Muscle Physiology II (Contraction) PDF

Muscle Physiology II Dr Panayiotis Avraamides MBBS Lon, BSc, FRCP Lon, FRCP Edin, FESC, FEACVI, FACC, FAHA, FSCAI, FHRS Clinical Professor of Cardiology Lecture at glance o Skeletal Muscle Mechanics o Skeletal Muscle Metabolism and Fiber Types o Control of Motor Movement Introduction ~600 ske...

Muscle Physiology II Dr Panayiotis Avraamides MBBS Lon, BSc, FRCP Lon, FRCP Edin, FESC, FEACVI, FACC, FAHA, FSCAI, FHRS Clinical Professor of Cardiology Lecture at glance o Skeletal Muscle Mechanics o Skeletal Muscle Metabolism and Fiber Types o Control of Motor Movement Introduction ~600 skeletal muscles Range in size (leg muscles vs external eye muscles) Connective tissue (penetrates from the surface into the muscle, divide the muscle into bundles, tendons that attach the muscles to bones) Tension is produced internally within the sarcomeres (contractile component) through cross-bridge activity and sliding of filaments Transmission of the tension to the bones through tendons Tendons have a certain degree of elasticity Series-elastic component of the muscle (Tendons) (vs titin=parallel-elastic component/contributes to muscle’s elastic recoil) Relationship between the contractile component and the series-elastic component in transmitting muscle tension to bone Flexion and extension of the elbow joint Origin=the end of the muscle attached to the stationary part of the skeleton Insertion=the end of the muscle attached to the skeletal that moves Types of contraction In an isotonic contraction, the load remains constant as the muscle changes length e.g. you lift an object. In an isokinetic contraction, the velocity of shortening remains constant as the muscle changes length (exercise machines). In an isometric contraction, the muscle is prevented from shortening, so tension develops at constant muscle length (supporting objects in a fixed position, maintaining posture). The same internal events occur in isotonic, isokinetic, and isometric contractions Concentric and eccentric contractions/Load-Velocity relationship Concentric contraction= the muscle shortens. The greater the load the lower the velocity at which a single muscle fiber shortens (load and velocity are inversely related) Eccentric contraction=the muscle lengthens (lowering a book to place it on a desk) (load and velocity are directly related ) Load–velocity relationship. This relationship between load and shortening velocity is a fundamental property of muscle. Load–velocity relationship in concentric contractions Lever systems of muscles, bones, and joints Lever=bones Fulcrum=joints=pivot points Disadvantage of this lever system=at the point of insertion the biceps muscle must exert a force 7 times greater than the load Lever systems of muscles, bones, and joints Motor units in a skeletal muscle Motor unit=the team of concurrently activated components (one motor neuron plus all the muscle fibers that innervates) One motor neuron innervates a number of muscle fibers, but each muscle fiber is supplied by only one motor neuron Twitch summation A single Action Potential in a muscle fiber produces only one twitch Two twitches from two AP added together sum to produce greater tension than that produced by a single AP WHY?The duration of the AP (1-2 msec) is much shorter than the duration of the resulting twitch (30-100 msec) Twitch summation and tetanus Length-tension relationship At the optimal muscle length, maximum tension can be developed Optimal muscle length (Io)=100%tension (maximum tension) Optimal overlap of thick filament cross- bridges and thin filament cross-bridges binding sites Because of the restrictions imposed by skeletal attachments, muscles cannot vary beyond 30% of their Io. Muscle fibers have alternate pathways for forming ATP Only limited stores of ATP are immediately available in muscle tissue, enough to power the first few seconds of exercise. 3 pathways supply additional ATP as needed during muscle contraction: (1) transfer of a high-energy phosphate from creatine phosphate to ADP, (2) oxidative phosphorylation (the electron transport system and chemiosmosis), (3) (anaerobic)glycolysis. Creatine phosphate Creatine phosphate is the first energy storehouse tapped at the onset of contractile activity. It contains a high-energy phosphate group, which can be donated directly to ADP to form ATP. Is catalyzed by the muscle cell enzyme creatine kinase, is reversible; energy and phosphate from ATP can be transferred to creatine to form creatine phosphate: Oxidative Phosphorylation Oxidative phosphorylation takes place within the muscle mitochondria if suffi cient O2 is present. This pathway is fueled by glucose or fatty acids, depending on the intensity and duration of the activity. Although it provides a rich yield of 32 ATP molecules for each glucose molecule processed, oxidative phosphorylation is relatively slow because of the number of enzymatic steps involved. 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. Pyruvate can be further degraded by oxidative phosphorylation to extract more energy. Glycolysis 2 advantages over the oxidative phosphorylation pathway: (1) glycolysis can form ATP in the absence of O2 (operating anaerobically), and (2)it can proceed more rapidly than oxidative phosphorylation. Activity that can be supported in this way is anaerobic or high-intensity exercise. Lactate production Muscle cells can store limited quantities of glucose as glycogen, but anaerobic glycolysis rapidly depletes these glycogen supplies. Anaerobic glycolysis can support muscle contractile activity for less than 2 minutes When the end product of anaerobic glycolysis, pyruvate, cannot be further processed by oxidative phosphorylation, it is converted to lactate (burning sensation of the muscle) Metabolic acidosis The three types of skeletal muscle fibers differ in ATP hydrolysis and synthesis Classified by their biochemical capacities, there are 3 major types of muscle fibers: 1. Slow-oxidative (type I) fibers 2. Fast-oxidative (type IIa) fibers 3. Fast-glycolytic (type IIx) fibers the 2 main differences: speed of contraction (slow or fast) and the type of enzymatic machinery they primarily use for ATP formation (oxidative or glycolytic). Muscle fiber types Fast twitch means higher ATP-splitting activity (myosin ATPase) Muscle fiber types Oxidative vs Glycolytic fibers depends on the ATP- synthesizing ability Increased capacity to form ATP through oxidative phosphorylation produces more ATP and is more resistant to fatigue Characteristics of Skeletal Muscle Fibers Muscle fiber types Genetic Endowment of Muscle Fiber Types The percentage of these various fibers not only differs among muscles within an individual but also varies considerably among individuals 2 types of changes can be induced in muscle fibers: changes in their oxidative capacity and changes in their diameter. Improvement in Oxidative Capacity Other changes Muscle Hypertrophy Testosterone promotes the synthesis and assembly of myosin and actin Interconversion Between Fast Muscle Types (fast-glycolytic fibers can be converted to fast-oxidative fibers and vice versa) Slow and fast fibers are not interconvertible Muscle atrophy Disuse atrophy Denervation atrophy Age-related atrophy, or sarcopenia Limited repair of the muscle (satellite cells (inactive muscle specific stem cells) myoblasts, mature muscle fiber)  Control of Motor Movement Each activated motor neuron triggers contraction of all of the skeletal muscle fibers within its motor unit. Motor activity can be classified as reflex, voluntary, or rhythmic Control of Motor Movement Somatic Reflex Responses: automatic responses brought about by skeletal muscle contraction that takes place without conscious effort Voluntary movements: most complex motor activity (Goal directed movements initiated and terminated at will) Rhythmic activities: stereotypical movements repeated in general pattern (e.g walking) The withdrawal Reflex Crossed extensor reflex A crossed extensor reflex is a postural reflex ensures that the opposite limb is in a position to bear the weight of the body as the injured limb is withdrawn from the stimulus. The crossed extensor reflex coupled 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 1. Elastic properties of the muscle and 2. minimal stimulation by motor neurons Muscle tone is regulated by postural reflexes and output from the multineuronal motor system THANK YOU FOR YOUR ATTENTION !