Voltage-Dependent Membrane Permeability

Question 1

Action potential

Answer 1

fundamental electrical signal generated by nerve cells and arise from changes in membrane permeability to specific ions

rapid and transient rise in Na followed by a slower, but more prolonged rise in K

Question 2

Why is Na sensitive to the membrane potential?

Answer 2

An increase in Na permeability occurs when the neuronal membrane potential becomes more positive than a threshold level. Thus the mechanism responsible for the increase in Na permeability is sensitive to the membrane potential.

Question 3

Voltage clamp method

Answer 3

Allows experimenters to control membrane potential and simultaneously measure the underlying permeability changes –> providing information needed to define the ion permeability of the membrane at any level of membrane potential

Question 4

Process of voltage clamp method

Answer 4

1. One internal electrode measures membrane potential and is connected to the voltage clamp amplifier.
2. Voltage clamp amplifier compares membrane potential with the desired (command) potential.
3. When membrane potential is different from the command potential, the clamp amplifier injects current into the axon through a second electrode. This feedback arrangement causes the membrane potential to become the same as the command potential.
4. the current flowing back into the axon, and thus across its membrane, can be measured here

Question 5

H+H first hypothesis that potential-sensitive Na and K permeability changes are both necessary and sufficient for the production of action potentials:

Answer 5

hyperpolarized: nearly instantaneous and short lived capacitative current
depolarized: following capacitative current, rapidly rising inward ion current which gives the way to a more slowly rising, delayed outward current

Question 6

2 types of voltage dependent ion currents

Answer 6

Changing the membrane potential to a level more positive than the resting potential produces 2 effects:

• early influx of Na into the neuron (producing a transient inward current)
• followed by a delayed efflux of K (producing a sustained outward current)

The differences in the time course and ion selectivity of the 2 fluxes suggest that 2 different ion permeability mechanisms are activated by changes in membrane potential.

Question 7

Pharmacological studies:

Answer 7

• tetrodotoxin
• tetraethylammonium

These drugs affect these two currents

Question 8

Conclusion 1: inactivation

Answer 8

While the Na and K conductance’s share the property of time-dependent activation, only the Na conductance exhibits inactivation. The time courses of the Na and K conductances are voltage-dependent, with the speed of both activation and inactivation increasing at a more depolarized potential.
More rapid membrane currents at more depolarized potentials.

Question 9

Conclusion 2:

Answer 9

Both the Na and K conductances are voltage-dependent. Both conductances increase progressively as the neuron is depolarized. Conductances are quite small at negative potentials, maximal at very positive potentials, and dependent on the membrane voltage at intermediate potentials.

Question 10

Process of activation and inactivation

Answer 10

○ This increases causes Na+ to enter the neuron - depolarizing the membrane potential, which approaches ENa
○ The rate of depolarization subsequently falls both because the electrochemical driving force on Na+ decreases and the Na+ conductance inactivates
○ At the same time, depolarization slowly activates the K+ conductance, causing K+ to leave the cell and repolarize the membrane potential toward EK
○ Because the K+ conductance is temporarily higher than it is in resting condition, teh membrane potential briefly becomes more negative (the undershoot)
This hyperpolarization of the membrane potential causes K+ to turn off

Question 11

Action potential propagation requires

Answer 11

the coordinated action of 2 forms of current flow:

• the passive flow of current
• active currents flowing through voltage-dependent ion channels