6. The Action Potential

Question 1

What is an action potential? 5

Answer 1

1. transient reversal of the membrane potential
2. form negative intracellular resting potential to positive
3. duration can vary from ms (nerve, skeletal muscle) to a few hundred ms eg. cardiac muscle
4. there is variety in the ions involved
5. It is an all or nothing process, does or doesn’t

Question 2

What are graded potentials? 6

Answer 2

1. Beyond threshold, action potential is fired and remains the same
2. can’t change size of action potential, only frequency
3. small, subthreshold stimulus means so AP
4. small stimulus-graded potentials represent a different way of exciting cells
5. both involve changes in membrane potential

Question 3

Describe the anatomy of an AP. 4

Answer 3

1. Depolarisation affects P(sodium channels open). More depolarized, more P(open)
2. above threshold, P(open) outweights P(closed)
3. this intoduces positive feedback
4. opening sodium ion channels causes depolarization, na+ flow into cell leading to more positive membrane potential.

Question 4

Do diagram 7

Answer 4

good effort

Question 5

Describe the diversity of APs. 5

Answer 5

1. pacemaker approx 200ms long, sets heart rate
2. cardiac muscle also about 200ms, remaines more depolarised for longer
3. the heart (sodium) ion channels are different
4. skeletal muscle is about 5ms wiht gradual repolarization
5. neurone is about 2ms with rapid repolarisation and overshoorting

Question 6

describe the structure of a sodium ion channel. 6

Answer 6

1. responsible for rising phase
2. longer alpha subunit can form a channel on its own
3. 24 transmembrane domains, split into four pseudosubinits, eaach with 6 transmembrane domains
4. dipping domains come together to form pore lining
5. 4 is the voltage sensor, it moves which is communicated to restof channel and more or less p(opening)
6. beta subunits surround and aid

Question 7

Describe the structure of a K+ channel. 2

Answer 7

1. 4 subunits are separate proteins which come together to form structure
2. similar structure and behaviour to Na+ ones

Question 8

Describe the knock on inactivation model of na+. 5

Answer 8

1. as negative ions move in, inside of cell becomes less negative, more positive
2. inactivation mechanism works like a ball and chain
3. swings in and plugs internal end of pore
4. sodium channels open rapidly but inactivate after 1ms
5. k+ channels open more slowly but don’t inactivate, simply close

Question 9

how is the balance restored after an AP? 4

Answer 9

1. overshooting due to very high Pk
2. sodium potassium pump kicks in and resotres resting potential membrane
3. during AP, Pna is very high, then drops on inactivation
4. raise in pk follows

Question 10

Descrive positive feedback in the AP. 4

Answer 10

1. positive membrane potential, about 1-1.5ms
2. na+ channles open and membrane depolarises
3. provides more stimulus so more sodium channels open
4. cycles until sodium channel inactivation limits it

Question 11

Describe negative feedback in the AP. 4

Answer 11

1. potassium channels open, leading to membrane repolarization
2. so fewer K+ channels open
3. we don’t need K+ channel inactivation, its self limiting
4. so effecive it leads to hyperpolarisation

Question 12

What is the refractory period? 4

Answer 12

1. absolute refractory is when applying further stimulation during an AP does nothing, can’t produce another AP
2. Relative refractory period, after an action potential, sodium ion channels are inactivated and recovering
3. cell is less excitable, larger stimulus needed
4. still lots of K+ channels open, trying to hyperpolarise

Question 13

How does the ion concentration inside the cell change during AP? 2

Answer 13

1. Ions go into layer inside membrane

2. Bulk con. of ions in neuron doesn’t change

Question 14

How does an AP spread? 3

Answer 14

1. APs are fired in response to NETS
2. spreads because it acts as a stimulus for other parts of neuron to respond to
3. action potentials are spread in other tissues too eg. cardiac