 98

Draw a graph illustrating the concept of the threshold stimulus and the all-or-nothing-law

Answer 98

Textbook page 59

Question 99

Once an action potential occurs it sweeps along the neurone as …

Answer 99

A nerve impulse

Question 100

Describe the process of nerve impulse propagation

Answer 100

• Action potentials ‘move’ very rapidly along neurones.
• In reality a wave of depolarisation moves rapidly along the neurone.
• Just as quickly the region immediately behind the depolarised zone becomes repolarised.
• As one part of the membrane becomes depolarised, it sets up local (electrical) circuits with the areas immediately adjacent on either side.
• Positive ions from the depolarised zone pass along the inside of the membrane towards the polarised zone immediately in front.
• A similar effect occurs on the outside of the membrane, where positive ions move back from the (as yet) still polarised zone into the depolarised zone.
• It is these processes occurring continuously that creates a wave of depolarisation that moves rapidly along the neurone.
• Similar circuits enable the resting potential to be restored directly behind the action potential.

Question 101

Draw a simple diagram illustrating the localised electrical circuits and nerve impulse propagation along the axon of a neurone

Answer 101

Textbook page 59

Question 102

What is the zone of depolarisation?

Answer 102

The zone of depolarisation is the part of the neurone where polarity is reversed, ie the inside is positive relative to the outside.

Question 103

Some textbooks refer to the propagation of the nerve impulse as being analogous to …

Answer 103

A ‘Mexican wave’ moving around a sports arena

Question 104

What factors affect the speed of the nerve impulse?

Answer 104

1. The presence or absence of a myelin sheath
2. The thickness of the axon
3. Temperature

Question 105

• Factors affecting the speed of the nerve impulse
  The speed of the nerve impulse is affected by a number of factors including the presence or absence of a myelin sheath and the thickness of the axon.

The myelin sheath acts as …

Answer 105

An electrical insulator in myelinated neurones.

Question 106

• Factors affecting the speed of the nerve impulse
  The speed of the nerve impulse is affected by a number of factors including the presence or absence of a myelin sheath and the thickness of the axon.

The myelin sheath acts as an electrical insulator in myelinated neurones. As an insulator it …

Answer 106

Prevents depolarisation in that part of the neurone

Question 107

• Factors affecting the speed of the nerve impulse
  The speed of the nerve impulse is affected by a number of factors including the presence or absence of a myelin sheath and the thickness of the axon.

The myelin sheath acts as an electrical insulator in myelinated neurones. As an insulator it prevents depolarisation in that part of the neurone. However, every 1-2 mm along the neurone the sheath is …

Answer 107

Disrupted

Question 108

• Factors affecting the speed of the nerve impulse
  The speed of the nerve impulse is affected by a number of factors including the presence or absence of a myelin sheath and the thickness of the axon.

The myelin sheath acts as an electrical insulator in myelinated neurones. As an insulator it prevents depolarisation in that part of the neurone. However, every 1-2 mm along the neurone the sheath is disrupted - these breaks are …

Answer 108

The junctions between adjacent Schwann cells.

Question 109

• Factors affecting the speed of the nerve impulse
  The speed of the nerve impulse is affected by a number of factors including the presence or absence of a myelin sheath and the thickness of the axon.

The myelin sheath acts as an electrical insulator in myelinated neurones. As an insulator it prevents depolarisation in that part of the neurone. However, every 1-2 mm along the neurone the sheath is disrupted - these breaks are the junctions between adjacent Schwann cells. At these points, called …

Answer 109

Nodes of Ranvier

Question 110

• Factors affecting the speed of the nerve impulse
  The speed of the nerve impulse is affected by a number of factors including the presence or absence of a myelin sheath and the thickness of the axon.

The myelin sheath acts as an electrical insulator in myelinated neurones. As an insulator it prevents depolarisation in that part of the neurone. However, every 1-2 mm along the neurone the sheath is disrupted - these breaks are the junctions between adjacent Schwann cells. At these points, called nodes of Ranvier, depolarisation can take place. The local circuits from …

Answer 110

Between the nodes only

Question 111

• Factors affecting the speed of the nerve impulse
  The speed of the nerve impulse is affected by a number of factors including the presence or absence of a myelin sheath and the thickness of the axon.

The myelin sheath acts as an electrical insulator in myelinated neurones. As an insulator it prevents depolarisation in that part of the neurone. However, every 1-2 mm along the neurone the sheath is disrupted - these breaks are the junctions between adjacent Schwann cells. At these points, called nodes of Ranvier, depolarisation can take place. The local circuits from between the nodes only, allowing …

Answer 111

Sections of the neurone to be bypassed

Question 112

• Factors affecting the speed of the nerve impulse
  The speed of the nerve impulse is affected by a number of factors including the presence or absence of a myelin sheath and the thickness of the axon.

The myelin sheath acts as an electrical insulator in myelinated neurones. As an insulator it prevents depolarisation in that part of the neurone. However, every 1-2 mm along the neurone the sheath is disrupted - these breaks are the junctions between adjacent Schwann cells. At these points, called nodes of Ranvier, depolarisation can take place. The local circuits from between the nodes only, allowing sections of the neurone to be bypassed. The action potentials …

Answer 112

‘Jump’ from one node to the next

Question 113

• Factors affecting the speed of the nerve impulse
  The speed of the nerve impulse is affected by a number of factors including the presence or absence of a myelin sheath and the thickness of the axon.

The myelin sheath acts as an electrical insulator in myelinated neurones. As an insulator it prevents depolarisation in that part of the neurone. However, every 1-2 mm along the neurone the sheath is disrupted - these breaks are the junctions between adjacent Schwann cells. At these points, called nodes of Ranvier, depolarisation can take place. The local circuits from between the nodes only, allowing sections of the neurone to be bypassed. The action potentials ‘jump’ from one node to the next in a process called …

Answer 113

Saltatory conduction

Question 114

What is the role of the myelin sheath in relation to the speed of the nerve impulse?

Answer 114

• The myelin sheath acts as an electrical insulator in myelinated neurones.
• As an insulator it prevents depolarisation in that part of the neurone.
• However, every 1-2 mm along the neurone the sheath is disrupted - these breaks are the junctions between adjacent Schwann cells.
• At these points, called nodes of Ranvier, depolarisation can take place.
• The local circuits form between the nodes only, allowing sections of the neurone to be bypassed.
• The action potentials ‘jump’ from one node to the next in a process called saltatory conduction.

Question 115

• Factors affecting the speed of the nerve impulse

The diameter of the axon also affects the speed of impulse. In general, …

Answer 115

The thicker the axon, the faster the impulse.

Question 116

• Factors affecting the speed of the nerve impulse

The diameter of the axon also affects the speed of impulse. In general, the thicker the axon, the faster the impulse. This is because …

Answer 116

There is proportionally less ‘leakage’ of ions in a neurone with a larger diameter.

Question 117

• Factors affecting the speed of the nerve impulse

The diameter of the axon also affects the speed of impulse. In general, the thicker the axon, the faster the impulse. This is because there is proportionally less ‘leakage’ of ions in a neurone with a larger diameter. If there is too much leakage, as can happen in …

Answer 117

Axons with very small diameters

Question 118

• Factors affecting the speed of the nerve impulse

The diameter of the axon also affects the speed of impulse. In general, the thicker the axon, the faster the impulse. This is because there is proportionally less ‘leakage’ of ions in a neurone with a larger diameter. If there is too much leakage, as can happen in axons with very small diameters, it makes it very difficult to …

Answer 118

Maintain the potential gradients required to form resting and action potentials.

Question 119

How does the diameter of an axon affect the speed of the nerve impulse?

Answer 119

• In general, the thicker the axon, the faster the impulse.
• This is because there is proportionally less ‘leakage’ of ions in a neurone with a larger diameter (ie surface-area-to-volume ratio).
• If there is too much leakage, as can happen in axons with very small diameters, it makes it very difficult to maintain the potential gradients required to form resting and action potentials.

Question 120

Draw a simple diagram illustrating saltatory conduction in a myelinated neurone

Answer 120

Textbook page 60

Question 121

In myelinated neurones, there are relatively few …

Answer 121

Ion channels under the fatty myelin sheath

Question 122

In myelinated neurones, there are relatively few ion channels under the fatty myelin sheath (they are concentrated at …

Answer 122

The nodes of Ranvier).

Question 123

In myelinated neurones, there are relatively few ion channels under the fatty myelin sheath (they are concentrated at the nodes of Ranvier). Consequently myelination tends to overcome …

Answer 123

The problems presented by neurones of small diameter

Question 124

In myelinated neurones, there are relatively few ion channels under the fatty myelin sheath (they are concentrated at the nodes of Ranvier). Consequently myelination tends to overcome the problems presented by neurones of small diameter, in addition to producing …

Answer 124

The faster speeds associated with saltatory conduction

Question 125

In myelinated neurones, there are relatively few ion channels under the fatty myelin sheath (they are concentrated at the nodes of Ranvier). Consequently myelination tends to overcome the problems presented by neurones of small diameter, in addition to producing the faster speeds associated with saltatory conduction, for example, …

Answer 125

100 m/s

Question 126

Comment on the distribution of ion channels in myelinated neurones

Answer 126

In myelinated neurones, there are relatively few ion channels under the fatty myelin sheath (they are concentrated at the nodes of Ranvier).

Question 127

Comment on the advantages of myelination of neurones

Answer 127

• In myelinated neurones, there are relatively few ion channels under the fatty myelin sheath (they are concentrated at the nodes of Ranvier).
• Consequently myelination tends to overcome the problems presented by neurones of small diameter (which are prone to leakage of ions), in addition to producing the faster speeds associated with saltatory conduction, for example, 100 m/s.

Question 128

How does temperature affect the speed of the nerve impulse?

Answer 128

• As with many metabolic processes, the speed of a nerve impulse is affected by temperature.
• As temperature affects the rate of diffusion of ions involved in neurone action, it affects the speed that neurones can conduct impulses.

Question 129

What are synapses?

Answer 129

Synapses are junctions between the axon of one neurone and the dendrite (or dendron/centron) of an adjacent neurone.

A synapse includes the synaptic bulb, the synaptic cleft and the post-synaptic neurone membrane.

Question 130

• Structure of synapses

Impulses are transmitted from one neurone to another by …

Answer 130

Chemicals called neurotransmitters

Question 131

• Structure of synapses

Impulses are transmitted from one neurone to another by chemicals called neurotransmitters that …

Answer 131

Diffuse across a very small gap

Question 132

• Structure of synapses
  Impulses are transmitted from one neurone to another by chemicals called neurotransmitters that diffuse across a very small gap, the …

Answer 132

Synaptic cleft

Question 133

• Structure of synapses
  Impulses are transmitted from one neurone to another by chemicals called neurotransmitters that diffuse across a very small gap, the synaptic cleft, which is _______ wide.

Answer 133

20-30 nm

Question 134

• Structure of synapses
  Impulses are transmitted from one neurone to another by chemicals called neurotransmitters that diffuse across a very small gap, the synaptic cleft, which is 20-30 nm wide. Both the pre-synaptic neurone, the neurone that …

Answer 134

Releases the transmitter

Question 135

• Structure of synapses
  Impulses are transmitted from one neurone to another by chemicals called neurotransmitters that diffuse across a very small gap, the synaptic cleft, which is 20-30 nm wide. Both the pre-synaptic neurone, the neurone that releases the transmitter, and the post-synaptic neurone, the …

Answer 135

Adjacent neurone that receives the diffusing neurotransmitter

Question 136

• Structure of synapses
  Impulses are transmitted from one neurone to another by chemicals called neurotransmitters that diffuse across a very small gap, the synaptic cleft, which is 20-30 nm wide. Both the pre-synaptic neurone, the neurone that releases the transmitter, and the post-synaptic neurone, the adjacent neurone that receives the diffusing neurotransmitter, are …

Answer 136

Specialised

Question 137

• Structure of synapses
  Impulses are transmitted from one neurone to another by chemicals called neurotransmitters that diffuse across a very small gap, the synaptic cleft, which is 20-30 nm wide. Both the pre-synaptic neurone, the neurone that releases the transmitter, and the post-synaptic neurone, the adjacent neurone that receives the diffusing neurotransmitter, are specialised for …

Answer 137

Their roles in synaptic transmission

Question 138

What is the pre-synaptic neurone?

Answer 138

The neurone that releases the neurotransmitter.

Question 139

What is the post-synaptic neurone?

Answer 139

The adjacent neurone that receives the diffusing neurotransmitter.

Question 140

Briefly state how impulses are transmitted from one neurone to another

Answer 140

Impulses are transmitted from one neurone to another by chemicals called neurotransmitters that diffuse across a very small gap (the synaptic cleft) causing depolarisation in the post-synaptic neurone allowing the nerve impulse to continue from one neurone to the next.

Question 141

How wide is the synaptic cleft?

Answer 141

20-30 nm wide

Question 142

How is the pre-synaptic neurone specialised for its role in synaptic transmission?

Answer 142

• The end of the pre-synaptic neurone is thickened into a synaptic bulb (knob).
• Synaptic bulbs contain large numbers of mitochondria (important in manufacturing the neurotransmitter) and synaptic vesicles in which the neurotransmitter is stored.
• The pre-synaptic membrane contains numerous calcium ion (Ca2+) channels

Question 143

How is the post-synaptic neurone specialised for its role in synaptic transmission?

Answer 143

• The membrane of the post-synaptic neurone contains receptors complementary to the type of neurotransmitter involved in that particular synapse.
• The membrane also contains specific membrane bound enzymes that hydrolyse the neurotransmitter preventing continuous action potential generation in the post-synaptic neurone.

Question 144

Draw a diagram of a typical synapse

Answer 144

Textbook page 61

Question 145

Outline the sequence of events that occur during nerve impulse transmission at synapses

Answer 145

1. When an impulse arrives at the end of a neurone (synaptic bulb), calcium ion (Ca2+) channels open allowing calcium ions to diffuse into the synaptic bulb.
2. The calcium ions cause the synaptic vesicles to move towards the pre-synaptic membrane.
3. The vesicles fuse with the pre-synaptic membrane, releasing the neurotransmitter (typically acetylcholine) by exocytosis into the synaptic cleft.
4. The acetylcholine diffuses across the synaptic cleft and binds to acetylcholine receptors in the post-synaptic membrane.
5. This causes the opening of ion (Na+) channels in the membrane of the post-synaptic neurone. As positive ions diffuse in, the membrane becomes gradually depolarised and an excitatory post-synaptic potential (EPSP) is generated.
6. If sufficient depolarisation can take place (dependent on the number of neurotransmitter molecules filling receptor sites), the EPSP will reach the threshold intensity required to produce an action potential in the post-synaptic neurone.
7. The enzyme acetylcholinesterase (attached to the post-synaptic membrane) breaks down the acetylcholine. The breakdown products, choline and ethanoic acid (acetyl), are released into the cleft. It is very important that the acetylcholine is broken down and does not continually remain in a receptor - this prevents it continuously generating a new action potential in the post-synaptic neurone.
8. The breakdown products diffuse across the cleft and are reabsorbed into the synaptic bulb. They are subsequently re-synthesised into acetylcholine which is stored in the synaptic vesicles to be used again. The ATP required is produced by the mitochondria.

Question 146

Compare the relative speed of the diffusion of chemicals across a short gap with the conduction of a nerve impulse along a myelinated neurone

Answer 146

• The diffusion of chemicals across a short gap is necessarily slower than the conduction of an impulse along myelinated neurones.
• However, due to the very short distances involved it is still very fast.

Question 147

• Function of synapses

What are the advantages to the nervous system of possessing synapses?

Answer 147

1. Synapses enable nerve impulses to pass from neurone to neurone - Synapses allow nervous communication to continue throughout the body, even though the nerve ‘hardware’ (neurones) is not continuous.
2. They ensure unidirectionality - Nerve impulses can only pass from the pre-synaptic neurone to the post-synaptic neurone, as the neurotransmitter is only made in the pre-synaptic neurone and neurotransmitter receptors are only in the membrane of the post-synaptic neurone.
3. They prevent the overstimulation of effectors (for example, muscles) - Too many impulses passing along the same neurone in a short period of time will exhaust the supply of the neurotransmitter more quickly than it can be built up - the synapses fatigue.
4. They provide integration - This may involve a number of pre-synaptic neurones forming junctions with one post-synaptic neurone. In effect, synapses provide flexibility - if there were no synapses, nervous activity would be little more than a series of reflexes with a particular stimulus producing an automatic and never-changing response. Integration is aided through the process of summation.
5. Synapses also have an important role in filtering out low-level background stimuli thus preventing overload and overstimulation.

Question 148

Label the structures evident on the TEM of a synapse in the brain. Textbook page 62

Answer 148

Pre-synaptic neurone
Post-synaptic neurone
Synaptic cleft
Synaptic vesicles
Mitochondrion

Question 149

In the TEM (transmission electron microscope) of a synapse in the brain on page 62 of the textbook, the axons of the two neurones extending beyond the synapses are not evident. Suggest an explanation for this.

Answer 149

This is because the axons were in different planes to the very thin section chosen for this photograph.

Question 150

Why is summation important?

Answer 150

Summation is important in providing the complexity and flexibility that synapses demonstrate.

Question 151

Give an example of why summation is important

Answer 151

For example, an infrequent action potential reaching a synapse may not be sufficient to cause an action potential in the adjacent post-synaptic neurone. However, a series of impulses travelling along the same neurone (temporal summation) or a number of pre-synaptic neurones operating in unison (spatial summation), each releasing neurotransmitter chemicals, may be enough to cause a sufficient EPSP to trigger an impulse in the post-synaptic neurone.

Question 152

The synapses discussed so far refer to …

Answer 152

Excitatory synapses

Question 153

What is the function of excitatory synapses?

Answer 153

In excitatory synapses neurotransmitter chemicals are released with the function of causing an EPSP and a subsequent action potential.

Question 154

What is the function of inhibitory synapses?

Answer 154

Inhibitory synapses have the function of making it more difficult for synaptic transmission to take place.

Question 155

How do inhibitory synapses make it more difficult for synaptic transmission to take place?

Answer 155

• The neurotransmitter they release makes it more difficult for an EPSP to form in the post-synaptic membrane.
• The inhibitory neurotransmitters lead to an influx of negative ions in the post-synaptic membrane, making the inside of the membrane even more negative, thus creating an inhibitory post-synaptic potential (IPSP) in the post-synaptic neurone which is even more negative than the normal resting potential.
• Consequently, this hyperpolarisation makes it even more difficult than normal for excitatory synapses to produce an EPSP that reaches the threshold level.

Question 156

What is summation?

Answer 156

Summation, which includes both spatial and temporal summation, is the process that determines whether or not an action potential will be generated by the combined effects of excitatory and inhibitory signals.

Question 157

What factors affect whether an impulse will actually take place in the post-synaptic neurone?

Answer 157

Whether an impulse will actually take place in the post-synaptic neurone depends on the relative contribution excitatory and inhibitory synapses make in promoting or inhibiting depolarisation.

Question 158

Why do we have inhibitory synapses in our nervous system?

Answer 158

They can help by reducing the input of background stimuli that would clutter up the nervous activity in the brain or may prevent some reflex actions.

Question 159

How do the effects of summation and the action of inhibitory synapses benefit the nervous system?

Answer 159

• The effects of summation and the action of inhibitory synapses provide integration and fine control through the synapse integrating all the different inputs (there may be hundreds) at synaptic junctions.
• Synapses also have an important role in filtering out low-level background stimuli thus preventing overload and overstimulation.

Question 160

What is the primary neurotransmitter substance in the central nervous system (CNS) of vertebrates?

Answer 160

Acetylcholine

Question 161

• Neurotransmitter substances

Acetylcholine is …

Answer 161

The primary transmitter substance in the central nervous system (CNS) of vertebrates

Question 162

• Neurotransmitter substances
  Acetylcholine is the primary transmitter substance in the central nervous system (CNS) of vertebrates. Name one other type of neurotransmitter.

Answer 162

Noradrenaline (typically used in involuntary nervous control, for example, the regulation of gut movements)

Question 163

How many types of neurotransmitter can each synaptic bulb produce?

Answer 163

Each synaptic bulb produces only one type of neurotransmitter.

Question 164

Give an example of a neurotransmitter that is released at inhibitory synapses

Answer 164

GABA (aminobutyric acid)

Question 165

What is GABA?

Answer 165

• GABA (aminobutyric acid) is an example of a neurotransmitter that is released at inhibitory synapses.
• It is the chief inhibitory neurotransmitter in mammals.

Question 166

How does GABA work?

Answer 166

GABA causes negative ions to flow into post-synaptic neurones, therefore causing hyperpolarisation.

Question 167

What is the role of GABA in mammals?

Answer 167

GABA has many roles including helping reduce anxiety and ‘panic attacks’ through a ‘damping down’ of the nerve pathways in the brain.

Question 168

What effect does the drug nicotine have on the nervous system?

Answer 168

Stimulates the release of acetylcholine and other neurotransmitters making action potentials more likely.

Question 169

What effect does the drug curare have on the nervous system?

Answer 169

Blocks receptors (at neuromuscular junctions) preventing synaptic transmission - loss of muscle function.

Question 170

What effect do opioids have on the nervous system?

Answer 170

• Block the calcium channels in the pre-synaptic neurone.
• Less transmitter substance is released and action potentials less likely.
• Opioids and related compounds can provide pain relief by reducing impulses coming from the pain receptors.

Question 171

Many examination questions involve what type of drugs?

Answer 171

• Drugs that either stimulate, or are antagonistic to, neurotransmitters.
• The questions often provide information concerning the function of the drug involved and you would be expected to deduce the effect it would have on synaptic transmission.