Topic 3: Nervous System 1 - Neuronal Physiology: AP

Question 1

What is an action potential? (2)

Answer 1

• a nerve impulse (signal)
• large change in MP that propagates along an axon with no change in intensity

Question 2

Where are action potentials triggered at? (3)

Answer 2

• trigger zone
• axon hillock of multipolar and bipolar neurons
• just past dendrites of unipolar neurons

Question 3

Action potential graph and explanation (5)

Answer 3

a) GP - membrane potential at the axon hillock reaches -55mV (threshold)
b) Depolarization phase of AP
c) Repolarization phase of AP
d) After hyperpolarization phase (Below RMP)
e) Once K+ channels close. MP returns to RMP

Question 4

What occurs in the depolarization phase of AP? (3 steps, 4 points)

Answer 4

1. voltage gated Na+ channels respond to MP change (ie. GP) and open to greatly increase Na+ permeability
2. As gates open, more Na+ diffuses in (further changing MP)
   - causes even more Na+ gates to open (positive feedback)
3. Na+ diffuses in, causing depolarization to +30mV where inside becomes positive

Question 5

What occurs in the repolarization phase? (2 steps, 4 points)

Answer 5

1. Na+ channels close, become inactivated (decrease Na+ permeability)
   - Na+ movement returns to resting levels
2. Voltage-gated K+ channels open (increase permeability)
   - K+ diffuses out (positive charge moves out, decreases MP)

Question 6

What occurs in the hyperpolarization phase?

Answer 6

• below RMP
  1. K+ channels are slow to close and remain open longer than necessary
  2. Na+ channels are reactivated and can respond to stimuli at this point
• Once K+ channels close, MP returns to RMP

Question 7

What is always working to maintain concentration gradients?

Answer 7

Na+/K+- ATPase

Question 8

How many AP does it taker to cause a measurable change in [ion] in the cell?

Answer 8

• 10 000s

Question 9

What are the two refractory periods of an AP?

Answer 9

1. absolute refractory period (prevent AP summation)
2. Relative refractory period (region d)

Question 10

What occurs in the absolute refractory period?

Answer 10

• NO AP can be generated, regardless of stimulus size

Question 11

Why does the absolute refractory period occur? (2)

Answer 11

• all Na+ channels being open (region b)
• or all Na+ channels being inactivated (cannot open until MP reaches RMP, region c)

Question 12

What is the relative refractory period?

Answer 12

• Period when an AP can be generated but only by a greater than normal stimulus

Question 13

What occurs in the relative refractory period? (2 steps, 4 points)

Answer 13

1. Na+ channels are reactivated when MP passes RMP
   - therefore they are closed but can be reopened if threshold is reached
2. K+ channels are open and membrane hyperpolarized
   - further to go to get to threshold and need larger stimulus

Question 14

What is the all or none principle of AP? (3)

Answer 14

All - if threshold is reached, AP is produced
- same every time (same max. depol. etc)
None - below threshold means no AP