The human body generates and uses bioelectricity, which differs significantly from the electricity that powers homes and devices. Bioelectricity is not a flow of electrons through a metal wire, but rather a complex, dynamic system based on charged particles. This biological voltage is created by the movement of ions—atoms carrying a positive or negative charge—across cell membranes. This energy system is fundamental to life, governing communication between cells and enabling functions from thought to movement.
The Origin of Electrical Charge in Cells
Every cell maintains a difference in electrical potential across its outer membrane, known as the resting membrane potential. This potential is established by an uneven distribution of positively charged ions, primarily Sodium (Na+) and Potassium (K+), inside and outside the cell. Potassium ions are kept high inside the cell, while sodium ions are concentrated in the surrounding fluid.
This chemical imbalance is actively maintained by the Sodium-Potassium pump (Na+/K+ ATPase), a specialized protein complex. This pump uses energy to constantly move three sodium ions out of the cell for every two potassium ions it brings in. Because more positive charges are expelled than imported, the cell’s interior maintains a net negative charge relative to the exterior, typically ranging from -70 to -90 millivolts (mV).
The cell membrane is also highly permeable to potassium due to numerous potassium leak channels, allowing K+ to slowly diffuse out of the cell. As positive potassium ions leave, they further contribute to the negative charge accumulating on the inner surface. This combination of the pump’s action and the membrane’s selective leakiness creates a stored electrical force, giving the cell the potential energy to generate a signal when stimulated.
How Nerve and Muscle Cells Transmit Signals
Nerve and muscle cells are “excitable” because they rapidly convert their stored electrical potential into a functional signal called an action potential. This action potential is a swift, momentary reversal of the resting membrane potential, serving as the primary communication method in the nervous system. The process begins when a stimulus causes the membrane potential to reach a specific threshold, triggering the opening of voltage-gated sodium channels.
The massive concentration gradient drives a rapid influx of positively charged sodium ions into the cell when these channels open. This sudden rush of positive charge causes the internal voltage to spike sharply, a phase known as depolarization, often reaching about +30 mV. The sodium channels quickly inactivate after this peak, halting the ion flow.
Almost immediately, voltage-gated potassium channels open, allowing positive potassium ions to flow rapidly out of the cell. This outward movement restores the negative charge inside the cell, a process called repolarization, which returns the membrane potential to its resting state. Once initiated, the action potential self-propagates along the nerve cell’s axon, triggering the same sequence of events in the adjacent segment.
This electrical signal is the basis for all thought, sensation, and physical movement. When a nerve impulse reaches a muscle cell, it triggers a corresponding action potential. This electrical event is the initial step that leads to the muscle fibers contracting, translating the rapid electrical signal into physical force and action.
Using Bioelectricity in Medical Diagnostics
Since bioelectricity is the operating language of the nervous system and muscles, measuring these electrical signals provides a non-invasive way to assess organ function. Clinical tools capture the aggregated electrical activity from large groups of cells to create characteristic wave patterns. Healthcare professionals use these patterns to diagnose a wide range of conditions by identifying deviations from normal electrical function.
Electrocardiogram (ECG or EKG)
The ECG is a common application that monitors the synchronized electrical depolarization and repolarization of heart muscle cells. Analyzing the ECG waveform helps detect heart rhythm irregularities (arrhythmias) and assesses the overall health of the cardiac tissue. The test involves placing electrodes on the skin to record the small voltage changes generated by the heart’s electrical system.
Electroencephalogram (EEG)
The EEG measures the electrical activity of the brain, reflecting the collective firing of millions of neurons. Electrodes placed on the scalp detect these electrical fluctuations. The resulting brain waves are analyzed to diagnose conditions such as epilepsy, sleep disorders, and other neurological issues. The EEG is a fundamental tool for visualizing the brain’s functional state in real-time.
Electromyography (EMG)
EMG specifically records the electrical activity generated by skeletal muscles at rest and during voluntary contraction. This technique helps differentiate between disorders originating in the muscle tissue itself and those stemming from a problem with the nerves controlling the muscle. Studying the electrical responses of muscles provides insight into the health of the peripheral nerves and the neuromuscular junction.