9. Electric Potential Mapping of the Human Body: Heart, Muscle, and Brain

The human body uses a complex system of electrical signaling to control various life functions. A network of nerve cells provide both sensory input and motor control. Our brains are complex webs of neurons able to outperform the most sophisticated computers in even the simplest tasks of recognition. Muscles conduct electricity as well as generate force. The heart should be singled out as the most notable muscle in the body, pumping blood by contraction of a series of muscles all controlled by the electrical activity of a pacemaker group of cells.

A number of medical technologies have been developed over many years to map the electrical activity of these various organs of the body. Here we will briefly describe three such technologies to map the electric potentials from muscles, the heart and the brain. The methods are known as electromyography (EMG of muscle), electrocardiography (ECG or EKG of the heart- the K rather than C appearing from the original Dutch), and electroencephalography (EEG of the brain). While we have little fundamental knowledge that would enable us to directly interpret the complex time and spatial patterns of such electric potentials, doctors have many years of empirical data allowing these methods to be used as indicators of normal or abnormal behavior.

The fundamental principles of the three techniques are the same: the mapping in time and space of the surface electric potential corresponding to electric activity of the organ. When a resting nerve or muscle cell, with a membrane potential of about -100 mV relative to the external medium, is stimulated, a wave of depolarization spreads over the surface of the cell. The resting cell has no dipole moment, but while the cell is undergoing depolarization it can be electrically represented by a time varying electric dipole moment that goes to zero after the resting membrane potential is restored in a process of repolarization. We will study some details of the depolarization and repolarization processes in connection with nerve conduction in the next chapter. For now, it is clear that such changes will lead to local variations in electric potential. In trying to map these changes in electric potential, only in EMG can an electrode be directly used to measure local potentials. Otherwise, for the heart and brain surface electrodes must be used. Implicit in their use is the notion that the body is a very good conductor, so that potential changes measured, for example in an EKG, between the ankle and the wrist reflect the potential differences directly across the heart.

In EMG, the simplest of the three techniques to be discussed, either surface electrodes on the skin or a needle electrode inserted into a muscle record the time variations in electric potential. Needle electrodes can probe a single muscle fiber and give a characteristic time record of electric potential with variations of several mV observed (Figure 17.23). Such recordings of voluntary muscle activity can check for normal functioning of nerve stimulation of muscle. More detailed information can be obtained with external electrical stimulation of the muscle since an entire group of muscle fibers can be simultaneously activated. Measurements at a number of distances along a muscle can determine conduction velocities along the stimulating nerve. While not as common as the EKG or EEG, the electromyogram can be more directly related to the depolarization of a single cell or small group of cells.

Figure 17.23 Contemporaneous EEG and EMG recordings when awake, in rapid eye movement (REM) sleep, about 20% of the time for an adult, and when in slow wave sleep (SWS).

The heart is composed of many individual muscles contracting in a synchronous fashion controlled by the pacemaker or sinoatrial (SA) node, located in the right atrium. Triggering of the pacemaker cells roughly once per second stimulates a wave of depolarization down across both atria, leading to their contraction and pumping of blood into the ventricles (as discussed in the last course in physics). Following the atrial contraction and repolarization, another wave of depolarization is initiated by the atrioventricular (AV) node, that lies between the two ventricles, leading to contraction of the ventricles and their subsequent repolarization. This entire sequence of events constitutes a heartbeat cycle and, just as in EMG, the waves of depolarization can be measured as surface electric potential changes, although in this case the potential waveform is quite complicated.

In its simplest form, an EKG can be imagined to measure the electric potential due to the heart being represented as a single time-varying electric dipole moment . In order to determine the value of , three independent measurements must be made so as to determine the three vector components of the dipole moment as functions of time. Accordingly, there are three required surface electrodes that must be used in EKG. These are attached at both wrists and the left ankle. Additional electrodes, usually a total of 12, are used to assist in the analysis, but these are not fundamentally required. An EKG recording gives information on the time sequence of potentials and the characteristic peaks and valleys are labeled in a standard manner (Figure 17.24). Newer computer-interfaced instrumentation can obtain high quality data and analyze EKGs for the amplitudes, durations, and areas under the primary peaks. These are then used for diagnostic purposes, with computers even able to point out potential problems. As we have already remarked, despite a lack of knowledge to interpret these potential mappings in detail, by simple numbers of recordings available, EKGs are very useful empirical tools for the diagnosis of various forms of heart disease.

Figure 17.24. A segment of an EKG signal showing the prominent features.

Since the first detection of electrical signals from the brain in 1929, doctors and scientists have been recording such signals in the form of EEGs in order to learn about the electrical activity of the brain. These signals are much weaker than those from the heart or muscle, typically less than 0.1 mV, and the patterns of voltage signals recorded are much more complex as might be expected from much more asynchronous firings of neurons compared to the heart. Some characteristic wave trains can be associated with various activities or abnormalities, including different stages of sleep, epileptic seizures, and visual or auditory excitation (so-called evoked responses). Frequency analysis of the wave trains divides signals into 4 frequency bands ranging from slow D waves at 0.5 to 3.5 Hz common during sleep, q waves in the range from 5 – 8 Hz common in newborns but indicating severe stress in adults, to normal a waves at 8 to 13 Hz from a relaxed brain and faster b waves at greater than 13 Hz from an alert brain; (Figure 17.25).

Figure 17.25 Lower EEG trace analyzed in terms of its frequency content in the upper spectrogram. Note the mixture of different types of waves based on frequency content.

A standard arrangement of 8 to 16 electrodes placed in a regular pattern around the head is used to record an EEG. Again, it should be emphasized that such measurements are not well understood, but are able to help in a diagnosis based simply on clinical studies of many individuals. Later we will see another new technique for studying the electrical activity of the brain by measuring the very small magnetic fields generated by the brain. This technique, known as magnetoencephalography – or MEG, is better able to localize electrical activity within the brain and has recently led to very interesting results. The method requires some quite specialized equipment but is a growing area of research and potential clinical use.