Chapter 4 - Neurones and Synapses Flashcards
1
Q
What are the functions of the nervous system?
A
- 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).
2
Q
Coordination in animals, as in plants, involves …
A
Hormones (called plant growth substances in plants)
3
Q
Coordination in animals, as in plants, involves hormones (called plant growth substances in plants). However, animals also have …
A
A nervous system
4
Q
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
A system of neurones (nerve cells) that transmit electrical nerve impulses throughout the body.
5
Q
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 …
A
A system of synapses (junctions) between neurones that control the nerve pathways involved.
6
Q
What are nerves?
A
Nerves are bundles of neurones (nerve cells) grouped together
7
Q
Do not confuse neurones with nerves. Nerves are bundles of neurones (nerve cells) grouped together (analogous to how …
A
Individual electrical wires are grouped in electrical cabling).
8
Q
In general, how does nervous control compare with hormone action?
A
In general, nervous control is faster and more precise than hormone action.
9
Q
What does nervous control involve?
A
Nervous control usually involves receptors and effectors with an interlinking coordinator.
10
Q
Where in the body can receptors be found?
A
In the eye, ear and nose
11
Q
What makes receptors specific in nature?
A
Each type of receptor is sensitive to a particular type of stimulus.
12
Q
What is a stimulus?
A
Something we see, hear or smell.
13
Q
What are effectors in the body?
A
Effectors are parts of the body that produce the response to the stimulus.
14
Q
In mammals effectors are often in the form of …
A
Muscle
15
Q
What does CNS stand for?
A
Central nervous system
16
Q
Coordination of the nervous system in animals invariably involves …
A
The central nervous system (CNS)
17
Q
What is the central nervous system (CNS) comprised of …
A
The brain and spinal cord
18
Q
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 …
A
The CNS from receptors
19
Q
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 …
A
The CNS to effectors
20
Q
Name the three main types of neurone
A
Motor neurones
Sensory neurones
Connector (relay, association or intermediate) neurones
21
Q
What are motor neurones?
A
Carry impulses from the CNS (brain or spinal cord) to effectors (muscles and glands).
22
Q
What are sensory neurones?
A
Carry impulses from receptors to the CNS.
23
Q
What are connector neurones?
A
Connect neurones within the CNS
24
Q
Give the similarities and differences between the three main types of neurones
A
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.
25
How do neurones in the body differ from diagrammatic representations found in textbooks
Neurones are much longer than they appear. Some neurones can have a thousand or more Schwann cells along the length of the axon.
26
Draw a diagram of a motor neurone
Textbook page 56
27
Draw a diagram of a sensory neurone
Textbook page 56
28
Draw a diagram of a connector neurone
Textbook page 56
29
In simple terms, a neurone can be said to consist of ...
A cell body and an extended nerve fibre
30
The cell body of a neurone is often referred as the ...
Centron
31
What does the cell body (centron) of a nerve cell (neurone) contain?
A nucleus, mitochondria and other organelles as well as Nissl's granules (large groups of ribosomes).
32
Terminology surrounding the nerve fibre depends on ...
Whether the part involved carries impulses to the cell body or away from it.
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 ...
An axon
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 ...
The entire fibre is an axon.
35
What is an axon?
A nerve fibre that transmits electrical nerve impulses away from the cell body (centron).
36
What is a dendron?
A nerve fibre that transmits electrical nerve impulses to the cell body (centron).
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 ...
Sensory neurones).
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 ...
A dendron
39
What is a dendrite?
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).
40
Axons terminate in ...
Synaptic bulbs (knobs).
41
Comment on the length of nerve fibres
Nerve fibres can range in length from less than a millimetre (some connector neurones) to over a metre.
42
In mammals, many nerve fibres (but not all) are ...
Myelinated
43
In mammals, many nerve fibres (but not all) are myelinated. What does this mean?
This means that their dendrons and axons are covered with an insulating myelin sheath.
44
Describe the composition of the myelin sheath
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.
45
Comment on the arrangement of Schwann cells along myelinated nerve fibres
* 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.
46
What is the function of the myelin sheath?
The myelin sheath is both protective in function and also serves to speed up nervous conduction.
47
Draw a diagram showing a longitudinal cross-section (LS) through a myelinated nerve cell axon
Textbook page 57, top diagram
48
Draw a diagram showing a transverse cross-section (TS) through a myelinated nerve cell axon which intersects a Schwann cell nucleus
Textbook page 57 diagram A
49
Draw a diagram showing a transverse cross-section (TS) through a myelinated nerve cell axon which does not intersect the Schwann cell nucleus
Textbook page 57 diagram B
50
Draw a diagram showing a transverse cross-section (TS) through a myelinated nerve cell axon which intersects a node of Ranvier
Textbook page 57 diagram C
51
What is the cell surface membrane of a neurone also referred to as?
Neurilemma
52
What do all myelinated and non-myelinated neurones have around their axons?
A protective connective tissue sheath
Textbook page 57 bottom photo
53
What are nerves?
Bundles of neurones protected within an outer protective layer.
54
What type of neurones can nerves contain?
Nerves can contain sensory neurones only, motor neurones only, or can be mixed and contain both types.
55
What makes neurones highly specialised cells?
Their ability to conduct electrical impulses.
56
The nerve impulse - The resting potential
| Being able to conduct electrical impulses, neurones are ...
Highly specialised cells.
57
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
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 ...
The neurones are polarised
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 ...
There is an electrochemical gradient across the membrane.
60
What is the resting potential?
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.
61
What causes the resting potential in neurones (nerve cells)?
* 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).
62
How is the resting potential maintained in neurones (nerve cells)?
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).
63
Draw a simple diagram showing the resting potential across the cell surface membrane of a nerve cell axon in transverse cross-section
Textbook page 58
64
- The action potential
| What happens when a neurone is stimulated?
When a neurone is stimulated the cell surface membrane becomes permeable to ions.
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 ...
Diffuse into the neurone down the concentration gradient.
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 (...
The outside +55 mV relative to the inside)
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 ...
A number of gated ion channels open
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 ...
The rate of diffusion of ions
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 ...
Depolarisation of the neurone.
70
Why do ions diffuse into a neurone once said neurone has been stimulated?
* 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.
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?
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.
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 ...
Positive relative to the outside
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 ...
Around 40 mV
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 ...
An action potential
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 ...
About 1 millisecond
76
What is the action potential?
The depolarisation and reversal of potential difference in neurones.
77
What is the potential difference across the cell surface membrane of a neurone at the peak of the action potential?
+40 mV relative to the outside
78
At the peak of the action potential (inside of neurone +40 mV relative to outside) the ...
Recovery phase starts
79
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.
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 ...
Restores the resting potential
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 ...
Largely impermeable again to Na+
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 (...
Refractory period)
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 ...
Further impulse cannot occur
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 ...
The gated ion channels are closed and the resting potential has not been fully restored.
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 ...
About 4 milliseconds
86
What happens at the peak of the action potential in a neurone?
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.
87
What happens during the recovery phase of an action potential?
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.
88
Draw a graph showing the changes in the potential difference across the axon membrane of a neurone as an action potential occurs
Textbook page 58
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.
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).
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.
Membrane becomes permeable and positive ions diffuse into the axon - membrane is starting to become depolarised.
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.
* 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.
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.
* 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.
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.
The resting potential is restored and the axon can conduct another nerve impulse if stimulated.
94
The sequence of events in the diagram on textbook page 58 representing the action potential describes the sequence ...
As it occurs at one point on the axon over a very short period of time (a few milliseconds).
95
What are the functions of the refractory period?
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.
96
What is the threshold stimulus?
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.
97
What is the all-or-nothing-law?
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.
98
Draw a graph illustrating the concept of the threshold stimulus and the all-or-nothing-law
Textbook page 59
99
Once an action potential occurs it sweeps along the neurone as ...
A nerve impulse
100
Describe the process of nerve impulse propagation
* 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.
101
Draw a simple diagram illustrating the localised electrical circuits and nerve impulse propagation along the axon of a neurone
Textbook page 59
102
What is the zone of depolarisation?
The zone of depolarisation is the part of the neurone where polarity is reversed, ie the inside is positive relative to the outside.
103
Some textbooks refer to the propagation of the nerve impulse as being analogous to ...
A 'Mexican wave' moving around a sports arena
104
What factors affect the speed of the nerve impulse?
1. The presence or absence of a myelin sheath
2. The thickness of the axon
3. Temperature
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 ...
An electrical insulator in myelinated neurones.
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 ...
Prevents depolarisation in that part of the neurone
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 ...
Disrupted
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 ...
The junctions between adjacent Schwann cells.
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 ...
Nodes of Ranvier
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 ...
Between the nodes only
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 ...
Sections of the neurone to be bypassed
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 ...
'Jump' from one node to the next
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 ...
Saltatory conduction
114
What is the role of the myelin sheath in relation to the speed of the nerve impulse?
* 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.
115
- 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.
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 ...
There is proportionally less 'leakage' of ions in a neurone with a larger diameter.
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 ...
Axons with very small diameters
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 ...
Maintain the potential gradients required to form resting and action potentials.
119
How does the diameter of an axon affect the speed of the nerve 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 (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.
120
Draw a simple diagram illustrating saltatory conduction in a myelinated neurone
Textbook page 60
121
In myelinated neurones, there are relatively few ...
Ion channels under the fatty myelin sheath
122
In myelinated neurones, there are relatively few ion channels under the fatty myelin sheath (they are concentrated at ...
The nodes of Ranvier).
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 ...
The problems presented by neurones of small diameter
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 ...
The faster speeds associated with saltatory conduction
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, ...
100 m/s
126
Comment on the distribution of ion channels in myelinated neurones
In myelinated neurones, there are relatively few ion channels under the fatty myelin sheath (they are concentrated at the nodes of Ranvier).
127
Comment on the advantages of myelination of neurones
* 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.
128
How does temperature affect the speed of the nerve impulse?
* 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.
129
What are synapses?
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.
130
- Structure of synapses
| Impulses are transmitted from one neurone to another by ...
Chemicals called neurotransmitters
131
- Structure of synapses
| Impulses are transmitted from one neurone to another by chemicals called neurotransmitters that ...
Diffuse across a very small gap
132
- 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
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.
20-30 nm
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- 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
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 ...
Adjacent neurone that receives the diffusing neurotransmitter
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 ...
Specialised
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 ...
Their roles in synaptic transmission
138
What is the pre-synaptic neurone?
The neurone that releases the neurotransmitter.
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What is the post-synaptic neurone?
The adjacent neurone that receives the diffusing neurotransmitter.
140
Briefly state how impulses are transmitted from one neurone to another
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.
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How wide is the synaptic cleft?
20-30 nm wide
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How is the pre-synaptic neurone specialised for its role in synaptic transmission?
* 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
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How is the post-synaptic neurone specialised for its role in synaptic transmission?
* 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.
144
Draw a diagram of a typical synapse
Textbook page 61
145
Outline the sequence of events that occur during nerve impulse transmission at synapses
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.
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Compare the relative speed of the diffusion of chemicals across a short gap with the conduction of a nerve impulse along a myelinated neurone
* 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.
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- Function of synapses
| What are the advantages to the nervous system of possessing synapses?
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.
148
Label the structures evident on the TEM of a synapse in the brain. Textbook page 62
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Pre-synaptic neurone
Post-synaptic neurone
Synaptic cleft
Synaptic vesicles
Mitochondrion
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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.
This is because the axons were in different planes to the very thin section chosen for this photograph.
150
Why is summation important?
Summation is important in providing the complexity and flexibility that synapses demonstrate.
151
Give an example of why summation is important
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.
152
The synapses discussed so far refer to ...
Excitatory synapses
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What is the function of excitatory synapses?
In excitatory synapses neurotransmitter chemicals are released with the function of causing an EPSP and a subsequent action potential.
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What is the function of inhibitory synapses?
Inhibitory synapses have the function of making it more difficult for synaptic transmission to take place.
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How do inhibitory synapses make it more difficult for synaptic transmission to take place?
* 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.
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What is summation?
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.
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What factors affect whether an impulse will actually take place in the post-synaptic neurone?
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.
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Why do we have inhibitory synapses in our nervous system?
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.
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How do the effects of summation and the action of inhibitory synapses benefit the nervous system?
* 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.
160
What is the primary neurotransmitter substance in the central nervous system (CNS) of vertebrates?
Acetylcholine
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- Neurotransmitter substances
| Acetylcholine is ...
The primary transmitter substance in the central nervous system (CNS) of vertebrates
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- Neurotransmitter substances
Acetylcholine is the primary transmitter substance in the central nervous system (CNS) of vertebrates. Name one other type of neurotransmitter.
Noradrenaline (typically used in involuntary nervous control, for example, the regulation of gut movements)
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How many types of neurotransmitter can each synaptic bulb produce?
Each synaptic bulb produces only one type of neurotransmitter.
164
Give an example of a neurotransmitter that is released at inhibitory synapses
GABA (aminobutyric acid)
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What is GABA?
* GABA (aminobutyric acid) is an example of a neurotransmitter that is released at inhibitory synapses.
* It is the chief inhibitory neurotransmitter in mammals.
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How does GABA work?
GABA causes negative ions to flow into post-synaptic neurones, therefore causing hyperpolarisation.
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What is the role of GABA in mammals?
GABA has many roles including helping reduce anxiety and 'panic attacks' through a 'damping down' of the nerve pathways in the brain.
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What effect does the drug nicotine have on the nervous system?
Stimulates the release of acetylcholine and other neurotransmitters making action potentials more likely.
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What effect does the drug curare have on the nervous system?
Blocks receptors (at neuromuscular junctions) preventing synaptic transmission - loss of muscle function.
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What effect do opioids have on the nervous system?
* 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.
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Many examination questions involve what type of drugs?
* 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.
