B - Nerve Transmissions

• Neurones


  Transmit electrical impulses
• Electrical impulses travel extremely quickly along the neurone cell surface membrane from one end of the neurone to the other
• These impulses are not a flow of electrons, unlike a normal electric current
• Action potentials


  Very brief changes in the distribution of electrical charge across the cell surface membrane
• Action potentials


  Caused by the rapid movement of sodium ions and potassium ions across the membrane of the axon
• Resting potential


  In a resting axon, the inside of the axon always has a slightly negative electrical potential compared to outside the axon, usually about -70mV
• Resting potential


  • The inside of the axon has an electrical potential about 70mV lower than the outside
• Channel proteins


  In the axon membrane that allow sodium ions or potassium ions to pass through
• Action potential stimulation


  1. Sodium ion channels in the axon membrane open

  2. Sodium ions pass into the axon down the electrochemical gradient

  3. Reduces the potential difference across the axon membrane as the inside of the axon becomes less negative (depolarisation)

  4. Depolarisation triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation (positive feedback)

  5. If the potential difference reaches around -50mV (threshold potential), many more channels open and many more sodium ions enter causing the inside of the axon to reach a potential of around +30mV

  6. An action potential is generated
• Conduction of action potentials

  1. Depolarisation of the membrane at the site of the first action potential causes sodium ions to diffuse along the axon, depolarising the membrane in the next section of the axon and causing sodium ion voltage-gated channel proteins to open there

  2. This triggers the production of another action potential in this section of the axon membrane and the process continues
• Conduction


  • Sped up by the presence of Schwann cells
• Repolarisation and refractory period

  1. Sodium ion voltage-gated channel proteins close, stopping further sodium ions diffusing into the axon

  2. Potassium ion voltage-gated channel proteins open, allowing the diffusion of potassium ions out of the axon, returning the potential difference to normal (-70mV) (repolarisation)

  3. Brief period of hyperpolarisation where the potential difference becomes more negative than normal

  4. Potassium ion voltage-gated channel proteins close and sodium ion channel proteins become responsive again (refractory period)
• Refractory period


  • Ensures 'new' action potentials are generated ahead (further along the axon), rather than behind the original action potential
  • Makes the action potentials discrete events and means the impulse can only travel in one direction
• The refractory period is essential for the successful and efficient transmission of nerve impulses along neurones