Resting Potentials and action potentials
Resting potential - The resting potential of a neurone is the potential difference across a neurone when it's at rest. At rest the potential difference is -60mv and the inside of the cell is negative compared to the outside of the cell. Different cells may have a different potential difference at rest.
Threshold potential - A threshold potential has a potential difference of -50mv. If the membrane is not depolarised to a value of -50mv (threshold potential) an action potential will not occur and vice versa.
Action potential - An action potential has a depolarised membrane with a value of +40mv. This means that the cell is positive on the inside compared to the outside of the cell. This action potential can be transmitted along a dendron or an axon.
- When at rest a neurone isn't transmitting an action potential, but the cells aren't truely at rest.
- ATP (Adenosinetriphosphate) is used to actively transport TWO K+ (Potassium ions) into the cell for every THREE pumped out of the cell.
- The cell membrane of neurones is more permeable to potassium ions that sodium ions. Therefore, potassium ions are likely to diffuse out of the cell again.
- Cell membrane also contains organic anions (negatively charged ions) which help to maintain the potential difference across the cell membrane (negative inside the cell).
- At this stage the cell membrane is said to be polarised.
- The potential difference across the membrane is negative compared to the outside and is around -60mv. This is known as the resting potential.
At rest the sodium ion channles are closed and two potassium ions are pumped in for every three sodium ions pumped out by active transport using ATP. However, the membrane is more permeable to potassium ions than Sodium ions and so a few potassium ion channels are open. This means the cell is polarised as there is a potential difference across a membrane of -60mv.
However, if some of the sodium ion channels are opened then this causes a movement of sodium ions into the cell as the sodium ions diffuse via faciliated diffusion down the concentration gradient.This causes a depolarisation of the membrane as it is lowering the potential difference (less negative). This makes the cell less negative on the inside of the cell compared to the outside of the cell.
In a generator region of receptor cells the energy changes in the environment cause the sodium ion channels to open. Therefore, receptors are known as energy transducers as they convert one form of energy…