Organisms Respond to Changes in their Internal and External Environments: Nervous Coordination - Nerve Impulses, Generating Action Potentials

Question 1

Define the term action potential

Answer 1

• The temporary change in electrical potential across the membrane of an axon in response to the transmission of a nerve impulse

Question 2

List the main stages of generating an action potential

Answer 2

• Stimulus/generator potential
• Depolarisation/action potential
• Repolarisation
• Hyperpolarisation
• Resting potential
• Refractory period

Question 3

What happens during the stimulus/generator potential stage?

Answer 3

• A stimuli excites the neurone cell membrane, causing Na+ ion channels to open
• Membrane becomes more permeable to Na+ ions
• Na+ ions diffuse into neurone
• This makes the inside of the neurone less negative
• Change in potential difference is known as the generator potential

Question 4

How does the size of a stimulus affect the generator potential?

Answer 4

• The bigger the stimulus, the bigger the generator potential

Question 5

Define the term threshold level

Answer 5

• A certain size of stimulus that is required to generate an action potential

Question 6

What happens if the generator potential reaches threshold level?

Answer 6

• If generator potential is big enough, it’ll trigger an action potential

Question 7

What happens during the depolarisation/action potential stage?

Answer 7

• If potential difference reaches threshold (around -55mV), voltage-gated Na+ ion channels open
• More Na+ ions diffuse into neurone and depolarise axon membrane, establishing an action potential

Question 8

Define the term depolarisation

Answer 8

• An increase in potential difference across a cell’s membrane, making it less negative than the resting potential

Question 9

What happens during the repolarisation stage?

Answer 9

• Once action potential of around 30-40mV has been reached, the voltage-gated Na+ ion channels close and voltage-gated K+ ion channels open
• Membrane is more permeable to K+ ions
• K+ ions diffuse out of neurone
• Starts restoration of resting potential

Question 10

Define the term repolarisation

Answer 10

• The re-establishment of the resting potential (-65mV)

Question 11

What happens during the hyperpolarisation stage?

Answer 11

• K+ ion channels slow to close so many K+ ions diffuse out of neurone
• Potential difference becomes more negative than resting potential

Question 12

Define the term hyperpolarisation

Answer 12

• A decrease in the membrane potential of an axon, so that it is even more negative than the resting potential

Question 13

What happens during the resting potential stage?

Answer 13

• Sodium-potassium pump restores resting potential by actively transporting 3 Na+ ions out and 2 K+ ions in

Question 14

Define the term refractory period

Answer 14

• The time period after an action potential during which further action potentials are prevented

Question 15

What happens during the refractory period?

Answer 15

• Voltage-gated for Na+ and K+ ions recover (closed)

- During this period, the membrane cannot be depolarised

Question 16

What is the importance of the refractory period?

Answer 16

• Produces discrete (separate) impulses

- Ensures action potentials are unidirectional

Question 17

What is the all or nothing response?

Answer 17

• All action potentials fire the same change in voltage, no matter how big the stimulus
• If threshold isn’t reached, action potential won’t fire

Question 18

How do bigger stimuli affect action potentials?

Answer 18

• Bigger stimuli won’t cause bigger action potentials but will increase the frequency of impulses