Action potentials and resting potential

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Resting potentials and action potentials
Neurones have very specialised membranes filled with many channel proteins that are
specific either to sodium or potassium ions.
Sodium-potassium pumps actively transports sodium ions out of the cell and potassium ions
into the cell.
There are also voltage-gated channels which are specific to ions: so only one gated channel
will only allow through sodium or potassium ions.
How the resting potentials is established
when a neurone is not transmitting an action potential it said to be at rest.
Sodium-potassium ion pumps use ATP to pump three sodium ions out of the cell for
every two potassium ions pumped into the cell.
The plasma membrane is more permeable to potassium ions than to sodium ions.
Potassium ions diffuse freely out of the cell.
The cell cytoplasm also contains large organic anions (negatively charged ions).
The inside of the cell is more negatively-charged than the outside of the cell.
The cell membrane is said to be polarised.
The potential difference across the cell membrane is -60mV. This is called the RESTING
How the action potential is established
1. The membrane starts in its resting state ­ polarised with the inside of the cell being
-60mV compared to the outside.
2. Sodium ion channels open and some sodium ions diffuse into the cell.
3. The membrane becomes depolarised (it becomes less negative with respect to the
outside of cell and reaches the threshold value of -50mV.
4. Voltage-gated sodium ions channels open and many sodium ions flood into the cell. As
more sodium ions enter, the cell becomes more positive.
5. The potential difference across the plasma membrane reaches +40mV. The inside of the
cell is positive compared to the outside of the cell.
6. The sodium ion channels close and the potassium ion channels open.
Damilola Fasoyiro A2 Biology

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Potassium ions diffuse out of the cell bringing the potential difference back to negative
inside the cell. This is called repolarisation.
8. The outflow of potassium ions overshoots the potential difference to just below -70mV,
which overcompensates the change in electronegativity ­ this is called hyperpolarisation ­
and then K+ channels close
9.…read more

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Transmission of an action potential
When an action potential arrives sodium channels open, which causes an influx of sodium
ions into the cell, depolarising that section of the membrane.
This disrupts the balance of ions in surrounding areas so that local currents are created in
the cytoplasm, causing sodium channels further along the membrane to open.
Local current are the movements of charged particles (ions) within a cell along a
concentration gradient.…read more

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Sodium and potassium ions cannot diffuse through this fatty area.
The ionic movements that create an action potential cannot occur over much of the
length of the neurone.
The gaps between the Schwann cells are called nodes of Ranvier.
The ionic changes that cause an action potential only occur at the nodes of Ranvier.
In myelinated neurones the local currents are elongated and sodium ions diffuse along
the neurone from one node of Ranvier to the next.…read more

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Damilola Fasoyiro A2 Biology
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