Action Potential In Cell

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Excitable Cells

* Muscle cells and
* Nerve cells (neurons) are excitable.

The color photo (courtesy of Julie H. Sandell and Richard H. Masland) is of a single interneuron in the retina of a rabbit. The cell has been injected with a fluorescent dye to reveal all its branches. Each of the small knobs at the tips of the branches makes a synapse with another cell in the retina.

The electric current

* In neurons is used to rapidly transmit signals through the animal.
* In muscles is used to initiate contraction. [Link]

The Resting Potential

All cells (not just excitable cells) have a resting potential: an electrical charge across the plasma membrane, with the interior of the cell negative with respect to the exterior. The size of the resting potential varies, but in excitable cells runs about -70 millivolts (mv).

The resting potential arises from two activities:

* The sodium/potassium ATPase. This pump pushes only two potassium ions (K+) into the cell for every three sodium ions (Na+) it pumps out of the cell so its activity results in a net loss of positive charges ...
... within the cell.

* Some potassium channels in the plasma membrane are "leaky" allowing a slow facilitated diffusion of K+ out of the cell (red arrow).

Ionic Relations in the Cell

The sodium/potassium ATPase produces

* A concentration of Na+ outside the cell that is some 10 times greater than that inside the cell
* A concentration of K+ inside the cell some 20 times greater than that outside the cell.

The concentrations of chloride ions (Cl-) and calcium ions (Ca2+) are also maintained at greater levels outside the cell EXCEPT that some intracellular membrane-bounded compartments may also have high concentrations of Ca2+ (green oval)
Depolarization

Depolarization

Certain external stimuli reduce the charge across the plasma membrane.

* Mechanical stimuli (e.g., stretching, sound waves) activate mechanically-gated sodium channels
* Certain neurotransmitters (e.g., acetylcholine) open ligand-gated sodium channels.

In each case, the facilitated diffusion of sodium into the cell reduces the resting potential at that spot on the cell creating an excitatory postsynaptic potential or EPSP.

If the potential is reduced to the threshold voltage (about -50 mv in mammalian neurons), an action potential is generated in the cell.

Action Potentials

The nerve impulse. In the resting neuron, the interior of the axon membrane is negatively charged with respect to the exterior (A). As the action potential passes (B), the polarity is reversed. Then the outflow of K+ ions quickly restores normal polarity (C). At the instant pictured in the diagram, the moving spot, which has traced these changes on the oscilloscope as the impulse swept past the intracellular electrode, is at position C.

If depolarization at a spot on the cell reaches the threshold voltage, the reduced voltage now opens up hundreds of voltage-gated sodium channels in that portion of the plasma membrane. During the millisecond that the channels remain open, some 7000 Na+ rush into the cell. The sudden complete depolarization of the membrane opens up more of the voltage-gated sodium channels in adjacent portions of the membrane. In this way, a wave of depolarization sweeps along the cell. This is the action potential (In neurons, the action potential is also called the nerve impulse.

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