The Heat-production in Prolonged Contractions of an Isolated Frog's Muscle - Assignment Example

1. Why is frog tissue used for the investigation of skeletal muscle function in these experiments? Ans: Frog tissue is used because it is a sturdy invitro preparation which functions at room temperature and the oxygen requirement is met from the atmospheric oxygen which gets diffused into the physiological saline solution irrigating the tissue known as Frog Ringer. Muscle and nerve cells are excitable i.e. they respond to the application of external stimuli by generating action potentials. In muscle cells, action potential results in contraction. Muscle fibres contract after a time span during which the action potential spreads across the surface and into the muscle. This time span is called latent period which is short and constant for skeletal muscle fibres such as the frog Gastrocnemius muscle. Hence it is the ideal tissue for such studies. 2. What are the components of a simple twitch? Ans: The contraction of a muscle fibre produced by a single action potential is called a twitch. A simple twitch is composed of three components – the latent phase, the contraction phase and the relaxation phase. The latter two phases have a measurable duration. The latent phase is the first phase where there is a delay due to the combination of initial inertia due to electric delay in the isolated sciatic nerve-Gastrocnemius muscle preparation and the stimulating external electric circuit as well as the inertia in the recording apparatus attached. The latent phase also exists in the in vivo conditions in the live animal where it is governed by the natural physiological status and is free from external influences in the experiment. Hence it is slightly shorter in the living animal. The contraction phase is the one when there is actual shortening of the muscle fibres as a response to the electrical and biochemical stimulus at the motor nerve end plate. The relaxation phase is the passive phase where the muscle recovers to its original state and shape by its endogenous elasticity. It is a mere mechanical process. The total time of a simple twitch lasts usually from 0.1 to 0.2 seconds. 3. Describe and give physiological explanations for the effect of temperature changes on the magnitude and duration of a skeletal muscle twitch and its components. Ans: The simple twitch when recorded at the room temperature (22°C) gave total cycle duration of 400 msec (milliseconds) which comprised of the contraction phase of 80 msec and the relaxation phase of 320 msec. At 4°C the total time was increased to almost double at 760 msec (160 msec contraction and 600 msec relaxation phase times). On increasing the temperature to 30°C, both the times decreased considerably (100msec contraction and160 msec relaxation phase times giving a total duration of the twitch at 260 msec). The results imply that the muscle responds more quickly at warmer temperatures. This is due to quick release of the neurotransmitter substance Acetylcholine at the neuromuscular junction or the motor end plate which makes the actin and myosin molecules slide against each other. There is increase in conduction velocity in the nerve and an increase in the rate of neuromuscular transmission. Muscle viscosity is decreased too. At cold temperature it takes longer for the release of the neurotransmitter substance and hence both the contraction and the relaxation phases of the twitch are prolonged. The relaxation phase was prolonged substantially to 600 msec in cold condition i. e. the muscle recovery to its original state took longer which is more of a physical rather than neuro-humoral phenomenon. However, the enzymatic activity has optimal performance at temperatures which are optimum for living organisms (37°C). The effect of temperature on the magnitude of contraction was not discernible at all ranges where it was the same (4.9 cm) at both room temperature and at 4°C and increased imperceptibly to 5.1 cm when the temperature was increased to 30°C. There was negligible change in the magnitude of contraction at different temperatures signifying that the contraction was more dependant on the action potential of the stimulating electrical current. However Frog’s muscle works better at room temperature as compared to mammalian muscles because its enzymes are less subject to variations due to temperature conditions in such an experimental setup. 4. Why does an increase in stimulus strength produce a larger twitch response over only a narrow range? Ans: The increase in stimulus strength produces a larger twitch response over only a narrow range because all muscle fibres are stimulated to their maximum strength, up to their upper limit. As the action potential crosses the threshold level of stimulation which elicits the first response and is increased in increments it increases the magnitude of contraction until an upper maximum level is reached. Any further increase in either amplitude or frequency of the electrical stimulus only leads to the development of a tetanic spasm in which multiple contractions are interpolated into each other. The first twitch is obtained at 150 mv (millivolts). The stimulus signal when increased by increments of 50 mv only increases the twitch height up to a certain extent after which it stabilizes at one point even when the stimulus signal reaches a value of 20 V. The tension developed during a twitch contraction depends on muscle length. There is an optimum length at which the maximum tension will be produced. Beyond that no more contraction is possible due to physical limitations. 5. What is a tetanic contraction and why is it stronger that a simple twitch? Ans: A tetanic contraction results when muscle twitches sum up to produce graded contractions. If a second twitch occurs before the first has relaxed, then the two twitches will sum. At high frequencies of action potentials, twitches sum to produce a smooth, sustained, maximum contraction called tetanus. Maximum tetanic contractions produce a force approximately 3-5 times a single twitch contraction. It is stronger than a simple twitch because all twitches sum up together and in individual twitches contributing to this summation don’t have adequate time to relax completely and are stimulated midway again during the relaxation phase. 6. Why is it essential to keep living tissue irrigated with physiological salt solution? Ans: It is essential to keep the living tissue irrigated with physiological salt solution in order to provide the muscle with the adequate simulated physiological status as it exists in the living animal. The ringer lactate is composed of the essential physiological salts like Sodium, Potassium and Calcium which are necessary for the normal physiological functioning of the isolated tissue. Normally the tissue in its living state is perfused by the animal’s blood which brings all essential nutrients and elements along with oxygen and carries away the waste metabolic products. In absence of physiological salt solution and lack of atmospheric oxygen the tissue would desiccate and lose all activity (die). The components of the physiological salt solution allow the metabolic activity to take place in the isolated preparation for a fair extent of time in which the experiment is conducted. 7. In what ways would you expect that frog Ringer solution would differ in composition from the intracellular fluid of the frog muscle? Ans: Frog Ringer solution is an ideal physiological solution for preserving the viability of tissue removed from an animal and contains all essential ions necessary for the functioning of the tissue. However it lacks the quality of carrying dissolved oxygen to the tissue which is obtained from the atmosphere. The intracellular fluid on the other hand carries dissolved oxygen and also has the capability of carrying away the waste metabolic products from the tissue which it perfuses. The Ringer solution just irrigates the tissue in vitro while the intracellular fluid is in a dynamic state and also carries the essential nutrients for the tissue. 8. Describe the results obtained for the afterload experiment and give physiological explanations for the shape of the load/work curve you have plotted. Ans: In the afterload experiment, the height of the contraction decreases and the latent period increases as the weight increases. The latent period increases due to lever inertia. As the muscle has to lift more loads the amplitude of contraction decreases leading to a decrease in the contraction phase and the duration of the active phase is thus decreased. 9. Describe the results obtained for the foreload experiment and give physiological explanations for the shape of the load/work curve you have plotted. Ans: In the foreload experiment, the height of contraction increases with increase in weight but within its physical limits. After the limit is reached amplitude begins to fall. The load acts on the muscle before it begins to contract thereby stretching it prior to contraction. The height of contraction increases because the stretching of the muscle in the free loaded condition increases the initial length of the muscle fibres. With increase in the muscle length prior to contraction, the interaction between the actin and myosin molecules increases but only up to the physical maximum stretch ability limits of the muscle. 10. Which type of loading enabled the muscle to perform most work? Ans: The fore loading enabled the muscle to perform most work as it was in a continuous ready state of working. 11. Explain why the muscle was able to perform more efficiently in this condition. Ans: The muscle was able to perform more efficiently because of more interaction between the actin and myosin fibres as the muscle length is already increased prior to contraction in the fore loading experiment. There is no change in the contraction time because the duration of the active stage does not change. 12. What factors limit the work a muscle can perform before it becomes fatigued? Ans: The factors leading to muscle fatigue are the depletion of the neurohumoral transmitter Acetylcholine which provides the stimulus for the muscle to contract, the formation and accumulation of lactic acid in the muscle. The initial state of the muscle is also an important factor, whether it is in a relaxed stage or already stretched. The number of actin and myosin proteins and the glycogen stored in the muscles also contribute to its ability to work prior to exhaustion. AFTERLOADING FORELOADING WEIGHT (KG) HEIGHT (METRES) CONSTANT l/L*g WORK IN JOULES WEIGHT (KG) HEIGHT (METRES) CONSTANT l/L*g WORK IN JOULES .001 .026 1.81 .00004706 0 .039 1.81 0 .002 .046 .00016652 .01 .036 .0006516 .003 .014 .00007602 .02 .035 .001267 .004 .012 .00008688 .03 .02 .001086 .005 .01 .0000905 .04 .019 .0013756 .006 .006 .00006516 .05 .016 .001448 .007 .006 .00007602 .06 .016 .0017376 .008 .005 .0000724 .07 .014 .0017738 .009 .005 .00008145 .08 .011 .0015928 .010 .005 .0000905 .09 .009 .0014661 .011 .005 .00009955 .10 .006 .001086 .012 .004 .00008688 .11 .004 .0007964 .013 .004 .00009412 .014 .003 .00007602 .015 .002 .0000543 Work done in After-loading experiment. X Axis: Work done in Joules Y Axis: Weight in Kgms Work done in Fore Loading experiment X Axis: Work done in Joules Y Axis: Weight increase in Kgms References: 1. A. V. Hill The heat-production in prolonged contractions of an isolated frogs muscle J. Physiol. 1913;47;305-324 2.. Carol A. Budd1, Russell F. Wells1, and Shelley Shreffler21 Department of Biology St. Lawrence University Canton, New York 136172 Biology Department Macalester College St. Paul, Minnesota 55105-1899 Fundamental Issues in Dissection: Muscle Physiology – A Case Study Chapter 9 3. M. J. BRIMBLE and C. T. MUSABAYANE Blood-Circulated Sciatic Nerve-Gastrocnemius Muscle Preparation in the Spinal Toad Department of Physiology University of Zimbabwe Harare, Zimbabwe, Read More