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Skeletal Muscle Lab Introduction: Motor neurons and muscle fibers are the building blocks of motor units and where they intersect is called the neuromuscular junciton. The region where the flattened end of a motor neuron transmits neural impulses to a muscle is the motor end-plate. The end plate potentials depolarizes skeletal muscle fibers caused by neurotransmitters binding to the postsynaptic membrane in the neuromuscular junction. The process of contraction of the muscle cell is called excitation-contraction coupling.

In this experiment, when we used a single electrical stimulus, it resulted in a muscle twitch with three phases: latent period, contraction phase, and the relaxation phase. Activity 1: Figure 1: Identifying the latent period [pic] The Latent Period of time that elapses between the generation of an action potential in a muscle cell and the start of muscle contraction. The length of the latent period was 2. 78msec. When we increased the stimulus voltage from 3v to 9v and then to 10v, the latent period remained the same at 2. 78msec.

Thus, the latent period does not change with different stimulus. Activity 2: Figure 2: Identifying the Threshold Voltage [pic] The threshold is the minimal stimulus needed to cause a depolarization of the muscle plasma membrane. It is the point at which sodium ions start to move into the cell for membrane depolarization. When we stimulated the voltage of 0, the active force showed a straight line such as depicted in figure 2. We kept on increasing the threshold hold voltage until it showed a value greater than 0. The threshold voltage value was . v to have the active force be greater than 0. The graph generated at the threshold differed from the graphs generated at voltages below the threshold because the active force was greater than 0 at . 8v compared to 0v. Activity 3: Figure 3: Effect of increasing the stimulus intensity [pic] |

Table 1: Activity 3 | | | | | | | | | | |Voltage |Length |Active Force |Passive Force |Total Force | |0. |75 |0 |0 |0 | |1 |75 |0. 15 |0 |0. 15 | |1. 5 |75 |0. 43 |0 |0. 43 | |2 |75 |0. 66 |0 |0. 66 | |2. 5 |75 |0. 87 |0 |0. 87 | |3 |75 |1. 04 |0 |1. 04 | |3. 5 |75 |1. 9 |0 |1. 19 | |4 |75 |1. 32 |0 |1. 32 | |4. 5 |75 |1. 42 |0 |1. 42 | |5 |75 |1. 51 |0 |1. 51 | |5. 5 |75 |1. 59 |0 |1. 59 | |6 |75 |1. 65 |0 |1. 65 | |6. 5 |75 |1. 7 |0 |1. | |7 |75 |1. 74 |0 |1. 74 | |7. 5 |75 |1. 78 |0 |1. 78 | |8 |75 |1. 81 |0 |1. 81 | |8. 5 |75 |1. 82 |0 |1. 82 | |9 |75 |1. 82 |0 |1. 82 | |9. 5 |75 |1. 82 |0 |1. 2 | |10 |75 |1. 82 |0 |1. 82 | | | | | | | [pic] In this part of the experiment, we increased the stimulus intensity to see if it affects the muscle response. The increase in voltage increased the peaks of the tracings. It also increased the active force. The voltage beyond no further increases in active force was 8. 5v, which was the maximum voltage. There is a maximum voltage because all the muscle fibers have been activated or contracted.

This does not follow the all or none principle because the response of a nerve or muscle fiber to a stimulus at any strength above the threshold is not the same. Activity 4: Figure 4: graph of Treppe [pic] Treppe is the increase in force when a muscle is stimulated at high frequency. We have noticed that this will twitch muscles and follow one another closely, each peak will be higher than the previous as depicted in figure 4. This is known as the staircase phenomenon. We observed that the peak increases with time and each twitch. Activity 5: Figure 5: summation graph [pic] Figure 6: single stimulus rise but not fall [pic]

Twitches overlap when they are stimulated within a short period of time. The active force of our contraction was 1. 83gms. After the second twitch, the active force was 3. 10gms. Thus, there was a change in the force generated by the muscle. In addition, when we clicked on the single stimulus again and allowed it to rise but not fall before clicking single stimulus again changed the force generated from 3. 10gms to 3. 17gms. The force has changed because the stimulus was clicked before the first stimulus had a chance to fall. When we decreased the voltage on the simulator it showed the same pattern in the force generated.

When we stimulated the muscle fast, the force increased to 5. 17gms. Activity 6: Figure 7: tetanus and complete tetanus [pic] Figure 8: maximal tetanic tension [pic] |Table 2: Acitivity 6 | | | | | | | | | | | | |Voltage |Length |Stimuli/sec |Act. Force |Pass. Force |Total Force | |8. 5 |75 |145 |5. 9 |0 |5. 9 | |8. |75 |146 |5. 95 |0 |5. 95 | |8. 5 |75 |147 |5. 95 |0 |5. 95 | |8. 5 |75 |148 |5. 95 |0 |5. 95 | |8. 5 |75 |149 |5. 95 |0 |5. 95 | |8. 5 |75 |150 |5. 95 |0 |5. 95 | [pic] In a quick succession, the muscle generated more force.

At around 80msec, the plateau began to be a constant force and this condition is call tetanus as depicted in the graph. When we increased the stimuli setting to 130, it increased the plateau and it started declining. This condition was called complete fused tetanus. When we increased the stimuli to 145, at 1. 46 stimulus frequency there was no further increase in force. This stimulus frequency is called maximal tetanic tension. Activity 8: Figure 10: Isometric Contractions [pic] Table 3: Activity 8 |Voltage |Length |Active Force |Passive Force |Total Force | |8. |50 |0. 11 |0 |0. 11 | |8. 2 |55 |0. 73 |0 |0. 73 | |8. 2 |60 |1. 2 |0 |1. 2 | |8. 2 |65 |1. 55 |0 |1. 55 | |8. 2 |70 |1. 75 |0 |1. 75 | |8. 2 |75 |1. 82 |0 |1. 82 | |8. |80 |1. 75 |0. 02 |1. 77 | |8. 2 |85 |1. 55 |0. 08 |1. 63 | |8. 2 |90 |1. 2 |0. 25 |1. 45 | |8. 2 |95 |0. 73 |0. 68 |1. 41 | |8. 2 |100 |0. 11 |1. 75 |1. 86 | [pic] In this part of the experiment, we left the voltage at 8. 2v and lowered the muscle length to 50mm.

We then increased the muscle length by 10mm until we reached 100mm. Based on this graph, the muscle lengths that generated the most active force was 70mm to 80mm. At 50mm to 75mm muscle length, the passive force begins to play less of a role in the total force generated by the muscle. Based on the graph, the passive force begins to play a role in the total force at 80mm to 100mm. The graph shows a dip at muscle length 90mm because the active force was decreasing and the passive force was increasing. The key variable in the isometric contraction was the muscle length.

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