6.4 Nerve impulses

Question 1

How is a resting potential established?

Answer 1

• Sodium potassium pump actively transports 3 Na+ out and 2 K+ into axon
• Creating an electrochemical gradient
• Membrane is more permeable to K+ which move out by facilitated diffusion
• Membrane is less permeable to Na+ as sodium ion channels are closed

Question 2

Stimulus

Answer 2

• Sodium ion channels open
• Na+ diffuse into axon down electrochemical gradient

Question 3

Depolarisation

Answer 3

• If threshold is reached an action potential is generated
• Voltage-gated sodium ion channels open
• So more Na+ diffuse in

Question 4

Repolarisation

Answer 4

• Voltage-gated sodium ion channels close
• Voltage-gated potassium ion channels open
• K+ diffuse out

Question 5

Hyperpolarisation

Answer 5

• Potassium ion channels are slow to close so there’s a slight overshoot

Question 6

What is the all-or-nothing principle?

Answer 6

• For an action potential to be produced, depolarisation must exceed threshold potential
• Action potentials are always same size but bigger stimuli increase frequency of action potentials

Question 7

Myelinated axon

Answer 7

Provides electrical insulation
Depolarisation at nodes of Ranvier only, resulting in saltatory conduction
Nerve impulses reach neuromuscular junction rapidly

Question 8

Non-myelinated neurone

Answer 8

No electrical insulation so ions may leak to other neurons
Depolarisation along whole length of axon, resulting in a wave of depolarisation
Nerve impulses take longer to reach neuromuscular junction

Question 9

What is the refractory period?

Answer 9

• Time taken to restore axon to resting potential
• No further action potential can be generated as sodium ion channels are closed
• Ensures impulses are discrete and uni-directional

Question 10

What factors affect speed of conductance?

Answer 10

• Myelination
• Axon diameter
• Temperature