Muscle Fatigue
The purpose of the muscle function lab is to become familiar with basic properties of the skeletal muscle: such as types of contractions and muscle twitch. The main method behind this lab is to dissect a gastrocnemius muscle of a frog and stimulate it. Various results will show a threshold stimulus and a maximal stimulus. Contraction length, summation, tetanus, and muscle fatigue are also shown in this specific lab. Also, through this electrical stimulation we can visually see the relationship between length and tension with the presence of an isotonic and isometric contraction. Chemically, calcium ions have an effect on muscle contraction. A prolonged stimulation of the muscle, causing muscle fatigue, as we learned also affects muscle contraction. In this experiment we used a physiologic stimulator. This device delivers something like an electric shock directly to the frog’s gastrocnemius muscle. A force transducer was used to convert mechanical movements produced into a form of electrical signals. In this particular experiment we used a string to help measure the force of the muscle contraction, and with the help of the force transducer we were able to see this movement in electrical signals. The MacLab/4 is computer hardware that we used to help display the data in which we encountered. And lastly we used a Macintosh Computer to read and analyze the data that was found with the help of the MacLab/4 program.
The second part of this experiment was a direct stimulation of the human forearm muscle and induction of finger movements. Once again, this involved stimulation of the muscle with a direct electrical current. We placed the electrical stimulator on the flexor digitorum sperficialis muscle of the forearm. When we did this experiment we found the minimal electrical stimulus required to achieve a movement and contraction of the FDS. During this experiment there was a time from when the electrical charge starts to move through the sarcolemma to the time when the muscle contraction begins. The brief period of time we learned is called the latent period. We also found that there is a greater latent period in the human than the frog. In this experiment we used a stimulator, which is the same device as earlier stated. The McADDAM is part of the computer, which moves the signals from the Physiogrip trigger to electronic signal. With these electronic signals, the computer is able to recognize what is happening with the muscle movement. The Physiogrip Transducer is shaped like a pistol trigger and the twitching finger will be placed on and the transducer will help display the mechanical force to an electrical signal for the Macintosh computer to read. Therefore, in both labs we were able to determine an intensity-tension relationship, length-tension relationship, and frequency-tension relationship.
This lab was an excellent source to learn about how the muscular system functions and helps us understand how some muscular disorders come into play in our lives.
For the physiology mid term paper, we are focussing on the muscle function lab. In this lab, we became familiar with skeletal muscle, by isolating a frog’s gastrocnemius muscle. As a class, we learned about the use of direct electrical stimulation of muscle, the relationship between intensity-tension, muscle-length, and frequency-tension, stimulus, and muscle fatigue.
When an area of the body is stimulated, the message is sent to the brain by an electrical signal. This happens due to the nervous system. It is made up of nerves, “which connect the central nervous system with the periphery” (Germann 782), and neurons, which are cells of the nervous system. These cells have many parts, which aid in the transmission of messages throughout the body. The soma (cell body) is where the nucleus is found. The dendrites branch out from the soma, and form synapses with other neurons. This is important because this is how other neurons receive messages. The axon of the neuron sends the information down the whole cell. It is mylenated to speed up the process. This is why it does not take a very long time to feel a pain sensation, or to move a muscle. Between the mylinated portions of the axon are nodes of Ranvier. These areas of the cell are the places that the neurons are able to become excited, and make an action potential. This simply means that sodium ions flow into the cell, depolarizing it, so that signals in the form of neurotransmitters can be sent. An action potential only occurs in excitable cells, and follows the all-or none rule. This simply means that an action potential will occur only if the neuron is stimulated to the threshold or above. If the stimulation does not reach the threshold, there will be no action potential, and no message sent. A graded potential differs from this because it does not follow the all-or-none rule. Graded potentials are smaller then action potentials and result from the opening or closing of an ion channel. A single graded potential will not elicit a response or action potential (Germann 186). When they overlap, however, they can add together. This can happen in two ways. In temporal summation, “stimuli are applied in such rapid succession that the graded potential from one stimulus does not dissipate before the next graded potential occurs” (Germann 186). Spatial summation is closely related to temporal summation, except is occurs in when different synapses are stimulated simultaneously.
Ion channels are places on the neuron where sodium (NA+) flows in to the cell, or potassium (K+) flows out. These channels open when they see a change of voltage along the neuron. As this happens the Na+ channels on the muscle open so the acytlecholine can bind to its receptors. When the distribution of these ions change, the cell either becomes depolarized or hyperpolarized. When a neuron has been depolarized and begins to be repolarized, it enters an absolute refractory period, in which any stimulus will not elicit a response. This is because the neuron has reached its maximum threshold. After repolarization, there is a chance that the action potential may be excited again. This happens during the relative refractory period. In this period, an extremely large stimulus may elicit a response.
The body has many different types of neurotransmitters that have an array of functions. Some are excitatory, and others are inhibitory. As stated before, when a neuron becomes excited, Na+ flows into the cell, depolarizing it. Inhibitory neurotransmitters have the opposite effect, and inhibit neuron activity. They are released when the neurons are hyperpolarized, and an influx of K+ flows out of the cell (Kapit, 87). In the nerve lab, the neurotransmitters that were being tested were acetylcholine and norepinephrine. Acetylcholine can be found at the neuromuscular junction. It is involved with the contraction and twitch of muscles. When added to the muscle, it should increase the contraction. Norepinephrine can be found in the nerve, and when added to the nerve it should also increase muscle contraction. When acetylcholine is released, it binds to the acetylocholine receptors on the post-synaptic neuron. This is how the neurotransmitters are sent from the brain to the muscle. As acetylcholine enters the synaptic gap, acetylcholineserase breaks it down into acetate and choline, in order for it to be able to bind to its receptors. This is important because without all of these chemicals muscles would not be able to contract.
As the frequency was increased, the muscle went through summation. The twitches added together until they reached their maximum, and tetanus was achieved. This is because the calcium ions did not have time to go back into the Sarcoplasmic Reticulum and restart the entire process. The sarcomeres are the smallest unit of muscle cell that are separated by elastic fibers called z-lines. We can find within each sarcomere two typed of myofilaments. These myofilaments are positioned along the sarcomere’s length. Thick myofilaments are made up of myosin, and thin myofilaments are made up of actin molecules. A sliding movement of the thin filaments with the thick filaments causes contraction. This results in the shortening of the sarcomere.
This act of the muscle tissue being stimulated directly with electric shocks will then send a release of calcium ions and activate the muscle’s contraction. A twitch of the muscle shows a single muscle fiber or a whole muscle fiber activated.
The skeletal muscle is a fixed muscle attached to bones and skin and it is responsible for all voluntary and involuntary movements. The skeletal muscles are controlled by the somatic nervous system (Vander, 288). The gastrocnemius is a whole skeletal muscle, comprised of many single muscle fibers. “The gastrocnemius muscle is one of the frog’s extensor muscles and it aids the frog in its jumping movement” (Meisami, 100). It is possible to do the same experiment on a single muscle fiber, as well as the whole muscle fiber.
In the experiment we dissected the gastrocnemius muscle, attached it to the force transducer, and studied the effects of the intensity-tension relationship first. While doing this part of the experiment, we had to conclude what was going to happen to the tension amplitude when we raised the voltage. In the next part of the experiment, we tested the relationship between length and tension. While doing this part of the experiment we studied the effects of what would happen to the tension when the length of the muscle was increased. The last relationship we tested in this lab was the relationship between frequency and tension. In this part of the experiment, we determined what would happen to the muscle tension when the frequency of stimulation was increased. One is able to learn by looking at the graph on page 115, in the Laboratory Manuel, that muscle tension gradually increases with increasing frequency of stimulation. The highest frequency that we were supposed to test our gastrocnemius muscle at is 16 Hz. Therefore, we can conclude that at 16Hz the state of muscle contraction is called tetanus, meaning the contraction becomes continuous (Meisami, 114). The last thing that we tested in this experiment was muscle fatigue. “When a skeletal muscle is repeatedly stimulated, the tension developed by the fiber decreases even though stimulation continues” (Vander, 309). This stimulation in a muscle can resolve in muscle fatigue. We tested muscle fatigue by increasing the frequency of stimulation until tetanus started to occur. After tetanus was occurring in the muscle for a short time, the muscle tension decreased. This decline in muscle tension brought about muscle fatigue.
The general objectives of the muscle function lab are “to familiarize you with some of the basic functional properties of the skeletal muscle. By use of a frog’s gastrocnemius muscle, calf muscle, you will learn the properties of muscle including muscle twitch and maximal contraction, the isometric and isotonic types of contraction and the relationship between length and tension. Summation of muscle twitches, occurrence of muscle tetanus and muscle fatigue and the effects of calcium ions on muscle contraction will also be studied” (Meisami 116). Furthermore, there are a few specific objectives listed in the lab manual or found below. These include understanding procedures for dissection and isolation of the frog and its muscle, understanding the use of direct electrical stimulation, understanding the concepts of isometric and isotonic forms of muscle contraction, study the different relationships, and study muscle fatigue after prolonged stimulation (116).
The Muscle Function Lab involves the usage of three different pieces of scientific equipment. These three scientific pieces include the physiologic stimulator, force transducer, and MacLab/4-Chart. The physiologic stimulator works to provide electric stimuli at various strengths that are measured in voltage, durations that are measured in milliseconds, and frequencies that are measured in hertz. The force transducer works to convert the movements into electrical readings. The string that is attached by the experimenter to the frog’s gastrocnemius aids this. Finally the MacLab/4-Chart takes the electrical readings and forms them on a chart onto paper. In the first part of the lab dealing with the intensity-tension relationship, the voltage delivered to the frog’s gastrocnemius muscle is changed to a higher voltage to help determine threshold intensity and maximal intensity.
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