6.2.1 Nerve impulses

• structure of myelinated axon:

  A) dendrite

  B) axon

  C) axon terminal

  D) node of ranvier

  E) myelin sheath

  F) cell body

  
• Describe resting potential

  Inside of axon has a negative charge relative to outside (as more positive ions outside compared to inside)
• Explain how a resting potential is established across the axon membrane in

  a neurone

  ● Na+/K+ pump actively transports:

  ○ (3) Na+ out of axon AND (2) K+ into axon

  ● Creating an electrochemical gradient:

  ○ Higher K+ conc. inside AND higher Na+ conc. outside

  ● Differential membrane permeability:

  ○ More permeable to K+ → move out by facilitated diffusion

  ○ Less permeable to Na+ (closed channels)
• Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential

  Stimulus

  • Na+ channels open, permeability to Na+ increases
  • Na+ diffuse into axon down electrochemical gradient

  Depolarisation

  • If threshold reached, action potential generated
  • more voltage-gated Na + channels open
  • more Na + diffuse in rapidly

  Repolarisation

  • Voltage-gated Na+ channels close
  • Voltage-gated K+ channels open; K+ diffuse out of axon

  Hyperpolarisation

  • K+ channels slow to close so slight overshoot – too many K+ diffuse out

  Resting potential

  • Restored by Na+/K+ pump
• establishment of action potential:

  A) stimulus

  B) depolarisation

  C) repolarisatioin

  D) hyperpolarisation

  E) resting potential

  
• Describe the all-or-nothing principle

  ● For an action potential to be produced, depolarisation must exceed threshold potential

  ● Action potentials produced are always same magnitude / size / peak at same potential

  ○ Bigger stimuli instead increase frequency of action potentials
• Explain how the passage of an action potential along non-myelinated and

  myelinated axons results in nerve impulses

  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 region depolarises

  Myelinated axon

  ● Myelination provides electrical insulation

  ● Depolarisation of axon at nodes of Ranvier only

  ● Resulting in saltatory conduction (local currents circuits)

  ● So there is no need for depolarisation along whole length of axon
• Suggest how damage to the myelin sheath can lead to slow responses and / or jerky movement

  ● Less / no saltatory conduction; depolarisation occurs along whole length of axon

  ○ So nerve impulses take longer to reach neuromuscular junction; delay in muscle contraction

  ● Ions / depolarisation may pass / leak to other neurones

  ○ 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 / inactive / will not open
• Explain the importance of the refractory period

  ● Ensures discrete impulses are produced (action potentials don’t overlap)

  ● Limits frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus)

  ○ Higher intensity stimulus causes higher frequency of action potentials

  ○ But only up to certain intensity

  ● Also ensures action potentials travel in one direction – can’t be propagated in a refractory region
• Describe the factors that affect speed of conductance

  Myelination

  ● Depolarisation at Nodes of Ranvier only → saltatory conduction

  ● Impulse doesn’t travel / depolarise whole length of axon

  Axon diameter

  ● Bigger diameter means less resistance to flow of ions in cytoplasm

  Temperature

  ● Increases rate of diffusion of Na+ and K+ as more kinetic energy

  ● But proteins / enzymes could denature at a certain temperature
• explain how a resting potential is maintained across the axon membrane in a neurone

  • higher concentration of potassium ions inside and higher concentration of sodium ions outside
  • membrane more permeable to potassium ions
  • sodium ions transported out and potassium ions transported in
• explain why the speed of transmission of impulses is faster along a myelinated axon than along a non-myelinated axon

  • myelination provides insulation
  • in myelinated, impulse moves by saltatory conduction
  • in non myelinated, depolarisation travels along whole length of axonn
• a scientist investigated the effect of inhibitors on neurones. she added a respiratory inhibitor to a neurone. the resting potential changed from -70mv to 0mv. explain why

  • less ATP produced
  • less active transport
  • electrochemical gradient not maintained