Action potentials

Action potentials

1. Long range and bidirectional

2. Refractory periods

   • the time in which an excitable cell is unable to generate a subsequent action potential
   • Absolute refractory period
     • Most Na channels are inactive
   • Relative refractory period
     • Enough Na channels are in a closed state
     • membrane is at a higher threshold and will require a greater stimulus to cause action potentials to fire

3. Conduction of APs

   • In unmyelinated axons, local depolarisation causes neighbouring Na channels to open
   • Saltatory conduction in myelinated axons
     • APs initiate at axon hillock/ axon initial segment
     • AP travel from node of Ranvier to node
     • Faster

4. Stages

   • 1. Resting state
     • Voltage-gated Na channels closed
     • Threshold = when sufficient voltage-gated Na channels open to initiate an action potential
   • 2. Depolarisation
     • Inward flux of Na causing rapid depolarisation
     • Rapid opening of VG Na channels
   • 3. Repolarisation
     • Na channels inactivate
     • VG K channels open
   • 4. Hyperpolarisation
     • VG K channels open
     • Na channels closed

5. Factors on AP velocity

   • Resistance of plasma membrane
   • Resistance of cytoplasm/axon diameter
   • Na channel density along axon
   • Temp.

6. Sensory input

   • Mechanoreceptors
     • Unmyelinated fibres in the skin sensitive to stretch/pressure
   • Mechanosensitive ion channels
     • Gates opened by the stretching of membrane
   • Dermatome
     • the area of skin with innervation supplied by one nerve