6.2-neurons

Question 1

What is resting potential in a neurone?

Answer 1

70mV

Question 2

How is resting potential maintained?

Answer 2

1. the Na+/K+ pump actively transports 3 Na+ ions out of the axon and 2 K+ ions into the axon
2. the membrane is more permeable to K+ ions (than to Na+ ions) so K+ ions can diffuse out of the axon (but Na+ cannot diffuse in)
3. there are negatively charged proteins in the axon

Question 3

Write a flow-chart to describe how an axon becomes depolarised:

Answer 3

1. Na+ ions enter the axon due to a stimulus
2. If enough Na+ ions enter the threshold is exceeded
3. This causes Na+ voltage-gated channels to open
4. So Na+ ions move into the axon by facilitated diffusion (down their electrochemical gradient)

Question 4

What is the definition of an action potential?

Answer 4

The movement of depolarisation down an axon

Question 5

Write a flow-chart to describe how an action potential is generated and propagated in an unmyelinated axon

Answer 5

1. The Na+ ions enter the axon causing the axon to become more positive inside
2. This causes the Na+ voltage-gated channels in the adjoining region on the axon to open
3. Therefore Na+ ions move into the adjoining region on the axon by facilitated diffusion and it depolarises

Question 6

Write a flow-chart to describe how an action potential is propagated in a myelinated axon

Answer 6

1. The Na+ ions enter the axon at the first Node of Ranvier causing the axon to become more positive inside
2. The influx of positive Na+ ions repels the positive ions already present in the axon so they move towards the next Node of Ranvier
3. This causes threshold to be reached at the next Node of Ranvier, so the Na+ voltage gated channels open and Na+ move in by facilitated diffusion
4. This process repeats down the axon
5. This is called salutatory conduction, because the action potential jumps from one node of ranvier to the next

Question 7

What is meant by the ‘all or nothing’ principal?

Answer 7

• A threshold value must be reached in order for an action potential to occur
• All action potentials are the same size

Question 8

How does the strength of a stimulus affect the frequency of action potentials

Answer 8

• The stronger the stimulus the higher the frequency of action potentials transmitted along a neuron.
• However, above a certain strength of stimulus the frequency of action potentials cannot get any higher

Question 9

What is the refractory period?

Answer 9

The period of time during which a new action potential cannot be generated

Question 10

What are the two factors which contribute to the refractory period?

Answer 10

• The sodium ion voltage gated channels become inactive immediately after they were opened. This means that for a short period of time they cannot open again
• The neurone is hyperpolarised so it is ‘further away’ from the threshold level required to generate an action potential

Question 11

How does the refractory period ensure that the impulses are kept discrete?

Answer 11

It prevents a new action potential being generated immediately after the previous one because the Na+ voltage gated channels will not open (they are inactive)

Question 12

How does the refractory period limit the frequency of impulse transmission?
When and why does this matter?

Answer 12

It prevents a new action potential being generated immediately after the previous one.
This is relevant because if a stimulus strength increases, the frequency of impulses increases. But because the frequency of impulses cannot exceed a certain level, it is not possible to differentiate between stimuli above a certain strength.

Question 13

Describe and explain how myelination and saltatory conduction affect the speed of conductance

Answer 13

Speed of conduction is higher in myelinated neurones
This is because the action potential can jump from one Node of Ranvier to the next by saltatory conduction
(Ions only diffuse in and out of the axon at the Nodes of Ranvier)

Question 14

Describe and explain how axon diameter affects the speed of conductance

Answer 14

The bigger the diameter, the higher the speed of conductance
This is because a bigger diameter means there is less resistance to the movement of the action potential down the axon
There is also proportionally less leakage of potassium ions

Question 15

Describe and explain how temperature affects speed of conductance

Answer 15

The higher the temperature, the higher the speed of conductance
This is because the ions have more kinetic energy so the rate of diffusion of ions into and out of the axon increases

Question 16

Write a flow-chart to describe what happens at a cholinergic synapse

Answer 16

1. Action potential reaches the pre-synaptic neurone
2. Calcium ion voltage gated channels open so Ca2+ ions move in by facilitated diffusion
3. The Ca2+ ions cause the synaptic vesicles to move towards the pre-synaptic membrane and fuse with it
4. The neurotransmitter (acetylcholine) is released (by exocytosis)
5. Acetylcholine diffuses across the synaptic cleft
6. Acetylcholine binds to the receptors on (the ligand gated Na+ channels) on the post synaptic membrane
7. Na+ channels open
8. Na+ ions move into the post synaptic neurone by facilitated diffusion (down their electrochemical gradient) causing it to depolarise
9. If threshold is exceeded Na+ voltage gated channels open, so more Na+ ions move in by facilitated diffusion and a new action potential is generated in the post synaptic neurone
10. Acetylcholinesterase hydrolyses the acetylcholine to form ethanoic acid and choline
11. The ethanoic acid and choline are reabsorbed into the pre-synaptic neurone
12. ATP is used to regenerate acetylcholine using the ethanoic acid and and choline

Question 17

What is meant by ‘unidirectionality’?

Answer 17

That the impulses only travel in one direction along axons and across synapses

Question 18

Describe how a synapse ensures unidirectionality of impulses

Answer 18

Only the pre-synaptic neurone contains synaptic vesicles with neurotransmitter
Only the post-synaptic neurone has receptors which the neurotransmitters can bind to

Question 19

What is an excitatory synapse?

Answer 19

• When Na+ ions move into the post synaptic neurone so it becomes more positive inside, making it more likely that an action potential is generated in the post-synaptic neurone

Question 20

What is an inhibitory synapse?

Answer 20

• When the post synaptic neurone becomes more negative
• Due to either K + ions moving out or Cl- ions moving in
• This means that more Na+ ions would be required in order for an action potential to be reached
• So it is less likely that an action potential is generated in the post-synaptic neurone

Question 21

What is spatial summation? Draw a diagram to represent what happens

Answer 21

• Spatial summation is when there is more than one pre-synaptic neuron synapsed to the same post-synaptic neuron.

Question 22

Describe how spatial summation makes it more possible for an action potential to occur in the post-synaptic neurone. (assume that it is an excitatory synapse)

Answer 22

• neurotransmitter from both pre-synaptic neurones bind to the receptors (on the ligand gated channels) on the post synaptic neurone
• so sufficient sodium ion ligand gated channels open
• so sufficient sodium ions move into the post-synaptic neuron by facilitated diffusion
• so threshold be exceeded and an action potential will be generated

Question 23

What is temporal summation?

Answer 23

• Temporal summation is when the pre-synaptic neuron has a high frequency of action potentials.

Question 24

Describe how temporal summation makes it more possible for an action potential to occur in the post-synaptic neurone. (assume that it is an excitatory synapse)

Answer 24

• A large amount of neurotransmitter is realised by the pre-synaptic neurone in a short space of time
• so sufficient sodium ion ligand gated channels open
• so sufficient sodium ions move into the post-synaptic neuron by facilitated diffusion
• so threshold be exceeded and an action potential will be generated

Question 25

Some drugs act as agonists to neurotransmitters. What is meant by an agonist?

Answer 25

* An agonist drug has a similar shape to the neurotransmitter and will bind to the same receptor that the neurotransmitter binds to * An agonist causes the receptor to respond in the same way as if the neurotransmitter itself was bound * E.g. if the neurotransmitter usually binds to a ligand gated channel and causes the channel to open, the agonist would bind to the channel and cause the channel to open even in the absence of the neurotransmitter

Question 26

Some drugs act as antagonists to neurotransmitters. What is meant by an antagonist?

Answer 26

* An antagonist drug has a similar shape to the neurotransmitter and will bind to the same receptor that the neurotransmitter binds to * An antagonist causes the receptor to respond in a different way to what would happen if the neurotransmitter itself was bound. * E.g. if the neurotransmitter usually binds to a ligand gated channel and causes the channel to open, the antagonist would bind to the channel (so the neurotransmitter cannot bind) and prevent the channel from opening