6.2.3 Nerve Impulse

• Action Potential = a brief electrical impulse that occurs when a neuron is stimulated by the rapid movement of sodium and potassium ions across the membrane of an axon
• Action Potential

  1. Stimulus
  2. Depolarisation
  3. Repolarisation
  4. Hyperpolarisation
  5. Resting Potential
• Stimulus

  1. Excites the neurone causing ligin gated sodium ion channels to open
  2. Allowing sodium ions to diffuse into the neurone down their electrochemical gradient bcause the membrane is more permeable to sodium ions
  3. This makes the inside of the neurone less negative
  4. Charge changes from -70 to -55
• Depolarisation

  1. If potential difference reaches threshold (-55mV), voltage gated sodium ion channels open.
  2. More sodium ions diffuse across the neurone
  3. Charge changes from -55 to 30
• Repolarisation

  1. Around 30 mV, sodium ion channels close and voltage gated potassium ion channels open because the membrane becomes more permeable to potassim ions
  2. Potassium ions diffuse out of the neurone down concerntration gradient.
  3. This gets the membrane back to its resting potential
  4. Charge changes from 30 to -70
• Hyperpolarisation

  1. Potassium ion channels are slow to close so there is a 'overshoot' where too many potassium ions diffuse out of the neurone
  2. Potential difference becomes more negative than resting potential
  3. Charge changes from -70 to -80
• Resting Potential

  1. Ion channels are reset
  2. Membrane is returned to resting potential by sodium-potassium pump until membrane is excited by another stimulus
  3. Charge changes from -80 to -70
• The all or nothing principle = For an action potential to be generated, the stimulus must be greater than the threshold value.