Neurones & The Resting Potential
- A myelinated motor neurone:
- 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
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:
