A-Level Biology AQA Notes

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Neurones & The Resting Potential

The resting potential is the difference in electrical charge across the membrane while the neurone is at rest
The sodium-potassium pump uses ATP to pump 3 sodium (Na^+) ions out of the cell and 2 potassium (K^+) ions into the cell. The membrane is permeable to K+ but impermeable to Na^+ ions. These factors allow an electrochemical gradient to be set up, with the cell negatively charged at -70mV.

Action Potentials

When the neurone receives an impulse from sensory receptors, sodium channels on the dendrites open, leading to the movement of Na+ ions into the cell causing depolarisation. If this depolarisation reaches the threshold potential it activates voltage-gated sodium channels causing an action potential. After Voltage-gated sodium ion channels close, and voltage-gated potassium channels open, causing Repolarisation as K+ ions leave the cell. Outward diffusion of K+ ions causes hyperpolarisation and the voltage-gated potassium channels close. Finally, the Sodium-potassium pump returns the cell to the resting membrane potential.
Action potentials are an all or nothing response because once the threshold is reached each action potential always depolarises the axon to the same voltage by voltage-gated sodium channels.
The refractory period is the period in an action potential where the axon can't be depolarised to initiate a new action potential. It limits the frequency of action potentials and ensures action potential are discrete & only travel in one direction.

Transmission of Action Potentials

Action potential are transmitted in non-myelinated axons because when a depolarisation happens, it causes voltage-gated sodium channels to open further down the axon. By the time the depolarisation has spread, part of the axon is repolarising
In myelinated axons, action potentials only occur at the nodes of Ranvier, with charge diffusing along the cell where myelin is present (saltatory conduction).
Factors affecting transmission speed:

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