Impulse Transmission

• 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