Neuronal communication snaprevise Flashcards
1
Q
what are neurones (neurones)?
A
Neurones (or nerve cells) are specialised cells that carry electrical signals in the nervous system.
2
Q
name the features of a neurone
A
- cell body
- dendrites
- axon
- synpases
3
Q
what is the function of the cell body of a neurone?
A
• Cell body — contains the nucleus and other organelles.It carries out all the normal functions of the cell (e.g. protein synthesis and ATP production)
4
Q
what is the function of the dendrites of a neurone?
A
• Dendrites — branches of the neurone membrane which receive signals from other neurones
5
Q
what is the function of the axon of a neurone?
A
• Axon — carries electrical signals away from the cell body, towards the synapses
6
Q
what is the function of the synapses of a neurone?
A
• Synapses — lie at the end (terminal) of the axon and pass the electrical signal on to the next cell
7
Q
what do neurones help maintain?
A
• Neurones maintain a potential differences across their plasma membrane
8
Q
what is a myelin sheath and what does it do?
A
The myelin sheath is a fatty layer which covers the axon providing electrical insulation as it prevents the movement of ions across the membrane.
9
Q
what are neurones with a myelin sheath called?
A
Neurones with this feature are called myelinated neurones
10
Q
what is the myelin sheath made of?
A
The myelin sheath is made up of layers of membrane from Schwann cells, cells which wrap themselves around neurones.
Non-myelinated neurones are associated, in groups, with just one Schwann cell so are only loosely wrapped in one layer of Schwann cell membrane.
11
Q
what are the main 3 types of neurone?
A
- motor neurones
- sensory neurones
- relay neurones
12
Q
describe the structure and function of a sensory neurone
A
- Sensory neurones transmit information from sensory receptors to the central nervous system (CNS)
- They have a long dendron and a short axon which extend from opposite ends of the cell body
- The dendron carries the action potential from the receptor to the cell body
- The axon then carries the signal from the cell body to the CNS
- Their cell bodies are located just outside the CNS
13
Q
describe the structure and function of a relay neurone
A
- Relay neurones carry electrical signals between sensory neurones and motor neurones
- They often have short, highly branched dendrites and axons
14
Q
describe the structure and function of motor neurones
A
- Motor neurones receive signals from relay or sensory neurones
- They have very long axons and transmit information to effectors. Effectors are cells that carry out the response (e.g. muscles or glands)
- Their cell bodies are found in the CNS
15
Q
what are sensory receptors?
A
Sensory receptors are specialised cells in the nervous system that can detect physical stimuli. Most sensory receptors are energy transducers — they convert one form of energy to another.
16
Q
what is the function of a sensory receptor?
A
• Sensory receptors convert (transduce) energy from the stimulus into electrical energy (i.e. a nervous impulse), called the generator potential
17
Q
how are sensory receptors specialised?
A
• Sensory receptors have specialised structures which means they can only respond to one type of stimulus (e.g. light, heat, pressure, etc.)
18
Q
define transducer cells
A
Cells that convert non-electrical signals (e.g. pressure, light, etc.) into an electrical (nervous) signal.
19
Q
define generator potentila
A
The depolarisation of the membrane of a receptor cell as a result of a stimulus.
20
Q
what type of receptors are pacinian corpuscles?
A
a type of sensory receptor
21
Q
where in the body are pacinian corpuscles and what do they do?
A
• Pacinian corpuscles lie deep under the skin and detect changes in pressure
22
Q
describe the structure of pacinian corpuscles?
A
• They are oval-shaped structures made up of concentric rings of connective tissue surrounding a nerve ending
23
Q
what is the sequence of events that occurs when there is pressure on the skin?
A
- They are oval-shaped structures made up of concentric rings of connective tissue surrounding a nerve ending
- Pressure on the skin deforms these layers of connective tissue, causing them to push against the nerve ending
- Pacinian corpuscles then transduce the mechanical energy from this stimulus into a generator potential
- Electrical signals are sent during the deformation, but when the corpuscle has been deformed for a period of time, the signal stops
24
Q
what happens with the pacinian corpuscle if there’s constant pressure on the skin?
A
• This means that the Pacinian corpuscle only responds to changes in pressure, not overall pressure
25
explain whether the membrane charged whilst at rest
Even when a neurone is at rest (not being stimulated) ions are still moving across its plasma membrane.
At rest, the number of positively charged ions inside the membrane is greater than the number outside.
In this state, the membrane is said to be polarised —there is a difference in charge (a potential difference or voltage) across it.
The potential difference across the membrane is measured in millivolts; at rest it is known as the resting potential.
26
what is resting potential?
The potential difference across the membrane while the neurone is at rest.
27
what is the resting potential of a neurone at rest?
Neurones have a negative resting potential (of –60mV) as the inside of the cell is more negatively charged than outside the cell.
28
what does the negative resting potential of a neurone mean?
This means that negative ions inside the cell try to leave the neurone, and positive ions outside the cell try to enter the neurone.
29
what enhances the negative resting potential of a cell?
The negative resting potential is enhanced by the presence of organic anions inside the cell; organic anions are negatively charged molecules inside the neurone.
30
how do ions cross the cell membrane?
The cell membrane is a phospholipid bilayer, thus is impermeable to ions.Therefore,ions leave via channel proteins in the cell membrane.
31
how is the resting potential created and maintained?
The resting potential is created and maintained by potassium ion channels, sodium ion channels and the sodium-potassium pump.
32
what does the potassium pump do?(overall)
The sodium-potassium pump acts to keep a larger number of positive ions outside the cell than inside.
33
what does the sodium-potassium pump do in order to maintain the negative resting potential?
* The sodium-potassium pump uses ATP to pump three sodium ions out of the cell and two potassium ions into the cell
* This results in a net loss of one positive charge from the cell each time
* This produces a neurone with a more positive charge outside the cell than inside, creating the negative resting potential
* When the neurone is at rest, the sodium-potassium pump is working to maintain the resting membrane potential
* The sodium-potassium pump results in a high potassium ion concentration inside the cell, and a high sodium ion concentration outside the cell
34
at rest, what is the membrane of neurones permeable and impermeable to?
• At rest, the membrane of neurones is permeable to potassium ions (through potassium channels), but not to sodium. Therefore, potassium ions tend to leak out of the cell by diffusion down their concentration gradient
35
what is an action potential?
An action potential is a rapid change in potential difference across a membrane which occurs when a neuron is firing a nerve impulse.
36
what needs to happen in order to trigger an action potential?
In order to trigger an action potential, sodium ions need to enter the neurone in order to depolarise the membrane.
37
what is an electrochemical gradient?
the concentration of sodium ions inside the neuron is lower than outside, creating an electrochemical gradient across the cell membrane
38
how is a membrane depolarised?
1. The neurone receives an impulse from sensory receptors
2. In response, sodium ion channels open in the generator region at the end of the dendrites
3. The concentration of sodium ions inside the cell is low, and the resting potential is negative so the sodium ions travel into the cell by facilitated diffusion through sodium channels
4. The diffusion of the sodium ions causes the depolarisation of the membrane — the inside of the neurone becomes less negative
39
what does the size of the depolarisation depend on?
the number of sodium channels that are open
40
is the resting membrane potential more or less negative after the generator potential?
After the generator potential, the resting membrane potential is slightly less negative due to more sodium ions being present inside the cell.
41
what happens if the depolarisation reaches the threshold potential?
it causes an action potential
42
what happens if enough depolarisation occurs due to the generator potential?
his activates voltage-gated sodium channels in the first section of the axon. These are sodium channels that open when the resting membrane potential becomes less negative.
1. Sodium rushes into the axon through the voltage-gated sodium ion channels causing even more depolarisation
2. This depolarisation causes even more voltage-gated sodium ion channels to open
3. So many sodium ions enter the cell that it becomes positively charged compared to the outside at +40 mV
43
is depolarisation in the action potential an example of positive or negative feedback?
This is an example of positive feedback — a small depolarisation of the membrane has caused a change that increases the depolarisation even further.
44
what is positive feedback?
A mechanism where a change moves the system further away from the optimum.
45
how is the membrane repolarised to return back to a negative value?
1. When the inside of the axon starts to become positive, voltage-gated potassium channels open. These are like voltage-gated sodium channels except that they open at a higher (more positive) voltage than the voltage-gated sodium channels
2. At the same time, the voltage-gated sodium channels start to close
3. The potassium concentration inside the cell is high, so potassium ions move out of the cell. This reduces the membrane potential — this is known as repolarisation
46
what is hyperpolarisation?
During repolarization, so many potassium ions leave that the membrane potential overshoots and becomes more negative than the resting potential — this is known as hyperpolarisation (there is high sodium ion concentration and low potassium ion concentration inside the neurone).
47
what happens after repolarisation?
After hyperpolarisation, the action of the sodium-potassium pump restores the ion concentrations, as well as restoring the membrane potential back to the resting potential.
48
what is the refractory period?
The period in an action potential where the axon can’t be depolarised to initiate a new action potential.
49
summarise all of the 8 steps involved when an action potential occurs?
1. Resting membrane potential is maintained by the sodium-potassium pump at–60 mV. At this point the membrane is polarised
2. The cell receives an impulse, triggering the voltage-gated sodium ion channels to open. Sodium ions enter, moving down their electrochemical gradient, and the membrane depolarises slightly — this is the generator potential. The potential difference across the cell membrane becomes less negative
3. If this depolarisation reaches the threshold value of –50 mV, it activates voltage-gated sodium channels in the first section of the axon. Positive feedback triggers nearby voltage-gated sodium channels to open. Sodium ions flood into the cell causing an action potential
4. As a result, the cell depolarises further and the potential difference increases (becomes even more positive) to around +40 mV
5. Voltage-gated sodium ion channels close, and voltage-gated potassium channels open
6. Potassium ions diffuse out of the cell through these channels, decreasing the potential difference and returning it to a negative value
7. Outward diffusion of potassium ions eventually hyperpolarises the cell as too many positive ions diffuse out.At this point the potential difference is more negative than the resting potential and the voltage-gated potassium channels close
8. Sodium-potassium pump works to return the cell to the resting membrane potential, before maintaining the potential difference at this level until the membrane is excited by another stimulus
50
what is meant by the all or nothing principle?
This means each action potential always depolarises the axon to the same voltage. If the depolarisation doesn’t reach the threshold potential, an action potential is not triggered.
* Weak activations of the neurone won’t produce an action potential
* Strong activations will produce the same magnitude action potential as a medium activation. The magnitude of an action potential is not proportional to the size of the stimulus. Instead, all action potentials have the same magnitude (+40mV)
* The neurone is either off (below the threshold potential) or on (in an action potential)
51
what determines the size of action potential triggered?
This means that all nerve impulses are identical (not graduated). The strength of the stimulus is not related to the size of the action potential it triggers.
A stimulus’ intensity is encoded by the frequency of action potentials arriving at the sensory region of the brain. When a stimulus is at a higher intensity, more sodium channels are opened in the sensory receptor, producing more generator potentials and therefore more frequent action potentials in the sensory neuron. Therefore action potentials enter the CNS more frequently.
52
how is an action potential transmitted across an axon?
AND
why do action potentials only travel in 1 direction?
* When depolarisation of the action potential happens, it causes voltage-gated sodium channels to open further down the axon
* By the time the depolarisation has spread further down the axon, the earlier parts of the axon have begun repolarising
* This ‘mexican wave’ of depolarisation and repolarisation spreads down the axon
* Behind the travelling wave, the axon is in its refractory period and can’t initiate any more action potentials. This ensures action potentials only travel in one direction
53
how does the structure of the cell body of a myelinated neurone relate to its function?
contains cell organelles such as many mitochondria and ribosomes and is associated with the production of protein
54
how does the structure of the dendron of a myelinated neurone relate to its function?
extensions of the cell body which branch into dendrites, which carry nerve impulses towards the cell body
55
how does the structure of the axon of a myelinated neurone relate to its function?
a single long fibre that carries nerve impulses away from the cell body
56
how does the structure of the schwann cells of a myelinated neurone relate to its function?
cells surrounding the axon, protecting it and providing electrical insulation. They also carry out phagocytosis (the removal of cell debris) and have a role in neuronal regeneration. Schwann cells wrap themselves around the axon many times so that layers of their membranes build up around it
57
how does the structure of the myelin sheath of a myelinated neurone relate to its function?
forms a covering of the axon and is made up of the membranes of the Schwann cell and prevents ions moving across the membrane. These membranes are rich in a lipid known as myelin
58
how does the structure of the nodes of ranvier of a myelinated neurone relate to its function?
constrictions between adjacent Schwann cells where there is no myelin sheath
59
what is the difference between myelinated and non-myelinated neurones?
* Myelinated neurones have their own Schwann cells, which wrap many times around the axon
* Non-myelinated neurones also have Schwann cells, but these Schwann cells wrap loosely around many neurones. Therefore, action potentials move along them as a wave
60
what is a local current?
When an action potential (A.P.) occurs, sodium ions diffuse into the cell through sodium ion channels. This creates a local current.
61
how is a local current formed?
1. When the action potential occurs, the sodium channels open at this point in the neurone
2. Sodium diffuses down the concentration gradient across the membrane into the neurone. This causes an increase in sodium ion concentration at this point in the neurone
3. Sodium ions continue to diffuse along the neurone, away from this region. This movement of charged particles is called the local current
4. The local current causes a slight depolarisation further along the neurone which causes the voltage-gated sodium ion channels to open,allowing rapid diffusion of sodium into the neurone. This causes a full depolarisation (action potential) further along the neurone
5. This process repeats and the action potential moves along the neurone
62
why are the sodium ions not attracted to the negative charge outside of the membrane?
• This fatty insulation also prevents sodium ions being attracted to the negative charge outside of the membrane
63
when can the movement of ions across the membrane create an action potential?
• The movement of ions across the membrane needed to create an action potential can only occur where no myelin is present
64
what is a saltatory conduction?
-The action potential jumps from one node of Ranvier to the next in myelinated neurones.
* Therefore sodium ions diffuse along the neurone from one node of Ranvier to the next, depolarising the membrane only at these sections. This increases the length of the local current — the charged particles must move further along the neurone before initiating the next action potential
* This means that the action potential seems to jump from one node to the next along the axon
65
why is saltatory conduction faster?
• Saltatory conduction is faster because the action potential moves down the axon in bigger jumps
66
why are myelinated neurones usually involved in transmission of action potentials over long distances?
Myelination increases the speed that action potentials are transmitted along a neuron through saltatory conduction.
As a result, myelinated neurones are usually involved in the transmission of action potentials over long distances.
67
beside myelination, what are other factors affect the speed of action potential transmission?
Besides myelination, other factors affecting the speed of action potential transmission include the AXON DIAMETER and AXON TEMPERATURE.
68
what is a synapse?
A synapse links two or more neurones together and allows the action potentials from one neurone to be communicated to the next.
69
how does information travel across a synapse?
Information travels from the pre-synaptic neurone, across the synaptic cleft (a small gap between the neurones), to the post-synaptic neurone.
70
how does action potential travel across the synpase?
The action potential itself cannot travel across the synapse.
Instead it causes the release of a chemical (neurotransmitter), which diffuses from the presynaptic neurone, across the synapse and initiates an action potential in the post-synaptic neurone.
71
what is the pre-synaptic bulb?
The pre-synaptic bulb (or knob) is a swelling at the end of the axon of the pre-synaptic neurone.
72
how does the structure of the pre-synaptic bulb relate to its function?
* It contains vesicles filled with neurotransmitter (e.g. acetylcholine)— a chemical which stimulates an action potential in the post-synaptic neurone
* These vesicles are made in the smooth endoplasmic reticulum (SER).The pre-synaptic bulb has an extensive SER
* There are many mitochondria in order to produce ATP for active processes
* The final feature of synaptic bulbs is the presence of voltage-gated calcium ion channels. These detect when an action potential arrives at the end of the axon
73
how is the synaptic cleft involved in the transmission of an action potential?
Neurotransmitters are released from the pre-synaptic bulb into the synaptic cleft, which is about 20-30 nm wide and contains enzymes that break down the neurotransmitter.
74
how is the structure of the post-synaptic neurone specialised for its function
Once the neurotransmitter diffuses across the synaptic cleft it reaches the membrane of the post-synaptic neurone.
On the membrane of the post-synaptic neurone there are neurotransmitter receptors.
These are specialised sodium ion channels that open or close when a neurotransmitter binds to a specific complementary receptor site.
These channels are made up of five polypeptide molecules, two of which make up the specialised receptor site.
75
what are the steps taken for the transmission of action potetials?
1. An action potential arrives at the pre-synaptic bulb, depolarising the membrane
2. The depolarisation causes voltage-gated calcium ion channels to open
3. As the concentration of calcium ions inside the cell is low, calcium ions enter, moving down their concentration gradient
4. The influx of calcium ions into the pre-synaptic neurone causes the synaptic vesicles to fuse with the pre-synaptic membrane.
5. Acetylcholine is released into the synaptic cleft via exocytosis and diffuses across the synaptic cleft
6. Acetylcholine molecules bind to receptor sites on the sodium ion channels in the post-synaptic membrane
7. The sodium ion channels open and sodium ions diffuse into the post-synaptic neurone
8. The sodium ions cause slight depolarisation of the post-synaptic neurone and a generator potential is produced
9. If a large enough generator potential is produced, the post-synaptic membrane reaches the threshold potential and a new action potential is generated in the post-synaptic neurone
76
what are cholinergic synpases?
Synapses that use acetylcholine as their neurotransmitter.
77
why is acetylcholine deactivated?
If acetylcholine is left in the synapse it will continue to stimulate the post-synaptic neurone, therefore the last stage in synaptic transmission is the inactivation of the neurotransmitter.
78
how and why is the neurotransmitter(acetylcholine) deactivated?
* The synapse contains the enzyme acetylcholinesterase. This enzyme hydrolyses acetylcholine into choline and ethanoic (acetic) acid. Broken down, these molecules can’t bind to acetylcholine receptors and therefore can’t stimulate the post-synaptic neurone
* The choline and ethanoic acid are then transported back into the pre-synaptic bulb where they are recycled; this requires ATP
* The reformed acetylcholine is then stored in vesicles, ready for another action potential
79
what are the 2 types of synpases?
- excitatory
| - inhibitory
80
what are excitatory synapses?
When an action potential reaches a synapse, it causes the release of a small amount of acetylcholine into the synaptic cleft.
The acetylcholine binds to the sodium channel receptors on the post-synaptic neurone and sodium ions diffuse into a neurone as a result of this binding.
The cell then undergoes a small amount of depolarisation, which on its own is not enough to cause an action potential in the post-synaptic neurone. We call this an excitatory postsynaptic potential (or EPSP).
81
what do inhibitory synapses do?
Stimulation of an inhibitory synapse hyperpolarises the post-synaptic neurone in response to neurotransmitter binding, instead of depolarising it.
This can decrease the resting membrane potential to as low as –80 mV which makes it much harder for an action potential to be initiated in the cell.
