Nervous transmission

• Resting potential stage 1


  K+ channels open

  both v-gated channels closed

  Membrane potential stays constant due to the sodium potassium pump
• Action potential

  Stimulus is applied- Na+ channels open- Na+ ions diffuse in- threshold- v-gated Na+ channels open- depolarisation- v-gated Na+ channels close and v-gated k+ channels open- k+ ions diffuse out- repolarisation- k+ channels closed hyperpolarisation- resting membrane potential restored.
• Resting potential

  Na+ ions are actively pumped out and k+ ions are actively pumped in by the sodium potassium pump.

  2:3 ratio

  So more Na+ ions outside the membrane

  More k+ ions inside the membrane

  Potassium ion channels are open so potassium ions diffuse out

  So more positively charged ions outside the cell.
• Propagation of action potentials

  Initial stimulus causes a change in the sensory receptor which triggers an action potential, so the 1st region of the axon membrane is depolarised.

  This acts as a stimulus for depolarisation of the next region of membrane- process continues forming a wave of depolarisation.

  • Once Na+ ions in axon are attracted by negative charge ahead and the concentration gradient to diffuse further along inside the axon triggering depolarisation in the next section.
  • Region of membrane will repolarize after action potential has passed and return to resting potential.
• Refractory period

  After an action potential there is a short period of time when the axon can't be exited again.

  • V-gated sodium ion channels remain closed.
  • Refractory period prevents the propagation potential backwards along the axon.
  • Makes sure the action potential are unidirectional and that they don't overlap and occur as discrete impulses.
• Saltatory conduction

  Myelinated axons transfer electrical impulses faster than non-myelinated.

  • Because depolarisation of the axon membrane only occurs at the nodes of ranvier where no myelin is present.
  • Here, Na+ ions can pass through the protein channels in the membrane, longer localised circuits arise between nodes.
  • The action potential jumps from one node to another which is faster than a wave of depolarisation along whole axon.
  • When channels open and ions move it takes time so reducing the number of times it happens speeds up the action potential.
• Continued:

  Saltatory conduction is more efficient because repolarisation uses ATP in sodium pump, so reducing the amount of repolarisation needed is more efficient.
• Factors that affect speed an action potentials travels

  -Axon diameter= bigger axon diameter, the faster the impulse is transmitted. Because there Is less resistance to the flow of ions in the cytoplasm, compared with those in a smaller axon.

  -Temperature= higher temperature, faster nerve impulse. Because ions diffuse faster at higher temperatures, more kinetic energy.

  -Myelination= increases speed of action potential. Depolarisation can only occur at nodes of ranvier so action potential jumps.
• All or nothing principle

  A certain level of stimulus, the threshold value, always triggers a response.

  If threshold is reached an action potential will be created.

  The same sized action potential will always be triggered.

  A larger stimulus will increase the frequency of action potentials.
• Action potential