Physiology of Nerve Conduction
- Neural membrane maintains a voltage difference of -60 to -90mV (positive outside the cell, negative inside the cell due to excess of Na+/K+ ions outside the cell)
- Potassium concentration outside the neuron is ~5mmol/L, while inside it is 150mmol/L
- Achieves this because at rest it is mostly impermeable to sodium ions but selectively permeable to potassium ions, therefore K+ moves down its concentration gradient out of the cell, making more positive ions outside
- Maintains this by the Na/K active pump, sustaining an ion gradient across the membrane by constant extrusion of sodium from insider the cell in exchange for potassium from outside in using ATP
- The nerve therefore acts like a potassium electrode according to the Nernst equation:
- Where Em is the membrane potential; Ek is the potassium equilibrium potential; R is the gas constant; T is temperature (Kelvin); F is Faraday‘s constant; and [K+ ] is the potassium concentration inside (i) and outside (o) the cell. For potassium, therefore,
- An action potential is the rapid change in transmembrane potential followed by a return to resting potential.
- An action potential is triggered when successive conductance increases to sodium and potassium ions causing a threshold potential of -50mV to be reached
- ACh is the most important chemical substance in enlarging Na+ channels and increasing Na+ permeability to sodium
- 1) The first step of the action potential is that the Na+ channels open allowing a flood of sodium ions into the cell. This causes the membrane potential to become positive.
- 2) At some positive membrane potential the K+ channels open allowing the potassium ions to flow out of the cell.
- 3) Next the Na+ channels close. This stops inflow of positive charge. But since the K+ channels are still open it allows the outflow of positive charge so that the membrane potential plunges.
- 4) When the membrane potential begins reaching its resting state the K+ channels close.
- 5) Now the sodium/potassium pump does it's work and starts transporting sodium out of the cell, and potassium into the cell so that it is ready for the next action potential.
- Saltatory conduction:
- Electronic potentials travel along sections of nerve surrounded by schwann cells, triggering action potentials at each node of ranvier
- This makes signal transmission much faster than pure action potential transmission, while sequentially triggering action potentials prevent dissipation of signal