Nerve Impulse Transmission (AQA A Level Biology): Revision Note

Nerve impulse transmission

• Action potentials occur at the point of stimulation, and can then be transmitted along the length of an axon as nerve impulses

Non-myelinated neurones

• Transmission of a nerve impulse in a non-myelinated neurone is as follows:

  1. the initial influx of sodium ions causes an increase in sodium ion concentration inside the axon

  2. sodium ions diffuse along the axon, depolarising the membrane in the next section of the axon

  3. voltage-gated sodium ion channels open in the new section of the axon and there is a new influx of sodium ions

     • As with the initial generation of an action potential, the voltage-gated sodium ion channels only open if the membrane depolarisation reaches the threshold potential; otherwise the initial action potential may not be transmitted any further

• The nerve impulse does not travel backwards due to the membrane behind the action potential being in a hyperpolarised state

Myelinated neurones

• Some neurones are surrounded by a fatty layer known as the myelin sheath

  • The myelin sheath is formed from Schwann cells

  • The regions of the axon in between each Schwann cell are known as the nodes of Ranvier

• The transmission of nerve impulses along myelinated axons occurs as follows:

  1. the initial influx of sodium ions causes an increase in sodium ion concentration inside the axon

  2. sodium ions diffuse along the axon

  3. in sections of the axon that are surrounded by myelin, depolarisation cannot occur

     • The myelin sheath stops the diffusion of sodium ions and potassium ions

  4. sodium ions diffuse along the length of each Schwann cell, from one node or Ranvier to the next in order to initiate an action potential at each node

     • This diffusion of charged particles sets up local currents, or local circuits

  5. Action potentials appear to jump from one node to the next, this is known as saltatory conduction

• Saltatory conduction allows the impulse to travel much faster than in an unmyelinated axon of the same diameter