Skeletal Muscle Contraction
BISC 316-010
Lab Group 1
Performed on: 9/18/13
Authored by: C. Posillico, et al.
Results
Experiment 1: Single Muscle Twitch
The single muscle twitch was observed in the isolated gastrocnemius muscle of a frog
through both direct and indirect stimulation. For direct stimulation, a muscle preparation was
used in which two pin electrodes were placed through each side of the muscle, and the force of
isometric contraction of the muscle was observed over varying stimulator settings as set with
PowerLab software. The threshold stimulus was determined by first setting the stimulator to
0.0V, stimulating the muscle, and incrementally increasing the voltage by 0.10V until a response
was recorded with the software. The threshold stimulus was recorded at 0.20V. Further
increasing the stimulation voltage caused a larger force of contraction by the muscle as noted by
increasing amplitude of the spikes on the program. The maximum response stimulus was
recorded to be 1.0V when the force of contraction did not increase after stimulating the muscle
beyond 1.0V.
For indirect stimulation, a nerve-muscle preparation was used in which the sciatic nerve
was stimulated instead, and the force of contraction of the gastrocnemius muscle was recorded
with the same software. Identical methods of determining the threshold stimulus were used for
the indirect stimulation, and it was recorded to be 0.36V. As seen in Table 1, the threshold
stimulus for direct stimulation was lower at 0.20V than the threshold stimulus recorded for
indirect stimulation at 0.36V.
The time courses of the single muscle twitch at maximum response stimulus for both the
direct and indirect methods of stimulation were determined using the Zoom Window function of
the PowerLab software. The three phases observed in the single muscle twitch were latency,
contraction, and relaxation. The starting point of stimulation was defined as t = 0 seconds on the
x-axis, and the force of contraction of the muscle was recorded and measured in grams on the yaxis over time.
For direct stimulation of the muscle, it took 0.012s from the point of stimulation to see an
increase in force of contraction which identified the latency. The time between this initial
increase to the peak at 71.98g of the force of contraction was 0.064s which marked the
contraction phase. Finally, the time between the peak of the force of contraction and its decrease
back down to 0.0g was 0.157s which defined the relaxation phase.
The latency, contraction, and relaxation phases of the muscle contraction under indirect
stimulation were determined in the same way. Respectively, they were recorded to be 0.010s,
0.047s, and 0.174s. As seen in Table 2, the latency and contraction phases with direct muscle
stimulation were longer than those from indirect stimulation, but the relaxation phase with direct
muscle stimulation was shorter than the corresponding phase with indirect stimulation.
Table 1. Threshold Response Stimuli for Direct and Indirect Stimulation of the Frog
Gastrocnemius Muscle.
Type of Stimulation (Preparation)
Threshold Response Stimulus (Volts)
Direct Stimulation (Muscle Preparation)
0.20
Indirect Stimulation (Muscle-Nerve Preparation)
0.36
Table 2. Duration of the Individual Phases of Muscle Contraction for Direct and Indirect
Stimulation of the Frog Gastrocnemius Muscle.
Type of Stimulation
(Preparation)
Latency (seconds)
Contraction (seconds)
Relaxation (seconds)
Direct Stimulation
(Muscle Preparation)
0.012
0.064
0.157
Indirect Stimulation
(Muscle-Nerve
Preparation)
0.010
0.047
0.174
Experiment 2: Length-Tension Relationship
In Experiment two, the length-tension relationship of the frog gastrocnemius muscle was
observed during isometric contraction. The length-tension relationship is the correlation between
the length of a sarcomere of the muscle and the force of contraction it produces. An isolated
gastrocnemius muscle was attached to the myograph (force transducer) with a string, and the
force of muscular contractions after stimulation at maximum response voltage (1.0V) was
recorded with the PowerLab software over a series of different sarcomere lengths. The sarcomere
length was stretched incrementally with successive ⅛ turns of the height dial of the force
transducer, which consequently raised the height of the baseline on the software. This change in
baseline determined the pre-load force of tension and was measured in grams. Contraction force
of the muscle after stimulation at different pre-loads was also measured in grams. Starting from
baseline of 0.0g, the first pre-load was measured at 4.91g, and the subsequent force of
contraction after stimulation was 188.90g. As sarcomere length increased, so did the force of
contraction (Figure 1). The transducer was raised to increase the pre-load tension and sarcomere
length until a decrease in the resulting force of contraction was observed. The peak tension of
234.8g occurred at a pre-load of 12.85g.
250 Force of Contraction (g)
240 230 220 210 200 190 180 170 160 150 0 2 4 6 8 10 12 14 Degree of Stretch (g)
Figure 1. Effects of Sarcomere Length on Force of Contraction of Frog Skeletal Muscle.
The effect of stretching the sarcomere on the force of contraction of frog skeletal muscle was
recorded using PowerLab and "Chart” software. The degree of sarcomere stretch was determined
by raising the force transducer to which the gastrocnemius muscle was attached and measuring
the resulting changes in baseline on the software in grams. The resulting force of contraction of
the muscle at maximum response stimulation was also measured in grams.
Experiment 3: Summation and Tetanus
Experiment three sought to determine the relationship between the frequency of
stimulation and the resulting contraction force of the frog gastrocnemius muscle. Summation,
incomplete tetanus, and complete tetanus were three features of muscle contraction that were
observed by gradually increasing the frequency of stimulation from 1 to 20 Hz. At 1Hz, the peak
of contraction force for a single twitch was recorded at 71.98g. The stimulation frequency that
first induced summation was determined by increasing the frequency until the peaks of the force
of each contraction per stimulation began to increase above 71.98g, and at which point the
muscle did not completely relax and return to the baseline between stimuli. Summation was
observed in the muscle at 2.3Hz. Incomplete tetanus was observed when the summated force of
contractions first showed signs of plateauing just below 7Hz. Complete tetanus was finally
observed when the force of contraction fully plateaued so that decrease in tension between
stimuli could not be detected. Complete tetanus was induced at 11Hz and caused a stable force of
contraction of 100.06g, which was much greater than the force of contraction of a single twitch,
aforementioned at 71.98g.
Experiment 4: Muscle Fatigue
In order to observe the onset of fatigue, the stimulator was set to complete tetanus as
determined in the previous experiment to be 11Hz. The plateaued force of contraction at 100.06g
did not decrease after the first five seconds of stimulation at this frequency, so frequency
stimulation was further increased until there was a detectable drop in force, which occurred at
14.5 Hz. After 24.6s of stimulation at this frequency, the force of the muscle contraction was
observed to be 50% less than the original measured contraction force of 100.06g.
Discussion
The prediction for the single muscle twitch experiment hypothesized that there would not
be a contraction response in the muscle until a threshold voltage was reached, at which point the
force of contraction would increase with further increases in voltage stimulation until a
maximum response was obtained. Results from this experiment support this hypothesis with a
recorded threshold response stimulus of 0.20V and a maximum response stimulus of 1.0V at
which point further increases of the voltage stimulation did not further increase the force of
contraction of the muscle. The prediction also postulated that direct stimulation of the muscle
would result in a lower threshold response stimulus than indirect stimulation. The results
reinforced this with a 0.20V threshold response stimulus for direct stimulation in comparison
with a 0.36V threshold response stimulus for indirect stimulation.
However, this prediction also hypothesized that direct stimulation of the muscle with
muscle preparation would result in shorter durations for the latency, contraction, and relaxation
phases of muscle contraction than indirect stimulation of the muscle with nerve-muscle
preparation. This hypothesis was not supported with the data; instead, the latency and contraction
phases of indirect muscle stimulation were shorter than those of direct muscle stimulation.
This data was unexpected because for direct stimulation, electricity only had to travel
from the stimulating electrodes directly into the muscle for it to be stimulated. On the other hand,
for indirect stimulation, the electricity had to travel from the electrode to the sciatic nerve which
then had to conduct the current over a longer distance before reaching the gastrocnemius muscle.
The distance that the stimulation had to travel alone should have resulted in a longer latency for
indirect stimulation, but instead it was recorded to be 0.010s in comparison to the direct muscle
stimulation latency which was recorded to be 0.012s.
Several factors could have contributed to this discrepancy, possibly the most influential
of which being that different frogs were used to obtain the results for the different muscle
preparations. Using two different frogs could vary the size of the gastrocnemius muscle as well
as the length and width of the sciatic nerve which would alter conduction velocity. Further, two
different force inducers, computers, and groups of researchers were used to obtain this data
which further increases the chance for error and incongruity between experiments that could
explain the unexpected results.
Additionally, the prediction for the length-tension relationship experiment proposed that
as tension on the muscle increased, force of contraction would increase until a point at which
additional tension on the muscle would cause a decrease in the force of contraction. The line
graph shown in Figure 1 most directly supports this hypothesis, graphically depicting just what
the prediction stated.
Finally, the prediction concluded in hypothesizing that increasing the frequency of
stimuli on the muscle would cause contractile force to summate and increase to a maximum
point without relaxation and remain there for a period of time before slowly showing decay in
force. The results from experiments three and four explain the occurrence of the phenomena of
summation, tetanus, and eventual fatigue of the muscle and the respective frequencies at which
each were observed.
Fortunately, the hypotheses for the majority of the experiments were supported by the
gathered data, and those that were not could be as a result of numerous uncontrolled variables
that may have ultimately rendered the comparison of that particular data unreliable.