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 great – 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 too small – 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.