Experiment 2: Skeletal Muscle
Frog skeletal muscle is used as an animal model to study muscle contraction. The objectives of this experiment is to demonstrate the physiological responses of skeletal muscle to electrical stimuli using frog gastrocnemius, to understand twitch, summation, tetanus and fatigue, to investigate the relationship between initial tension and force of contraction, to explore the differences between human and frog skeletal muscle. The threshold voltage is 0.4V. The minimum voltage required to give a maximal response is 5V. Stimulus voltages and muscle response shows a linear relationship from 0.4V to 5V and reaches plateau at 6V. The optimal initial tension at which maximum force of contraction could be achieved is 45 g. Maximum contraction occurs when maximum number of cross-bridges are formed. The duration of twitch contraction is 233 ms, which is considerably longer than the duration of action potential (1-2 ms). Summation of twitches, subteanus, and complete tetanus occurred at frequency of 5 Hz, 10 Hz and 20 Hz respectively. The duration of muscle fatigue was longer than nerve fatigue during prolonged stimulation at 100 Hz. After addition of d-tubocurarine and the stimulation of the muscle and nerve, full muscle response was seen, but no nerve response was observed because d-tubocurarine blocks the signal pathway by competing with acetylcholine for its nicotinic receptor binding sites. Human muscle had higher threshold voltage than the frog muscle because humans have larger body mass and thus higher muscle mass. However, tetanus and summation of human muscle occur at same frequency as those of the frog muscle. INTRODUCTION
Skeletal muscles are the engines of the body. They are attached to the bones of the skeleton and so serve to produce movements or exert forces. They are essential for positioning and the movement of the skeleton . They consist of muscle fibers that contain sarcoplasmic reticulum and the t-tubule . Skeletal muscle contractions are driven by the release of acetylcholine (Ach) at neuromuscular junctions (NMJ) between motor neuron terminals and muscle fibers [2-4]. The binding of Ach with the motor end plate changes the permeability of the muscle fiber, resulting in an action potential (AP) . Once an AP is initiated, it is conducted into the muscle cell through the T-tubule, which triggers the release of stored calcium ions (Ca2+) from the sacroplasmic reticulum (SR) [1-4]. Consequently, the released Ca2+ bind to the troponin on the thin filaments and tropomyosin rolls over to uncover the binding sites on actin. Exposed actin sites attach to myosin head to form cross bridges [1-4]. The myosin begins to pull the actin filaments toward the centre of sarcomere, which is powered by energy provided by adenosine triphosphate (ATP) [1-4]. When there is no action potential, Ca2+ is actively taken up by SR and the binding sites of actin are blocked by the tropomyosin [1-4]. Thin filaments slides back to resting position and the contraction terminates [1-4].
A positive relationship exists between the tension developed by each contracting fiber and the contraction of the whole muscle [1-4]. Twitch summation is defined as the increase in tension associated with repetitive summation of the muscle fiber [1-4]. If the muscle fiber is stimulated very rapidly that it does not have time to relax between stimuli, a maximal contraction known as tetanus takes place [1-4]. Long stimulation of muscle at high frequencies will cause the muscle to fatigue, a condition in which the muscle is unable to generate the expected twitch strength [1-4]. The tension generated is relative to the number of cross-bridges breaking down ATP [1-4].
In this study, frog’s gastrocnemius muscle is stimulated electrically to study the relationship between intensity of stimulus and muscle response as well as the relationship between initial tension and contraction strength. The causes of fatigue...
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