Skeletal Muscle Muscle Contraction and Relaxation

							JOHNSON COUNTY COMMUNITY COLLEGE Human Physiology Ateegh Al-Arabi, Ph.D.

Skeletal Muscle Muscle Contraction and Relaxation
In order for the skeletal muscle to contract and relax, the following steps should take place: 1. 2. 3. 4. 5. 6. 7. 8. The muscle membrane (sarcolemma should be depolarized). This requires acetylcholine to be released by a motor neuron at the neuromuscular junction. When the sarcolemma has been depolarized, this depolarization will spread to the membrane of the sarcoplasmic reticulum (SR) and particularly to the cisternae of the SR. Depolarization of the cisternal membrane results in the opening of the voltage gaited Ca++ channels. When the Ca++ channels are open, Ca++ will diffuse into the sarcoplasm (the cytoplasm of the muscle fiber). Ca++ will then react with the C subunits of the troponin molecule which in turn will activate the other two subunits (I and T). The T subunits of the troponin will dissociate the tropomyosin away from the binding sites of the actin molecules. Now the binding sites of the actin filaments are available to react with myosin heads. The myosin heads after reacting with the actin binding sites will bend back, pulling with the actin filament, and the sliding of the actin over the myosin will start. This results in the dissociation of the ADP molecule and phosphate group (see step 11) from the myosin heads. The myosin heads react again with a new ATP molecule. Step 9 results in the myosin heads losing affinity to and leaving the binding sites of the actin. After leaving the actin binding sites, the myosin heads extend back, a process which results in the splitting of the ATP molecules into ADP + P. But, these two molecules do not dissociate from the myosin heads. The myosin heads are then said to be energized and ready to react again with new binding sites on the actin molecules. Steps 7 to 12 (the contraction steps) will then be continuously repeated as long as depolarization of the membrane exists and Ca++ is abundant in the sarcoplasm. If depolarization is lost by the process of repolarization due to the lack of acetylcholine, the calcium pump in the cisternal membrane will pump Ca++ back into the cisternae and step 6 will be reversed. Therefore, the binding sites of the actin are no longer available to react with myosin heads. This causes the actin to slide back to its original relaxation position and the muscle is then called relaxed.

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Characteristics of the Cardiac Muscle Fiber
1. All muscle fibers of the heart are connected to each other forming a syncytium or a network. This is due to the presence of special connecting membrane structures known as intercalated discs. Within these structures are found gap junctions. These gap junctions which are physical passages allow an easy flow of ions between the adjacent fibers. This means that depolarization of any muscle fiber in the heart results in the spread of this depolarization to all muscle fibers. Therefore, the group of muscle fibers that can reach action potential first will be designated as the pacemaker of the heart contraction. Self-Excitation: Due to the presence of a significant number of Na+ leak channels (not gaited) in the sarcolemma of the cardiac muscle fiber, sodium causes spontaneous depolarization of the membrane. These leak channels are eventually assisted by the opening of slow Ca++/Na+ channels (voltage gaited) which leads to the opening of fast voltage gaited Na+ channels. This means that the cardiac muscle (in general) can eventually reach action potential without the presence of a nerve impulse. Classified into three functional groups: a. Excitatory (pacemaker) Normally the SA node. This group of muscle fibers is located in the posterior wall of the right atrium just below the opening of the superior vena cava. The SA node fibers are characterized by their less-negative resting membrane potential of -60 to -65mv, compared to the other fibers which are having a resting membrane potential ranging from 75mv in the AV node to -90mv in the ventricular muscle fibers. b. Conductive Fibers • Fast Conductive (transatrial pathways, AV bundle of His and its branches, and the Purkinje fibers). These fibers are very large, not only in diameter, but also in length, and have a larger number of gap junctions in their intercalated discs. These two unique features help them to conduct electrical signals in the form of ions very fast. Slow Conductive (AV node). These fibers in the AV node are slow in their conduction of electrical signals. This is due to the fact that they have exactly the opposite features of the fast conductive ones. They are small and have much fewer gap junctions than any other fibers in the heart.

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c. Contractile Fibers All of the previously described muscle fibers do not make a significant contribution to the physical contraction of the heart. This function is performed by the rest of the muscle fibers in the heart and particularly strong by the ventricular muscle fibers.

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The cardiac muscle fibers are 100% aerobic. This characteristic makes the cardiac muscle fibers very sensitive to the lack of oxygen. Therefore they need a continuous supply of oxygenated blood. For this reason, they also contain a much larger number of mitochondria than the average skeletal muscle fibers. Less Developed Sarcoplasmic Reticulum and Lack of Cisternae: This means that they don’t have a defined Ca++ reservoir like in the case of the skeletal muscles. Their supply of Ca++ (required for contraction) comes directly from the T tubules of the SR. Since the T tubules are directly affected by the interstitial environment, the cardiac muscle fibers are directly affected by the changes in the level of Ca++ in the interstitial fluid. Hypercalcemia or hypocalcemia in the interstitial fluid may result in malfunctioning of the whole heart. Presence of Plateau in the Action Potential Curve: Due to the presence of slow Ca++/Na+ channels in the membrane which usually fail to immediately close when action potential is reached, Na+ and Ca++ continue to influx into the muscle fibers for a short time. During this time the potassium (repolarizing) channels are also open. Therefore an almost equal exchange of positive ions takes place during this period. This prevents or slows down the repolarization process and a plateau appears on the action potential curve. The duration of the plateau therefore depends primarily on the number of the slow channel present in the membrane of each group of fibers. The plateau is more significant in the ventricular muscles and almost absent in the SA node and the rest of the atrial fibers.

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Isotonic and Isometric Contraction
Isotonic Contraction: It is the contraction of a muscle when a constant load is moved through the range of motions possible at a joint. The Two Types of Isotonic contractions: 1. Concentric contraction: When the muscle shortens to move against a bone to reduce the angle of the joint (e.g., picking up an object). 2. Eccentric contraction: When the muscle lengthens to increase the angle of the joint (e.g., lowering the object back to place). Eccentric contractions produce more damage to the muscle and more delayed onset of muscular soreness. Isotonic contraction is aerobic (e.g., jogging, aerobics, dancing), and thus increases the blood flow to the muscle. Isometric Contraction: A muscle contraction in which tension on the muscle increases, but there is only minimal muscle shortening so that no movement is produced. Isometric contraction is anaerobic (e.g., weight lifting, sprinting), stimulates synthesis of muscle protein and thus results in muscle hypertrophy.