Question 3(p.2)

Outline the physiology of excitation and conduction in nerve axons (60% of marks). List the factors which delay axonal conduction (40% of the marks).

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College Answer

The following points were expected to be outlined in this question
1. The resting membrane potential (RMP) and its physiological basis
2. How the RMP changes rapidly after a stimulus e.g. electrical or chemical and reaches a
threshold potential and an all or none action potential results
3. The ionic basis of the action potential
4. How the action potential is propagated
5. Factors that delay axonal conduction such as fibre size, myelination and electrolyte
abnormalities e.g. hypermagnesaemia.
Syllabus G2a
Reference: Power and Kam 1st edition p 6-9

Discussion

  • Physiological basis of the resting membrane potential
    • The resting membrane potential is the voltage (charge) difference between the intracellular and extracellular fluid, when the cell is at rest 
    • For neurons, this is ~70 mV
    • It is maintained by electrochemical gradients between major ions in the intracellular fluid and the extracellular fluid
  • Threshold potential is the transmembrane potential which must be achieved to produce depolarisation of the membrane,  ~55 mV 
  • All-or-none effect is the finding that a subthreshold stimulus will produce no response, whereas all suprathreshold stimuli will produce an identical and maximal response.
  • Depolarisation occurs as the result of voltage-gated sodium channel opening
    • The result is an influx of sodium ions into the cell
    • This rapidly depolarises the membrane (over 0.5-1.0 msec)
  • Repolarisation occurs due to potassium channel opening and sodium channels closing
    • Sodium channels enter a refractory period and cannot be activated again
    • Potassium channels permit an outward potassium current, repolarising the cell
  • Propagation of the action potential along a neuron occurs because the current generated locally by depolarisation changes the transmembrane potential in adjacent areas of membrane, also depolarising it.
  • Factors which affect neuronal conduction include:
    • Myelination: (myelinated fibres conduct faster)
    • Thickness of the fibre: the thicker the fibre, the faster the conduction
    • Properties of the membrane: capacitance and resistance (the lower they are, the faster the conduction)
    • Properties of the extra-axonal environment (eg. electrolyte derangement):
      • Hyponatremia, hypermagnesemia, acidosis and hypothermia all decrease the velocity (and sometimes amplitude) of nerve conduction