Solve all 5 questions in first two pages using the data listed ?? Frog Gastrocne
ID: 3512860 • Letter: S
Question
Solve all 5 questions in first two pages using the data listed ?? Frog Gastrocnemius Muscle Lab Report Name Graphs: Make sure you collect data to construct the following graphs. Part 1.Stimulus (volts) vs Tension developed (grams) Part 2 Stimulus frequency (Hz) vs Tension developed (grams) Part 1. Twitch Responses Threshold volitage 1V Maximal voaeV Force generated ~ S Duration of twitch contraction cuo C Latent period (time from stimulus to twitch) 0.1( latent period a Force generated o a Duration of twitch contraction ase c sac 1. Describe the events taking place that result in the "latent period," the time from stimulating the nerve to muscle contraction. 2. What do you think is happening as you increase voltage from threshold to the maximal voltage? Why does the force generated by the whole muscle increase? (Remember the sciatic nerve is a bundle of axons).
Explanation / Answer
1. A muscle twitch has 3 phases: 1) the latent period; 2) the contraction period; 3) the relaxation period. The latent period is the time between the initiation of electrical stimulus and the initiation of muscle contraction. It is required for electrical coupling to occur. In electrical coupling, the entry of Ca2+ into the muscle fibers activates actin-myosin cross-bridges.
i) action potential moves across the sarcolemma.
An impulse from the nerve reaches a motor unit through the motor neuron at the neuromuscular junction. The nerve releases calcium into terminal buttons that results in release of acetylcholine (Ach). Ach bind with the receptors on the sarcolemma and opens up the sodium channels in the sarcolemma, resulting in a change in resting potential. The sarcolemma will depolarize.
b) Depolarization of sarcolemma traverses down the muscle fiber via the transverse tubule system. As a result, ryanodine receptors open. Now, the sarcoplasmic reticulum release calcium into the cytoplasm. This action increases calcium level in cytosol. Calcium binds to troponin C, and removes tropomyosin that was blocking the binding sites on actin. There is a change in troponin, which results in shifting of trophomyosin and active sites on actin are unblocked. This will allow myosin heads to involve in cross bridge formation. The muscle is now ready for cross-bridge formation.
2. When the intensity is increased beyond the threshold, the muscle contraction will increase. Threshold is the level of stimulation required to trigger the smallest measurable contraction. This is a result of excitation and contraction of the first few muscle fibers. However as the stimulus reaches a particular maximal level, muscle contraction will remain constant. This stimulus is the maximal stimulus where all of the fibers in the muscle are stimulated. These muscles respond in all-or-none fashion. Supramaximal stimuli are stimuli above the maximal stimulus while subthreshold stimulus is stimulus below the threshold where there is no mechanical response.
The stimulation from 0.13-0.24 V increased the muscle tension from 11-20.5 g. Muscle tension is affected by sarcomeres length, frequency of stimulation, motor unit size, and recruitment of motor unit. It is the force generated by the contraction of the muscle. As the stimulus increases, the number of motor units stimulated increases. Each motor unit has a motor neuron associated with a number of muscle fibers. When a motor unit is stimulated, all the motor fibers associated with it will respond. The sciatic nerve is a bundle of axons. As the voltage is increased, more and more impulses are generated by stimulation of the axons. Hence, the frequency of stimulation is also increased. The impulses will reach the neuromuscular junction faster and release increasing amounts of acetylcholine. There are more motor units recruited as stimulation increases with more axons being involved as voltage increases. As a results, more motor fibers get stimulated and the force generated in the muscle increases.
3. An increase in the frequency of muscle stimulation increases the strength of contraction. Wave Summation occurs when one more than one stimulus administered close in time. As a result, the relaxation of the preceding stimulus overlaps with the stimulus of the next contraction. Hence, the relaxation phase of the muscle twitch becomes shorter. This causes the contractions to become more powerful.
When the stimulus frequency was increased (as voltage increased), the contraction of first stimulus starts merging with the contraction of the next and so on. As a result, the relaxation starts to decrease with increasing voltage. Due to wave summation as frequency went on increasing, the force will increase due to less and lesser relaxation phases. The strength of the contraction depends on frequency of stimulation. Presence of calcium ions is responsible for the decrease in relaxation of the muscle.
4. Tetanus is the state when there is no relaxation of the muscle. When the strength is at a maximum rate, the myocytes will remain in a persistent contracted state, which is known as tetanus. A limit exists to the maximum capacity of the muscle to contract. This is due to physical limitation of the actin-myosin cross bridging. At above the tetanizing frequency, no further muscle stimulation will occur. At tetanus, the muscle is responding to stimulus at a high rate. When muscle is stimulated at high frequency, the calcium ions in sarcoplasm increases. When the levels of calcium in sarcoplasmic reticulum reach its maximum level, it results in tetanus tension, allowing no relaxation of the muscle.
5. Fatigue is the decline in the ability of the muscle to respond to repeated stimulus. Frequency of the impulse affects muscle fatigue. Action potentials in neurons and myocytes are all or none phenomena. They occur with the same amplitude. The frequency of the impulse affects the release of neurotransmitter. A decline in frequency will cause less neurotransmitter to be released. If the frequency of impulse decreases, there will be fatigue due to less neurotransmitter.
Secondly, the neuron and myocytes interact through synapses at the neuromuscular. If there is no replacement of neurotransmitter containing vesicle at the pre-synaptic membrane, a post-synaptic impulse in the myocytes will decrease, causing fatigue.
Several factors contribute to muscle fatigue:
1) Oxygen depletion and Lactic acid accumulation: As muscle is contracting faster, the amount of ATP required is higher. Hence, the muscle utilizes anaerobic breakdown of glucose as amount of oxygen supply to the muscle is far less than what is required. The breakdown of glucose for anaerobic respiration (due to less oxygen supply) will lead to accumulation of lactic acid. This will cause damage to muscle fibers and result in fatigue.
2) The myocytes can less responsive with prolonged stimulation by either decrease in Ca++ availability or decrease in ATP. However, calcium depletion has less severe effect that ATP depletion.
3) Creatine phosphate is broken down to provide inorganic phosphate during muscle contraction. The amount of inorganic phosphates increase which results in muscle fatigue.
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