Ion Channel Gating

Question 1

What is ionic current measured for?

Answer 1

1. Permeability properties of channels

2. Mechanisms by which channels open and close

Question 2

Point of voltage clamp?

Answer 2

To measure ionic currents
Membrane current is addition of ionic current (I flowing though conductance) + a component of capacitive current
- But if we measure current, when the voltage is steady, the membrane current = the ionic current

Question 3

What is current

Answer 3

rate of flow of a particular ionic species is the conductance of the membrane to the species multiplied by the driving force for that species.

Question 4

What is driving force?

Answer 4

(Em – Eion

Question 5

What is conductance in terms of resistance?

Answer 5

g = 1/R

Question 6

Current in terms of conductance?

Answer 6

I = g * (Em – Eion)

NB: Ohms law = V = IR

Question 7

Difference between conductance and permeability

Answer 7

Conductance depends on membrane potential, the type of ion and the concentration of the ion (on either side of membrane). Permeability on all but not the concentration of the ion.

Question 8

What is inward current?

Answer 8

• positively charged ions moving into cell
• downward deflection
• depolarizes cell under voltage clamp (takes positive into a negative cell)

Question 9

What is outward current?

Answer 9

positive charges leaving the cell (like K)

Question 10

What does Hodgkin & Huxley prove?

Answer 10

Early inward current associated with Na+

Late outward associated with K+

Question 11

Which channel shows inactivation Na+ or K+

Answer 11

Only Na+

Question 12

What is Na+ channel inactivation dependent on?

Answer 12

Time and voltage

Question 13

Population movement of channels

Answer 13

Greater proportion move to closed/open state. Remember channels randomly opening and closing all the time

Question 14

Structure of K channel

Answer 14

4 monomers held together

• 6 transmembrane domains each
• loop between S5+S6 -sticks in middle
• S4 is voltage sensor?

Question 15

What is N-type inactivation?

Answer 15

Following activation by changing trans-membrane potential some channels show time-dependent inactivation even though trans-membrane potential is maintained. This is known as rapid inactivation or N-type inactivation.
An indication of the mechanism underlying this process arose from application of proteolytic enzymes to the intracellular surface of the membrane.