- There is a difference in charge between the inside and outside of the cell
- The inside is negatively charged relative to the outside
- There is a voltage (potential difference) across the membrane
- Resting potential refers to this potential difference
- The resting potential is generated by ion pumps and ion channels
- Active transport using Sodium-Pottasium pump (3Na+ actively pumped out of the membrane, and 2K+ are actively transported into the axon)
- Na+ diffuses back in whilst K+ diffuses back out
- Creates a sodium ion electrochemical gradient because there is a more positive sodium ion outside and outside the cell
- When the cell is at rest most potassium ion channels are open so that the membrane is permeable to potassium ions so more diffusion back out through the potassium ion channels.
- The cell membrane is excited when a stimulus is detected and becomes permeable which allows more ions to move in and out of the cell (which therefore alters the potential difference)
- This change in potential difference due to a similes is the generator potential
- A bigger stimulus excited the membrane more resulting in a bigger movement of ions and an even greater change in potential difference
- Stimulus causes te membrane at one part of the neurone to increase in permability to Na
- Na voltage gated channels open and na enters the axon down their electrochemical gradient by diffusion
- This results in resiting potential
- If the generator potential exceeds the threshold it will trigger an action potential (an electrical impulse along a neurone)
- It is only triggered if the generator potential reaches the threshold level.
- Action potentials are all one size, therefore strength of stimulus is measured by frequency of action potentials
- If the stimulus is too weak the generator potential will not reach the threshold level and so there is no action potential
- When depolarisation reaches +30mV the Na+ ion channels close and potassium ion channels open
- The diffuse membrane is more permeable to potassium so K+ diffuses out of the neurone down the potassium ion concentration gradient
- This starts to get the membrane back to its resting potential
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