An introduction to electrical circuits, and their use in physiology
More advanced material – please note that you will be taught this as part of the HOM or PoO courses!
Capacitance and nerve cell membranes
A nerve cell (or indeed any cell) is surrounded by a plasma membrane, made of phospholipid. The cell can be seen as two electrically-conducting regions (the cytoplasm and the extracellular fluid, both electrolyte solutions), separated by a thin layer of insulator (the plasma membrane). The membrane therefore acts as a capacitor! For this reason, we portray charge as lined up on either side of the membrane, just as charge builds up on each plate of a capacitor.
Because of the capacitance of the membrane, any change in voltage across the membrane, as for example the depolarization associated with an action potential, will take time to occur: the membrane has a time constant, τ. This will affect the velocity at which the action potential is propagated.
KEY POINT: THE CAPACITANCE OF AN EXCITABLE CELL MEMBRANE WILL AFFECT THE VELOCITY OF ACTION POTENTIAL CONDUCTION.
If you think of myelin as effectively increasing the thickness of the cell membrane, you would expect this to decrease its capacitance (thicker insulating layer). Although this is a gross oversimplification of what is really happening, it turns out that myelination does have the effect of reducing membrane capacitance. However, myelination also increases the membrane resistance, so overall the time constant (= RC) might not actually change much. Myelination increases conduction velocity mainly because the increased membrane resistance increases the length constant, λ. The length constant is another story...