Biomedical Engineering Reference
In-Depth Information
Table 3.1
Uses of Bioelectrical Flow Analysis
Body composition and body hydration by measuring electrical
characteristics
of biological tissue.
Transcutaneous electrical nerve stimulation (TENS), a noninvasive
electroanalgesia used in physiotherapy to control both acute and chronic
pain arising from several conditions.
At the overall body level
Electrocardiogram (ECG or EKG), pacemakers, defibrillators.
Electroencephalogram (EEG): records spontaneous neural activity in the
brain and understand brain functions with time and spatial resolution.
Electroneurogram (ENG): records neural activity at the periphery.
Electromyogram (EMG): records activity in the muscle tissue.
Electrogastrogram (EGG): records signals in the muscles of the stomach.
Electroretinogram (ERG): detects retinal disorders.
At the tissue level
At the cell level
Understanding cellular mechanisms in transport of various molecules,
developing novel therapeutic molecules.
3.2 Membrane Potential
The bilayered cell membrane functions as a selectively permeable barrier separat-
ing the intracellular and extracellular fluid compartments. The difference in the ion
concentration between the intracellular and extracellular compartments creates an
electrical potential difference across the membrane, which is essential to cell sur-
vival and function. The electrical potential difference between the outside and in-
side of a cell across the plasma membrane is called the membrane potential (
ΔΦ
m
).
The electric potential difference is expressed in units of volts (named after Alessan-
dro Volta, an Italian physicist), which is sometimes referred to as the voltage. Under
resting conditions, all cells have an internal electrical potential (
Φ
i
), usually negative
Φ
o
) (Figure 3.1). This is because of a small excess of negative ions inside
the cell and an excess of positive ions outside the cell. Convention is to define the
internal electrical potential referenced to the external fluid, which is assumed to be
at ground (or zero) potential. When the cell is not generating other electrical signals,
the membrane potential is called the resting potential (
to outside (
ΔΦ
rest
) and it is determined
by two factors:
Figure 3.1
A part of the membrane of an excitable cell at rest with part of the surrounding intra-
cellular and extracellular media. The direction of electrical and diffusional fl uxes is opposing. In the
fi gure, the assumption is that cell membrane is permeable to only K
+
ions.
