Potential Difference Across a Cell Plasma Membrane

• Sodium-Potassium Pump

  A) Na^+

  B) K^+

  C) P_i

  D) ATP

  E) ADP

  F) K^+

  G) P_i

  H) Na^+

  I) P_i

  
• 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^+

  
• Potential Difference across a Membrane

  • 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)
  • Pumps in membrane actively transport 3 Na^+ out to 2 K^+ in
  • Against concentration gradient
  • Na^+ and K^+ voltage-gated protein channels may be open or shut, they affect potential difference across plasma membrane
  • Membranes have potential difference across them, -70mV
  • It is only in neurone where potential difference can change
• Resting Potential

  • When receptor is not stimulated
  • Neurone inside is less positive charge than outside
  • So there is voltage (potential difference) across membrane
  • Voltage usually measured in millivolts (mV)
  • Membrane is polarised; there is charge difference, ~-70mV
  • Maintained by Na+/K+ pumps (use ATP), move 3 Na+ out and 2K+ in
  • 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 is 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
• Action Potential

  • Voltage reaches -55mV, causing depolarisation wave (electrical impulse); occurs on all or nothing principle
  • Same size + only occurs if specific value reached
  • More voltage-gated Na+ channels open, making it more positive than out until reaching maximum (40mV)
  • 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
• Graph

  • All action potentials cause same membrane potential changes (70-40mV)
  • So action potential frequency increases with stimulus strength (not size)
  • Threshold potential is potential difference across membrane
  • Reached during depolarisation
  • Achieve action potential (~-55mV)