3 - Basis of Excitability

Question 1

how is repol caused?

Answer 1

• more K channels open and cause K efflux
• • mv inside
• repol INACTIVATES Na leak channels

Question 2

what is ion conductance and what does changing it result in?

Answer 2

• permability for that ion [gK or gNa]

g is equal to no. of channels open for that ion

changing it will cause change in Em

Question 3

how do ion conductances change to cause an action potential?

Answer 3

• depol cause Na voltage gated channels to open [gNa increases]
• Na influx [+mv]
• Em approaches ENa
• Na channels INACTIVATE despite maintained stimuli
• K voltage gated channels open [delayed]
• K efflux
• Em approaches EK
• K v-gated close
• Em returns to normal by leak channels not pumps!

Question 4

how will a Na influx only cause depolarisation?

Answer 4

if the amount of Na influx [v-gated] is greater than K efflux [leak]

Question 5

what is the threshold?

Answer 5

the Em value when Na influx by voltage gated entry is greater than the k efflux and a depol is caused

Question 6

different between absolute and relative refractory periods?

Answer 6

absolute

• no stimulus can cause an AP
• Na channels are inactivated and K still open

relative

• stronger stimuli to reach increased threshold than original can cause another AP
• enough to open inactivated Na voltage gated channels