Muscle Contraction - Weber State University

Muscle Contraction

Background Information: A skeletal muscle is a collection of muscle cells, or muscle fibers. Each muscle fiber is

composed of a cell membrane, cytosol, mitochondria, and a whole bunch of myofibrils (bundles of contractile and elastic proteins that carry out the work of contraction). Each muscle cell also contains an extensive sarcoplasmic reticulum for calcium storage, and release.

A Myofibril is comprised of several types of proteins: two contractile proteins, myosin and actin, two regulatory proteins, troponin, and tropomyosin, and a few accessory proteins to boot.

The contraction of a muscle is a complex process, involving many ions, proteins, and certain regulatory mechanisms. Myosin is motor protein that converts chemical bond energy from ATP into mechanical energy of motion. Your textbook describes the molecular basis for contraction, the power stroke, a six-step process very well (Please read it). Actin is associated with the regulatory proteins that prevent the myosin heads fro m completing the power stroke. Tropomyosin is a long polymer that wraps around the actin filament, directly blocking the myosin head binding site, and inhibiting actin and myosin sliding. Ca++ inhibits troponin so filament sliding can occur. Ca++ plus troponin, cause a tug on tropomyosin, pulling it deeper into the groove between actin strands. This uncovers active sites, which react with cross bridges on myosin strands causing the sliding process (power stroke). ATP breaks these reaction bonds and forms new ones (power stroke), and is required for relaxation. You should be able to explain how the following components work to gether to produce a controlled skeletal muscle contraction: Actin, tropomyosin, troponin, myosin, myosin head (crossbridge), Ca++(salts) and ATP. The control of contraction is summarized in Chapter 12 of your text for a full discussion.

Relaxation of the muscle occurs when the sarcoplasmic reticulum pumps Ca++ back into its lumen using a Ca++ -ATPase. As the free cytosolic calcium decreases, Ca++ releases from troponin, tropomyosin slides back to block the myosin binding site and the fiber relaxes.

The basic unit of contraction in an intact skeletal muscle is the motor unit. One motor unit is comprised of one neuron and the muscle fibers it innervates. If the action potentials of a motor unit continue to stimulate the muscle fiber repeatedly at short intervals (high frequency), relaxation between contractions diminishes until a muscle fiber reaches a maximal level of contraction called tetanus. Look at the figures describing summation of contractions leading to maximum tension, and eventually fatigue.

The term fatigue describes a condition in which a muscle is no longer able to generate or sustain the expected power output. Fatigue can be caused by a variety of factors, such as depletion of neurotransmitter, ATP, oxygen, varying ion concentrations, and a build up of lactic acid to name a few.

The purpose of today's laboratory activities is to reinforce the concept that many chemicals are required for skeletal muscle contraction to occur, and to gain a better understanding of the process. In addition, you will have an opportunity to observe both skeletal muscle fatigue and summation to tetanus.

Experimental Protocol: Activity A 1.) In this activity, you will be using glycinerated skeletal muscle to witness contraction. 2.) Obtain a microscope, 3 slides, a ruler, 2 glass teasing needles.

3.) Carefully use the teasing needles to retrieve a fragment of muscle from the glycerine. Take care to not touch the muscle fragment with your fingers.

4.) Using the glass teasing needles, tease the muscle fragment you obtained into separate fibers. The goal is to get single muscle fibers (i.e., muscle cells), although this may be difficult. Examine these fibers using the microscope, observing the striations of the resting muscle.

5.) Place three single muscle fibers with a small amount of the glycerine on each of the 3 microscope slides. The glycerine simply ensures that the muscle fibers do not dry out.

6.) Label three of the slides with an A, B, or C. 7.) Arrange the fibers in a parallel fashion on the slide. 8.) Measure the length of each fiber by placing the slide on top of the ruler under the

microscope. 9.) Wick away the glycerine on the slides using a KimWipe. Rinse each of these fibers by

placing a drop or two of distilled water on the fibers. 10.) Wick away the water on slide A with another KimWipe. 11.) Flood the fibers with ATP. Immediately observe the fibers under the microscope. 12.) After 2 minutes, re- measure and record the length of the fibers.

13.) Wick away the water on slide B with another KimWipe. 14.) Flood the fibers on slide B with Salt solution (containing K+ and Mg++). Immediately

observe the fibers under the microscope. 15.) After 2 minutes, re- measure and record the length of the fibers. 16.) Wick away the water on slide C with another KimWipe.

17.) Flood the fibers with ATP + Salt solution. Immediately observe the fibers under the microscope.

18.) After 2 minutes, re- measure a nd record the length of the fibers. 19.) Calculate the percent contraction for each of the nine fibers using the following formula:

% contraction = Initial length (mm) ? Contracted length (mm)

100

Initial length (mm)

Activity B 1.) In this activity, you will directly observe the effects of muscle fatigue. 2.) Record the time as your laboratory partner stands facing you with his/her arms held

straight out, parallel with the floor, with palms facing upward toward the ceiling. Stop recording when his/her arms begin to shake with fatigue. Determine the duration of time it took until this fatigue was reached. 3.) Record the time as your laboratory partner repeats step 2, but this time, place a magazine on his/her palms when you begin recording. Stop recording when your lab partner's arms begin to shake with fatigue, and determine the duration of time it took until this fatigue was reached. 4.) Record the time as your lab partner repeats steps 2 and 3, but this time, place a text book on his/her palms when you begin recording. Stop recording when his/her arms begin to shake with fatigue. Determine the duration of time it took until this fatigue was reached. 5.) Weigh the magazine and the textbook.

6.) Based upon the results obtained, draw a hypothesis regarding the effect of `load' (i.e., weight) on muscle fatigue.

Activity C 1.) In this activity, you will have the opportunity to measure skin-surface voltages associated

with skeletal muscle contraction through a procedure known as electromyography (EMG). EMG will be used to measure the voltages associated with contractions of increasing force. 2.) For this activity, it will be necessary to calibrate the BioPac for each individual. 3.) To calibrate the BioPac machine, attach three electrodes to your dominant forearm as shown in the picture below. [If your lab partner timed YOU in Activity B, above, then it will be necessary that your lab partner perform this Activity C].

4.) Open the BioPac program and select Lesson L01 (EMG-1). 5.) While sitting still, have your lab partner select "Calibrate". This will begin the process

whereby the computer will automatically establish parameters to record the data properly. 6.) Follow the instructions given by the BioPac program. Clench your fist and relax as

instructed; the program will automatically stop the recording. The recording should look similar to that in the picture below; if it does not, select "Redo Calibration" and repeat the procedure.

7.) Have your lab partner select "Record " and follow the instructions. Clench your fist softly for 2 seconds, then relax for two seconds, then clench harder for two seconds, then relax for two seconds, then clench even harder for two seconds, then relax for two seconds, and finally clench with maximum strength for two seconds then relax. When you have finished with all four `clench-relax' cycles, have your lab partner select "Suspend".

8.) The result should be a series of four clenches of increasing intensity, as shown in the figure below. If the data look very different, have your lab partner select "Redo" and repeat step 7. If the data look similar, select "Stop", followed by "Yes" (the latter in response to the question, `Are you finished with both forearm recordings'), and finally "Done". Remove the electrodes from your forearm.

9.) Select "Analyze Current Data File " and "OK" to analyze the results. 10.) Observe the Raw EMG recording and the computer calculated Integrated EMG. The raw

EMG is the actual voltage recordings (in mV) at each instant in time while the integrated EMG reflects the absolute intensity of the voltage from baseline. 11.) To analyze the data, set up the first four pairs of the channel/measurement boxes at the top of the screen by selecting the following channels and measurement types from the drop down menus:

12.) Use the arrow cursor and click the `I-beam' cursor box on the lower right side of the screen to activate the `area selection' function. Using the activated I-beam cursor, highlight the first EMG cluster, representing the first clenching event.

13.) The computer will automatically calculate the minimum value, the maximum value, the `peak to peak' value (p-p; the difference between min and max), and the mean (average value) for the selected area.

14.) Record each of these values in the table below. Round to the nearest 1/100 mV.

15.) Repeat the analysis for each of the remaining 3 clench events, and record the data in the table.

Clench 1 Clench 2 Clench 3 Clench 4

MIN

MAX

P-P

MEAN

Activity D 1.) In this activity, skeletal muscle contraction to the point of fatigue will be visualized

using EMG on the BioPac. Gross muscular contraction depends upon the gradual and coordinated activation of multiple muscle cells--at minimum, all of those cells in a single motor unit. In most cases, multiple motor units are recruited simultaneously. The force of contraction of an entire muscle increases with the involvement of a larger number of motor units, as well as with an increase in the frequency of nerve impulses delivered to these motor units. In this way, the muscle can produce graded contractions. For example, the biceps muscle will activate a greater number of motor units when lifting a 10kg object than when it lifts a 2kg object. In addition, when lifting the heavier object, the motor neuron of each active motor unit will increase its frequency of action potentials delivered to the cells within that unit. This eventually results in a phenomenon called tetanus, which is a smooth, sustained muscle contraction. When all of the motor units within a muscle are activated to a state of tetanus, maximum force for that muscle is achieved. 2.) For this activity, it will be necessary to calibrate the BioPac for each individual. 3.) To calibrate the BioPac machine, attach three electrodes to your dominant forearm as

shown in the picture above for activity C. 4.) Open the BioPac program and select Lesson L02 (EMG-2). Enter a file name that you

will remember. 5.) With the hand dynamometer at rest on the table, select "Calibrate", then select "OK". 6.) When instructed, grasp the hand dynamometer with your do minant hand as close to the

crossbar as possible without actually touching the crossbar. The crossbar should be held parallel to the floor, not at an angle. When this is so, select "OK".

7.) Follow the instructions given by the BioPac program. When instructed to do so, wait two seconds, then squeeze the hand dynamometer as hard as possible for two seconds, then relax. Continue to follow instructions until calibration is complete. The computer

will automatically stop the calibration. The calibration should look similar to that shown in the picture below. If it does not, select "Redo Calibration" and repeat the procedure.

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