nervous transmission

• How does an electrical impulse travel along a neurone in myelinated neurones?

  • Between each adjacent Schwann cell there’s a node of ranvier
  • Creates gaps in myelin sheath
  • Gaps occur every 1-3mm
  • because sheath is an electrical insulator and the impulse jumps from one node to the next across the gap, the impulse is transmitted faster than with non-myelinated neurones
  • In non-myelinated, impulse doesn’t jump, just travels continuously so it much slower
• which two states can the membrane of an axon be in at all time?

  • resting potential
  • Action potential
• What is a resting potential?

  • When a neurone isn’t transmitting an action potential
  • The outside of the membrane is more positively charge than the inside
  • The membrane is polarised as theres a pd across it
  • Resting potential is -70mV
• What is meant by polarised?

  A region of negative electrical potential
• How does resting potential occur?

  It’s the result of the movement of sodium and potassium ions across the axon membrane
• Why can’t sodium and potassium ions simply diffuse across the axon membrane?

  The hydrophobic tails in the phospholipid bilayer prevents the ions from diffusing across the membrane
• How do sodium and potassium ions travel across the axon membrane? Describe the two ways they can

  • Channel proteins
  • Some are voltage gated channel proteins- must be opened to allow specific ions to pass theough them
  • Others remain open all the time allowing the ions to simply diffuse through them
• Describe the events which result in resting potential

  • Sodium ions actively transported out the axon
  • k ions actively transported into axon
  • By intrinsic protein called Na/K pump
  • 3 sodium ions out, 2 K ions in
  • More sodium ions out than in axon cytoplasm, so they diffuse back in down electrochemical gradient
  • But most gated sodium ions channels are closed, preventing many sodium ions moving in again but k ions channels remain open- they can diffuse out the axon
  • Overall, more k ions inside the axon than outside cytoplasm and more sodium ions outside the axon than inside
• What is depolarisation?

  Change in pd from negative to positive
• What is repolarisation?

  Change in pd from positive back to negative
• What elicits the occurrence of an action potential?

  • Voltage gated ion (protein) channels change shape as a result of the change in voltage across the axon membrane
  • Causes the channels to open or close

  
• Describe the process of an action potential

  • Resting potential- some k ions channels open, sodium ones closed
  • Energy of stimulus triggers sodium voltage gated ions channels to open
  • Sodium ions diffuse into axon down electrochemical gradient
  • Causes more Na ion channels to open, allowing more sodium to diffuse into axon (+ve feedback)
  • Called depolarisation- axon potential reaches +40mV- depolarised
  • Na channels close, K channels open, K diffuses out and cell becomes more -ve
  • Hyperpolarisation- reaches below resting potential
  • Resting potential restored by sodium potassium pump
• explain what is meant by the all or nothing response of a neurone to a stimulus

  Certain level of stimulus (threshold value) always triggers a response. If threshold is reached, action potential will be generated, no matter how large the stimulus is, the AP is always the same size- a stronger stimulus only makes action potentials generated more frequent, but still the same size. If the threshold isn’t reached, no action potential is triggered
• Why is saltatory conduction useful?

  repolarisation uses ATP from the sodium pump, so by reducing the amount of repolarisation needed the conduction of impulses is more efficient
• What is a refractory period?

  • during this time, sodium voltage gated ion channels are closed, preventing the movement of sodium ions into the axon, preventing propagation of further action potentials until resting potential is reached again
  • Prevents propagation of an action potential backwards along the axon (unidirectional)
  • Ensures that action potentials don’t overlap and that they occur as discrete impulses
• What is saltatory conduction?

  where an action potential is transmitted from one node (of Ranvier) to another
• Why are myelinated axons faster at transmitting action potentials than non myelinated neurones?

  • Depolarisation can only take place at the nodes of ranvier where no myelin is present, as sodium ions can pass through protein channels in membrane
  • So the action potential has to jump from node to node to avoid the myelin sheath
  • Reduces the number of action potentials across the neurone- reduces the amount of times ion channels need to open and this takes time
• describe the process of an action potential

  • Resting potential (-70mV) - some k ions channels open, sodium ones closed
  • Energy of stimulus triggers sodium voltage gated ions channels to open
  • Sodium ions diffuse into axon down electrochemical gradient
  • Causes more Na ion channels to open, allowing more sodium to diffuse into axon (positive feedback)
  • Called depolarisation- axon potential reaches +40mV- depolarised
  • when +40mV is reached Na ion channels close, voltage-gated K ion channels open, K diffuses out and cell becomes more -ve
  • Hyperpolarisation- reaches below resting potential. Voltage gated K ion channels now close to leave the normal K ion channels open
  • Resting potential restored by sodium potassium pump (repolarisation)

  
• how is an action potential propagated across a neurone?

  • begins at one end of the neurone and is propagated to the other
  • first region of the axon membrane is depolarised
  • this acts as a stimulus for the depolarisation of the next region of the neurone
  • process continues along the axon, forming a wave of depolarisation
  • once sodium ions are inside the axon, the are attracted by the negative charge ahead
  • diffuse further along the axon along electrochemical gradient, triggering depolarisation of the next section

  
• what is saltatory conduction?

  • Depolarisation can only occur at the nodes of ranvier where no myelin is present (as here sodium ions pass through the protein channels in the membrane)
  • Action potential jumps from one node to the next (saltatory conduction) much faster than a wave of depolarisation along the axon membrane
  • reducing the places at which the sodium ion channels open reduces time as the movement of ions across the membranes at each region along the axon takes time

  
• apart from myelination, what two other factors affect the speed at which an action potential travels?

  • axon diameter- the bigger the diameter, the faster the impulse as there's less resistance to the flow of ions in the cytoplasm
  • temperature- the higher it is, the faster the nerve impulse. This is because ions diffuse faster at higher temperatures. But this only occurs up to about 40 degrees as the proteins in the membrane will become denatured
• true or false? The larger the stimulus, the more frequent action potentials are generated

  TRUE