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BIOB32H3 Lecture Notes - Lecture 8: Nerve Conduction Velocity, Gastrocnemius Muscle, Sciatic NerveExam

Biological Sciences
Course Code
Steve Joordens
Study Guide

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1. Understand the basic operation of a kymograph and how it can be used to assess muscle contraction.
2. Understand the relationship between muscle stimulation frequency and muscle contraction properties.
3. Measure the kinetics of muscle twitches.
4. Measure the force exhibited by muscles during twitches and tetanus.
5. Examine the effect of preload on active, passive, and total muscle force.
6. Examine the effect of afterload on force production and velocity of muscle shortening, and to determine maximal
velocity of shortening (Vmax) and maximal force production (Fmax).
In previous years, students have conducted this laboratory session using a freshly-prepared sciatic-
nerve/gastrocnemius muscle preparation from recently killed frogs. While working with such
preparations is preferred for several reasons (e.g., it permits students to appreciate natural variability
among specimens), it has recently become impossible for us to acquire healthy frogs for use in this
experiment. Fortunately, a few years ago, Joanne and Chris, our wonderful laboratory technicians,
video-recorded some of the preparations, and you can use these recordings to complete this laboratory
session. In addition, we will distribute copies of kymograph recordings from several frog species collected
in previous years so you can see the degree of natural variability.
In last weeks laboratory session, you measured the conduction velocity of a nerve from one of the walking legs of a crab,
which is comprised of unmyelinated axons. Today, you will be provided with copies of oscilloscope recordings from the
sciatic nerve of a frog, which is the nerve the innervates the gastrocnemius, and which is comprised of myelinated axons.
Use the recordings to calculate the conduction velocity of the sciatic nerve and compare it to that measured last week for
the crab leg nerve. This will allow you to visualize the effect of myelination on nerve conduction velocity.
frog sciatic nerve conduction velocity = _ ____________________m/sec
Force production during isotonic muscle contraction, i.e., muscle contraction resulting in the shortening of the muscle, can
be determined using a kymograph (Figure 1). The muscle is fixed in place at one end and is attached to an unfixed lever at
the other end. A kymograph pen is attached to end of this unfixed lever and placed in contact with a rotating kymograph
drum, which has kymograph paper (similar to chart paper) attached to it. When the muscle is stimulated to contract
either directly or through its associated nerve, via stimulating electrodesthe contraction of the muscle will displace the
lever and kymograph pen upward, with the amplitude of the deflection being proportional to the force generated by the
muscle during its contraction. In order to calibrate the force of muscle contraction, weights can be attached to the lever
prior to muscle contraction, which will cause a downward deflection of the lever and kymograph pen. The amplitude of
the downward displacement can be calibrated to the magnitude of the weights used. [Remember, weight is a force, being
equal to the product of mass and acceleration due to gravity.]

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Figure 1. Measuring force production during muscle contraction with a kymograph.
Two different versions of Video 1 are available on Quercus: regular speed and slow-motion.
Using the Regular Speed Video:
In this video, you will see how muscle stimulation frequency affects muscle contraction characteristics, especially
summation and force production. Ideally, the stimulator would have one output to the muscle and one output to the
kymograph so that the time of stimulation could be observed on the kymograph trace in addition to the time and
magnitude of muscle contraction and relaxation. This was not done in the available videos. For this reason, we are unable
to measure the latent period, that is, the time between the stimulus and the start of muscle contraction.
Open the video, but before pressing play:
1. Measure the diameter (thickness) of the gastrocnemius muscle at its widest point.
2. Calculate the cross-sectional area of the muscle, which is proportional to its maximal force production. For the purposes
of this calculation, we will assume that the muscle is a cylinder. When you take a cross-section of a cylinder, you get a
circle. Using the equation for the area of a circle (A = πr2), calculate the cross-sectional area of the gastrocnemius muscle.
A stimulator was used to directly stimulate muscle contraction by the frog gastrocnemius muscle in this video. The
stimulator itself is not visible in the video, but the electrodes leading to the muscle are visible. Therefore, it is clear that
the muscle is being stimulated directly in this video, rather than indirectly via the sciatic nerve.
To elicit muscle twitches, the experimenter set “Pulse Duration Setting to 0.5ms, “Pulse Number Setting to 1, and
Voltage Setting” to 2V, which delivers a stimulus capable of eliciting maximal twitch force. The experimenter stimulated
the muscle several times, eliciting several muscle twitches.
Press play on the video:
3. You will see a series of muscle twitches. Measure the height of the peak corresponding to each muscle twitch and
calculate the mean twitch peak height.
Pause the video, and examine the printed copy of the kymograph data:
[NOTE: There are a few different printed copies available, corresponding to different muscle preparation from previous
years. All were collected using a similar procedure, so your group may want to examine several different copies in order
to assess the variability among frog muscle preparations. What is likely to account for this variability?]
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