Nerve impulses

• Oscilloscope trace:

  1. Resting potential - polarised = -70mv
  2. Depolarisation = action potential = up to +40mv
  3. Repolarisation = "reset the membrane" = from +40 to -70mv
  4. Hyperpolarisation = go below resting potential = approx. -90mv

  Threshold potential = -55mv = "all or nothing law"

  Time is in milliseconds

  Potential difference/voltage is in millivolts

  
• Proteins in the neurone cell membrane:

  1. Na+/K+ pump - requires ATP = active transport
  2. K+ leakage channel = facilitated diffusion
  3. Na+ (voltage) gated channel = facilitated diffusion
  4. K+ (voltage) gated channel = facilitated diffusion

  
• Resting potential:

  There are large protein anions and organic phosphates e.g. ATP4- in the cytoplasm that have a negative charge.

  1. Na+/K+ pump pumps out of the cell 3Na+ and pumps into the cell 2K+
  2. There is a high concentration of Na+ outside the cell
  3. The membrane is impermeable to Na+ because the Na+ gated channel is closed
  4. K+ can diffuse out of the cell through the K+ leakage channel
  5. The membrane is more permeable to K+ than Na+

  
• Depolarisation:

  • A stimulus causes Na+ gated channels to open
  • Na+ ions rapidly diffuse into the cell
  • The membrane is now more permeable to Na+ than K+
  • Enough Na+ diffuse in to change the voltage from -70 to -55mV = Threshold potential reached
  • +40mV is reached
  • Membrane is depolarised
  • Na+ diffuse in the cytoplasm from a high to low concentration. They increase the charge from -70 to -55mV
  • This causes Na+ voltage gated channels to open
  • More Na+ rapidly diffuse into the cell/axon.
  • Inside of the cell now = +40mV and outside = -40mV
  • Action potential/impulse is propagated along the neurone.

  
• Repolarisation:

  • At +40mV Na+ voltage gated channels close.
  • K+ voltage gated channels open.
  • The membrane is relatively impermeable to Na+ but very permeable to K+
  • K+ rapidly diffuse out of the cell/axon.
  • Membrane is repolarised.

  Hyperpolarisation:

  • If too many K+ diffuse out of the cell, the membrane hyperpolarises to approx. -90mV
  • To restore the resting potential at -90mV the K+ voltage gated channels close
  • Na+/K+ pump restores the resting potential
  • This state is known as the refractory period

  
• Refractory period:

  • For the first millisecond following repolarisation the resting potential has not been restored, it is therefore impossible for another action potential to take place. This brief time is called absolute refractory period; it has an important role because it ensures that each wave of action potentials travel as a discrete impulse and that the impulse travels in one direction only
• Threshold potential + all or nothing law

  Low/small stimulus:

  • fewer Na+ gated channels open
  • Fewer Na+ rapidly diffusing into the cell
  • The membrane polarises slightly but threshold potential is not reached
  • no action potential
  • no impulse through the nerve
  • This is called failed initiation
• Threshold potential + all or nothing law

  High/big stimulus:

  • more Na+ gated channels open
  • more Na+ rapidly diffuse into the cell
  • Threshold potential is reached = -55mV
  • action potential will occur
  • impulse will be propagated along the neurone