Neuro – EP

Master answer

Classification of Nerve Fibres
(Erlanger and Gasser Classification)

Action potential generation and propagation

“Action Potential” (AP) → large rapid change in membrane potential that occurs in excitable cells

• AP = electrical response of neurons and other excitable tissues during which membrane potential rapidly ↑ and ↓
• All or nothing phenomenon
• Allow rapid signalling within excitable cells over long distances
• AP results from brief ↑ in membrane conductance to Na+, followed by slower ↑ in membrane conductance to K+
• Key parameters
  • RMP -70mV
  • Threshold potential -55mV
  • Peak potential (depolarisation) +20-40mV
  • Duration of AP 1-2ms

Events of an AP:

• Phase 1 – threshold potential: depolarisation stimulus reaches neuron → CM reaches -55mV → activation of voltage gated Na+ channels → Na+ influx > K+ efflux
• Phase 2 – AP: rapid influx of Na → further depolarisation → +ve feedback → rapid upstroke → drives membrane potential to Nernst potential for Na of ~+50mV → peak potential +30mV
• Phase 3 – repolarisation: AP never reaches theoretical max (+50mV) due to 2 events:
  • Inactivation of voltage gated Na+
    channels → ↓ membrane permeability to Na+
  • Delayed activation of voltage gated K+
    channels: ↑ membrane K+ permeability → K+
    efflux → membrane potential driven back towards Nernst for K+ of ~-90mV
  • Membrane potential briefly more –ve than RMP = “after hyperpolarisation” – due to gradual closure of voltage-gated K+
    channels
• Phase 4 – restoration of RMP
  • -70mV maintained by: Na/K ATPase (EC gradient) + Na/K pump

Refractory period

• Time following an AP
• 1. Absolute Refractory Period:
  • AP cannot be triggered whatever the size of the stimulus
  • starts from when voltage-gated Na+ channels open → continues until repolarisation 1/3 complete
• 2. Relative Refractory Period:
  • Repolarisation: K leak channels + voltage gated K channels open = K permeability is highest
  • AP only with ↑↑stimulus to counteract ↑K+ efflux
• Important for 2 reasons:
  • Ensure unidirectional propagation of APs
  • Limiting frequency of APs

Saltatory conduction

• Saltatory conduction: propagation of AP along myelinated axons, whereby wave of depolarisation “jumps” from one rode of Ranvier to the next
• Mechanism of saltatory conduction
  • Depolarisation of a node → influx of Na ions → creating a sink (area of –ve charge at the surface)
  • +ve charge on nodes ahead flows into sink → ↓ polarity inside the membrane → AP → propagating current activates fast Na channels → wave of
    depolarisation down axon
  • minimal electrical signal degradation as axon is insulated by myelin sheath
  • AP reaches next node of Ranvier → continues down myelinated fibre
  • Nerve impulses appear to rapidly jump from one node to the next

Bianca / Kerr 2016

Answer by Bianca, Kerr