Nerve Impulses

• Schwann Cells

  • Wrap around the axon to form the myelin sheath which is a lipid so does not allow charged ions to pass through it
  • Myelin sheath has small uninsulated sections along its length called Nodes of Ranvier
  • Presence of schwann cells means electrical impulses jump from node to node (saltatory conduction).
• Cell body - where proteins and neurotransmitters are made
• Dendrites

  • Carry action potentials to surrounding cells
  • Connect to other neurones and receive impulses from them
• Maintaining a Resting Potential

  • Maintained by a sodium-potassium pump involving active transport, ATP

  1. The pump moves 2K+ ions in and 3Na+ ions out of the axon
  2. Movement of ions via sodium-potassium pump establishes an electrochemical gradient
  3. The potassium ion channel uses facilitated diffusion of K+ ions out of the neurone down their concentration gradient
• Depolarisation

  • Stimulus causes Na+ ion channels to open causing the all-or-nothing threshold of -55mv to be reached
  • Subsequently opens voltage gated Na+ channels
  • Na+ ion diffuses down its electrochemical gradient into the axon
  • This causes the potential difference to increase (depolarisation)
• Repolarisation

  • Once voltage reaches +40mv the Na+ ion voltage gated channels close
  • The K+ ion voltage gated channels open
  • Allows K+ to diffuse out of the axon down their electrochemical gradient
• Hyperpolarisation

  • Voltage gated K+ channels stay open for longer as they are slow to close
  • Allows many more K+ to diffuse out of the axon
  • This means that the potential difference across the membrane continues to decrease below -70mv
  • The voltage gated K+ channels close and the sodium-potassium pump re-establishes the resting potential
• Explain why action potentials do not overlap (3 marks)

  • AP have a refractory period
  • During this period ion channels are recovering and cannot be made to open
  • This means that no more sodium ions can diffuse into the neurone to trigger another AP
• Importance of a refractory period

  1. Makes sure AP do not overlap but instead pass along discrete separate impulses
  2. Makes sure they are unidirectional do they do not travel in two directions and prevent a response
  3. Limits the number of impulse transmissions. Important to prevent overreaction to a stimulus
• All or Nothing Principle

  • Once threshold is reached an AP will always fire with the same change in voltage
  • A bigger stimulus will not causes a bigger AP but will cause more frequent waves of depolarisation
• What affects the speed of conduction of an AP ?

  1. Myelination
  2. Axon diameter
  3. Temperature
• Myelinated vs Non myelinated

  • Myelinated - Depolarisation only happens at the nodes of Ranvier
  • Non-myelinated - Depolarisation along the whole length of the membrane
  • Slower than saltatory conduction
• Axon Diameter

  • AP are conducted quicker along axons with bigger diameters
  • Less resistance to the flow of ions
  • Less resistance means depolarisation reaches other parts of the neurone quicker
• Temperature

  • Speed of conduction increases as temperature increases
  • Ions diffuse faster
  • The speed will only increase up to around 40 degrees. Ion channels will denature and the speed decreases
• Explain what causes the conduction of impulses along a non-myelinated axon to be slower than a myelinated axon (3 marks)

  • Non-myelinated next section of the membrane is depolarised
  • Myelinated instead depolarisation only at nodes
  • Impulse jumps from node to node