H&H

Question 1

What are voltage gated channels?

Answer 1

Ion channels whose conductance varies as the voltage varies

Question 2

What kind of gate is the potassium channel?

Answer 2

A persistent gate. One type of closed and one type of open

Question 3

What kind of gate is the Sodium channel?

Answer 3

One open state and two types of closed gate

Question 4

What makes up the potassium gate?

Answer 4

Four independent sub-gates which must be open to allow potassium ions through.

Question 5

gk=gkbar * n^4

describes what?

Answer 5

The overall potassium conductance. N is the probability of a subgate being open and gkbar is the conductance

Question 6

What is the following equation?

taun(V)*dn(t)/dt=n_inf(V)-n(t)

Answer 6

The dynamical equation for n.
If taun(V) and n_inf(V) were constants, n(t) would decay to n_inf with a timescale of taun.

Question 7

For the dynamical equation of n., we can see that n is small when the voltage is near the resting value but climbs towards 1 as V increase.

Answer 7

n isnt equal to n_inf, rather it decays towards it with a time constant given by taun, potassium channels open as the voltage increases.

Question 8

Is the flow of potassium determined by gkV?

Answer 8

No, because there are more potassium ions inside the cell than outside and they would flow out even if V=0.

Question 9

What is this equation representing ?

Ik=gk(E-V)

Answer 9

Reversal potential calculated by Nernst equation.

Question 10

What is the Sodium channel called?

Answer 10

Transient channel because it has two closed and one open state.

Question 11

Sodium gate continued

Answer 11

The part of the gate that is closed to give the initial closed state is very like the potassium gate, but with three subgates; the probability of these subgates being open is usually called m; the other part, the gate that closes to give the second closed state is different in that it is not made of subgates, its probability of being open is usually called h and its asymptotic value, h_inf, is near one for lower V and near zero for larger V

Question 12

What is the value of reversal potential for sodium?

E_Na=

Answer 12

50mV

Question 13

I_Na=g_Na(E_Na-V)
g_Na=g_Na(bar)m^3h
What does this pair describe?

Answer 13

The sodium current

Question 14

We can now give a rough description of how spikes are formed. The time constants tau for the three gating probabilities are given in Fig. 4, these are quite complicated, but the key thing is that tau is very small, no matter what the value of V is. This means that m stays very close to its asymptotic value m_inf. As V approaches the threshold of about -55 mV, m increases towards one, with m^3 increasing even more dramatically. Opening the sodium gates ..

Answer 14

allows sodium to flood the cell, increasing the V further and further opening the gates. This gives the rapid upswing in voltage, the rising part of the spike. The other two gating probabilities have slower dynamics and it takes n and h a while to catch up with n1 and h1. However, as h decreases, it closes the sodium gates again, preventing more sodium getting in to the cell; n increases opens the potassium gates, potassium flows out reducing the V again, back towards -70 mV. This gives the downswing of the spike.