#7: Membrane Potential & Action Potential

Question 1

Resting Membrane Potential

Answer 1

Electrical potential difference between exterior and interior of the cell.

Determined by:

• difference in ion concentrations of the intracellular and extracellular fluids.
• relative permeability of the cell membrane to the different ion species.

Inside is more negative than outside. Slightly more negative ions inside than outside. Ions cannot diffuse through cell membrane, so they instead use ion channels. Although many ions are present, Na+, K+, and Cl- are by far the most abundant, therefore play biggest role in generating resting membrane potential.

Question 2

Reversal/Nerst/Equilibrium Potential

Answer 2

Electrochemical gradient creates the driving force, moving the ion across the cell membrane for reaching the equilibrium.
- There is abundant K+ inside the cell. That creates a chemical gradient which ones to drive K+ out of the cell if channels are open. At the same time, there is a negative charge from the amino acids inside the cell that wants to keep K+ in. So the outcome is decided by voltage factors, and we use nerst equation to calculate that voltage at which electrical gradient will equal chemical gradient.

Nernst equation for Eion:

E[ion] = [(R x T)/(z x F)] x ln [(ion)o/(ion)i]

For monovalent cations: E[ion] = 58 log [(ion)o/(ion)i]
For monovalent anions: E[ion] = 58 log [(ion)i/(ion)o]

When electrical gradient equals -90 mV, it opposes chemical gradient for K+, and the net flow of K+ will become 0. If more positive, chemical gradient will win and more K+ will exit. If more negative, electrical gradient will win, and more K+ will enter.

Question 3

Parallel Conductance Equation

Answer 3

Calculates how much each ion contributes to the resting membrane potential.

E = (EnaGna + EkGk + EclGcl)/(Gna + Gk + Gcl)

G = conductance

Question 4

Ion Flow

Answer 4

Given a typical neuron with Ek+ = -90mV and Vm = -70mV, determine the direction of K+ flow when the membrane is forcibly set (voltage clamp) to the following voltages.

1) When Vm = 0mV
2) When Vm = -70mV
3) When Vm = -90mV
4) When Vm = -120mV

The resting membrane potential (-70) is more positive than -90, so you don’t have enough negative ions inside the cell to prevent potassium from leaving, so K+ will leave cell due to stronger chemical gradient if K+ channels open during resting state. Will be true for any voltage more positive than -90, so situations 1 and 2.

In situation 3, the electrical gradient is strong enough to oppose the chemical gradient, so the movement of K+ reaches equilibrium. This is why -90mV is the equilibrium potential for K+.

In situation 4, the electrical gradient wins. So K+ will actually be attracted into the membrane, and its concentration inside will grow.

Question 5

Action Potential

Answer 5

Action potentials reflect changes in membrane potential of one individual cell.

Excitable cells like nerve and muscle cells generate these.

The membrane of neurons have channels that are selective for different ions. When ion moves through channel, it carries a charge, and is able to change the membrane potential.

Question 6

Depolarization Phase

Answer 6

Phase of action potential. Can happen but injecting a current experimentally, or when neurotransmitters binds to and opens the Na+ channels. Positive charges then enter the cell causing a depolarization phase.

Cells become excited, meaning membrane potential becomes more positive or negative, this voltage change can cause more Na+ channels to open (voltage-gated) and more Na+ rushes into the cell causing rapid depolarization.

As membrane potential becomes more positive, Na+ become inactivated and voltage gated K+ channels open, thus K+ floods out of neuron. Makes the inside of cell more negative.

Question 7

Hyperpolarization Phase

Answer 7

Happens after depolarization phase. Since more K+ channels are open, compared to resting membrane potential, it results in hyperpolarization (making inside more negative), as voltage dependent K+ channels slowly close. The membrane potential returns to its resting state.

Question 8

Develop a Hypothesis

Answer 8

In this week’s lab, you will be asked to develop a hypothesis explaining the phenomena you observed. This part of the lab requires you to apply the scientific method, a process by which we construct an accurate, reliable, consistent and non-arbitrary representation of the world.

There are several critical steps in this process:

1. Observe and describe a phenomenon or a group of phenomena.

In this week’s lab, you will examine how some neurotoxins work. In this case, you will need to pay attention to how each of they affects the shape of an AP.

2. Formulate an hypothesis to explain the phenomena.

Recall what we have learned in the previous tutorials, especially what ion channels are involved in the generation of an AP.

3. Perform additional experiments to test the predictions by several independent experiments.

In your experiment, consider other contributing channels and manipulate them to see how it further change/or not change the generation of an AP.

4. Analyze your data and Draw a conclusion.

If the experiments results support the hypothesis, it may come to be regarded as a theory or law of nature. If the experiment results do not support the hypothesis, it must be rejected or modified.

Now, let’s practice how you will develop a hypothesis. Keep in mind a good hypothesis has to
- be testable. To support or reject your hypothesis, you need to do experiment and make observations. - These experiments should be repeatable as well.
be a statement, not a question.
- be simple and clear.
- keep the variables in mind. What changes during the testing.
- be specific. So it can be addressed using a single experiment.