Biomedical Engineering Reference
In-Depth Information
CHAPTER 2
Passive Membranes
Electrophysiology is the study of the electrical properties of biological materials from the molecules to
the entire body. Although all materials in the body can be characterized by their electrical properties, the
nervous and muscular system in particular use electrical impulses to communicate information between
cells. In this chapter, we will lay down the basic principles of cellular electrophysiology which may be
applied to any cell in the body. In Ch. 3, we will consider the more specialized electrical properties of
neurons.
2.1 CELLULAR ELECTROPHYSIOLOGY
As we are primarily concerned with electrical properties, we must first define what voltages (difference
in potential) and currents (flow of charged particles) mean in the context of a cell. All of the normal
principles of electricity apply, however, most electrical texts consider the movement of negatively charged
electrons. In biological systems, currents are a flow of ions (e.g., Na + , Cl ) and voltages are differences
in potential created by different ionic concentrations.
2.1.1 Cellular Voltages
The transmembrane voltage , V m , is defined as the difference in potential across the cell membrane (Fig. 2.1)
and is typically measured in mV .
e
i
Intracellular
Extracellular
Figure 2.1: Definition of membrane voltage, V m .
V m =
φ i
φ e =−
φ
(2.1)
where φ i is the potential inside the cell and φ e is the potential outside the cell. Surprisingly, the trans-
membrane voltage will reach a steady state called the resting membrane voltage , V rest
m
, that is not zero. In
most neurons V rest
m
60 mV , meaning that at rest the inside of the cell has a more negative potential
than the outside. Any positive change in V m
≈−
is called a depolarization (Fig. 2.2) and may occur because
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