Conduction of Nerve Impulses

Question 1

Describe the charge on the inside and outside of the axon at resting potential.

Answer 1

Inside of axon - Negative
Outside of axon - Positive

Question 2

What is the potential difference of the inside of the axon at resting potential?

Answer 2

-70mV

Question 3

How many Na+ pumped out of the cell for every 2K+ pumped into the cell at resting potential?
What type of gradient does this produce?

Answer 3

3
This produces an electrochemical gradient.

Question 4

Why is the inside of the axon negative at resting potential?

Answer 4

There is more movement of positive ions outside of the cell (3Na+) than positive ions into the cell (2K+)

Question 5

What happens to movement of K+ when the membrane is more permeable to K+ at resting potential?

Answer 5

K+ diffuses out of the axon in facilitated diffusion.

Question 6

What happens when the membrane is less permeable to Na+ at resting potential?

Answer 6

Na+ channels close so it remains in the cell.

Question 7

Generally, what are action potentials caused by?

Answer 7

Rapid movement of Na+ and K+ across the membrane of the axon.

Question 8

What is another word for action potential?

Answer 8

Depolarisation.

Question 9

At action potential, what happens to Na+ ion channel proteins?
This leads to movement of Na+ into the axon by which transport process?

Answer 9

They open.
Na+ enters the axon by facilitated diffusion.

Question 10

Why do Na+ ions want to diffuse into the axon during action potential?

Answer 10

The positive ions are attracted to the negatively charged inside of the axon.

Question 11

Name the type of gradient that sodium ions diffuse down into the axon.

Answer 11

Electrochemical.

Question 12

Why does an influx of positive sodium ions lead to depolarisation?

Answer 12

They reduce the potential difference of the axon from -70mv to +40mV. This is because the ions are positive, so the influx raises the positive charge.

Question 13

Explain the positive feedback in the axon during action potential.
Explain how this links in with threshold potential.

Answer 13

Depolarisation causes more sodium channel proteins to open, so more sodium ions can diffuse in leading to more depolarisation.
If the influx has raised the potential difference to -50mV, this means the threshold is reached so more channels can open, and more Na+ diffuses in.

Question 14

How does conduction of an action potential occur?

Answer 14

Na+ ions from the first site of action potential diffuse down the axon which leads to depolarisation of the next section of membrane and the opening of sodium ion channels there.

Question 15

What does repolarisation mean?

Answer 15

Resting potential of -70mV restored.

Question 16

What happens to sodium ion voltage-gated channel proteins after an action potential has occurred in this section of membrane?

Answer 16

They close, but only in this region. This stops anymore sodium ions from entering.

Question 17

When sodium channels shut after action potential, which channels now open?

Answer 17

Potassium ion voltage gated channels.

Question 18

Describe the transport process used to remove potassium ions from the axon after action potential.

Answer 18

K+ leaves the axon in facilitated diffusion down the concentration gradient.

Question 19

Loss of K+ from axon after depolarisation leads to what process occurring?
This process changes potential difference to what?

Answer 19

Repolarisation. This causes potential difference to return back to -70mV.

Question 20

What is the refractory period?

Answer 20

A period where a section of the axon membrane is unresponsive to any new stimuli, so an action potential is not produced.

Question 20

What is hyperpolarisation?
- This is a result of which ion moving out?
- Is the new potential difference higher or lower than resting potential?

Answer 20

Where there is an overshoot in potential difference as a result of excess K+ ions diffusing out, causing the potential difference to become lower than the resting potential.

Question 21

Why is the refractory period important?

Answer 21

It ensures action potentials will be generated ahead of the original one. This means the impulse will travel in one direction only.

Question 22

Why does the refractory period prevent new action potentials from being generated?

Answer 22

Na+ channels are closed. There is no voltage present to open them.

Question 23

What does ‘discrete impulse’ mean?

Answer 23

Action potentials do not overlap.

Question 24

Name a type of membrane protein that leads to the restoration of resting potential.

Answer 24

Na+/K+ pump.

Question 24

What is the all or nothing principle?

Answer 24

Depolarisation must exceed the threshold potential for the action potential to be conducted through the axon.

Question 25

The all or nothing principle states that all action potentials have the same…
Therefore, a bigger stimulus will…

Answer 25

Magnitude/size/peak.
Therefore, bigger stimuli will only increase the frequency of action potentials, not the size.

Question 26

Compare how action potentials pass in myelinated and unmyelinated neurones.

Answer 26

Myelinated - Action potentials generated at each node.
Unmyelinated - Action potentials pass as a wave of depolarisation through the whole axon as sodium ions diffuse down.

Question 27

Explain how less saltatory conduction leads to symptoms of multiple sclerosis.

Answer 27

Depolarisation occurs down the whole length of the axon so nerve impulses take longer to reach the neuromuscular junction, delaying muscle contraction.

Question 28

Explain how damage to the myelin sheath could lead to ions leaking to other neurones and cause symptoms of multiple sclerosis.

Answer 28

Ions could leak out to other neurones which may lead to the wrong muscle fibres contracting.

Question 29

How does myelination increase speed of conductance?

Answer 29

Depolarisation occurs at nodes of Ranvier only, therefore it uses saltatory conduction. This means that the impulse doesn’t need to depolarise the whole length of the axon.

Question 30

How does a bigger diameter of the axon affect conduction?

Answer 30

Bigger diameter means less resistance to flow of ions, so conduction speed is faster.

Question 31

How does a higher temperature affect speed of conduction down the axon?
What are the risks of higher temperatures?

Answer 31

Higher temperature increases rate of diffusion of Na+ and K+ because they have more kinetic energy.
However, this may denature proteins and enzymes.