Impulse Transmission

• Graph: All action potentials cause same membrane potential changes (70mV to 40mV) so action potential frequency increases if stimulus strength increases (not size)

  A) Action Potential

  B) Threshold

  C) Depolarisation

  D) Stimulus

  E) Repolarisation

  F) Refractory Period

  G) Resting State

  H) Failed generator potentials

  I) Na^+

  J) K^+

  
• Hyperpolarisation + Repolarisation

  • Action potential achieved with max PD +40mV
  • Na+ voltage-gated channels close
  • K+ voltage gated channels open
  • More K+ diffuse out, down electrochemical gradient
  • PD fall, restore negative PD
  • Slight K+ overshoot, too many diffuse out so PD more negative than normal, e.g -75mV (hyperpolarised)
  • Na+ and K+ ions on opposite sides, action potential cannot occur (until more Na+ ions out, more K+ in)
  • Refractory potential:
  • Time taken (milliseconds) after action potential peak to establish true axon resting potential
  • Adjusts ions to correct positions
• Resting Potential

  • When receptor not stimulated
  • Neurone inside is less positive charge than outside
  • Voltage (potential difference) across membrane
  • Potential Difference
  • Plasma membrane has unequal ion distribution across
  • Large anions (negative-charge ions) in cytoplasm too large to pass
  • Phospholipid bilayer; impermeable to Na^+ and K^+ (charged)
  • Membrane is polarised; there is charge difference, ~-70mV
  • Maintained by Na+/K+ pumps (use ATP), move 3 Na+ out + 2K+ in
  • Na^+ + K^+ voltage-gated protein channels open or shut
  • Voltage usually measured in millivolts (mV)
  • Voltage-gated Na+ channels closed so can't diffuse back
  • Leaky K+ ion channels allow facilitated K+ ion diffusion out down electrochemical gradient
  • More positive ions move out, in remains more negative
  • Polarised membrane: higher Na+ concentration out and K+ in
• Generator Potential

  • When stimulus detected
  • Membrane more permeable
  • Causing voltage change across plasma membrane
  • Reduces difference (depolarised)
  • Change known as generator potential
  • Bigger stimulus move more Na+ ions cause larger voltage change
  • If over threshold (-55mV), it cause action potential
  • Voltage gated Na+ ion channels open, more permeable
  • Cause influx
  • Making positive ions on in more positive/less negative
  • All action potentials cause same membrane potential change (70-40mV)
  • Action potential frequency inc w/ stimulus strength (not size)
• All or Nothing

  • Action potential peaks at +40MV
  • Stimulus over threshold does not cause larger action potential value
  • Either full action potential or nothing
  • But, impulses/action potentials frequency can vary
  • Larger stimulus increase frequency

  High Temp affects Speed

  • Kinetic energy increased so ion diffuse faster
  • Speeds up depolarisation + refractory periods
  • Only true up to 40C
  • Above protein ion channels denature
  • Disrupts membrane fluidity, decreasing speed
• Action Potential

  • Voltage reaches -55mV, causing depolarisation wave (electrical impulse); occurs on all or nothing principle
  • Same size + only occurs if specific value reached
  • Action potentials transmitted along axon plasma membranes
  • Positive ion diffusion, down electrochemical gradient
  • To where no action potential
  • More voltage-gated Na+ channels open, making it more positive than out until reaching maximum (40mV)
  • Next membrane polarised
  • Rapid, brief + temporary depolarisation
  • Calculate time
  • From 1st membrane potential increase
  • To resting potential resume
  • For action potential length
  • Repolarisation:
  • Eventually Na+ channels close
  • K+ channels open
  • More K+ ions diffuse out down gradient
  • Back to resting potential