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LAB 7 - The Human Ulnar Nerve


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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.


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The physiological stimulator generates electrical stimuli, used here to initiate action potentials in the ulnar nerve.

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An Oscilloscope displays voltage changes in an X Y axis. The oscilloscope used for this lab was an OLD Tektronix 502A (vintage 1970).

Settings - 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. Hookup.jpg (31854 bytes)

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
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The Wrist Stimulus
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