Bioelectricity

Living cells maintain unequal concentrations of ions across their cell membranes. Important ions include: sodium `Na⁺`; potassium `K⁺`; chloride `Cl⁻`; calcium `Ca²⁺`. The cell membrane acts as a selective barrier. Differences in ion concentration and membrane permeability produce a membrane potential. Many excitable cells maintain an electrical potential difference across their membrane while at rest.

Nerve and muscle cells can undergo temporary changes in membrane potential. These electrical changes propagate as biological signals.

A cell membrane can be modelled approximately as a capacitor: conductive fluids exist on both sides; the lipid membrane acts as an insulating layer. Thus electrical charge separation can occur across the membrane.

Bioelectric signals contribute to technologies such as measurements of electrical activity from the heart or nervous system. The important physics idea is: **ionic charge movement → potential differences → measurable electrical signals**

Electrical potential difference concept: **ΔV=ΔUq Membrane-capacitance approximation: Q=CV**