Force of Muscle Contraction Lab Report PDF

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Force of Muscle Contraction Lab Report for Animal Physiology Lab...

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Force of Muscle Contraction Lab Report

INTRODUCTIO N A muscle contraction occurs when the thin filaments in a muscle fiber slide past the thick filaments, generating a force in the muscle. When the muscle is relaxed, the filaments only overlap in certain locations. When a stimulus is received, the filaments begin to slide past each other (Hoehn Marieb 285). A single stimulus produces a twitch in a muscle, which is a quick, jerky contraction. If a second stimulus is applied to the muscle before it has fully relaxed, a summation of contraction occurs, which is known as wave summation. Sustained contractions occur when a high frequency stimulus is constantly applied, a phenomenon known as tetany. Fatigue occurs in the muscle when it is in a continued state of tetany (Force of Muscle Contraction 129-132). Skeletal muscles attach to the skeleton and are responsible for voluntary body movements. The plasma membrane of muscle fibers is known as the sarcolemma, and the cytoplasm is called the sarcoplasm. The contractile unit of the muscle is the sarcomere, and they contain the myofilaments which are responsible for muscle contractions. Each sarcomere has five important locations: the A band, I band, H zone, M line, and Z disc. The distance between two Z discs represents a sarcomere. The H zone and M line are located in the middle of that space. The myofilaments are split in two categories, thin and thick. The thin filaments contain actin, and the thick filaments contain myosin. During contraction, the globular heads of myosin form cross bridges with the globular actin subunits on actin and generate force. The sarcoplasmic reticulum and T-tubules help regulate contraction. Calcium is stored in the sarcoplasmic reticulum and released when the muscle is stimulated. The T-tubules conduct impulses to the myofibrils and ensure that they will all contract at the same time (Hoehn Marieb 278-284).

Force of Muscle Contraction Lab Report To produce a muscle contraction, the muscle fibers need to first be stimulated by a motor neuron. The action potential in the motor neuron causes the neurotransmitter acetylcholine to be released, which then attaches to receptors on the sarcolemma of the muscle cell. This opens ligand gated ion channels on the membrane, and Na+ diffuses into the cell while K+ diffuses out. The depolarization of the sarcolemma causes the membrane voltage to reach threshold, generating an action potential that propagates along the membrane and down the T-tubules. This causes Ca2+ channels on the sarcoplasmic reticulum to open, and Ca2+ flows into the sarcoplasm. Calcium binds to troponin, exposing the active binding sites on actin. Myosin heads then attach to the binding sites on actin and form cross bridges. As the filaments slide past each other and generate a contraction, the excitation-contraction coupling phase is over. When the stimulus ends, calcium levels return to normal and myosin-actin binding is inhibited by the tropomyosin complex. Contraction ends, and the muscle enters the relaxation phase (Hoehn Marieb 286-292). A twitch is a muscle’s response to a single stimulus. The three phases of a muscle twitch are the latent, contraction, and relaxation periods. In the latent period, excitation-coupling is occurring but there is no measurable muscle tension produced. The contraction period lasts ten to one hundred milliseconds and actin-myosin cross bridges are active. During this time muscle tension is measurable, and if the tension overcomes the resistance of the load, the muscle shortens. The relaxation period begins when Ca2+ diffuses back into the sarcoplasmic reticulum. Contraction ends and muscle tension drops down to zero. The length of a twitch is different in all muscles, for example, the extraocular muscle of the eyes has a fast contraction while the soleus muscle contracts more slowly (Hoehn Marieb 293-294). When a second contraction occurs before the muscle is completely relaxed a summation of contraction occurs. Calcium ions are pumped into the sarcoplasm at a faster rate than they are

Force of Muscle Contraction Lab Report pumped back into the sarcoplasmic reticulum. The increase of Ca2+ causes the actin-myosin cross bridges to form in greater levels. This results in smooth muscle movements, as opposed to the jerky movements of an isolated twitch. Complete tetanus is when all the contractions fuse into a smooth plateau. A sustained state of tetanus leads to muscle fatigue. During fatigue, the muscle is unable to contract or produce tension even though it may still be receiving stimuli (Hoehn Marieb 294-300). Five experiments were performed on the amphibian gastrocnemius muscle. Recording a twitch was the first experiment performed, the threshold stimulus and phases of contraction were recorded. The second experiment was measuring the effects of stretch on the force of muscle contractions. The third experiment measured wave summation by stimulating the muscle with equal pulses at different intervals. The fourth experiment examined tetany, and the last one measured the effects of high frequency stimuli on muscle fatigue (Force of Muscle Contraction 127-142).

M AT E R I A L S A N D M E T H O D S The equipment used to perform the dissection of the gastrocnemius muscle included scissors, blunt probes, forceps, and bone shears. Ringer’s solution was also used to keep the muscle from drying out. The program used to record data was PowerLab. Materials used to record data also included stimulating electrodes, a micropositioner, myograph cables, a force transducer and a muscle holder. Before the dissection, the bullfrog’s spinal cord was severed from its brain to prevent the animal from having any sensations during the surgery. The first step in dissecting the gastrocnemius muscle was to cut around the mid-abdominal skin of the frog. Then, the skin was pulled off,

Force of Muscle Contraction Lab Report stripped off the animal’s torso all the way down to the ankles. Cutting away at the connective tissue in between the skin and underlying tissue made this process easier. The frog was constantly sprayed with ringer’s solution to keep the tissues from dying. Once the skin was peeled off and the muscles were exposed, the three muscle groups closest to the knee were cut. The frog’s Achilles tendon was tied off with a piece of string. The frog’s leg was removed by cutting through the femur, and then the gastrocnemius muscle was separated from the leg by tearing away at the tissues holding it in place. Finally, the Achilles tendon was cut in the location distal to where it was tied off. Once the muscle was completely separated from the frog, it was placed in a beaker of ringer’s solution. For more detailed instructions concerning the dissection of the gastrocnemius muscle, please refer to Chapter 8: Force of Muscle Contraction in the Animal Physiology Lab Manual (Force of Muscle Contraction 127-143). To record data, first the BNC connectors of the stimulating electrodes were plugged into the outputs on the PowerLab equipment. Then, the muscle was placed in the muscle holder of the recording apparatus and connected to the stimulating electrodes. The free end of the string tied to the Achilles tendon was tied to the hook of the force transducer. For further instructions on setting up the PowerLab equipment and collecting data, please refer to Chapter 8: Force of Muscle Contraction in the Animal Physiology Lab Manual (Force of Muscle Contraction 127143).

Force of Muscle Contraction Lab Report R E S U LT S

Figure 1

Force (N)

Stimulus v. Force 0.550 0.500 0.450 0.400 0.350 0.300 0.250 0.200 0.150 0.100 0.050 0.000 -0.20 0.00

0.20

0.40

0.60

0.80

1.00

1.20

Stimulus Voltage (V)

Figure 1: The stimulus voltage at .31 V marks the threshold stimulus.

Figure 2

Effects of Stretch on Contraction Force 2.000 1.800

Force (N)

1.600 1.400 1.200

Net Force (N)

1.000

Resting Force (N)

0.800

Twitch Force (N)

0.600 0.400 0.200 0.000 0

1

2

3

4

5

6

7

8

9

Change in Length (mm)

Figure 2: Both the resting force and twitch force started producing increasingly stronger forces when the muscle was stretched 5 mm.

10

Force of Muscle Contraction Lab Report Figure 3

Wave Summation

Force (N)

0.300 0.280 0.260 0.240 0.220 Force 1 (N) Force 2 (N)

0.200 0.180 0.160 0.140 0.120 0.100 0

50

100

150

200

250

300

350

400

Interval (ms)

Figure 3: The second force decreased a lot at 100 ms, while the first force remained almost constant. Figure 4

Tetany 0.600 Maximum Force (N)

0.500 0.400 0.300 0.200 0.100 0.000 0

50

100

150

200

250

Interval (ms)

Figure 4: The muscle produced its highest force at 20 ms.

Table 1

Fatigue and

Contracture

300

350

400

Force of Muscle Contraction Lab Report

Section 505 Group 4

Max Force (N)

Force at End (N)

Decline in Force (%)

0.708

0.239

66.2

Table 1: The force of the muscle declined by 66.2% from the first stimulus to the last.

DISCUSSIO N During the latent phase of a muscle twitch no measurable muscle tension is produced, until the stimulus applied reaches the threshold stimulus, then the cell enters the contraction phase. When a slightly stronger stimulus than the threshold stimulus is applied, the muscle produces increasingly stronger force because more muscle fibers are being recruited to undergo contraction. It is the act of a group of muscle fibers that increases muscle force and tension, not a single fiber. Increasing the stimulus will not increase the individual tension generated by a single fiber because response occurs in an all or none manner. By increasing the strength of the stimulus more muscle fibers are recruited causing a greater amount of fibers to contract, which increases muscle tension. If the stimulus is increased enough it will reach the supramaximal level, where tension does not increase. By now, the maximum number of muscle fibers have been recruited, and they are all contracting so tension remains constant (Lab #9 Muscle Physiology). One of the factors that influences the force produced by a muscle is its length. A barely stretched muscle will produce a weak contraction, and therefore a weak force, because opposing thing filaments overlap with one another. The ends of the Z discs overlap and this creates conflict in the formation of cross bridges. Less cross bridges are formed, leading to relatively weak tension generated by the muscle. A moderately stretched muscle will produce maximal contraction. The muscle is stretched enough to where the filaments overlap perfectly and the maximum amount of

Force of Muscle Contraction Lab Report cross bridges are formed. This increase in tension generates a stronger force produced by the muscle. If the muscle keeps on being stretched, it will become over stretched and tension will decrease. The filaments cannot form cross bridges because they are pulled to a point where they cannot interact. The thin filaments are at the ends of the thick filaments, producing very small tension because only a slight amount of cross bridges can participate in contraction. Therefore, when a muscle is overly stretched the force produced greatly decreases (Contraction of Whole Muscle). When stimuli are applied at a high frequency, the muscle does not have time to completely relax and their contractions are added together, producing a wave summation. The calcium levels in the sarcoplasm become even higher with high frequency stimuli, causing a larger number of cross bridges to form. Calcium ions attach to troponin and expose the myosin binding sites, so when there is a greater amount of Ca2+ present, more sites become active and can form cross bridges. The graded response generated by the muscle is an increase in force (Burggren et al. 391-392). When the muscle is stimulated at a high frequency and does not relax at all in between stimuli, contraction is sustained at a constant level. This is known as tetany, and it is how skeletal muscles produces smooth movements. While one motor unit may be entering the relaxation phase of a twitch, another may be barely starting to contract, due to the high frequency of the stimulus. This occurs in many collective motor units and generates sustained tension. This summation of contraction produces greater force in the muscle than an average wave summation (Lab #9 Muscle Physiology).

Force of Muscle Contraction Lab Report The muscle fatigues when tetany is sustained for too long. Muscle tension decreases due to a decreasing amount of ATP, and when there is none left the muscle enters a state of contracture. Contracture can be observed in rigor mortis because there is no way of replenishing ATP once the heart stops beating but calcium still leaks into the sarcoplasm. This results in muscle contraction where the cross bridges cannot detach from one another, producing a sustained contraction (Force of Muscle Contraction 132). Muscle fatigue is evident when there is a shortage of oxygen being received by the muscle cells. Without oxygen the cell cannot generate ATP, so it turns to alternative pathways to fight fatigue. During fatigue there is a decrease of calcium ions being released by the sarcoplasmic reticulum. The presence of inorganic phosphate is one proposed explanation for the decreased calcium levels in muscle fatigue. Inorganic phosphate binds with calcium ions in the sarcoplasmic reticulum to create a calcium phosphate precipitate, and this reduces the amount of free calcium available to diffuse out of the reticulum and into the sarcoplasm. Cross bridges form at a lower level and this generates a weaker force by the muscle (Allen Westerblad).

CONCLUSION In the first experiment, a twitch of the amphibian gastrocnemius muscle was observed. In the second experiment, the effects of stretch on muscle force was examined. Then, high frequency stimuli were applied to the muscle to observe a wave summation. Tetany was tested in the fourth experiment by stimulating the muscle with a sequence of stimuli at varying frequencies. Lastly, fatigue was observed in the last experiment. The muscle was stimulated until it was no longer responsive.

Force of Muscle Contraction Lab Report Studying muscle contraction is a relevant field of study because muscles are used in everyday body movements. Understanding how they generate contractions is valuable information for not only physicians but also body trainers and coaches. Muscular disorders are extremely devastating, so knowing how muscles work is vital for treatments and cures.

Force of Muscle Contraction Lab Report

L I T E R AT U R E C I T E D Allen, D.G., Westerblad, H. “Role of Phosphate and Calcium Stores in Muscle Fatigue” The Journal of Physiology. 2001. 2. Print.

Burggren, W. French, K. Randall, D. “Muscles and Animal Movement.” Animal Physiology Mechanisms and Adaptations. Ed. J. Noe, M. Ryan, J. O’Neill. China: W.H. Freeman and Company, 2002. 361-423. Print.

“Contraction of Whole Muscle.” University of Dayton. Web. November 5, 2013.

“Force of Muscle Contraction.” Animal Physiology Lab Manual. Ed. A. Curran. Denton, TX: University of North Texas, 2013. 127-143. Print.

Hoehn, K. Marieb, E. “Muscle and Muscle Tissue.” Human Anatomy and Physiology. Ed.S. Beaurparlant, G. Puttkamer, S. Cutt. United States of America: Pearson, 2013. 280-323. Print.

“Lab #9 Muscle Physiology.” Indiana University. Web. November 5, 2013....