Chapter 6 The Nervous System 205
What Research Tells Us
...about
Measuring Nerve Impulses
How do scientists and clinicians
measure the speed and function
of nerve impulses? One common
method is the use of a nerve
conduction velocity (NCV) test.
Conducting and Interpreting
NCV Tests
The NCV test begins with the
attachment of three small, fl at,
disc-shaped electrodes to the skin.
The electrodes are attached over
the nerve being studied and over
the muscle supplied by the nerve.
The third electrode is attached
over a bony site, such as the elbow
or ankle, to serve as an electrical
ground.
The technician then
administers short, tiny electrical
pulses to the nerve through the fi rst
electrode and records the time it
takes for the muscle to contract (as
sensed by the second electrode).
Computer software calculates
the NCV as the distance between
the stimulating and sensing
electrodes divided by the elapsed
time between stimulation and
contraction.
Placing stimulating electrodes
at two or more different locations
along the same nerve makes it
possible to determine the NCV
across different segments of the
nerve. To test for sensory neuron
function, the stimulating electrode
is placed over a region of sensory
receptors such as a fi ngertip. The
recording electrode is then placed
at a distance up the limb.
How are the results of a
clinical NCV test interpreted? An
NCV that is signifi cantly slower
than normal suggests that damage
to the myelin sheath is likely.
Alternatively, if NCV is slowed but
close to the normal range, damage
to the axons of the involved
neurons is suspected. Evaluation
of the overall pattern of responses
can serve as a diagnostic tool in
helping a clinician determine the
likely pathology involved in an
abnormal NCV.
Microneurography
Scientists have used a
similar but more sophisticated
procedure called microneurography
(MIGH-kroh-noo-RAHG-ra-fee)
to record electrical activity from
single sensory fi bers. Figure 6.8
shows that the technique involves
the direct insertion of fi ne-tipped
needle electrodes into the nerve
being studied.
Through the use of
microneurography we have
developed our current
understanding of the sympathetic
nervous system. Topics studied
include various refl exes,
interactions within the sympathetic
nervous system, metabolism,
hormones, and the effects of
drugs or anesthesia during
operative procedures. Sympathetic
recordings have also been used to
study the effects of performance at
high altitudes, as well as in space.
Taking It Further
1. Working with a partner,
research nerve damage
further. Develop a report for
the class on the more common
causes.
2. Investigate and report to the
class on technologies, in
addition to NCV tests, that
are used for diagnostic and
therapeutic purposes to treat
nerve disorders.
Figure 6.8 Microneurography is
a technique involving insertion of
fine wire electrodes into a nerve
for direct recording of electrical
impulse activity.
The fi nal step in communication between
nerves at a synapse is the removal of the
neurotransmitter, usually by an enzyme, to
prevent ongoing stimulation of the receptor cell.
Acetylcholine, for example, is deactivated by the
enzyme acetylcholinesterase (a-SEE-til-KOH-
leen-EHS-ter-ays).
Refl exes
Refl exes are simple, rapid, involuntary,
programmed responses to stimuli. The transmission
of impulses follows a refl ex arc that includes
both PNS and CNS structures (Figure 6.7). There
are two categories of refl exes.
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