Nerve impulse

• Potential Difference (p.d.)


  Measured in mV, inside is always measured relative to outside
• Resting potential


  Normal resting state of an axon, p.d. = -70mV, membrane is polarised with inside of cell positive/negative relative to outside
• Action potential


  The p.d. changes to ~+35mV, membrane is depolarised with inside of cell positive/negative relative to outside
• Changes in potential difference

  1. Resting potential

  2. Depolarisation

  3. Repolarisation

  4. Hyperpolarisation
• Resting potential


  • Inside of axon is negatively charged, no voltage-gated channels open
• Depolarisation


  • Voltage-gated Na+ channels open, voltage-gated K+ channels closed, inside of axon becomes more positive, p.d. increases to +35mV
• Repolarisation


  • Voltage-gated Na+ channels close, voltage-gated K+ channels open, inside of axon becomes negatively charged
• Hyperpolarisation


  • Voltage-gated K+ channels remain open, inside of axon becomes too negative
• Threshold


  Stimulus must be greater than this value to generate an action potential
• Once threshold is reached, action potential generated is equal size regardless of strength
• Weak stimulus


  No action potential
• Strong stimulus


  +35mV action potential, frequency increased relative to medium stimulus
• Refractory period


  Prevents an action potential being generated, regulates nerve impulses, nerve impulses move one-way only, limits number & frequency
• Absolute refractory period


  • 1ms after action potential, no further impulse possible in region, stimulus strength irrelevant
• Relative refractory period


  • 5ms after action potential, action potential may be generated but only if stimulus exceeds higher than normal threshold value
• Myelinated neurons


  • Action potential moves faster, nodes of Ranvier, saltatory conduction
• Multiple sclerosis affects the myelin sheath in the central nervous system
• Axon diameter


  • Larger axons transmit faster impulses, smaller axons harder to generate action potential
• Higher temperature


  Faster speed of impulse
• Resting potential is negative inside the axon
• Resting potential is maintained by diffusion of potassium ions
• Influx of sodium ions causes the action potential
• A stronger stimulus causes a higher frequency of action potentials
• The period when an axon can't be excited right after an action potential is called the refractory period