Neural Signaling: How a Message Moves Along A Neuron

There is a -0.070 volt (V) potential difference across a cell membrane.
Translated into normal language: the inside of the cell is negative with respect to the outside; it is not neutral!

Nature of the Resting Potential

The resting potential is due to the nature of the cell membrane.
- The membrane keeps large negative ions (e.g. protein molecules) inside.
- It is impermeable to sodium ions and potassium ions, so they stay in their place.
  - This is a bit of lie since potassium is a bit leaky in the outward direction and sodium is a bit leaky in the inward direction.
  - They both want to diffuse down their concentration gradients.
There is a special channel for potassium and another for sodium.
- They are called gated ion channels because they are a hole with a gate.
- The gates are closed at this time.
The sodium–potassium pump:
- Pumps sodium out of the cell.
- Pumps potassium into the cell.
- More sodium is pumped out than potassium is pumped in.
Result of the sodium–potassium pump's actions:
- The membrane is more negative on the inside: -0.070 V.
- This is called the resting potential of the nerve cell.
- We say the membrane is polarized; i.e., it is not neutral.
- This is true of all nerve cells that are at rest (not stimulated).

Figure 48.5, page 965, Campbell's Biology, 5th Edition

A graded potential is a stimulus which causes the membrane to change slightly from its normal -0.070 V resting potential.
Two possibilities:
- Hyperpolarization: more negative than the resting potential
- Depolarization: more positive than the resting potential
Called “graded” because the amount of polarization depends on the strength of the stimulus.
- A greater stimulus will cause a greater change from the resting potential of -0.070 V.

Figure 48.6(a) and 48.6(b), page 967, Campbell's Biology, 5th Edition

Action potential
- A membrane will exhibit graded potential characteristics until a threshold is reached.
- At that point the membrane becomes radically depolarized.
- The threshold is 0.015 to 0.020 V more positive than the resting potential (i.e., -0.055 to -0.050 V).
- Reaching the action potential is an all-or-nothing event (it is non-graded).
Exhibits a characteristic pattern as shown in the graph.
- Depolarization phase
- Repolarization phase
- Hyperpolarization phase (undershoot)
- Back to resting potential

Figure 48.6(c), page 967, Campbell's Biology, 5th Edition

Resting state
- Sodium channels are closed.
- Potassium channels are closed.
- Sodium–potassium pump is pumping sodium out and potassium in.
- -0.070 V potential is maintained (this is the resting potential).
Depolarization
- A sufficient stimulus causes enough depolarization of the membrane that the threshold potential is reached.
- Sodium gates are opened and sodium rushes into the cell.
- Potassium gates stay closed.
- Inside of the membrane becomes more positive than the outside.
Repolarization
- Sodium gates are closed.
- Potassium gates are opened.
- Potassium ions rush out of the cell.
- Loss of potassium causes the inner cell membrane to become more negative than the outside of the cell.
Hyperpolarization
- Sodium gates remain closed.
- Potassium gates have not had time to close yet.
- Potassium continues to leave the cell.
- Cell becomes even more negative than during the resting state, hence the term hyperpolarization.
Eventually the potassium gates are closed and the membrane returns to its resting state by the action of the sodium–potassium pump.
Refractory period: time during which the cell membrane cannot form a new action potential

Figure 48.7, page 968, Campbell's Biology, 5th Edition

Figure 48.8, page 969, Campbell's Biology, 5th Edition