Action Potential

Question 1

How is resting potential created and maintained?

Answer 1

• Na+/K+ pump in the axon membrane actively transports 3Na+ out of the axon and 2K+ into the axon, against a concentration gradient
• K+ ion channels transport many K+ out of the axon down a concentration gradient by facilitated diffusion
• This makes the membrane potential inside of the axon more negative (-70mv)

Question 2

What is a nerve impulse?

Answer 2

the passage of an action potential along an axon

Question 3

What is an action potential?

Answer 3

• a rapid change in voltage across the axon membrane
• due to changes in the cell surface membrane permeability to Na+ and K+

Question 4

Describe depolarisation.

Answer 4

• localised depolarisation causes Na+ channels to open
• Na+ diffuses into the axon down a concentration gradient by facilitated diffusion
• this stimulates more Na+ channels to open so more Na+ diffuses in
• This causes embrace potential to become less negative, reaching +40mV

Question 5

Describe repolarisation.

Answer 5

• Na+ channels close as the permeability of the membrane to Na+ decreases
• K+ channels in the membrane open, so K+ diffuses out of the axon
• as K+ are positively charged this returns the membrane potential to the resting value (-65mV)

Question 6

Why does hyper-polarisation occur?

Answer 6

• voltage gated K+ channels need to close, but they close slowly
• sp the membrane potential drops to below -70mV

Question 7

What is the all or nothing principle?

Answer 7

An action potential is only initiated of the threshold is passed

Question 8

Describe what changes with strong and weak stimuli.

Answer 8

frequency of action potential increases as stimulus strength increases

Question 9

What is the refractory period?

Answer 9

the rest period after one ion channel opens and the next time it reopens

Question 10

What are the importances of the refractory period?

Answer 10

• produces discrete impulses in one direction, which don’t merge
• limit to frequency of impulses
• action potentials are unidirectional

Question 11

Why do nerve impulses travel faster along myelinated neurones than unmyleinated neurones?

Answer 11

• saltatory conduction takes place as impulses jump from node to node
• as the myelin sheath acts as an electrical inculcator so prevents the axon from be depolarised