Action potential

Question 1

Def: action potential.

Answer 1

• Action potential: voltage increase in neurone beyond resting potential, generates nervous impulse.

Question 2

What is increase in neurones voltage from resting potential called? Why does this happen?

Answer 2

• Neurone increase in voltage: depolarisation.
• Neurone membrane becomes more permeable to Na+.
• So, more Na+ diffuse into axon memrbane –> increase voltage.

Question 3

What 3 proteins within axon membrane are responsible for generating an action potential?

Answer 3

1.) Voltage-gated Na+ channel proteins.
2.) K+ channel protein permanently open.
3.) K+ channel protein that opens/ closes.

Question 4

Describe 1st stage of generating action potential.

Answer 4

1.) Depolarisation.
2.) Stimulus, if reaches threshold voltage, gives some voltage-gated Na+ channel proteins energy to open.
3.) Na+ ions diffuse into axon.
4.) K+ channel that opens/ closes remains closed.
5.) K+ diffuse out of axon through K+ channel protein that’s constantly open.

Question 5

List stages of generating action potential.

Answer 5

STIMULUS.
1.) Depolarisation.
2.) Repolarisation
3.) Hyperpolarisation.

Question 6

What value does action potential peak at? Why?

Answer 6

• Peaks: +40mV
• Why: voltage-gated Na+ channels close once +40mV voltage inside axon.

Question 7

Describe 2nd stage of generating action potential.

Answer 7

1.) Repolarisation.
2.) K+ channels (that open/ close) are opened.
3.) Na+ voltage-gated channels are closed.
4.) K+ ions diffuse out of axon through K+ channels that open/ close + permanently open K+ channels.
5.) This reduces positive voltage in axon.

Question 8

Describe 3rd stage of generating action potential.

Answer 8

• Hyperpolarisation.
• Too many K+ ions diffuse out of axon through permanently open channel/ channel that opens + closes.
• Inside action becomes more negative than resting potential.

Question 9

A stimulus will cause an axon to be depolarised once the axon reaches a certain threshold voltage.
What is this threshold voltage?

Answer 9

Threshold: -55mV

Question 10

Explain

All or nothing principle.

Answer 10

1.) If depolarisation doesn’t exceed threshold (-55mV), action potential/ electrical impulse not produced.

2.) If stimulus triggers depolarisation to above -55mV, will always peak at same MAXIMUM VOLTAGE (+40mV.)

3.) Bigger stimuli increase frequency of action potentials NOT the peak.

Question 11

True or False

Bigger stimuli will increase the peak voltage of action potential.

Answer 11

• False.
• Bigger stimuli increase FREQUENCY of action potentials.

Question 12

Why is “all-or-nothing” principle important?

Answer 12

• Ensures animals only respond to large enough stimuli.
• Ensures animals don’t respond to slight change in environment –> would overwhelm them.

Question 13

How does action potential move across motor neurone?

Answer 13

• Action potential @ each Node of Ranvier
• “Jumps” from node to node –> mexican wave.
• Known as: **saltatory conduction. **

Question 14

True or False

Action potentials only move along myelinated axons.

Answer 14

• False.
• Action potentials can also move along non-myelinated axons, stimulating nervous impulses.

SPEC point.

Question 15

What happens to sodium voltage-gated channels in the refractory period?

Answer 15

• Closed.
• Means that membrane cannot be stimulated during the period of 1st action potential and going back to resting potential.

Question 16

2 advantages of refractory period.

Answer 16

1.) Ensures discrete impulses: action potential can’t be generated immediately after, each is separated (no overlapping impulses!)
2.) Limits frequency of impulse transmission. Prevents overreaction to stimulus/ overwhelming the senses.

() - extra info

Question 17

3 factors that affect speed of conductance along axon.

Answer 17

1.) Myelination/ saltatory conduction.
2.) Axon Diameter.
3.) Temperature.

Question 18

What is advantage of saltatory conduction?

Answer 18

• Action potential travels along axon faster.
• Don’t have to generate action poitential along entire length, just at Nodes of Ranvier

Question 19

How does increasing axon diameter affect conductance along axon?

Answer 19

• Wider diameter, speed of conductance increases.
• As wider diameter means less leakage of ions –> action potentials travel faster.

Question 20

How does a higher temp affect speed of conductance along axon? Give 2 reasons why.

Answer 20

• Higher temp increases speed of conductance.
  Why?
  1.) Ions diffuse faster.
  2.) Enzymes involved in respiration work faster. More ATP for active transport via Na+/K+ pump.