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 W