Bioelectricity refers to the electrical potentials and currents produced by or occurring within living organisms. Unlike metallic conductors where electrons carry current, bioelectric signals arise from the movement of ions (such as , , , and ) across semi-permeable cell membranes through specialised protein channels.
The cell membrane has a structure analogous to a parallel-plate capacitor:
| Capacitor Component | Biological Equivalent |
|---|---|
| Conducting plates | Intracellular and extracellular ionic solutions |
| Insulating dielectric | Lipid bilayer (~7–10 nm thick) |
Because the lipid bilayer is a thin insulator separating two conducting ionic media, the membrane stores separated charge — exactly as a capacitor does. The typical membrane capacitance is approximately .
The capacitance of the membrane is defined as:
where is the charge stored and is the potential difference (membrane potential) across it.
In the resting state, a neuron maintains a stable potential difference across its membrane called the resting membrane potential, approximately:
The negative sign indicates the inside of the cell is negative relative to the outside. This potential arises because:
When a neuron is stimulated beyond a threshold, a rapid, transient reversal of membrane potential occurs — this is the action potential (nerve impulse).
| Stage | Event | Potential |
|---|---|---|
| Resting | leak maintains gradient | |
| Depolarisation | Rapid influx through voltage-gated channels | rises to |
| Repolarisation | channels close; efflux | returns toward |
| Hyperpolarisation | Slight overshoot below resting potential | briefly |
| Recovery | Na⁺-K⁺ pump restores ion gradients |
The total change in potential:
The bioelectric signals generated by organs can be detected by electrodes placed on the skin and recorded as diagnostic traces:
| Quantity | Value |
|---|---|
| Resting membrane potential | |
| Peak action potential | |
| Membrane capacitance | |
| Na⁺-K⁺ pump ratio | 3 Na⁺ out : 2 K⁺ in |