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The purpose of this lab was to measure the conduction velocity of action potentials
in the A alpha axons in the human ulnar nerve. A nerve is a bundle of axons, which carry
different information either to the central nervous system (Afferents) or from the central
nervous system (Efferents).
Myelination is a vertebrate innovation, which permits rapid conduction in axons of
relatively small diameter. For example, a 100-foot blue whale getting commands from its
brain to muscles in the rear. Myelination allows action potentials to skip from node of
Ranvier to node of Ranvier. Older invertebrates have had to come up with giant axons in
order to perform rapid movements and perception because they do not have ability of
myelination. This means they sacrifice fine control because they cannot pack a lot of
giant axons into a reasonably sized nerve.
The physiological stimulator generates electrical stimuli, used here to initiate action
potentials in the ulnar nerve.
An Oscilloscope displays voltage changes in an X Y axis. The oscilloscope used for this
lab was an OLD Tektronix 502A (vintage 1970).
- Oscilloscope sweep speed was set at 5 msec/cm, the vertical amplifier at 0.5 mV/cm, and
the stimulator settings with duration at 0.2 msec, delay at 10 msec, frequency at 3/sec,
and voltage increasing from 0 up to c. 140 V.
|Setup - One electrode was attached over the palmaris
brevis muscle with another on the back of the subject's hand. Another electrode was
attached to the upper arm to serve as a ground with a second attached there as one member
of the stimulating pair of electrodes. The mobile electrode was used to probe for the
ulnar nerve at the elbow and wrist while increasing the voltage and observing the EMG
signal on the screen.
The calculation was done by - Measuring the distance
from the stimulus artifact to the beginning of EMG, for the case of stimulation at the
wrist, and then converting it to a time (1 cm horizontally = 5 msec). The distance from
stimulus artifact to beginning of EMG for the case of stimulation at the elbow was also
measured, and converting to msec. The first time (= time for action potentials to travel
from wrist to p. brevis muscle via ulnar nerve + the time required for transmission
between the motor neurons and the skeletal muscle fibers of the p. brevis muscle) was
subtracted from the second. This equals the time required for action potentials to travel
from the elbow to the wrist along the ulnar nerve motor axons. To get the conduction
velocity (distance/time), we measured the distance (in m) from the elbow to the wrist and
divided this number by the time taken for action potentials to travel from the elbow to
the wrist. Typically, the value should be in the range of 40-60 m/sec.
Oscilloscope sweep speed set at 5 msec/cm; vertical amplifier at 0.5 mV/cm;
stimulator settings: duration at 0.2 msec, delay at 10 msec, frequency at 3/sec, voltage
increase from 0 up to c. 140 V
|The Elbow Stimulus
The Wrist Stimulus
ZOOLOGY 310 GUIDE TO HUMAN ULNAR NERVE