L08 - HB_A_P- Muscle Fatigue (1).pptx

BIOL 221 – Muscle III Cristofre Martin Department of Biochemistry St. George’s University  Learning Objectives Describe muscle fatigue Define oxygen debt Describe a single muscle twitch Describe normal muscle tetanus Distinguish between isometric and isotonic contractions Define refractory pe...

BIOL 221 – Muscle III Cristofre Martin Department of Biochemistry St. George’s University  Learning Objectives Describe muscle fatigue Define oxygen debt Describe a single muscle twitch Describe normal muscle tetanus Distinguish between isometric and isotonic contractions Define refractory period  Fatigue Fatigue refers to the use-dependent decrease in a muscle’s ability to generate force. It results mostly from the contraction of the muscle under anaerobic conditions, causing a build-up of lactic acid in muscle and blood. Increased acidity: 1.Alters the activity of glycolytic enzymes 2.Interferes with the cross-bridge (actin: myosin) cycle.  Fatigue can involve depletion of glycogen stores in the liver as well as in the muscle that is being exercised. As glycogen is depleted, the body utilizes fat as its sole energy source. The rate of energy production from fat is about half that from glucose, so a substantial decrease in muscle performance  Oxygen debt Oxygen debt occurs and remains until excess lactic acid is metabolized into CO2 and H2O or (within the liver) converted to glucose. Increased temperature in heart and skeletal muscle immediately after exercise contribute to temporary excess O2 use.  Phases of muscle contraction 1. Latent period: period between stimulation (passage of neurotransmitter) and contraction. events…? 2. Period of contraction: onset of shortening to the peak of tension development. 3. Period of relaxation: initiated by reentry of Ca++ into sarcoplasmic reticulum. One muscle twitch  Refractory period While the sarcolemma is repolarizing, it cannot respond to a second stimulus. This is the refractory period. Repolarization is accomplished while much of the Ca++ is still outside the SR. If a second stimulus is received at that time, the muscle will respond more strongly to this stimulus: WAVE Force of contraction Would the peak force of the second contraction in (b) be larger or smaller if the second stimulus was applied a few milliseconds later? Tetanus As stimuli are repeated at brief intervals, the relaxation time becomes shorter and shorter, until finally all evidence of relaxation disappears and the contractions fuse into a smooth sustained contraction called tetanus. (see next diagram). This is the usual pattern of muscle contraction in the body.  Clostridium tetani, a bacterium, produces a toxin that causes continuous stimulation of muscles and therefore continuous contracture (“lock jaw”). It can end in death from respiratory failure. C. tetani probably derived its name from the fact that muscles are in a permanent state of tetanus. However, fused tetanus contractions in skeletal muscle are normal, and should not be confused with this abnormal state. Recruitment A motor unit is a neuron and all the fibers that it innervates. Weak or precise movements involve few, small units. Powerful contractions require many large motor units. Utilizing many motor units is known as recruitment.  Stimulus A threshold stimulus is a stimulus of sufficient strength to cause the first visible evidence of contraction. The maximum stimulus is the largest stimulus that results in increased force.  Isometric vs Isotonic Isometric contractions: the length of the muscle remains the same, the tension increases. Ex., press against an immovable object. Isotonic contractions: the tension remains the same, the length decreases. Ex., lifting a weight. The muscle continues to shorten  Isometric vs. Isotonic II When a weight is held steady, the muscle length remains constant, but the tension increases (isometric). Most actions combine isometric and isotonic activity. When lifting a weight, the pull of gravity opposes the tug of muscles. While the weight stays in place, the action is isometric. When enough fibers are recruited to move the weight, the action becomes isotonic.  Cardiac muscle Cardiac muscle cells are coupled to their neighbors electrically by gap junctions, forming a functional contractile unit. Note: The atria and ventricles have separate contractile units. Electrical impulses begin spontaneously in the muscle cells in a specific region of the myocardium known as the pacemaker. These are cardiac myocytes that have the ability to generate electrical impulses. Contraction of heart chambers pumps blood out. Relaxation allows the chambers to refill.  The atria contract first, and then the ventricles contract. Cardiac muscle has a much longer refractory period than skeletal muscle. Fused, sustained contractions would not be practical for the heart! Smooth muscle Actin and myosin are not arranged in sarcomeres. Actin is held in place by intermediate filaments. Sarcoplasmic reticulum is absent. Ca++ enters from the extracellular fluid to initiate contraction. Ca++ binds to calmodulin (similar to troponin) initiating the process of cross bridge formation. Visceral smooth muscle cells can produce electrical impulses spontaneously which spread via gap junctions. (Nervous stimulation can also activate smooth muscle  Single unit (Visceral) Multi-unit Single unit: nerve impulses spread via gap junctions, and cells contract as a single unit. Ex.,muscles of visceral organs, heart, blood vessels. Multi-unit: each muscle cell has its own nerve supply. Ex.muscle of iris.  Aerobic warm-up Warm up: transition between rest and high performance; it should last 10-15 minutes. In the beginning, circulatory and respiratory systems are not working hard, some ATP will come from anaerobic respiration. Recall: anaerobic respiration uses only glycolysis; small quantities of glucose are stored in the muscles. Anaerobic respiration produces lactic acid, making muscles sluggish, contractions painful. Therefore, a good warm-up starts  Warm up... 1. The amount of CO2 in the blood increases. It is detected by the medulla oblongata( brain), which stimulates heart rate and more rapid, deeper breathing. 2. The adrenal glands secrete epinephrin(adrenalin). Epinephrin: Dilates air passages Constricts blood flow to inactive regions Dilates blood vessels to active muscles Stimulates release of stored blood from  Warm up (continued) 3. At first blood is shunted away from the skin, increasing heat retention, muscle temperature. This accelerates O2 diffusion; softens ligaments and cartilage, increasing joint flexibility. 4. Cartilage within joints absorbs water, expands by 12%, increasing cushioning effect. 5. By the end of warm-up, blood is again going to the skin, stabilizing body temperature.  Physiological measures: rest vs. exercise Action measured rest exercise cardiac output l/min 5 30 pulse 70 180 blood pressure 120 175 blood flow to muscles l/min 1.2 12.5  Types of muscle fibers: Slow oxidative twitch (red) Fast glycolytic twitch (white) Fast oxidative (red)  The citric acid cycle, which accesses a large quantity of energy, occurs only aerobically. Glycolysis, which accesses a small amount of energy, can occur anaerobically (net gain: 2 ATP vs 36 ATP per glucose molecule)  Slow oxidative twitch muscle (red) Slow twitch- muscle fibers take longer to reach maximum tension, but can work for a longer period without fatigue. Such fibers have a high capacity for aerobic respiration due to many mitochondria, many aerobic respiratory enzymes, a rich capillary supply and a high concentration of myoglobin.  Fast glycolytic twitch fibers (white): Reach maximum tension quickly. Are adapted to anaerobic respiration. Have a high concentration of glycogen. Have fewer capillaries and mitochondria, less myoglobin than slow oxidative twitch fibers. Fast twitch fibers are adapted for rapid generation of power and grow thicker (more myofibrils) and stronger in response to weight training. …..Age and muscle…..  Fast oxidative (red) Humans also have an intermediate form of fast twitch fibers with a high oxidative capacity, which are more resistant to fatigue. Endurance training can somewhat increase the proportion of these fibers in muscles.  All muscles in the body have fast and slow fibers. The ratio varies with the muscle. Arm muscles have a higher % of fast fibers. Marathon runners have a high % of slo oxidative fibers; so do migrating birds. Sprinters have a higher % of fast glycolytic fibers.  Postural muscles have many slow oxidative fibers. Postural muscles are almost always partially contracted. The nervous system alternates activation among the motor units, reducing the length of time any one set of fibers is contracted. Prolonged contraction can result in muscle fatigue due to the depletion of ATP and dissipation of ion gradients required for normal electrical signaling.  Abnormal contractions Spasm- a sudden involuntary contraction of a large group of muscles. Tremor is a rhythmic, involuntary, purposeless contraction of opposing muscle groups. Tic - a spasmodic twitching made involuntarily by muscles usually under voluntary control. Ex., eyelid or facial muscle. Tics are usually psychological in origin. Cramp: a muscle contracts spasmodically, but does not relax completely. This is due to a lack of ATP, which is needed to break the actin:myosin cross bridges and send the  Myasthenia gravis MG is a weakness of skeletal muscles caused by an abnormality at the neuromuscular junction. This autoimmune disorder produces antibodies that bind to receptors in the sarcolemma for acetylcholine (a neurotransmitter). As the disease progresses, more receptors are affected, and muscles become weaker. Onset: most common  Muscular dystrophy M.D. encompasses several muscle- destroying diseases, characterized by degeneration of individual muscle fibers, leading to atrophy of the skeletal muscle. The protein dystrophin makes up only.002% of normal muscle protein. Dystrophin is absent in Duchenne muscular dystrophy, a wasting disease that mostly affects young boys. Dystrophin binds to the inner surface of the sarcolemma, supporting it against the powerful force of contraction. Without this  Parkinson’s disease- cells in certain areas of the midbrain no longer secrete sufficient dopamine; this interferes with initiation of muscle movements and causes muscle rigidity. About 5% of population over 85 yrs. has Parkinson’s disease. A very small % of young people have this and in their cases, it is genetic. It has been tied to defective mitochondrial function. Transplanting stem cells improves muscle function in animals, and there is speculation