What is an action potential?
Change in charge across an axon membrane when a nerve impulse passes through.
The inside is more positive compared to the outside.
How does the membrane become depolarised?
- A stimulus causes an increase in membrane permeability with Na gates opening.
- Na ions diffuse rapidly into the axon down an electrochemical gradient, making the membrane more positive, DEPOLARISING the membrane. This causes more gates to open and more Na to enter.
- The action potential is now formed, with the charge being 40 mV.
How does the membrane become repolarised?
- The membrane is less permeable to Na as the gates close and is more permeable to K as the K gates open.
- Positively charged K ions diffuse out, making the membrane negative again (more ions outside the membrane than in). This REPOLARISES the membrane.
What is hyperpolarisation?
The membrane becomes hyperpolarised - more negative than the resting potential.
This is because the membrane has increased permeability to K ions.
How is a resting potential restored?
The K gates close and the resting levels of permeability are re established (normal concentrations of ions on each side of the membrane), along with the action of the sodium-potassium pump.
What is the refractory period?
A period after an action potential has been generated where the membrane can’t be stimulated because the sodium channels are closed and can’t be stimulated (membrane can’t become repolarised again).
Important because:
- Ensures nerve impulses are only passing in one direction.
- Limits number of impulse transmissions stopping the over reaction to a stimulus and overwhelming the senses.
How does an action potential move down an unmeylinated neuron?
- Action potentials are transmitted by a wave of depolarisation
- A stimulus leads to sodium ions diffusing in, depolarising a part of a neuron/
- Local electrical currents cause sodium gated channels to open further along the membrane while the part of the neuron before depolarises.
What is saltatory conduction?
- Action potentials occur at the nodes of Ranvier and jump from one node to the other.
This speeds up transmission as ionic movements in an action potential happen LESS FREQUENTLY so less time is taken to pass an impulse.
