Biological signals, or bio signals, are space, time, or space-time records of a biological event such as a beating heart or a contracting muscle. The electrical, chemical, and mechanical activity that occurs during this biological event often produces signals that can be measured and analyzed. Bio signals, therefore, contain useful information that can be used to understand the underlying physiological mechanisms of a specific biological event or system and that may be useful for medical diagnosis.
Biological signals can be acquired in a variety of ways—for example, by a physician who uses a stethoscope to listen to a patient’s heart sounds or with the aid of technologically advanced biomedical instruments. Following data acquisition, biological signals are analyzed in order to retrieve useful information. Basic methods of signal analysis, such as amplification, filtering, digitization, processing, and storage, can be applied to many biological signals. These techniques are generally accomplished with simple electronic circuits or with digital computers. In addition to these common procedures, sophisticated digital processing methods are quite common and can significantly improve the quality of the retrieved data. These include signal averaging, wavelet analysis, and artificial intelligence techniques.
Physiological Origins of Bio signals
Bioelectric Signals
Nerve and muscle cells generate bioelectric signals that are the result of electrochemical changes within and between cells (see Chapter 5). If a nerve or muscle cell is stimulated by a stimulus that is strong enough to reach a necessary threshold, the cell will generate an action potential. The action potential, which represents a brief flow of ions across the cell membrane, can be measured with intracellular or extracellular electrodes. Action potentials generated by an excited cell can be transmitted from one cell to adjacent cells via its axon. When many cells become activated, an electric field is generated that propagates through the biological tissue. These changes in extracellular potential can be measured on the surface of the tissue or organism by using surface electrodes. The electrocardiogram (ECG), electrogastrogram (EGG), electroencephalogram (EEG), and electromyogram (EMG) are all examples of this phenomenon (Figure 11.1).
Bio magnetic Signals
Different organs, including the heart, brain, and lungs, also generate weak magnetic fields that can be measured with magnetic sensors. Typically, the strength of the magnetic field is much weaker than the corresponding physiological bioelectric signals. Biomagnetism is the measurement of the magnetic signals that are associated with specific physiological activity and that are typically linked to an accompanying electric field from a specific tissue or organ. With the aid of very precise magnetic sensors or SQUID (superconducting quantum interference device) magnetometers, it is possible to directly monitor
FIGURE 11 (a) Electrogram recorded from the surface of a pig’s heart during normal sinus rhythm. (b) Electrogram recorded from the surface of the same pig’s heart during ventricular fibrillation (VF). (Sampled at 1,000 samples/s.)
magnetic activity from the brain (magnetoencephalography, MEG), peripheral nerves (magnetoneurography, MNG), gastrointestinal tract (magnetogastrography, MGG), and the heart (magnetocardiography, MCG).
