Axon Initial Segment

Axon Initial Segment

1. Action Potential

   • Rate of firing
     • Voltage dependent channels
       • E.g. Channel responsible for LTP where Mg2+ is ejected out of channel to allow Ca2+ to enter; Ca2+ voltage gated channel etc.
   • Mediated by voltage gated Na+/K+ channels
     • Ion selectivity pore
     • Voltage pedal open/close
       • Contains a positively charged string of amino acids. Depolarized - VP is repelled from interior of cell Hyperpolarized - VP is attracted to the interior of cell Movement opens/close ion channels.
     • Na+ channel
       • Drives depolarization of cell
       • Opens/close rapidly
       • Gating arm
         • Na+ is mediated by 2 gating mechanisms i) Voltage pedal ii) Gating arm
         • 3 states: Open, closed & inactivated
           • Relative refractory period
             • Na+ channel remains inactivated until hyperpolarized. A stronger than normal stimulus is required to elicit an AP during this period. 
           • Absolute refractory period
             • Unable to fire an AP during this period
             • Results in unidirectional AP
     • K+ channel
       • Opens/close slowly
       • Causes hyperpolarization of the cell
   • All or nothing
   • Length constant
     • The greater the length, the further the potential will travel.
     • Myelination of axon
     • Diameter of axon

2. AP generation

   • Bidirectional: Axon & Soma
   • Region has high density of voltage gated ion channels
     • NAv1.6
       • Bigger Na+ conductance
       • Higher sensitivity to changes in voltage
       • Localised to AIS via Ankyrin G (AnkG)
         • Ankyrin G is linked to the actin of the AIS via BIV spectrin. Nav1.6 has a protein sequence that enables it to bind to AnkG which is why it is high in density in the AIS.
         • AnkG bound to AIS actin via BIV spectrin

3. Integration and regulation of graded potentials

   • AIS has synaptic inputs too. Loss of AIS has been linked to epilepsy.