Biomedical Engineering Reference
In-Depth Information
stimulation of the muscle since an entire group of muscle fibers is simultaneously
activated. Measurements at a number of distances along a muscle determine con-
duction velocities along the stimulating nerve. While not as common as the ECG or
EEG, the electromyogram is more directly related to the depolarization of a single
cell or small group of cells. In the recorded signal, the stimulation artifact normally
strongly dominates in relation to the electrical stimulation response. If the response
signal and the artifact overlap, the response signal is lost due to overdrive of the
recording amplifier. Every rising edge of the stimulation impulse leads to a charging
current and every falling edge leads to a discharging current in the capacitive part
of the load impedance. The two edges of a same phase stimulation pulse produce
a slowly decreasing discharge current through the tissue after the second (falling)
edge.
When measuring biopotentials (say, ECG), everything else (even other biopo-
tentials such as EEG, EMG, EOG) and power line interferences are noise sources.
Although some noises have characteristic frequencies and are minimized using
band pass filters, one has to understand and develop sophisticated signal processing
methodologies. If at a particular measurement site, the resistance of the conductor
(tissue) is higher or the conductor size (finger compared to arm) is decreased, then
the electrical potential increases. Artifacts also arise due to changes in the amount
of intervening tissue (tissue filter function and volume conductor effects) but espe-
cially from the dead skin cell layer. Further, movement of the patient causes the skin
to stretch, creating a disturbance of the charge distribution at the electrode/elec-
trolyte boundary. This is commonly referred to as the motion artifact and removal
of motion artifact is a major task of the monitoring system's filtering algorithms.
Complex algorithms try to manipulate incoming signals to produce output signals
based solely on the electrical activity of the tissue of interest. However, motion
artifact persists as the major component of signal degradation, even with complex
algorithmic filtering. Hence, computer algorithms still have a difficult time identi-
fying poor signals due to motion artifact.
Problems
3.1
Determine the Nernst potentials for potassium ions and chloride ions between
the inside and the outside of the cell membrane. Use the concentration values
shown in Figure 3.2.
The Nernst potential for magnesium ions (Mg
2+
) is
3.2
50 mV. The extracellular
concentration of magnesium
C
out
is 0.6 mM. What is the intracellular concen-
tration of magnesium (in mM)?
−
Voltage-dependent L-type Ca
2+
channels (
I
Ca,L
) play vital roles
for cardiac
functions, including triggering cardiac contraction and pacemaker activity in
nodal
cells. Using the values given for Ca
2+
, calculate the resting potential
across a cell.
3.3
Assume that a membrane is selectively permeable to SO
4
−2
. At equilibrium,
what is the membrane potential if the concentration of Na
2
SO
4
is 120 mM on
one side and 10 mM on the other side?
3.4
