Visualize how bioelectric dipole sources propagate through conductive body tissue to the surface
ECGEEGEMG
📍 Body Cross-Section — Voltage Distribution
Drag the dipole ● to reposition
−V max+V max
NegativeZeroPositive
Max Surface V
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Min Surface V
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Peak-to-Peak
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Source Depth
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Current Dipole Formula
V(r) = ( p⃗ · r̂ ) / ( 4π σ r² )
p = dipole moment [A·m] | σ = conductivity [S/m]
r = source–point distance [m]
V at cursor: hover over body
Volume conduction is the passive spread of electric potentials through conductive biological tissue. When excitable cells (cardiac, neural, muscular) depolarize, they act as current dipoles — current flows from sink to source through the tissue. The body behaves as a 3D resistive volume; these currents obey Ohm's law and produce measurable surface potentials. The pattern depends on dipole strength, orientation, depth, and tissue conductivity.
p (magnitude) — Combined effect of cell count and alignment. Larger p → stronger surface signal, scales linearly with V. θ (angle) — Dipole orientation; determines which regions see positive vs. negative voltage. The dipole's axis defines the null line (zero crossing). Depth — Deeper sources produce weaker, spatially smeared potentials. Signal falls off as 1/r². σ (conductivity) — Higher σ → lower voltage magnitude. Bone (0.02), fat (0.04), muscle (0.4), blood (0.7 S/m). 2-Layer σ ratio — A low-conductivity layer (like skull) can significantly attenuate and blur signals.
ECG: The cardiac dipole (heart vector) is the bioelectric source. Different electrode pairs capture different projections of this dipole — forming the 12 standard leads.
EEG: Patches of synchronously firing cortical neurons create dipole layers. These propagate through CSF, skull, and scalp — the skull's low conductivity severely attenuates and smears signals.
EMG: Motor unit action potentials create propagating dipoles along muscle fibers. Surface electrodes measure their volume-conducted sum.
⚙ Parameters
Medium Model
Dipole Magnitude (p)1.00 mA·m
Strength of the equivalent current dipole source
Dipole Orientation (θ)90°
Direction of the dipole vector (0° = right, 90° = up)
Source Depth30%
Radial distance from body center (% of body radius)