Biomedical Engineering Reference
In-Depth Information
CHAPTER 3
Bioelectrical Phenomena
3.1 Overview
As described in the previous chapter, there is a difference in the composition of
electrolytes in the intracellular and extracellular fluids. This separation of charges
at the cell membrane and the movement of charges across the membrane are the
source of electrical signals in the body. Exchange of ions through ion channels in
the membrane is one of the primary signal mechanisms for cell communication and
function. For example, normal function in the brain is a matter of the appropriate
relay of electrical signals and subsequent neurotransmitter release. Through elec-
trical signals, neurons communicate with each other and with organs in the body.
Neurons and muscle cells were the first to be recognized as excitable in response to
an electrical stimulus. Later, many other cells were found to be excitable in associa-
tion with cell motion or extrusion of material from the cells, for example secretion
of insulin from the pancreatic beta cells. A few uses of evaluating bioelectrical phe-
nomena are summarized in Table 3.1.
It is important to understand the conductivity of electricity in the body and
time-dependent changes in bioelectrical phenomena. Abnormal electrical activity in
the tissue leads to many diseases including that in the heart, brain, skeletal muscles,
and retina. Substantial progress in research and clinical practice in electrophysiol-
ogy has occurred through the measurement of electrical events. Understanding that
the body acts as a conductor of the electrical currents generated by a source such as
the heart and spreads within the whole body independently of an electrical source
position led to the possibility of placing electrodes on the body surface noninva-
sively (i.e., without surgical intervention) to measure electrical potentials. ECG has
become a common diagnostic tool to monitor heart activity. Understanding the
movement of ions, relating the ion transfer to the function of cells with knowledge
about mechanisms of tissues and organ function, and the complex time-dependent
changes in electrical signals, and propagation in excitable media is important for
development of advanced computational tools and models. This chapter provides
an introduction to these concepts.
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