Nerve impulses

• Define the resting potential and the value

  The state of a neuron when not transmitting a nerve impulse

  Value = -70mV inside the axon when compared to the outside
• Describe the components of a myelinated motor neuron.

  Dendrites - branches of the cell body that reieve nerve impulses from relay neurons

  Cell body - contains organelles

  Axon - elongation of cytoplasm that transmits nerve impulses to the other end

  Nodes of ranvier - gaps in the axon without myelin

  Schwann cells - surround axons, making the myelin that covers them

  Myelin - glycolipid sheath coating axons that speeds up transmission of nerve impulses

  Axon terminals - branches of an axon that pass nerve impulses into effectors
• Describe how the resting potential is established

  • Na+/K+ pump actively transports 3Na+ out of the axon and 2K+ into the axon using energy from ATP hydrolysis
  • This establishes an electrochemical gradient for Na+ to diffuse back in and K+ to diffuse back out
  • The axon is more permeable to K+ so it diffuses back out but Na+ can't diffuse back in

  This means the inside of the axon has an overall negative charge (membrane potential) of -70mV compared to the outside.

  The axon membrane is said to be polarised.
• Define an action potential

  Electrical currents that develop in one region of an axon at a time, and then spreads sideways
• Describe the depolarisation stage of an action potential

  • A stimulus opens voltage gated Na+ channels
  • Na+ diffuses into the axon down an electrochemical gradient
  • Membrane potential increases to +40mV
• Name the 3 stages of an action potential

  1. Depolarisation
  2. Repolarisation
  3. Hyperpolarisation
• Describe the repolarisation stage of an action potential

  • At +40mV, voltage gated Na+ channels close and voltage gated K+ channels open
  • K+ diffuses out of the axon down an electrochemical gradient
  • Membrane potential decreases to -70mV again
• Describe the hyperpolarisation stage of an action potential

  • At -70mV, voltage gated K+ channels close slowly, so an excess of K+ diffuses out
  • Membrane potential decreases to -80mV
  • The Na+/K+ pump is activated to restore resting potential
• Describe the propagation of an action potential along an axon

  1. When Na+ enters the first region of an axon, some diffuse through into the adjacent region
  2. This causes adjacent voltage gated Na+ channels to open, and more Na+ to diffuse in
  3. This continues until the last region of the axon, creating a wave of depolarisation
• Describe the all-or-nothing principle

  • Action potentials are always the same size regardless of the strength of the stimulus
  • The stimulus just needs to be large enough to reach the threshold of depolarisation
  • Stronger stimuli are perceived by an increase in the frequency of action potentials
• Define the refractory period

  The time when it is impossible to produce another action potential as the voltage gated Na+ channels are shut and the voltage gated K+ channels are open
• Describe the importances of the refractory period

  1. Keeps action potentials separate to prevent merging
  2. Limits frequency of action potentials
  3. It ensures action potentials are only transmitted in one direction
• What are the three factors affecting the speed of action potential transmission?

  1. Myelination
  2. Temperature
  3. Axon diameter
• Describe how myelination affects the speed of action potential transmission

  • Myelinated axons transmit action potentials rapidly using saltatory conduction
  • This is where the nodes of Ranvier are depolarised as myelin is an electrical insulator
  • In non-myelinated axons (continuous conduction), the whole axon needs to be depolarised so transmission is slower
• Describe how temperature affects the speed of action potential transmission

  As temperature increases, Na+ and K+ have more kinetic energy, so they diffuse into and through the axon at a faster rate
• Describe how axon diameter affects the speed of action potential transmission

  • As axon diameter increases, there is more space away from the organelles
  • As a result, Na+ can diffuse into the next axon region at a faster rate as there is less resistance