What Is a Twitch Muscle Contraction

If necessary, the maximum number of motor units in a muscle can be recruited at the same time, creating the maximum contraction force for that muscle, but it may not take very long due to the energy required to maintain the contraction. To prevent complete muscle fatigue, not all motor units are usually active at the same time, but some motor units rest while others are active, allowing for longer muscle contractions. The nervous system therefore uses recruitment as a mechanism to effectively use skeletal muscle. In this part of the activity, the question for students is, “What happens when the electrical stimulus of a nerve powering a muscle is gradually increased?” Contraction of muscle contractions: The time between stimulation and contraction is called the latency period. After contraction, the muscle relaxes to a level of resting tension. Together, these three periods form a single muscle contraction, three phases are recognized for a muscle contraction. The first phase is the latency period, during which the action potential spreads along the sarcolemma and Ca++ ions are released from the sarcoplasmic reticulum. This is the phase where excitation and contraction are coupled, but contraction has not yet taken place. The contraction phase occurs when the muscle creates increasing tension; Ca++ ions in sarcoplasm are bound to troponin, tropomyosin has moved away from actin binding sites, bridges have formed, and sarcomeres are actively shortening. The final phase is the relaxation phase, in which the tension decreases when the Ca++ ions are pumped from the sarcoplasm into the sarcoplasmic reticulum and the muscle fibers are returned to their resting state.

In a dormant vertebrate muscle, the available intake of ATP can maintain maximum muscle work for less than a second. So the muscle has to continually replenish its ATP memory, and this happens in different ways. A mechanism for the formation of ATP works so quickly that for a long time, scientists could not detect a change in the amount of ATP in the muscle as a result of contraction. This immediate reconstruction of ATP is achieved through the reactions of compounds called phosphagens. All of these compounds contain phosphorus in a chemical unit called phosphoryl group, which they transfer to ADP to produce ATP (these compounds are also known as high-energy phosphates). When a sarcomere is stretched beyond the ideal length (more than 120%), the thick and thin filaments do not completely overlap, resulting in lower tension. If the muscle is stretched to the point where the thick and thin filaments do not overlap at all, no transverse bridge can be formed and no tension is generated. This amount of stretching usually does not occur because accessory proteins and connective tissue stand in the way of extreme stretching. Although a person may experience a “contraction” of skeletal muscles, a single contraction does not produce “useful” activity in a living body. Instead, a rapid set of action potentials sent to muscle fibers is needed for muscle contraction that can produce work. By varying the rate at which a motor neuron triggers action potentials, the amount of tension generated by innervated muscle fibers can be altered; This is called a graduated muscle response. This is an opportunity for students to review and analyze their muscle contraction recruitment records.

Verify students` understanding by asking questions such as: Question: Why does the variation in stimulus strength affect the power of contraction? The nerve impulses that ultimately lead to muscle contraction appear as an action potential on the sarcolemma, the membrane that surrounds the muscle fiber. This electrical signal is communicated to the myofilaments in the fiber in the following way. When the action potential reaches the opening of the transverse tubules (channels that open through the sarcolemma to the space outside the fiber; see above Myofibrill) on the surface of the fiber, it migrates along the tubular membranes that are continuous with the surface membrane to a fraction of a micrometer of each functional contractile unit in the fiber (Figure 7). In the frog muscle, the transverse tubules surround the myofibrils at each Z line, and in the mammalian muscles they are located at the edge of the A and I bands. In triads (the three-element complex consisting of a transverse tubule and two tanks, which are enlarged bag-shaped membranes), the transverse tubular walls are located near the membranes of the terminal tanks of the sarcoplasmic reticulum. If an additional action potential were to stimulate muscle contraction before a previous muscle contraction had fully relaxed, this would add to that previous contraction and increase the total amount of tension generated in the muscle. This addition is called a summation. In a muscle sum, it can occur through the motor units to recruit more muscle fibers, and also in the motor units by increasing the frequency of contraction. The strength developed by a muscle, whether it contracts or rests, depends heavily on the length of the muscle. Resting skeletal muscles do not exert less force than the normal length of resting muscle in the body. However, when the resting skeletal muscles are extended slightly beyond the normal length of the muscle, passive force begins to prevail. The exact length in which this passive force occurs depends on the particular muscle.

This force is characterized as passive because it is developed in non-contracting or inactive muscles by the elastic elements of the muscle. A skeletal muscle fiber produces some force when the stimulus is strong enough to reach the threshold of muscle contraction. This is called the all-or-nothing law. Let`s say we stimulate a muscle fiber electrically. We start with a small amount of stimulation that does not reach the threshold to create a contraction. The muscle fiber reacts by remaining relaxed, it will not contract. Now, when we increase the stimulation so that enough is produced to reach the threshold, the muscle fiber reacts by contraction. Finally, if we continue to increase the stimulus so that it far exceeds the threshold, the fiber will react by contracting with the same force when we have just reached the stimulus.

The muscle does not contract with greater force when the stimulus is greater. The muscle responds to stronger stimuli by generating the same strength. In skeletal muscle, a motor neuron can innervate many muscle fibers. This is called a motor unit. There are many motor units in skeletal muscles. The motor units act in a coordinated manner. A stimulus affects all muscle fibers that are innervated by a particular motor unit. Observations: Now that students have observed muscle contraction, can they explain what they see in terms of EMG and strength? Question: When the current has reached the following stages, what proportion of the fibers in the muscle has contracted?: Most of the body`s actions are the result of a combination of isotonic and isometric contractions that work together to achieve a wide range of results. These muscular activities are under the control of the nervous system.

A crucial aspect of controlling the nervous system of skeletal muscles is the role of motor units. Note: These activities involve the application of electrical currents to the muscle through electrodes placed on the skin. People who have pacemakers or who suffer from neurological or heart diseases should not volunteer for these activities. Muscle contraction and sensations (such as tingling or short discomfort) may be associated with nerve stimulation. If the volunteer feels very uncomfortable during the activities, stop the exercise immediately. Muscle tone is achieved through a complex interaction between the nervous system and skeletal muscles, which leads to the activation of certain motor units simultaneously, most likely cyclically. In this way, the muscles never get tired completely, because some motor units are in a state of recovery, while others actively create tension. Skeletal muscle responds to a single electric shock of sufficient strength with rapid and intense contractions called phasic contractions. When the ends of a frog sartorius muscle (at 0 ° C) are fixed to avoid shortening, the tension increases for about 200 milliseconds, and then begins to decrease quite quickly at first, and then more slowly. During this mechanical response to a single stimulation called contractions, more things happen than the voltage recording suggests. Skeletal muscles interact to create movement through anatomical positioning and coordinated summation of innervation signals. When a weak signal is sent by the central nervous system to contract a muscle, the smaller motor units, which are more excitable than the larger ones, are first stimulated.

As the signal strength increases, more (and larger) drive units are excited. The largest motor units have up to 50 times the contractile strength of the smallest; Thus, as more and more motor units are activated, the muscle contraction force becomes stronger and stronger. .