Abstract:
Skeletal muscle's impressive property of elasticity and contractibility brings forth the power for us to move around and interact with the world all around us. In this test we studied the contractile behavior of the frog's gastrocnemius muscle to be able to extrapolate the studies and gauge an idea of perhaps the way the individual skeletal muscle's properties. The first part of the this experiment dealt with calculating the muscle twitch contraction in presence of an individual electric stimulus of increasing level beginning with 0 volts to 5 volts with 0. 25 volt increments. With each increment in stimulus intensity, the contractions were more powerful till 2. 25 volts-after which any further increase in power had no matching upsurge in contraction. The next area of the experiment investigated how the muscle contraction occurs in occurrence of multiple stimuli close with time to the other person with constant 2 volt stimulus. The happening of summation and tetanus were witnessed at frequencies of 4 and more hertz, with complete tetanus happening at 30Hz.
Introduction:
The skeletal muscle system is an essential and quality area of the vertebrates. The key benefit of this system is that it offers ability to go in addition to cover, defence and aggression (to victim, for example). The skeletal muscles are composed of skeletal muscles cells (also known as muscle fibres). The skeletal muscles are voluntarily managed by anxious system using neuronal innervations of the muscle fibre. Skeletal muscle fibres are long, slim, includes several nuclei nearby the sarcolemma (the plasma membrane of the muscle cells) (Hoehn & Marieb, 2007). The muscle fibres are in turn manufactured from myofilament composition of actin and myosin, which is the useful device of muscle fibre known as sarcomere (Hoehn & Marieb, 2007). The actin and myosin filaments slide against each other to form contraction of sarcomeres leading to muscle contraction in existence of a power stimulus which in turn causes action potential over the muscle fibre. The action potential sets off the discharge of Ca2+ ions from sarcoplasmic reticulum (Moyes and Schulte, 2008). The Ca2+ ions bind to troponin which in turn causes change in tropomyosin setting and exposes the myosin bind sites on actin filament. The binding of myosin on actin combined with the usage of ATP molecule triggers myosin head to move on actin to cause electric power heart stroke (Moyes and Schulte, 2008). Combination of such power strokes are the functional basis of contraction observed in the whole muscle (Lombardi et al. , 1992).
The amount of pressure generated is dependent on the distance of the muscle fibre and the strain placed on it. There can be an optimal insert which extends the muscle at an best size, allowing the muscle to perform the maximum work (Wassenbergh et al. 2007). At the optimal duration muscle, in sarcomeres, the actin and myosin cross-bridging is in a way that neither excessive overlapping nor inadequate from it occurs to ensure sufficient contractile space (Moyes and Schulte, 2008). Hence extended muscle will have better potential for producing higher contractile force than unstreched muscles as stretches could cause fibres to be put optimally at optimal lengths in comparison to an unstreched muscle (Dou et al. 2008).
There are two types of muscle contractions: Spatial and Temporal. Spatial summation of the twitch contractions escalates the amplitude or durability of contractions by recruiting more fibres (Staud et al. , 2007). Higher strength causes increased amount of Ca2+ to be released and therefore increasing variety of crossbridges being made (Kargo and Rome 2008). Temporal summation is achieved by increasing consistency of the excitement which causes upsurge in amplitude of contraction and successive reduction in relaxation stage (Staud et al. , 2003).
The purpose of this test is to look at response of the muscle by changing parameters of durability and regularity of stimulus. The latency, contractile, relaxation stages and amplitude of the twitch is the information that may allow us to comprehend muscle response with regards to parameters of stimulus. The muscle may also be tested to see if there is spatial and temporal summation in conditions of its contractions.
Methods and Materials:
Physiology Laboratory 1: Skeletal muscle lab manual was followed to conduct the experimental steps. There have been no deviations or modifications made from the prescribed laboratory method as given in the lab manual.
Results:
Part 1: Stimulus Response
Table 1: Muscle response in presence of stimulus in terms of its twitch amplitude, contraction, rest and latency periods
Stimulus
Amplitude
(V)
Muscle Twitch
Amplitude
(mV)
Contraction time
(ms)
Relaxation time
(ms)
Latency time
(ms)
0
0
0
0
0
0. 25
0
0
0
0
40
55
65
0. 75
45
50
75
49
50
90
1. 25
55
55
100
1. 5
63
50
115
1. 75
66
60
125
73
55
130
2. 25
73
55
135
2. 5
75
60
135
15
2. 75
76
55
140
75
55
145
3. 25
70
60
135
15
3. 5
76
60
125
15
3. 75
75
60
145
15
76
65
135
15
4. 25
72
60
125
15
4. 5
75
60
140
15
4. 75
76
60
160
15
78
65
175
In this stand, muscle twitch amplitude, contraction time, leisure time and latency time is assessed for each particular stimulus voltage increment of 0. 25 volt beginning with 0 volt to 5 volt. After each stimulus, there is a lag or wait period of about 20-30 seconds before another stimulus is delivered to the muscle to permit the muscle to get back its contractile power. Amount 1 is representative of muscle twitch in existence of a single stimulus of 2V. The latency period is the period from the initiation of the electric stimulus till the muscle contraction initiation. The contraction period is from the initiation of muscle contraction to the peak of muscle contraction accompanied by the relaxation period till the initial muscle position is achieved.
Figure 1: Muscle Twitch & its relevant phases (intervals)
Figure 2: Muscle twitch regarding increasing stimuli with 0. 25 V increments from 0 to 5 volts.
Figure 3: Contraction time Vs Stimuli
Figure 2 and 3 represents that with increase in stimulus, there can be an increase in twitch amplitude and contraction time.
Part 2: Summation and Tetanus
Table 2: Stimulus with differing frequency of application but continuous volt and its influence on amplitude and passive tension
Stimulus
Frequency
(Hz)
Amplitude
1st twitch
(mV)
Maximum
Amplitude
(V)
Change in
Passive stress (V)
Summation/
Tetanus
0. 5
68
0. 071
0. 001
No
1
70
0. 072
0. 001
No
2
74
0. 074
0. 005
No
3
68
0. 072
0. 006
No
4
73
0. 073
0. 010
Summation
5
74
0. 078
0. 022
Summation
10
75
0. 081
0. 069
Incomplete tetanus
20
90
0. 098
0. 081
Incomplete tetanus
30
97
0. 108
0. 090
Complete tetanus
Figure 4: Mechanical Summation
Figure 5:Incomplete Tetanus at 2V-20HzFigure 6: Complete tetanus at 2V-30Hz
When several stimulus was delivered at a rate of 5 Hz as in Body 2, the muscle summation was detected relaxation phase was much shorter. With increasing speed of administration of the stimulus, the leisure phase further decreased to give imperfect and complete tetanus as seen in amount 3 and 4 respectively. This data are displayed in Stand 2 where increasing stimulus consistency till 3 Hz got no summation or tetanus; 4 and 5 Hz showed summation evident from its change in passive anxiety and at higher frequencies the tetanus was seen (imperfect at 10 and 20 Hz; complete at 30 Hz).
Discussion:
The muscle twitch is the basic product of contraction (Moyes and Schulte, 2008). The twitch observed in our experiment occurs via electrical stimulus administered using an electrode. This stimulus produces the same action potential in the muscle mass as neuronal excitation would via electric motor end dish (Hoehn and Marieb, 2007). As a neuron requires a certain threshold for an action potential to be made, muscle, too, needs a certain amount of depolarization before it can flame an action potential across the complete tissue. This also has its biological profit as energetically it might be expensive for muscles cannot afford to written agreement at every single stimulus. As observed in figure 1, a single contraction-relaxation twitch has three phases: Latency, Contraction and Leisure. Latency period is the stage between your initiation of the electric stimulus and the initiation of the muscle contraction. The occurrence of this lag or wait in muscle contraction can be explained by the time needed for the excitation-contraction (EC) coupling to take place (Moyes and Schulte, 2008). EC coupling is an activity involving the accessibility of Ca2+ in to the muscle fibres following a stimulus, and activation of actin-myosin cross-bridges (Moyes and Schulte, 2008). The latency period of the muscle with solo stimulus delivery is between 15msec to 20 msec (refer Table 1).
Contraction phase begins by the end of latency period when the action potential goes by across the muscle fibre and through the T-tubules releasing the calcium stores from the sarcoplasmic reticulum into the sarcomere (Moyes and Schulte, 2008). The release of calcium causes troponin to improve the conformation of tropomyosin and unbinds the actin brain for myosin to add (Moyes and Schulte, 2008). Simultaneously, the myosin or the solid filament uses ATP to get into high energy conformation from low energy conformation. The binding of myosin to actin head and use of ATP means that power stroke can take place-moving the actin and myosin across each other and shortening the sarcomere. The mixed effect of such shortening of sarcomeres in the muscle fibre triggers collective shortening of the muscle. As long as there is availability of calcium in the sarcomere, the muscle will stay contracted scheduled to actin myosin binding.
Relaxation phase begins by the end of contraction period when the calcium sequestered in the sarcomere is re-taken in to the sarcoplasmic reticulum via dynamic transportation by Ca2+-Na+ antiporter (Kargo and Rome 2008). The lowered level of calcium in sarcomere causes tropomyosin to job application it's point out of covering the actin binding site and the sarcomere lengthens because of unbinding of myosin and actin filaments. Enough time needed for most of the calcium re-uptake is reflective of the lengthening of the sarcomere to its normal length-this time needed reflects as leisure period of the muscle fibre (Kargo and Rome 2008).
Increasing the stimulus heightens muscle contraction till 2. 5 V. Raising stimulus above 2. 5 V had no additive effect on the muscle contraction. This is explained at the sacromere level with the muscle level. At low levels of stimulus, e. g. 0-1V, the depolarization is temporary. However, whenever there are high levels of stimulus, ie above 2. 25-5 V, the depolarization is long lived and calcium release is large, which is why rate of rest for the solitary high voltage stimulus is increasing. Moreover, with high voltage, the number of muscle fibres employed for the contraction increases. Hence, using high stimulus single supervision brings forth better, serious contractions. However, muscles has a certain degree of contractility due to the physical barrier of actin-myosin crossbridging (Hoehn and Marieb, 2008). Hence, muscle cannot indefinitely long term contract at higher stimulus.
Summation of the contraction occurs when there are more than one stimulus given so close in time that the preceeding leisure is overlapped with the stimulus to deal again. Such stimuli makes the relaxation period of the muscle twich shorter and the contractions more powerful (Staud et al. , 2007). This is seen when 2 volt stimulus were given at 4-20Hz where contractions superimposed over each other creating summation and unfused tetanus as seen in physique 4 and 5. At 30 Hz, the contractions are at maximum potential and leisure is nearly zero. This is actually the condition of fused tetanus as seen in physique 6.
The trend of summation and tetanus can be explained by the contraction initiated by the action potential before the relaxation began. That is, prior to the calcium re-uptake, a novel stimulus appeared that depolarized reticulum further release a calcium and cease the uptake (Kargo and Rome 2008). Prolonged presence of calcium in the fibres carried out the contraction to the fullest and resulted in decreased leisure period. Within the summation and additive impact whereby the second circular of contraction builds on the first circular of contraction or the incomplete relaxation; in doing so increasing the strength of contractions. However, as discussed earlier, there is a limit to the maximum capacity of the muscle to contract as a result of physical limitation of the actin-myosin crossbridging-after maximal crossbridging, further excitement of the muscle will haven't any increase in contractile power.
The results obtained via this experiment provided a useful insight into how the muscle twitch contraction occurs. It provided useful and quantifiable data to comprehend about latency, contraction and leisure period, summation and tetanus of the skeletal muscle fibre. The contraction strength increased as the stimulus strength increased. Mechanical summation eventually fused to create the unfused and fused tetanus and produced a single stronger contraction for long term time frame.
