1) What is the distribution on Na⁺ and K⁺ in a neuron at rest?

There is more Na⁺ outside than inside the cell and more K⁺ inside, but the difference is greater for Na⁺ than K⁺, which more positive ions are on the outside than inside.

2) What does the Na⁺-K⁺ pump do?

It uses ATP to pump 3 Na⁺ out of and 2 K⁺ into the cell.

3) What causes the passive diffusion of Na⁺ and K⁺ when the neuron is at rest?

A concentration and electrical gradient is established by the Na⁺-K⁺ pump.  Some Na⁺ diffuses back into the cell down its concentration and the electrical gradient. Much more K⁺ diffuses down its concentration gradient, but it goes against the electrical gradient, so eventually reaches an equilibrium of unequal concentrations.

4) Why is there more K⁺ diffusion then Na⁺ diffusion?

More K⁺ diffuses because there are many more K⁺ passive channels than Na⁺ passive channels.

5) Why is the electric potential “–“70 mV?

Because the inside is negative.

1) Where do they occur?

In the receptive region of a neuron (usually dendrites and sometimes cell bodies)

2) Why are they called “local”?

They only travel a short-distance.

3) What is meant by describing them as decremental and graded?

Decremental means that the potential weakens as it travels away from the point of stimulation.

Graded means that the potential can vary in its magnitude (some are stronger than others)

4)  What possible changes occur in these local potentials & why?

Hyperpolarization – potential becomes more negative because either K⁺ flows out or Cl^- inward

1) Where do action potentials start?  Why do they start there?

AP start at the axon hillock or trigger zone (for unipolar neurons) because there is a high concentrations of voltage-regulated gates.

2) During the resting state, what voltage-regulated gates (VRG) are open and which are closed?

Both Na⁺ and K⁺ VRGs are closed when axolemma is at rest.

3) How does a local potential initiate an action potential?

If a local potential is strong enough to exceed the threshold (-55mV) at the trigger zone, an action potential is generated.

4) What happens to the axolemma VRGs when the membrane potential reaches threshold (–55mV)?

First, Na⁺ VRGs will open and then K⁺ VRGs will slowly open.

5) What ions are moving and in what direction during the early depolarization phase?

When Na⁺ VRGs open, Na⁺ rushes into the cell down its concentration gradient and the electrical gradient.  This causes the membrane potential to increase.

6) What VRGs are opening and which are closing during the late depolarization phase?

When the membrane potential passes 0mV, the cell is now positive on the inside. Na⁺ VRGs will start to close and K⁺ will be fully opened by end of phase.

7) What ions are moving and in what direction during the late depolarization phase?

When the K⁺ VRGs open, K⁺ exits the cell down its concentration and the now reversed electrical gradient.  This will slow down and eventually reverse the membrane potential.

8) What is the maximum voltage during an action potential?

+35 mV

9) What ion channels are opening and which are closing during the repolarization phase?

K⁺ VRGs continue to open and Na⁺ are all closed.

10) What ions are moving and in what direction during the repolarization phase?

K⁺ continue to flow out of the cell and the membrane potential decreases, so that the inside of the cell becomes more negative than the outside.

11) Why does hyperpolarization occur?  What is happening with the potential during this hyperpolarization phase?

K⁺ are slow to close so that K⁺ continues to flow out causing the membrane potential to fall below -70 mV.

12) How does the resting membrane potential get restored?

Na⁺-K⁺ pump and diffusion continue to work to restore the membrane potential back to -70 mV.

13) How does the action potential move away from its point of origin? 

As the Na⁺  VRGs open and depolarize the membrane in the axon hoillock it causes adjacent VRGs to open toward the axonal terminals.  Depolarization there causes the adjacent gates to open and so on.

14) Why does it move in only one direction?

Na⁺ VRGs open only if they are fully closed.  After depolarization occurs, the Na⁺ VRGs are inactive for a brief period.  That makes the AP irreversible and it will always go toward the axonal terminals.

15) Why is an action potential an all-or-none response?

Because once a local potential reaches threshold, the membrane potential will always go all the way to 35 mV and travel down the entire axolemma at 35 mV.  if threshold is not reached an AP is not generated.