Terms to know about cell firing

 

Na+ (sodium)

The major positive ion in the fluid outside the cell: it can leak into the cell.

K+ (potassium)

The major positive ion inside the cell. It can leak in and out.

Na+/K+ ATPase, or Na+/K+ pump

An enzyme that uses ATP for fuel, to pump the Na+ back out of the cell and pump the K+ back into the cell

Cell resting potential (also called Em)

This is the charge inside the cell when it is at rest. It is around -90 millivolts, because the cell contains negatively charged proteins.

Ion channels or ion gates

Proteins in the cell membrane that let particular ions through

Ligand-gated Na+ channels

These are gates for Na+, and they will open when a chemical from outside the cell binds to them (like a molecule you're smelling or tasting). Then Na+ can go into the cell.

Voltage-gated Na+ channels

These are gates for Na+ that will open when the cell charge becomes more positive (for instance, when you get an electric shock). Then Na+ can go into the cell.

Depolarization

This is when Na+ gates open and Na+ goes into the cell, making it become more positive.

Threshold potential (Et)

As the cell gets more positive, it reaches a charge at which the voltage-gated Na+ channels will open. This is the threshold potential. It is around -50 to -70 mV.

Action potential (Ea)

When Na+ has flooded into the cell, the cell becomes as positive as it can. This is its action potential.

Voltage-gated K+ channels

These K+ channels open when the cell reaches action potential, and let the K+ flow out of the cell.

Repolarization

This is when the K+ leaves the cell, making the cell become negative again.

Refractory period

This is a short period after the cell repolarizes, when it can't fire again until it has 'reset' itself.

 

 

Hyperpolarized

A cell whose resting potential is too negative - that is, instead of being -90mV, it might be -120 mV. This cell will not fire as easily as usual.

Hypopolarized

A cell whose resting potential is not negative enough - instead of being -90 mV, it might be -70 mV. This cell will fire easily.

Sensitivity

How easily a cell fires. This depends on how hard it is to raise the cell's charge from its resting potential to the threshold potential.

K+ effect on resting potential

Increasing the amount of K+ outside the cell will make the resting potential more positive and hypopolarize the cell. Decreasing the amount of K+ outside the cell will make the resting potential more negative and hyperpolarize the cell.

Extracellular Ca2+

When Ca2+ is outside the cell, it can block the Na+ channels. This makes it harder for the cell to fire.