control and coordination

• Sodium–potassium pumps


  Membrane proteins that use energy from the hydrolysis of ATP to move sodium ions (Na+) out of the axon, and potassium ions (K+) into the axon, against their concentration gradients
• Sodium–potassium pump operation


  1. 3 sodium ions removed from the axon

  2. 2 potassium ions brought into the axon

  3. 1 molecule of ATP hydrolysed
• Potential difference


  The difference in electrical potential between two points; in the nervous system, between the inside and the outside of a cell surface membrane
• Resting potential


  The difference in electrical potential that is maintained across the cell surface membrane of a neurone when it is not transmitting an action potential; it is normally about –70 mV inside and is partly maintained by sodium–potassium pumps
• The presence of many organic anions inside the cell, such as negatively charged proteins
• The impermeability of the membrane to ions; sodium ions cannot diffuse through the axon membrane when the neurone is at rest
• The phospholipid bilayer has a hydrophobic core which does not permit the movement of ions
• Channel proteins


  Respond to changes in the potential difference across the membrane
• Channel proteins


  • Closed so sodium and potassium ions cannot diffuse through them
  • Described later as voltage-gated channel proteins
• Membrane


  • Has protein channels for potassium and for sodium which are open all the time
  • Far more of these for potassium than for sodium
• More potassium ions could diffuse out


  Compared with the sodium ions diffusing in
• Many large, negatively charged molecules inside the cell


  Attract the potassium ions reducing the chance that they will diffuse out
• The result is an overall excess of negative ions inside the membrane compared with outside
• Axon membrane


  • Relatively impermeable to sodium ions
  • Two things influence the inward movement of sodium ions during an action potential: steep concentration gradient, and inside of the membrane is negatively charged which attracts positively charged ions
• Electrochemical gradient


  A 'double' gradient like this, of concentration and electrical potential
• Action potentials


  1. Axon stimulated with a very brief, small electric current

  2. Potential difference across the cell surface membrane of the axon suddenly switches from -70 mV to +30 mV

  3. Swiftly returns to normal after a brief 'overshoot'

  4. Whole process takes about 3 milliseconds
• Action potential

  Rapid, fleeting change in potential difference across the membrane
• Voltage-gated channel proteins


  Channel proteins that open and close depending on the electrical potential (or voltage) across the membrane
• Action potential generation

  1. Electric current used to stimulate the axon causes the opening of the voltage-gated channels in the cell surface membrane

  2. Allows sodium ions to pass through

  3. Greater concentration of sodium ions outside the axon than inside