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How students receive directions for lab reports will vary from teacher to teacher and class to class. College teachers will probably expect prior mastery of basic writing skills and some familiarity with the "how-to's" of lab report writing. If in doubt, always ask for clarification. For the lab report assignment from which three selections of student writing appear below, students received a list of procedures to follow in the lab without any additional written directions for writing the report itself. (To see the list, click here. )
Regardless of how thorough or sketchy the directions, students should begin by clarifying for themselves exactly what the writing assignment asks them to do. In the lab discussed in this workshop, students were asked to illustrate the relevance of their data in frog neuromuscular studies. The study included the observation of nerve and muscle characteristics: conduction velocity, muscle twitch, fatigue, and the stimulus strength-duration relationship.
One of the first pieces of information to note was that the frog we used was quite large. It is because of this, we hypothesized, that the threshold was quite large. The muscle had good mass and responded well. Through the experiments the importance of the complexity of the muscle became apparent. The ability for it to react differently to different types of stimulus i.e. strength changing over duration of stimuli. The results for the threshold were higher to be expected, but can be explained through the size of the frog. The strength-duration relationship shown on the graph illustrates the smooth graph that was expected and taught before in class.
The fused tetni that resulted was expected to occur as it did, the only thing that needed to be acquired was the actual frequency at which the stimuli fused for our given frog. The data printed out for the different frequency clearly shows that which was to be expected, the gradual fusing of the stimuli into a single artifact.
When recording a single twitch the chart speed needed to be fast, the chart speed recorded a clear representation of a twitch. Allowing for the extrapolation of the contraction-relaxation time.
OBJECTIVE: The fundamental properties of skeletal muscle were studied by electrically stimulating the gastrocnemius muscle of a frog contract. The effects of stimulus strength and frequency show what occurs when variables are manipulated.
DISCUSSION: The experiment demonstrated many of the fundamental properties of skeletal muscle. The results varied from expected observations for unknown reasons. There were also problems with the apparatus during the course of the experiment that need to be noted.
The value obtained for maximal stimulus strength was three times as high as the expected value of 10 volts. This could have been accurate; however, it is probably due to error caused by handling.
Temporal summation occurred when the frequency of stimulation was increased. The force of the twitches observed, both fused and unfused, did not increase as was expected. They did summate, but the force of the contraction decreased. This is most likely due to slipping in the system. Either the femur or string must have been moving which caused the tension to be recorded inaccurately.
The experiment shows how stimulation causes more force than resistance on the muscle and therefore a contraction is obtained. As the amplitude or duration of a twitch is increased, there are more motor units activated. At the biochemical level, the interaction of myosin and actin explains the amount of force produced by a contraction. The greater number of interactions corresponds to larger amount of force. These basic properties of skeletal muscle were observed and studied in this experiment.
OBJECTIVE: To demonstrate how direct electrical stimulation of the gastrocnemius muscle can cause contraction. The contraction of the muscle then allows observations to be made about various stimulous strengths, durations, frequencies and contraction and relaxation periods.
DISCUSSION: The muscle, similar to the nerve, has a particular threshold that must be reached in order for the reaction, a contraction, to occur. It also is composed of many individual muscle fibers that all have different levels of excitability. Therefore a minimal stimulus will create a weak contraction because not all the fibers have reached threshold. As the stimulus increases, the number of fibers recruited has increased so the contraction is stronger. Furthermore, like the nerve there is a particular point, maximal voltage, where that force of the contraction does not significantly increase with the stimulus voltage. This is a sign that all muscle fibers have been recruited, or stimulated.
The strength-duration relationship is an inverse exponential relationship, meaning as the duration is increased, the strength of the stimulus needed to cause a twitch decreases.
The effect of frequency on the muscle demonstrates temporal summation. Increasing stimulus frequency ultimately results in a fused tetanus. As shown in figure #3 when a certain frequency is reached the separation between each stimulus is no longer visile and a plateau-like signal is recorded. This fused tetanus occurs because the fibers do not have enough time to relax before they are stimulated again. This is due to the accumulation of free calcium that cannot be sequestered back into the sarcoplasmic reticulum quickly enough.
The final part of the experiment was measuring the times of contraction and relaxation periods. After recording a twitch, the time from the beginning of the upstroke to the peak is the contraction period and then from the peak back to the baseline is the relaxation period. Because of the equipment we were using, we were not able to label or calculate the latent period. This is the time between the point of stimulation until the contraction actully begins.
Fortunately, we did not run into any recording or technical problems. We were fortunate to have a good enough muscle and preparation to get through almost all of the required data.