Basis of neuronal excitability

Question 1

What is the voltage at the peak of depolarisation?

Answer 1

+40mV

Question 2

What is called when the line goes down after the peak?

Answer 2

repolarisation

Question 3

What is it called in the graph when the line goes below -70mV?

Answer 3

hyperpolarisation

Question 4

How long does an action potential take?

Answer 4

1-2msec

Question 5

What is depolarisation caused by?

Answer 5

influx of Na+ (increasing PNA)

Question 6

What is repolarisation caused by?

Answer 6

• K+ influx (increasing PK by opening more channel)

- terminating activity of extra Na+ channels (decrease in PNa)

Question 7

What is gK?

Answer 7

the conductance of potassium

Question 8

What does the membrane act like?

Answer 8

an electrical resistor (R)

Question 9

What is the equation for conductance

Answer 9

g=1/R

Question 10

What happens to voltage gated channels during depolarisation and repolarisation?

Answer 10

• depolarisation=they open

- repolarisation=they close

Question 11

What causes the initial depolarisation that opens the odium voltage gated channels?

Answer 11

synapses and generator potentials

Question 12

Explain how an action potential occurs

Answer 12

1. Initial stimulus (depolarisation) causes:
2. Opening of V-gated Na+ channels (increase in gNa)
3. Na+ entry causes further depolarisation increasing gNa further
4. Em approaches ENa (PNa (gNa) has increased 1000x)
5. Na+ channels inactivate (decrease gNa)
6. Na+ influx stops despite maintained stimulus
7. Opening of V-gated K+ channels (increase in gK)
8. K+ exit => repolarisation (but gK increases after Em returns to rest)
9. Em approaches Ek (hyperpolarisation)
10. V-gates K+ channels close (decrease in gK)
11. Em returns to resting value (leak channels)-NOT PUMPS

Question 13

What has to happen for depolarisation to occur?

Answer 13

• Na+ influx (leak and voltage-gated) > K+ efflux (leak)

- therefore threshold has to be reached

Question 14

In the absolute refractory period can an action potential be established?

Answer 14

no further AP by any stimulus because Na+ V-gated channels are mostly inactivated and the K+ V-gated channels are open

Question 15

Can an action potential occur in the relative refractory period?

Answer 15

yes but you need a stronger stimulus to open sufficient Na+ channels to increase gK (threshold higher) this happens because Na+ v-gated channels are recovering from inactivation and some of the K+ channels are still open

Question 16

Why can’t action potentials move backwards?

Answer 16

because the membrane is refractory

Question 17

How does propagation of an action potential occur in a myelinated neutron?

Answer 17

• saltatory conduction

- ions flow from node of ranvier to another node of ranvier

Question 18

What is myelin composed of?

Answer 18

glial cells

Question 19

How does the current clamp experiment work?

Answer 19

1) current injected by electrode
2. current injection causes depolarisation and the stimulus to open channels
3. Record the change in Em in neurone during action potential

Question 20

In the current clamp experiment, what does a brief injection cause?

Answer 20

a single action potential

Question 21

What does a long injection cause?

Answer 21

multiple action potentials