Nerve impulses are caused by the movement of ions which trigger changes in the membrane potential of nerve cells (neurons)

• An impulse will occur when the membrane potential is changed from a resting potential to an action potential
  

Resting Potential

• When a neuron is not sending an impulse, sodium-potassium pumps maintain a resting potential of –70mV

• These pumps use active transport to shuttle three sodium ions outside of the cell for every two potassium ions internalised

• This creates a difference in charge across the membrane, with the inside being negative relative to the outside

Action Potential

• Neurons fire when ion channels change the resting potential into an action potential (roughly +30mV)

• An action potential will only occur if an external stimulus triggers a sufficient change in membrane polarity (threshold potential = –55mV) 

• Action potentials consist of intervals of depolarisation followed by repolarisation:

  • Depolarisation – Voltage-gated sodium channels open and the influx of sodium ions makes the membrane potential more positive 

  • Repolarisation – Potassium channels then open and the efflux of potassium ions returns the membrane potential to a negative value
    

• After each action potential, the resting potential must be re-established by sodium-potassium pumps before the neuron can fire agan (refractory period)