Muscle Physiology

Background reading: see especially chapter 8 p.268-72

Objectives

1) Understand and define the following terms: muscle twitch, motor unit, latent period, contraction time, relaxation time, summation, tetany and fatigue.

2) Understand the ways muscle twitches can be manipulated, i.e., increasing voltage, changing frequency, etc...

3) Be familiar with the Lab View program that allows us to measure muscle twitches.

Introduction

The muscle responds to a single stimulus (neural stimulation or electric shock) with a quick contraction and relaxation. Together, this is called a muscle twitch (Figure 1 ). A muscle is made up of individual muscle fibers (you will get this in more detail in lecture).

Figure 1: From Sherwood, L. Human Physiology From cells to Systems. Figure 8-14 4th ed. Brooks/Cole, CA, 2001. p.251

The stimulus that results in a muscle twitch will be varied in two ways in this lab: increasing the magnitude of the stimulus (voltage), and increasing the rate at which stimuli are applied (frequency). These manipulations are intended to illustrate that the muscle contraction is actually made up of many muscle fiber groups contracting in unison.

The strength of the twitch increases with the increase in the magnitude of the shock. Therefore, the strength of the twitch is said to be graded (or incrementally increased). This is due to the recruitment of increased numbers of muscle fibers that are involved in the twitch.

If you increase the frequency of the shock, eventually the second stimulus will be applied before the first muscle twitch is over, then the second stimulus will build upon the previous contraction and add to that response.This is called twitch summation (Figure 2).

Figure 2: From Sherwood, L. Human Physiology From cells to Systems. 4th ed. Figure 8-17 a and b Brooks/Cole, CA, 2001. p.271

As you continue to increase the frequency of shocks, eventually no relaxation will be allowed and the muscle contraction will increase smoothly up to a point of maximum strength where indvidual twitches are not observed. This is called tetany. Tetany is the way by which normal muscle contractions occur. Do not confuse this term with tetanus, a disease where muscles contract continuously due to bacterial infection and release of tetanus toxin which prevents the neuromuscular synapse from relaxing.

Figure 3: From Sherwood, L. Human Physiology From cells to Systems. 4th ed. Figure 8-17 Brooks/Cole, CA, 2001. p.271

 

Materials and Methods

A. Hardware Description

Physiogrip: This is a trigger with a transducer attached to it. A transducer converts a physiological event, such as a muscle contracting, to an electrical signal. This signal is sent to a computer where our LabView program will interpret it for us.

Stimulator: This sends out square waveform voltage pulses, of varying strength (volts), duration(msec), and frequency(Hz=cycles/second), and is used to stimulate muscle in human subjects.

B. Connecting the Subject

NOTE: Students with a history of heart disease should not serve as subjects. Also, before stimulating the subject, you must first become familiar with the stimulator controls. Giving your classmate an electrical shock is not something to be taken lightly, so be careful when setting controls.

There are controls for stimulus strength, duration, and frequency; make sure you understand the differences among these variables. There are also switches that allow you to multiply the values of the variables by any number; make sure you choose the correct level. Do not change variable too quickly or start at high levels; inform your subject of any changes you will make. You should never need the multiplier buttons for this lab, so steer clear of those controls.

1. Refer to Figure 4. Coat the inner surface of the flat plate electrode with electrode jelly, and secure it to the back of the subject's hand with a rubber strap. Put a dab of electrode jelly on the end of the stimulator probe. Try not to put too much because excess jelly will diffuse the signal and give the subject a tingling sensation on all areas of the skin covered by the electrode jelly.

2. With the stimulator switch to off, set the voltage to 10-5 V, a frequency of 2 shocks/sec, and a duration of 10 msec .

3. Locate the flexor digitorum superficialis (FDS) muscle by observing the forearm as the subject voluntarily flexes and extends the fourth (ring) finger of the hand with the plate electrode attached.

4. Press the electrode firmly above the fds muscle. Move the probe around over the surface of the skin while pressing firmly until the fourth (or third) finger flexes strongly. You may want to mark the spot for future reference. The voltage may have to be increased gradually if no response is obtained.

 

Figure 4: Placement of electrodes on the forearm

C. Muscle twitch experiments

1. Single Muscle Twitch

Stimulator Settings
Voltage Set for a response that gives a measureable muscle twitch.

Frequency 1 shock/sec

Duration 10 msec

Mode Continuous

a. Click the white run arrow, on the top left of the screen.

b. Perform muscle stimulation experiment with Physiogrip.

c. Push yellow "stop" button.

d. A graph of displacement of trigger (mm) over time (sec) will appear.

e. To calculate the duration, strength, and velocity of any particular muscle twitch:

Duration

Strength

Velocity

 

 

Figure 5: Example of measuring the muscle twitch

2. Effect of Stimulus Strength.

Stimulator Settings

Voltage sufficient for a minimal twitch

Frequency 1-2 shock/sec

Duration 10 msec

Mode Continuous

a. Use the data from Part 1. Single Muscle Twitch as your first data point for Stimulus Strength (voltage vs. displacement).

b. Continue increasing the voltage in small increments (2-5 Volts) to obtain 4 other data points, but not to the point of invoking real pain. If the trigger is reaching the end of its travel, increase the spring tension or substitue a stiffer spring, then repeat the experiment.

c. Measure the strength (displacement), contraction duration, and relaxation duration for each twitch. Record these numbers along with the voltage of each twitch measured.

 

3. Temporal Summation and Tetany

Stimulator Settings

Voltage sufficient for a minimal twitch

Frequency 1-2 shock/sec

Duration 10 msec

Mode continuous

 

a. Grasp the pistol grip as before. Use a voltage that gives a minimal, but recordable, twitch.

b. Gradually (over a period of about 4 seconds) increase the stimulus FREQUENCY (NOT THE VOLTAGE) until no relaxation period is recorded for about 1-2 seconds. The experiment may have to be repeated several times to get it right.

c. Determine the time interval between stimuli as measured from the peak of one twitch to the peak of the next twitch (seconds) before summation occurred. Record this number. Determine the time interval between stimuli measured from the peak of one twitch to teh peak of the next twitch (seconds) just before tetany occured. Record this number.

d. Measure the displacement (strength) of one twitch that occurred before summation. Record this number.

e. Measure the displacement (strength) of the tetanic contraction. Record this number.

 

4. Fatigue and Muscle Relaxation

Stimulator settings:

Voltage Sufficient for ~5 mm displacement

Frequency 1 shock/sec

Duration 10 msec

Mode Continuous

a. Grasp the pistol grip as before.

b. Click the run arrow, then quickly turn the stimulator to a tetanizing frequency, as determined earlier. Immediately turn off the stimulator BUT CONTINUE COLLECTING INFORMATION ON THE COMPUTER until the muscle tension returns to baseline, click the "stop data" button.

c. Determine the length of time for relaxation of the muscle. Record this number.

d. Rest for a few minutes.

e. Have the subject contract the same finger rapidly and vigorously for 2 minutes. Grasp the pistol grip as before.

f. Click the white run arrow and quickly turn the stimulator to a tetanizing frequency, as determined earlier.

g. Immediately turn the stimulator off, but CONTINUE collecting data until the muscle tension returns to baseline.

h. When the muscle has relaxed to the baseline, stop collecting data.

i. Determine the length of time for relaxation of the muscle. Record this number.

 

5. Fatigue and Muscle Contraction - You will not be using the stimulator for this experiment

Stimulator Settings

OFF

a. Install the STRONGEST SPRING onto the pistol grip.

b. Grasp the pistol grip with the INDEX FINGER on the trigger.

c. Click the run arrow and immediately begin squeezing the trigger all the way in and all the way out twice every second. Continue rhythmically squeezing until the trigger can no longer be pulled all the way in.

d. Record the time required for the onset of fatigue.

e. Rest about 5 minutes, then inflate a sphygmomanometer cuff around the subject's arm to about 140 mm Hg and repeat the above experiment and record the time required for the onset of fatigue.

LINK TO MUSCLE WORKSHEET