25 Nerve Conduction–Electrocardiograms

Nerve Conduction

Every thought, sensation, muscle contraction, and heartbeat depends on tiny electrical signals generated by specialized cells. Unlike current in a metal wire, electrical signaling in the body occurs through the controlled movement of ions across cell membranes. Although the biological details are more complex, the underlying physics is familiar: electric forces, electric potential, diffusion, and current all play central roles.

Nerve conduction is the transmission of electrical signals along nerve cells, or neurons. It is one example of bioelectricity, the study of electrical phenomena produced by living organisms. Neurons allow the nervous system to receive information from the environment, process it in the brain and spinal cord, and send commands to muscles and organs.

A typical neuron is shown in Figure 25.1. Signals are received through branching extensions called dendrites or across specialized junctions called synapses. If the incoming signals are strong enough, the neuron generates its own electrical signal, which travels along the long extension called the axon toward another neuron, muscle, or gland.

The electrical signals produced by neurons arise from the movement of charged ions rather than free electrons. The most important ions involved are sodium ions ([latex]\text{Na}^+[/latex]), potassium ions ([latex]\text{K}^+[/latex]), and chloride ions ([latex]\text{Cl}^-[/latex]). Their movement across the cell membrane creates voltage differences that allow information to be transmitted throughout the nervous system.

Resting Membrane Potential

In its resting state, a neuron maintains a small voltage difference across its cell membrane. This voltage is called the resting membrane potential. It exists because the concentrations of ions inside and outside the cell are different and because the membrane is semipermeable, meaning that some ions cross more easily than others.

As shown in Figure 25.2, potassium ions can diffuse out of the cell more easily than sodium ions can diffuse in. Because potassium ions carry positive charge, their movement leaves the inside of the membrane slightly negative and the outside slightly positive. Chloride ions also contribute to the charge separation under certain conditions.

This separation of charge creates an electric field across the membrane. As more charge separates, the electric force increasingly opposes further diffusion. Eventually, diffusion and the electric force balance, producing a stable voltage across the membrane.

The resting membrane potential of many neurons is approximately [latex]-70~\text{mV}[/latex] to [latex]-90~\text{mV}[/latex], where the inside of the cell is negative relative to the outside. Although this voltage is small, it exists across a membrane only about [latex]8~\text{nm}[/latex] thick. The resulting electric field is enormous:

Such strong electric fields help control the opening and closing of ion channels in the membrane. Maintaining the ion concentration differences also requires energy. The sodium-potassium pump uses cellular energy to move sodium ions out of the cell and potassium ions into the cell, helping preserve the conditions needed for bioelectric signaling.

Action Potentials

An action potential is a brief voltage pulse produced when a stimulus changes the permeability of the cell membrane. If the stimulus is strong enough, voltage-gated sodium channels open, allowing [latex]\text{Na}^+[/latex] ions to rush into the cell. This influx of positive charge makes the inside of the membrane less negative and then briefly positive. This stage is called depolarization.

Shortly afterward, sodium channels close or inactivate, and potassium channels open. Potassium ions then move out of the cell, carrying positive charge outward and returning the membrane voltage toward its resting value. This stage is called repolarization. The full sequence produces the voltage pulse shown in Figure 25.3.

Only a small fraction of the ions near the membrane move during a single action potential, so a neuron can fire many times in rapid succession without immediately depleting its ion concentration differences. Over longer times, however, active transport processes such as the sodium-potassium pump are needed to maintain these concentration gradients.

Propagation of the Nerve Impulse

An action potential begins at one location on a cell membrane, but a nerve signal must travel along the axon. This happens because depolarization at one region changes the electric field and voltage near adjacent regions of membrane. That neighboring region then reaches threshold and generates its own action potential.

In this way, the action potential is regenerated again and again along the membrane. The signal is not a single group of ions traveling all the way down the axon. Instead, it is a moving pattern of depolarization and repolarization, much like a wave traveling along the membrane.

In unmyelinated axons, this process may occur at speeds of about [latex]1~\text{m/s}[/latex]. This is slow compared with electrical signals in metal wires, but it is fast enough for many biological functions. Other neurons use myelin to increase signal speed dramatically.

Myelinated Axons

Some axons are covered by insulating layers called myelin sheaths. These sheaths are made from specialized cells containing fatty material that acts as an electrical insulator. The myelin is interrupted at small gaps called nodes of Ranvier, as shown in Figure 25.5.

Myelin improves nerve conduction in several ways. First, it reduces electrical leakage across the membrane, allowing the signal to travel farther and faster along the axon. Second, action potentials occur mainly at the nodes of Ranvier rather than along the entire membrane. The signal appears to “jump” from node to node, a process called saltatory conduction. Third, because fewer membrane regions need to actively exchange ions, myelinated axons use less energy than unmyelinated axons.

Damage to myelin can seriously impair nerve signaling. In multiple sclerosis (MS), for example, the immune system attacks myelin in the central nervous system. This disrupts the propagation of nerve impulses and can cause symptoms such as fatigue, weakness, vision problems, numbness, tingling, and loss of coordination.

Bioelectricity Beyond Neurons

Neurons are not the only cells that use electrical signals. Muscle cells also depolarize, and this depolarization triggers contraction. This is why nerve impulses can control skeletal muscles. Heart muscle is especially important because its cells combine properties of nerve and muscle tissue, allowing electrical signals to coordinate rhythmic contractions.

Some animals use bioelectricity in dramatic ways. Electric eels, for example, have specialized cells that act somewhat like biological batteries. Many of these cells are arranged so their voltages add together, producing a large enough voltage to stun prey or deter predators.

Electrocardiograms

The same electrical principles that allow neurons to transmit signals also allow the heart to function as an efficient pump. Each heartbeat begins with an electrical impulse that spreads through the cardiac muscle, triggering coordinated contraction of the atria and ventricles. Because this electrical activity creates small voltage differences on the surface of the body, it can be measured noninvasively using electrodes placed on the skin.

An electrocardiogram (ECG) records these changing voltages over time. Rather than measuring the mechanical pumping of the heart directly, an ECG measures the electrical signals that cause the contractions. It is one of the most common diagnostic tools in medicine because it provides rapid information about heart rate, rhythm, and the electrical conduction system of the heart.

The electrical impulse that initiates each heartbeat normally begins in the sinoatrial (SA) node, often called the heart's natural pacemaker. The depolarization wave then spreads across the atria, passes through the atrioventricular (AV) node, and finally travels throughout the ventricles, causing them to contract. This sequence is illustrated schematically in Figure 25.7.

Modern clinical ECG systems typically use 10 electrodes to generate 12 different leads. Each lead measures the electrical activity of the heart from a different viewpoint, allowing physicians to determine not only the timing of cardiac activity but also the approximate location of abnormal electrical conduction or damaged heart tissue.

One of the most commonly displayed recordings is the Lead II ECG, shown in Figure 25.8. Its characteristic features correspond to specific electrical events during the cardiac cycle.

• P wave: depolarization of the atria, causing the atria to contract and pump blood into the ventricles.
• QRS complex: rapid depolarization of the ventricles, initiating ventricular contraction. Because ventricular muscle is much larger than atrial muscle, this is the largest feature of the ECG.
• T wave: repolarization of the ventricles, preparing the heart muscle for the next heartbeat.

The mechanical pumping of blood follows these electrical events by a short delay. As the ventricles contract immediately after the QRS complex, arterial blood pressure rises and reaches its maximum value (the systolic pressure).

Because every person's heart differs slightly in size, shape, and orientation within the chest, normal ECGs vary somewhat between individuals. For this reason, physicians often compare a patient's current ECG with previous recordings when looking for subtle changes that may indicate disease progression or recovery.

Modern ECG technology has become highly portable. Compact battery-powered monitors can continuously record a patient's heart rhythm during normal daily activities, in ambulances, athletic training, intensive care units, and even remote environments such as space missions. An example of a portable monitoring system is shown in Figure 25.9.