Biomedical signals originate from different sources. Bioelectric signals are generated by nerve and muscle cells. Each single cell under proper condition may generate an action potential, which is measured at single cell level or with more gross methods, where the electric field generated by the action of many cells is measured with surface electrodes at the body surface.
Bioimpedance signals are generated within tissue when excited by AC currents. The measurement is conventionally performed with four point method. Impedance changes are result of a tissue composition, blood volume and distribution, endocrine activity, sympathic nervous system activity, and other phenomena influencing body impedance.
Biomagnetic signals are generated by various bio-systems and organs. They generate weak magnetic fields which provides information not obtainable from other biosignal sources, like bioelectric signals. Biomagnetic signals has very poor signal to noise ratio
Some biomedical processes generate bioacoustic signals. These signals are usually acoustic noise. The measurement of noises provides information about underlying system. Typical noises are generated by blood flow of through the heart or hearts valves and blood vessels. Measurement of acoustic sounds generated by air flow through airways and lungs is one of the most important vital signs monitoring methods used also for special monitoring procedures. Characteristic sounds are also generated by digestive tract, joints, muscles or during pregnancy.
Biologic systems may generate Biooptical signals naturally or as a result of optical stimuli. Blood flow or blood oxygenation may be measured by light transmission and reflection at different wavelengths. Propagation of the fluids may be estimated when fluids are diluted with dye and dye appearance is measured with minimally invasive sensors like optical fibres.
Biomechanical signals are signals generated by displacement, pressure, tension, flow and others. Main difference between biomechanical and other signals is the biomechanical phenomenon does not propagate like electric and magnetic fields or acoustic signals through the tissue. This enforces invasive measurement techniques.
Chemical reactions between tissue or other live samples and other reagents usually result in different ion concentrations. Biochemical signals are result of such ion concentration deviation measurements performed with dedicated ion electrodes. Changes induced by biochemical reactions are in range of several seconds or minutes and could be treated as ”almost” DC signals.