6.2.1 nerve impulses

• structure of a myelinated motor neurone

  dendrite

  cell body (soma)

  axon

  myelin sheath (made of schwann cells)

  node (of ranvier)

  axon terminal
• describe resting potential

  inside of axon has a negative charge relative to outside (as more positive ions outside compared to inside)
• explain how 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 due to higher K+ concentration inside AND higher Na+ concentration outside

  differential membrane permeability - more permeable to K+ so moves out by facilitated diffusion and 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 so membrane permeability to Na+ increases

  Na+ diffuse into axon down electrochemical gradient (causing depolarisation)
• explain how changes in membrane permeability lead to depolarisation and the generation of an action potential (depolarisation)

  if threshold potential reached an action potential is generated

  as more voltage-gated Na+ channels open (positive feedback effect)

  so more Na+ diffuse in rapidly
• explain how changes in membrane permeability lead to depolarisation and the generation of an action potential (reploarisation)

  voltage-gated Na+ channels close

  voltage-gated K+ channels open and K+ diffuse out of axon
• explain how changes in membrane permeability lead to depolarisation and the generation of an action potential (hyperpolarisation)

  K+ channels slow to close so there's a slight overshoot (too many K+ diffuse out)
• explain how changes in membrane permeability lead to depolarisation and the generation of an action potential (resting potential)

  restored by Na+/K+ pump
• 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

  a bigger stimuli would instead increase frequency of action potential
• explain how the passage of an action potential along a non-myelinated axon results in nerve impulses

  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
• explain how the passage of an action potential along a myelinated axon results in nerve impulses

  myelination provides electrical insulation

  depolarisation of axon only at nodes of ranvier

  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 so depolarisation occurs along whole length of axon

  so nerve impulses take longer to reach neuromuscular junction so 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 action potentials

  but only up to a certain intensity

  also ensures action potentials travel in one direction so can't be propagated in a refractory region
• describe factors that affect speed of conductance (myelination)

  depolarisation only at nodes of ranvier so saltatory conduction

  impulse doesn't travel/depolarise whole length of axon
• describe factors that affect speed of conductance (axon diameter)

  bigger diameter means less resistance to flow of ions in cytoplasm
• describe factors that affect speed of conductance (temperature)

  increases rate of diffusion of Na+ and K+ as more kinetic energy

  but proteins/enzymes could denature at a certain temperature