Impulse Transmission

• Hyperpolarisation + Repolarisation

  • Action potential achieved with max PD +40mV
  • Na+ voltage-gated channels close
  • K+ voltage gated channels open
  • More K+ diffuse out, down electrochemical gradient
  • PD fall, restore negative PD
  • Slight K+ overshoot, too many diffuse out so PD more negative than normal, e.g -75mV (hyperpolarised)
  • Na+ and K+ ions on opposite sides, action potential cannot occur (until more Na+ ions out, more K+ in)
  • Refractory potential:
  • Time taken (milliseconds) after action potential peak to establish true axon resting potential
  • Adjusts ions to correct positions
• Action potentials along axon plasma membrane

  • Action potentials transmitted along axon plasma membranes
  • Positive ion diffusion, down electrochemical gradient
  • To where no action potential
  • Na+ ions diffuse in, positive ion conc high, compared to regions nearby
  • Next membrane is polarised
  • Much lower Na+ concentration
  • So Na+ ions diffuse down electrochemical gradient in
  • As more Na+ ions diffuse to next region
  • PD increases
  • Na+ voltage gated channels open; cause action potential
  • Impulse only transmitted one direction, region behind in refractory period
• 'All or Nothing’ Response

  • Action potential peaks at +40MV
  • Stimulus over threshold does not cause larger action potential value
  • Either full action potential or nothing
  • But, impulses/action potentials frequency can vary
  • Larger stimulus increase frequency
• Myelinated Neurone

  • Enclosed by insulating myelin sheath
  • Made largely of phospholipids (80% lipid + 20% protein)
  • Form by Schwann cells that wrap whole axon length
  • Schwann cells spiral round, enclosing axon in many plasma membrane layers
  • Spaces between cells are Nodes of Ranvier; every 1-3mm
  • Speeds nerve impulse transmission
  • Action potentials, can only occur at Nodes of Ranvier
  • Jump node to node
  • Impulse conduction is saltatory conduction
• Unmyelinated Neurone

  • Action potentials have to occur along whole length
  • Conduction speed along unmyelinated is much slower than myelinated
  • Diameter also effects speed
  • Wider axon, faster, less ion leakage + resistance; unmyelinated wider
  • Peripheral fibre severed
  • Myelin sheath provides track regrowth
  • Unmyelinated fibres and myelinated axons of mammalian CNS do not regenerate
• At high temp

  • Kinetic energy increased so ion diffuse faster
  • Speeds up depolarisation + refractory periods
  • Only true up to 40C
  • Above protein ion channels denature
  • Disrupts membrane fluidity, decreasing speed