Nerve Impulses

• Describe the structure of a myelinated motor neuron:

  • Dendrites
  • Cell body (Soma)
  • Axon
  • Myelin sheath - made of Schwann Cells
  • Nodes of Ranvier
  • Axon terminal
• How is a resting potential established across the axon membrane in a neuron:

  • Na+ / K+ ATPase actively transports 3 Na+ out of axon into the tissue fluid and 2 K+ into axon from the tissue fluid
  • An electrochemical gradient for both K+ and Na+ forms
  • K+ and Na+ channels are open so K+ and Na+ move through the channels via facilitated diffusion
  • More diffusion of K+ and less of Na+ as there are more K+ channels along axon
  • The membrane is more permeable to K+ and less permeable to Na+
  • Less Na+ is being diffused back across so the build up causes a bigger charge to form on the outside of axon
• Describe the process of Depolarisation:

  • When a stimulus is applied and and it exceeds the threshold value of -40mV an action potential is generated
  • As more Voltage-gated Na+ channels open
  • Causing a gradient to form and Na+ to diffuse in rapidly
  • Which causes the charge to build up inside the axon
• Describe the process of Repolarisation:

  • When +40mV is reached repolarisation occurs
  • Voltage-gated Na+ channels close
  • Voltage-gated K+ channels open
  • K+ diffuses across membrane and into the tissue fluid
• What is the Refractory Period?

  • All protein channels close apart from Na+ / K+ ATPase
  • Na+ / K+ ATPase returns conc. of Na+ and K+ back to normal and on the right side of membrane
  • Resting Potential is restored
• What is Hyperpolarisation?

  K+ channels are slow to close so there is a slight overshoot as too many K+ diffuse out
• Describe the All or nothing Principle:

  • For an action potential to be produced, depolarisation must exceed the threshold value of -40mV
  • Action potentials produced are always the same magnitude / size / peak at same potential
  • Bigger stimuli instead increase the frequency of action potentials
• Explain the passage of an action potential along a non-myelinated axon:

  • Action potential passes as a wave of depolarisation
  • Influx of Na+ in one region increases permeability of adjoining region to Na+ by causing voltage-gated Na+ channels to open so adjoining regions depolarise
• Explain the passage of an action potential along a Myelinated axon:

  • Myelination provides electrical insulation
  • Depolarisation of axon at Nodes of Ranvier only
  • Resulting in saltatory conduction
  • So there is no need for depolarisation along whole length of axon
• Suggest how damage to to Myelin sheath can lead to slow responses and / or jerky movements:

  • Less / no saltatory conduction; depolarisation occurs along whole length of axon - so nerve impulses take longer to reach neuromuscular junction; delay in muscular contraction
  • Ions / depolarisation may pass / leak to other neurons - Causing wrong muscle fibres to contract
• Describe the nature of the refractory period:

  • Time taken to restore axon to resting potential when no further action potential can be generated
  • As Na+ channels are closed
• Explain the importance of the refractory period:

  1)Ensures discrete impulses are produced (action potentials don't overlap)

  2)Limits frequency of impulse transmission at a certain intensity (prevents overreaction to stimulus)

  • Higher intensity stimulus causes higher frequency of action potentials
  • But only up to a certain intensity

  3)Also ensures action potentials travel in one direction - can't be propagated in a refractory period
• Describe how Myelination can affect speed of conduction:

  • Depolarisation oat Nodes of Ranvier only -> Saltatory Conduction
  • Impulse doesn't travel / depolarise whole length of axon
• Describe how Axon diameter can affect speed of conduction:

  • Bigger diameter means less resistance to flow of ions in cytoplasm
• Describe how Temperature can affect speed of conductance:

  • Increases rate of diffusion of Na+ and K+ as more kinetic energy
  • But proteins / enzymes could denature at a certain temperature