6.2.1 Nerves Impulese

• Structure of a myelinated motor neurone

  dentrile, cell body, axon (myelin sheath and node of ranvier), axon terminal

  
• resting potential- At resting potential:

  • inside of axon has a negative charge relative to outside
  • As more positive ions outside compared to inside

  • Na*/K* pump actively transports:

  • (3) Na* out of axon

  • (2) K* into axon

  • Creating an electrochemical gradient:

  • Higher K* conc. inside

  • Higher Na* conc. outside

  • Differential membrane permeability:

  • More permeable to K* → move out by facilitated diffusion

  • Less permeable to Na* (closed channels)

  
• The all-or-nothing principle

  Bigger stimuli increase frequency of action potentials

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

  ● Action potentials produced are always the same magnitude / size / peak at same potential
• 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
• Application: damage to myelin sheath eg. multiple sclerosis → slow responses / 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
  • lons / depolarisation may pass / leak to other neurones
  • Causing wrong muscle fibres to contract
• The nature and importance of the refractory period

  • Ensures discrete impulses are produced ( action potential doesn’t overlap

  • 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

  In the second half of the refractory period an action potential can be produced but requires greater stimulation to reach threshold
• Refractory period: time to restore axon to resting potential when no further action potential can be generated because Na* channels are closed / inactive / will not open
• The importance of refectory period

  • Ensures discrete impulses are reproduced so 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
• In the second half of the refractory period an action potential can be produced but requires greater stimulation to reach threshold
• Factors affecting 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

  • enzymes could denature at too high temperatures