THE ORIGIN OF BIO-POTENTIALS:
· Bioelectric phenomenon is of immense importance to biomedical engineers because these potentials are routinely recorded in modern clinical practice.
· ECG (Electrocardiogram), EMG (Electromyogram), EEG (Electroencephalogram), ENG (Electroneurogram), EOG (Electro-oculogram), ERG (Electroretinogram), etc. are some examples of biopotentials. We will briefly look at origin of ENG, EMG and ECG in this talk.
· As engineers, we should have a good physical insight into the nature of electromagnetic fields generated by bioelectric sources. Therefore we could contribute to quantitative solution of biological problems.
To understand the origin of biopotentials we need to focus on:
· Bioelectric phenomena at the cellular level
· Volume conductor fields of simple bioelectric sources
· Volume conductor fields of complex bioelectric sources
· Volume conductor fields as a necessary link between cellular activity and gross externally recorded biological signals
1. ELECTRICAL ACTIVITY OF EXCITABLE CELLS
· Biopotentials are produced as a result of electrochemical activity of excitable cells: i.e., nervous, muscular (cardiac and smooth) and glandular cells
Factors influencing the flow of ions across the cell membrane
· Diffusion gradients
· Inwardly directed electric field (inside negative, outside positive)
· Membrane structure (availability of pores; K+, Na+and permeability of membrane to different ions)
· Active transport of ions across membrane against established electrochemical gradients
· When appropriately stimulated, they generate an action potential (flow of ions across the cell membrane and generation of a propagating wave of depolarization along the membrane)
2. BIOELECTRIC PHENO MENA AT THE CELLULAR LEVEL
important topic in electrophysiology is the relationship between intracellular
and extracellular potentials, especially in nerve or muscle fibres .
The relation between extracellular potentials (A-E), transmembrane potential Vm(part F) and the second derivative of Vm(part G).
The relationship between trans membrane (action potential) Vm (monophasic) and volume conductor (extracellular field) potential d2Vm/dt2(triphasic). Students interested in the biophysics of this topic should refer to: Bioelectic Phenomena by Robert Plonsey
Trans membrane (Et= Vm) an d extracellular action potentials (Ex) obta ined from different excitable tissues. Note the monophasic and triphasic shapes.
a. Frog semitendinous m uscle
b. Toad sartorius muscle
c. Rabbit atrium
d .Squid giant axon.