5 Electricity in Biological Systems

Learn how ion movement creates electrical signals in cells, how circuit models explain membrane behavior, and how instruments measure bioelectric activity.

Fields, voltage, and ion movement

Living tissue conducts electricity mainly through charged ions moving in watery fluids and across cell membranes. This differs from a metal wire, where electrons carry most of the current. In biological systems, common charge carriers include $Na^+$ , $K^+$ , $Cl^-$ , and $Ca^{2+}$ .

An electric field describes force per unit charge, $\mathbf{E}=\mathbf{F}/q$ . Voltage difference describes a change in electric potential energy per unit charge. Across a roughly uniform field, the field magnitude is approximately $E=\Delta V/d$ , where $d$ is the distance over which the voltage changes.

Cell membranes are only a few nanometres thick. Consequently, a voltage of tens of millivolts across a membrane can create a strong local electric field. That field acts on ions and contributes to their movement through membrane channels.

Circuit models of membranes

A cell membrane can be modeled as a in parallel with pathways through ion channels. The lipid bilayer separates charge like an insulating layer, while channels provide routes for particular ions to cross. Channel conductance describes how readily current passes through a channel; conductance is the inverse of resistance.

The circuit time constant is $\tau=RC$ , where resistance and capacitance determine how quickly voltage changes in response to current. A simplified membrane current balance is

C_m\frac{dV_m}{dt}=I_{app}-\sum I_{ion}.

Here, $C_m$ is membrane capacitance, $V_m$ is membrane voltage, and $I_{app}$ is applied current. The sign convention treats outward ionic current as positive. Each ionic current depends on channel opening and on the difference between membrane voltage and that ion’s reversal potential.

This circuit analogy helps describe membrane behavior, but it does not mean ions travel through cells in the same way electrons travel through a metal wire.

Membrane voltage and ion equilibrium

The is the voltage inside a cell relative to the outside: $V_m=V_{inside}-V_{outside}$ . In many resting neurons it is negative and often near $-70\,\text{mV}$ , though its value varies with cell type and conditions.

The potential results from two linked factors: ions are distributed unevenly across the membrane, and the membrane is selectively permeable to them. Ion pumps help maintain concentration differences, while channels allow ions to move down their electrochemical gradients.

For one ion species at electrochemical equilibrium, the gives its equilibrium potential:

E_{ion}=\frac{RT}{zF}\ln\left(\frac{[ion]_{outside}}{[ion]_{inside}}\right).

In this expression, $R$ is the gas constant, $T$ is absolute temperature, $z$ is the ion’s charge number, and $F$ is Faraday’s constant. At $E_{ion}$ , the electrical force balances the concentration-driven tendency for that ion to diffuse. Because real membranes are usually permeable to multiple ions, the resting is not generally equal to the Nernst potential of any one ion.

How nerve signals travel

Neurons receive graded voltage changes from their inputs. When depolarization at the spike-initiation region reaches threshold, an begins. Voltage-gated sodium permeability rises rapidly, sodium ions enter, and the membrane depolarizes. Sodium channels then inactivate while potassium permeability rises; potassium ions leave and help repolarize the membrane. A brief refractory period limits immediate re-firing.

An is regenerated along the axon rather than traveling as a fading voltage change. Local currents from an active region depolarize the next region of membrane. In myelinated axons, excitation is regenerated mainly at gaps in the myelin called nodes of Ranvier, making the signal appear to jump from node to node. At nerve terminals, electrical activity can trigger chemical signaling to another cell.

Measuring bioelectric signals

Bioelectric instruments measure voltage differences between electrodes. In surface recordings, electrodes detect signals conducted through body tissues; they do not directly measure the membrane voltage of an individual cell. A measures the difference between two electrode potentials and rejects much of the voltage common to both. High input impedance helps prevent the instrument from drawing substantial current from the tissue.

Common measurement methods include:

ECG/EKG: surface electrodes record voltage differences associated with the heart’s electrical activity.
EEG: scalp electrodes record voltage fluctuations associated with brain activity.
EMG: electrodes record electrical activity associated with skeletal muscle.
Patch clamp: a fine electrode measures current through a small membrane patch or controls membrane voltage to investigate ion-channel behavior.

Recorded signals can be affected by electrode contact, movement, and electrical interference. Electrode placement matters because surface recordings reflect activity as it spreads through tissue to the measuring sites.

Takeaway: Cells create electrical signals through ion gradients and selective membrane permeability. Circuit models describe membrane charging and channel currents; nerve signals arise from coordinated changes in those currents; and electrodes measure potential differences that usually combine activity conducted through tissue.