Action Potential and RMP

Relative Refractory Period

A period after firing when a neuron is returning to its normal polarised state and will fire again only if the incoming message is much stronger than usual.

Only some ion channels can be activated.

Absolute Refractory Period

The period immediately following the firing of a neuron when it cannot be stimulated no matter how great a stimulus is applied.

No ion channels can be activated.

Refractory Period Function

Prevents impulses from travelling backwards or firing too frequently.

Action Potential Steps

• Cell is depolarised to -55mV
• Voltage-gated Sodium Channels open
• Sodium ions enter
• Cell reaches +30mV
• Sodium channels close
• Voltage-gated Potassium channels open
• Potassium leaves
• Cell repolarises, then hyperpolarises
• Sodium-Potassium Pump restores RMP (-70mV)
• Charges trigger AP along the axon
• Signal is passed to receiving cell

Types of Action Potential

Excitatory - Promotes AP in receiving cell
Inhibitory - Inhibits AP in receiving cell

Sodium-Potassium Pump Properties

Transmembrane Protein
Keeps RMP
Uses ATP for Active Transport

Sodium-Potassium Pump steps

• 3 Na+ ions inside the cell bind to the pump. A phosphate group from ATP also binds to the pump.
• Pump changes shape transporting 3 Na+ ions across the cell membrane. Na+ ions are released outside the cell.
• 2 K+ ions outside the cell bind to the pump. The K+ ions are transported across the cell membrane.
• Phosphate group is released, the 2 K+ ions are released into the cell.