K1vi: The physiology of excitation and conduction in nerve axons.

The factors which delay axonal conduction.

  • A neuron is a specialised cell transmitting nerve impulses
  • They transmit information from one cell to another by conducting an electrical impulse & secreting chemical NTs
  • Ions flow across nerve cells through 3 types of ion channels:

1. Non-Gated Ion Channels

  • Always open
  • Important for establishment of RMP
  • Found throughout neuron
  • Govern flux of Na+/K+ when neuron is in resting state

2. Ligand Gated Ion Channels

  • Activated by neurotransmitters
  • Found at dendrite/soma of neuron

3. Voltage Gated Ion Channels

  • Required to initiate & propagate an AP
  • Found on axons + axon terminals

At rest

  • RMP -60mV
  • Reflects the steady state described by the Goldman Equation
  • Extrac [Na+] > intrac [Na+]
  • Intrac [K+] > Extrac [K+]
  • Cell permeability K+ >>> Na+ because there are many more non-gated (leaky) K+ channels cf. Na channels
  • ∴RMP of neuron is much closer to Equilibrium Potential for K+ (EK)
  • In a neuron:
    • EK = -100mV
    • ENA = +70mV
  • Neuron RMP -60mV ∴ closer to EK
  • Because sodium is far from its equilibrium potential, there is a large driving force on Na+ when Na+ channel open

Ap Initiation & Propagation

  • Axon hillock with high density of voltage gated Na+ & K+ channels generates an AP
  • Depolarisation of Axon hillock to threshold causes generation + propagation of AP
  • Membrane depolarised → voltage gated Na+ channels open → Na+ channels (+ve feedback)
  • Explosive activation of many voltage-gated Na+ channels when threshold reached
  • If the initial depol does not reach threshold (-55mV) → NO AP generated
  • ∴ Initiation of AP is an ‘ALL OR NOTHING EVENT’
  • Threshold (-55mV) reached → activation gate of Na+ channel open → Na+ influx
  • Voltage gated K+ channel closed (initially) ∴Na+ influx unopposed
  • Later K+ channels open → K+ efflux → ∴no permeability to Na+
  • K+ channels are still open & K+ efflux continues
  • Eventually both channels close & membrane potential returns to RMP
Ap Initiation & Propagation

Refractory Period

  • Impossible to evoke another AP during the Absolute Refractory Period when Na+ channels are open
  • Followed by Relative Refractory Period where a supramaximal stimulus can evolve an AP

Factors Delaying Axonal Conduction

Fibre Size

  • Large diameter axons conduct quicker because they leave less resistance ∴permit a greater flow of ions

Myelin

  • Promotes Saltatory conduction because AP jumps from one NoR to the other
  • ∴AP travels faster

Temperature

  • At higher temps, molecules move faster
  • ∴↓temp = ↓rate of diffusion & slows axonal propagation

Electrolytes

  • Hyponatraemia → ↓[ ] of diffusion by Fick’s Law
  • Hypermagnesemia
    • Mg2+ prevents release of ACh @ NMJ
    • ∴↑Mg2+ = ↓neuromuscular transmission → muscle weakness, loss of deep tendon reflexes, flaccid paralysis
    • Ca2+ antagonises effects of Mg2+ @ NMJ ∴effects of ↑Mg2+ is exaggerated by ↓Ca2+
Membrane