action potentials

what is an action potential?

rapid impulse that travels along a neurone, causing changes in membrane potential

what are the 3 main stages of generating an action potential?

1. depolarisation

2. repolarisation

3. hyperpolarisation

describe and explain how an action potential is generated:

1. resting neurone at resting potential - some K⁺ voltage gated channels are open, all Na⁺ channels are closed

2. stimulus arrives at neurone, causing voltage-gated Na⁺ channels to open ∴ Na⁺ diffuses into the axons down an electrochemical gradient, making it less -ve

3. if the membrane potential reaches the threshold potential of -55 mV, more voltage-gated Na⁺ channels open - this influx of Na⁺ causes depolarisation

4. when enough Na⁺ enters the axon, membrane potential reaches +40 mV - this is action potential

5. when action potential has been reached, all voltage-gated Na⁺ channels close and voltage gated K⁺ channels open - this means that K⁺ start diffusion down the electrochemical gradient out of the axon

6. the diffusion of K⁺ out causes a temporary overshoot of the resting potential - this is hyperpolarisation - to restore resting potential, voltage-gated K⁺ channels close and Na-K pump actively transports 3Na⁺ out and 2K⁺ in

what is depolarisation?

a reversal in membrane potential

what voltage is action potential?

+40 mV

what voltage is the threshold potential?

-55 mV

how does the action potential move along the neurone?

as a wave of depolarisation

how does the speed of action potential transmission change with axon diameter?

• larger axon diameter means there is less resistance to ion flow

• ∴ wave of depolarisation travels faster

how does the speed of action potential change with temperature?

• higher temp → faster diffusion of ions

• ∴ faster action potential transmissions

• over 40^oC - proteins denature → slower action potential transmission due to membrane damage

explain the importance of the refractory period:

• ensures action potentials are discrete (i.e. don’t overlap)

• limits the freq of impulses by setting a minimum time period between action potentials

• ensures impulse travels in 1 direction

describe and explain the all-or-nothing principle:

• once the threshold is reached, an action potential will always fire w/ the same change in voltage, no matter how big the stimulus is

• if the threshold isn’t reached, an action potential won’t fire

• a bigger stimulus won’t cause a bigger action potential, but it will cause them to fire more frequently

complete this table:

A = closed

B = open

C = open

D = closed (but leaky)

E = closed

F = open