Control of Muscle Tension and Muscle Metabolism PDF

Control of muscle tension and muscle metabolism Dr. Isehaq Al-Huseini  Objectives Describe how frequency of stimulation affects muscle tension and how muscle tone is produced. Distinguish between isometric and isotonic contractio...

Control of muscle tension and muscle metabolism Dr. Isehaq Al-Huseini  Objectives Describe how frequency of stimulation affects muscle tension and how muscle tone is produced. Distinguish between isometric and isotonic contraction. Distinguish between Concentric and eccentric contraction Describe how muscle tension and length is monitored by sensory receptors Describe the reactions by which muscle fibers produce ATP. Distinguish between anaerobic and aerobic cellular respiration Define muscular fatigue and list factors that contribute to muscle fatigue. Compare the structure and function of oxidative and glycolytic skeletal muscle fibers and compare the effects of exercise on them Describe the effects of exercise on different types of skeletal muscle fibers and changes that occur with disuse and denervation.  Control of muscle tension Whole-muscle tension depends on the number of muscle fibers contracting and the tension developed by each contracting fiber. Factors influence the extent to which tension can be developed: 1. Frequency of stimulation 2. Length of the fiber at the onset of contraction 3. Extent of fatigue 4. Thickness of the fiber  Twitch summation and tetanus Single action potential produces only one muscle twitch. Repeated fibers stimulation produces longer and greater tension. Stimulation of muscle fiber by a second stimuli before it has completely relaxed causes a second contractile response which is added on top of the first twitch. A process known as Twitch summation. Tetanus is a continues contraction without relaxation due to rapid stimulation (3-4 times stronger than a single twitch) Twitch summation is possible because the duration of action potential is (1-2 msec) is much shorter than the duration of the resulting twitch (30-100 msec). During the initiation of first action potential, a brief refractory period occurs, during which another action potential can not be initiated. Twitch summation and tetanus  Types of contraction  Isotonic contraction: The load remains constant as the muscle changes the length. Used for body movements and for moving objects Picking a book up off a table  Isometric contraction: the muscle is prevented from shortening, so tension develops at constant muscle length. Holding a book steady using an outstretched arm  Concentric and eccentric contraction Concentric contraction: Muscle shortening while contracting Eccentric contraction: The muscle fiber is lengthening, but are still actively contracted in opposition to being passively stretched by the load.  Muscle tone Muscle tone refers to an ongoing, involuntary, low-level state of tension in a muscle even at rest. Skeletal muscle tone is important in maintaining postural stability. Due to elastic properties of the muscle ( resist passive stretching) and continues minimal stimulation by motor neurons (produce constant partial muscle contraction). Muscle tone is regulated by postural reflexes and output from the multineuronal motor system, namely the spinocerebellum, basal nuclei, and brain stem. Small groups of motor units are alternatively active and inactive in a constantly shifting pattern to sustain muscle tone. Muscle tone keeps skeletal muscles firm Keeps the head from slumping forward on the chest  Muscle sensory receptors Extrafusal muscle fibers Proprioceptors are found in the joints and the muscles themselves. Alpha motor neuron They provide information about the location of body parts Muscle spindle relative to one another and about body movement. Golgi tendon organ Tendon 2 types of muscle proprioceptors : Golgi tendon organ: detect changes in muscle tension. Muscle spindles: monitor muscle length. Both are activated by muscle stretch  Muscle spindles External muscle= skeletal muscle that produce tension innervated by alpha motor neuron. Internal muscle = skeletal muscle that contains muscle spindles and innervated by sensory neurons and other types of motor neurons. Muscle spindles (l0) Muscle reflexes help prevent damage  Production of ATP in Muscle Fibers For contractile activity to continue, ATP must constantly be supplied. There are 3 pathways supply additional ATP as needed during muscle contraction: 1. Creatine Phosphate 2. Oxidative phosphorylation 3. Glycolysis  1. Creatine Phosphate Creatine phosphate is the first source for supplying additional ATP when exercise begins. By donating phosphate group directly to ADP to form ATP. Catalyzed by the muscle cell enzyme creatine kinase (is reversible) Creatine kinase Creatine phosphate + ADP creatine + ATP Because only one enzymatic reaction is involved in this energy transfer, ATP can be formed rapidly by using creatine phosphate.  2. Oxidative phosphorylation Oxidative phosphorylation takes place within the muscle mitochondria if sufficient O2 is present. (aerobic) This pathway is fueled by glucose or fatty acids, depending on the intensity and duration of the activity. Oxidative phosphorylation is relatively slow because of the number of enzymatic steps involved, but provide a rich yield of 32 ATP molecules for each glucose molecule processed. This pathway is used more in aerobic or endurance-type exercise. Increased O2 is made available to muscles during exercise by several means: 1. Rapid and forceful contraction of the heart to pump oxygenated blood. 2. Dilation of blood vessels 3. Hemoglobin molecules release more O2 4. Some type of muscles have more myoglobin which can store small amount of O2 and increases the rate of O2 release from the blood into the muscle tissue. Oxidative phosphorylation  3. Glycolysis When O2 delivery or oxidative phosphorylation cannot keep pace with the demand for ATP formation as the intensity of exercise increases, the muscle fibers rely increasingly on glycolysis to generate ATP. During glycolysis, a glucose molecule is broken down into 2 pyruvate molecules, yielding 2 ATP molecules in the process. Two advantages of Glycolysis when compared with oxidative phosphorylation: 1. Can form ATP in the absence of O2 (anaerobically). 2. Can proceed more rapidly than oxidative phosphorylation. Glycolysis supports anaerobic or high-intensity exercise.  Lactate production Using glycolysis pathway has two consequences: 1. Because it is less efficient (each glucose molecule makes 2 ATP, it requires large amount of glucose. Anaerobic high-intensity exercise can be sustained for only a short duration because of rapid depletion of stored glycogen in the muscle. 2. When the end product of anaerobic glycolysis, pyruvate, cannot be further processed by oxidative phosphorylation, it is converted to lactate. Lactate accumulation has been implicated in the acute muscle soreness  Fatigue Fatigue causes muscle tension to decrease. There are two types of fatigue: 1. Muscle fatigue: Occurs when an exercising muscle can no longer respond to stimulation with the same degree of contractile activity. Considered as a defense mechanism that protects a muscle from reaching a point at which it can no longer produce ATP. The primary factors: local increase in inorganic phosphate, leakage of Ca2+ from SR, and depletion of glycogen. 2. Central fatigue: Occurs when the central nervous system (CNS) no longer adequately activates the motor neurons supplying the working muscles. Central fatigue often is psychologically based.  Increased O2 consumption is necessary to recover from exercise. Why do we continue to breathe deeply and rapidly for some time after exercising? This known as oxygen debt because of a cumulative deficit of oxygen available for oxidative metabolism that develops during periods of intense bodily activity. Oxygen is needed for recovery of the energy systems. How? Fresh supplies of ATP are formed by oxidative phosphorylation New ATP is used to resynthesize creatine phosphate to restore its reserves. Converting back the accumulative lactate to pyruvate that used by the mitochondria for ATP production or converted back into glucose in the liver. (pyruvate require O2)  Types of muscle fibers There are three major types of muscle fibers: 1. Slow-oxidative (type I) fibers 2. Fast-oxidative (type IIa) fibers 3. Fast-glycolytic (type IIx) fibers Two main differences among these fiber types are their speed of contraction (slow or fast) and the type of enzymatic machinery they primarily use for ATP formation (oxidative or glycolytic).  Two factors determine the speed with which a muscle contract: 1. The load( load-velocity relationship) 2. The myosin ATPase activity of the contracting fibers (fast or slow twitch)  Effects of exercise on the muscle Aerobic exercise: long endurance exercises promotes metabolic changes within oxidative muscle fibers by increasing the number of mitochondria and number of capillaries supplying blood to these fibers. High intensity: anerobic, short duration and high intensity resistance training  Muscle atrophy When muscle is not used, its actin and myosin content decreases, its fibers become smaller, and the muscle accordingly atrophies (decreases in mass) and become weaker. Muscle atrophy can take place by three ways: 1. Disuse atrophy: when a muscle is not used for along period with intact nerve supply) 2. Denervation atrophy: when the nerve supply to a muscle is lost. 3. Age-related atrophy or sarcopenia: naturally with aging. Thank You