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Nerve Impulse Transmission
Neurones have a specialised cell surface with many sodium and potassium channels.
One type is a sodium-potassium pump, which is constantly actively transporting sodium ions
out of the cell, and potassium ions into the cell.
Voltage-gated channels can open and close to control the flow of ions through it.
The membrane of the neurone is polarised because of the potential difference across it.
The inside of the membrane is more negatively-charged than the outside, although they both
have a negative charge.
There is a high concentration of sodium ions outside the cell, and a high concentration of
potassium ions inside the cell.
Resting potential When the neurone is not being stimulated, there are more positive ions
outside of the cell than inside of the cell. This is due to K+ diffusing out of the cell faster than
Na+; however they both diffuse into the cell at the same rate. This is resting potential, and its
charge is about -70mV.
Action potential Action potential is reached when an impulse from a stimuli causes
depolarisation (due to the influx of sodium ions into the cell), and the charge reaches +40mV.
When repolarisation (Na+ channels close and K+ channels open to allow K+ out of the cell)
occurs, the sodium channels close and the potassium channels open. There is a refraction
period, where the charge will drop below the resting state, then returns to the resting state
again after this period. Resting potential is restored, which is a constant state until another
impulse triggers the sodium channels to reopen.
Threshold potential This is the potential difference across the membrane at about -55mV. If
the stimulus causes the charge to go above this threshold, then action potential will be
reached. However, if the stimulus does not have the power to reach the threshold, then the
initiation will fail, and resting potential will be restored.
Refractory period During the refractory period, the flow of K+ out of the membrane slightly
overcompensates for the fall in charge, making the inside of the membrane even less
negative, before the membrane will restore the original balance of ions inside and outside of
the cell, at which point resting potential will be restored.
The direction in which the impulse travels along the membrane is controlled by the locking of
sodium channel gates. During the refractory period, the gate on the sodium channel will lock,
so the impulse will be unable to trigger that gate to open. This will prevent the impulse from
stimulating the channel that it has already triggered, and therefore prevents the impulse
from travelling backwards, and only forwards.