Functionality of Cardiac and Skeletal Muscles Essay

Abstract:

Lab exercises 8 and 9 were performed to learn about the physiological properties of skeletal and cardiac muscles. In lab exercise 8, the gastrocnemius muscle was isolated from a bullfrog and several experiments were performed with a kymograph to test the effect of stimulus intensity on a muscle, the timing of muscular contraction, and testing titanic contraction.

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In lab exercise 9, a bullfrog heart was used to understand the properties of cardiac muscle. Cardiac muscle is very unique, because it can only be found in the heart organ. Experiments in lab exercise 9 included controlling the heart rate with four different agents: adrenalin, cold ringer’s solution, warm ringer’s solution, and acetylcholine. Additionally, the initiation and transmission of cardiac contraction was tested by stopping the SA node and finally looking at the reaction properties of cardiac muscle with the assistance of a kymograph (Stefaniak pg 84).

Introduction:

Skeletal and Cardiac muscles can be very similar. They possess the ability to contract due to the actin and myosin sliding filaments, which is a trait in every type of muscle tissue. They are also both striated and when stimulated, can cause action potentials. This means that they both follow the sliding-filament model, which is based on the interactions between the proteins myosin and actin. Actin makes up the thin filaments of a myofibril, while myosin makes up the thick filaments and together with the help of ATP, causes a muscles to physically contract (Campbell pg 1106). Despite having the same basis for contraction, cardiac and skeletal muscles are very different.

Cardiac and skeletal muscles can only be found in certain places of the body. Skeletal is only found on bones, while cardiac can only be found in the heart. Skeletal muscles voluntary contract, which means you can control the contraction and is mediated by the nervous system. There are two mechanisms that the nervous system produces graded contractions.

The first is that the number of individual muscle fibers that contract can be increased in response to different levels of stimuli, known as recruitment. The heavier the force is on the muscles, the greater the stimulus and thus the more fibers stimulated to contract. The other mechanism is increasing the rate at which stimuli are sent to the muscle fibers. The more stimuli sent will mean that the muscle will have less time to relax from its contraction. This gives increased force to the tendons and muscles. When a muscle is in a sustained contraction due to a high rate of stimuli impulses, it is called tetanus (Campbell pg 1105)

Cardiac muscles are involuntarily, which means that a person cannot control them. Hearts are myogenic, so it is not controlled by the nervous system like skeletal muscles, but rather by the heart tissue itself. The pacemaker, or sino-atrial node is responsible for the contractions in the heart. Gap junctions, in intercalated disc regions of the heart allow signals that commence contractions to progress throughout the whole heart. Additionally, unlike skeletal muscles, heart muscles do not have summation or tetanus, because of a long refractory period.

Hypothesis 1: Up to a certain point, increasing stimulus intensity (after it passes the subthreshold) will give increasingly stronger contractions for skeletal muscles.

Hypothesis 2: Rapid succession of stimuli to skeletal muscles can create tetanus.

Hypothesis 3: The effects of adrenalin and acetylcholine on cardiac muscle are integral to controlling the heart rate.

Hypothesis 4: Increasingly stronger stimulus intensity, after it passes the subthreshold, will not give a stronger cardiac muscle contraction, but it would rather all be equal.

Materials and Methods:

Hypothesis 1:

The materials used in order to test this hypothesis are a kymograph, stimulators and electrodes, a ring stand, clamps, a muscle lever, kymograph paper, gastrocnemius muscle (isolated from the frog leg), ink reservoir, and some thread. The ring stand and clamps are used to place and hold the gastrocnemius muscle as it is being tested. A piece of thread should be attached to the end of the muscle at the tendon. The other part of the thread should attach tightly to the muscle lever, which is used to record the movements of the muscle as it contracts. A pen should then be attached the end of the muscle lever and a steady flow of ink from the ink reservoir should be seen as it is tested on the kymograph paper. After all these materials have been setup, attach an electrode to the muscle. The electrode should be connected to a stimulator that will produce electric shots sent to the muscle. Single shocks of 1, 2, 5, 10, 15, 20, 25, 30, 40, and 50 volts should be applied one at a time to produce your data and observations (Stefaniak pg 83).

Hypothesis 2:

The materials used should be the same as in hypothesis 1 except for one change. That is, instead of having a single stimulus sent to the skeletal muscle from the stimulator, have a continuous supply of 20 volts going to it. The settings for the stimulator controls should be: the mode set on continuous (as opposed to single), numbered dial on 0, multiplier slider to X1, duration set to .2 ms, and amplitude/volts set to 20. Also, let the kymograph drum revolve at a sensible speed to record the muscle contractions and results (Stefaniak pg 83).

Hypothesis 3:

The materials used to test this hypothesis are ring stand clamps, insect hook, a kymograph, kymograph paper, a ring stand, adrenaline, acetylcholine, a functional bullfrog heart, a heart lever, thread, and clay. Attach the heart lever to the ring stand and place clay on each side of the lever in order to counter balance it. Then a length of thread should be tied tightly from the heart lever, to the end of an insect hook that should hook through the tip of the heart ventricle.

Finally, a pen should be attached to the end of the muscle lever, which will record the heart rate on the kymograph paper. A baseline heart rate should be recorded on to the kymograph paper that can be used as a comparison to the heart rates after adrenaline or acetylcholine has been added. The kymograph must be adjusted so that one full rotation will equal one minute. This is to measure the amount of heart beats per minute. Record the baseline heart rate and then add three drops of adrenalin to the sinus venosus area of the heart. After the data has been recorded wash off the adrenalin with room temperature ringer’s solution. Then record another baseline tracing and afterward apply 20 drops of acetylcholine directly to the heart (Stefaniak pg 93).

Hypothesis 4:

The materials used to test this hypothesis are ring stand clamps, insect hook, a kymograph, kymograph paper, a ring stand, electrode and stimulator, a functional bullfrog heart, a heart lever, thread, and clay. These materials should have the same setup in hypothesis three, but no acetylcholine or adrenaline should be used. Instead hook an electrode that is connected to a stimulator to an isolated heart which is no longer beating. The settings for the stimulator are: the mode is set to off, the events/sec should not be used, and the duration set to .2 ms. While the drum is turned off give a single stimulus to the ventricle of the heart of 2, 5, 10, 15, 25, and 50 volts. After each shock, move the drum a little bit to give space for the next contraction to be recorded (Stefaniak pg 94)

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Data collected after the experiment was complete of hypothesis one are shown below in table 2-1

Table 2-1

Based off of table 2-1, the skeletal muscle could not be stimulated enough to contract when 1, 2, or 5 volts were applied to it. When the voltage was increased to 10, 15, 20, 25, 30, 40, and 50 respectively, it made the muscle contract. Additionally, based off the kymograph paper, as the voltage level went up, so did the amplitude of each response.

Observations done on hypothesis two showed that a continuous amount of stimuli on a skeletal muscle can cause it to have one sustained contraction, instead of individual fast contractions. The kymograph showed that the amplitude of a rapid contraction exceeds that of any individual contraction. This can be compared with the data obtained from hypothesis one, because the amplitude is greater than any individual stimuli. At the end of the hypothesis two experiment, the contractions and responsiveness of the frog leg began to decline due to fatigue.

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Discussion:

Table 2-1 measures the effects of increasing stimuli on skeletal muscle contractions. Since no contractions occurred on the skeletal muscle after it was stimulated with 1, 2, and 5 volt shocks, but contracted with a 10 volt shock shows that the subthreshold value for that muscle was 10 volts. This is due to the fact that 10 volts is the minimum amount of stimulation needed in order to make the muscle contract. The amplitude on the kymograph shows how powerful each of the muscular contractions are.

As the amplitude rises, so does the force of each muscle twitch. This is due to the fact that electrical impulses sent to the muscle fibers depolarize the cell membrane to cause calcium ion channels to activate (Kimball). The calcium binds to troponin-C, which is present on actin containing thin filaments of myofibrils. This causes myosin to bind to special actin sites. This allows for the release of ADP from the myosin, which is used as energy as well as cause the myosin-actin filaments to slide. The sliding filaments are what cause a contraction. The stronger a stimuli impulse is will cause for a greater recruitment of muscle fibers, which creates a stronger contraction. This proves that hypothesis 1 is true.

The action potential that was created by the stimuli caused the muscle to have a high tension. This is because the stimuli reactivated the ion channel gates during the falling phase causing the muscle to have stronger contraction instead of relaxing. Additionally as the amplitude increased, the refractory period decreased. When the stimuli are separated by intervals of time a little greater than the refractory period, summation will occur and be accompanied by the union of a sequence of contractions (Stefaniak pg 84). When the individual contractions of a skeletal muscle form into one big and sustained contraction that is known as tetanus and thus proving that hypothesis two is credible.

Hypothesis three results showed that after adding adrenaline and acetylcholine to the heart, the heart rate did change from the baseline rate. Adrenaline caused a heart rate increase of 12.5%, while acetylcholine caused a heart rate increase of 15.4%. During this experiment a lot of human error occurred unfortunately. After the adrenaline tests were done, the second baseline reading was 44 as opposed to 21. The effects of the adrenaline have clearly not worn off by this time, which then gave very inaccurate results for the effects of acetylcholine. Acetylcholine, which slows the membrane depolarization by blocking the potassium currents during the process of an action potential, slows the beat of a heart (Kimball). During the lab, the heart rate increased instead of having an expected decrease.

The results for hypothesis four show that as the stimuli intensity increased, the force of the cardiac contractions stayed the same. The kymograph paper showed that the amplitude, when the heart was stimulated with various amounts of volts, all measured out to 20 mm. The heart would not contract any harder or any lighter after it passed the subthreshold value. This can be explained by the fact that cardiac muscles have relatively long refractory periods that are much longer than that of skeletal muscles. In order for stronger contractions to occur, the intervals of time for the stimuli shocks have to be slightly greater than that of the refractory period. However, a situation like that to occur to cardiac muscle on a normal daily basis is highly unlikely (Johnson).

Acknowledgements:

I would like to thank my teaching assistant, Dean Able, for helping explain to me each of the lab procedures of how to dissect and isolate the frog calf muscle as well as explain about action potentials to me. My lab partners Scott Hoover, Alex Laffoon, and Christian Erickson, were also a great help to me and without them experiment exercises 8 and 9 would be of greater difficulty to complete. Together, all of us were able to actively engage in the labs and understand very important muscle and contraction concepts.

References:

Campbell, N and Reece, J. (2009). Biology Eighth Edition. Pearson Education, Inc., publishing as Benjamin Cummings, San Francisco, CA.

Frankel, Williams, Irish, and Stefaniak (2009). Principles of Biology I Laboratory Manual Sixth Edition. Pearson Custom Publishing, Boston, MA.

Johnson, John K. Muscles. Saint Michael’s College Department of Biology. Web. 13 Apr. 2010. <http://www.cartage.org.lb/en/themes/sciences/zoology/animalphysiology/muscles/muscles.htm>.

Kimball, John W. “Muscles.” RCN D.C. Metro | Digital Cable TV, High-Speed Internet Service & Phone in the D.C. Metro Area, including Washington, D.C., Bethesda and Silver Spring in Maryland and Falls Church in Virginia. Web. 14 Apr. 2010. <http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Muscles.html>.

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