Muscle Physiology - Part 1 - Skeletal Muscle PDF

Part 1 – Skeletal Muscle MUSCLE PHYSIOLOGY Marcus Machado, MSc, PhD [email protected] Learning Objectives At the end of the lecture, students should be able to: w Understand the basic components of skeletal muscle, the muscle fiber, and the myofibril. w Give examples of muscle functions. w Und...

Part 1 – Skeletal Muscle MUSCLE PHYSIOLOGY Marcus Machado, MSc, PhD [email protected] Learning Objectives At the end of the lecture, students should be able to: w Understand the basic components of skeletal muscle, the muscle fiber, and the myofibril. w Give examples of muscle functions. w Understand the organization of the skeletal muscle. w Describe the types of muscles and muscle fibers. w Describe the physiological events of muscle contraction. Types of Muscles Skeletal (40% of the body) w w w w Voluntary muscle; controlled consciously Striated muscle Attached to the skeleton Locomotion, posture, body temperature. Smooth (10% of the body) w w w w Involuntary muscle; controlled unconsciously Non-striated muscle In the walls of blood vessels and internal organs Respiration, digestion, blood circulation. Cardiac w w w w Controls itself with help from nervous and endocrine systems Striated muscle Only in the heart Blood circulation Muscle Properties MUSCLES HAVE FOUR SPECIFIC PROPERTIES THAT ENABLE THEM TO PERFORM THEIR FUNCTIONS EFFECTIVELY. Contractility - The ability to contract or shorten, allowing muscles to generate tension. Extensibility - Is the property of muscles that allows them to be stretched or lengthened beyond their resting length. Elasticity - The ability to return to its original shape after being stretched or contracted. Excitability - The capacity to receive and respond to stimuli (from the motor neuron, neurotransmitters, hormone, etc). Key Points General Characteristics of a Muscle Fiber w There are three types of muscle: Skeletal muscle; Smooth muscle; and Cardiac muscle. w Muscle contraction is essential for a wide range of physiological functions, including, locomotion, respiration, blood and lymph circulation, etc. w Muscle fibers have four specific properties: Contractility; Excitability; Extensibility; Elasticity Skeletal Muscle Primary function is to enable movement of the body – Attached to the bones of the skeleton – Skeletal muscles work in coordination with the skeletal system to produce voluntary movements, such as walking, running, and lifting objects. – Most segments have one or more muscles on both sides either to increase or decrease its angle. – Is stimulated by a motor neurone – Under voluntary (conscious) control Skeletal Muscle Structure EPIMYSIUM – A fascia of fibrous connective tissue that surrounds the entire muscle. Tendon FASCICLE - A small bundle or cluster of muscle fibers Bone Epymysium (Surrounds the muscle/muscle belly) PERIMYSIUM - Connective tissue extensions from the epimysium that surround each fascicle. ENDOMYSIUM - Connective tissue extensions from the perimysium that surround the muscle fibers and are attached to the sarcolemma. Perimysium (Surrounds the fascicle) MYOFILAMENTS - Are responsible for muscle contraction. Composed of thin and thick filaments. Fascicle (Bundle of muscle fibers) Endomysium (Surrounds the Muscle fiber) MYOFIBRILS - Each muscle fiber contains several hundred to several thousand myofibrils. Myofibril (Muscle cell) Myofilaments (Thin and thick filaments) SARCOLEMMA – It is a thin bilayer lipid membrane enclosing a muscle fiber (cell) – The sarcolemma invaginates into the sarcoplasm of the muscle cell, forming an organelle called T-tubules. T-tubules will carry the depolarization from action potentials to the interior of the fiber. T-tubules – Tubules arranged transversely to the myofibril – Periodic invaginations of the sarcolemma – Carry the depolarization from action potentials to the interior of the fiber. – Presence of voltage-sensitive receptor (dihydropyridine receptor) attached physically to the ryanodine receptor voltage-gated in the sarcoplasmic reticulum. Level of Organization in Skeletal Muscle MUSCLE FIBERS – Bundle of myofibril – elongated shape – Contain the basic contractile unit (sarcomere) SARCOMERE – Contain the myofilaments, basic contractile unit of striated muscle fibers (Found between Z LINES or Z discs) – Their arrangement gives the striations pattern. Myofilaments Myofilaments are responsible for muscle contraction – THIN FILAMENT Actin, Troponin complex, Tropomyosin – THICK FILAMENT Myosin A Myosin Filament (Thick Filament) Myosin heads Heavy Chain Actin binding site Light Chain ATP binding site Myosin tail Flexible hinge Are composed of multiple MYOSIN molecules – Myosin molecule contains a TAIL of intertwined helices and 2 globular HEADS that can bind both ATP and ACTIN Functions as an ATPase enzyme – uses ATP as an energy source for contraction – Aproximately 500 myosin heads of a thick myosin filament form cross-bridges that interact with actin to shorten the sarcomere An Actin Filament (Thin Filament) Troponin C I Troponin T Are composed of ACTIN, TROPOMYOSIN, and TROPONIN COMPLEX – 2 helical strands of actin protein – 2 helical strands of tropomyosin protein All intertwined together as a large helical complex – Troponin is a complex of three globular protein subunits (C, I and T) TnC – Calciun binding subunit TnI – Inibitory subunit TnT – Tropomyosin binding subunit Organelles of the Muscle Cell (Fiber) MITOCHONDRIA – Power plant of ATP – Provides myofibrils with large amounts of energy allowing muscle contraction – Slow-Twitch fibers (red) have more numbers of mitochondria Organelles of the Muscle Cell (Fiber) SARCOPLASMIC RETICULUM – Is a specialized endoplasmic reticulum – Very important for muscle contraction – Regulates calcium storage, release and reuptake Key Points The Muscle Fiber ▪ The skeletal muscle is surrounded by several layers of connective tissue (epimysium, perimysium, and endomysium) that provide strength and stability to the muscle and prevent it from ripping while contracting. ▪ A muscle fiber is enclosed by a plasma membrane called the sarcolemma. ▪ The sarcoplasmic reticulum (SR) is a specialized organelle responsible for storing, releasing, and reuptake ion calcium. ▪ T-tubules allow for rapid transmission of the action potential into the cell and play an important role in regulating cellular calcium concentration. Key Points The Myofibril w Myofibrils are made up of sarcomeres, the smallest functional units of a muscle. w A sarcomere is composed of filaments of two proteins, myosin and actin, which are responsible for muscle contraction. w Myosin is a thick filament with a globular head at one end. w An actin filament is composed of actin, tropomyosin, and troponin. It is attached to a Z disk. EVENTS LEADING TO MUSCLE CONTRACTION – ACTION POTENTIAL - It is a rapid change in membrane potential that travels rapidly along the membrane. - It is caused by the inflow of Na+ (highly concentrated outside the cell) and the outflow of K+ (highly concentrated inside the cell), causing a change in the voltage of the cell, producing a spike in the action potential. The Sliding Filament Model Proposed in the early 1950s. Two British biologists, Hugh Huxley and Andrew Huxley. The theory proposes that a muscle shortens or lengthens because thick and thin filaments slide over each other without changing length. Professor Hugh E. Huxley (25/02/1924 – 25/07/2013) Sliding Filament Model of Muscle Contraction Before muscle contraction begins Myosin heads bind with ATP (low energy configuration) – The ATPase activity immediatly cleaves the ATP in ADP and Pi – Cleavage products are kept bound to the head – Head becomes energized in a “cocked position” Sliding Filament Model of Muscle Contraction When calcium ions bind the troponin-tropomyosin complex, active sites of the actin filaments are uncovered – Myosin heads bind to these sites – The cross bridge is formed – The phosphate ion and ADP is released Sliding Filament Model of Muscle Contraction The cross bridge causes a conformational change in the head – Myosin heads bend toward the center of the sarcomere, causing the actin to slide toward the M line – POWER STROKE – The energy that activates comes from the stored ADP – Another ATP molecule will take place causing detachment of the myosin head from the actin filament to begin a new cycle Key Points Muscle Fiber Action w Muscle action is initiated by a nerve impulse. w If the cell receives the right stimulus, an action potential occurs which releases stored Ca2+ ions. w Ca2+ ions bind with troponin, which lifts the tropomyosin molecules off the active sites on the actin filament. These open sites allow the myosin heads to bind to them. w Energy for muscle action is provided when the myosin head binds to ATP. ATPase on the myosin head splits the ATP into a usable energy source. Key Points Muscle Fiber Action w Once myosin binds with actin, the myosin head tilts and pulls the actin filament so they slide across each other. w Muscle action ends when calcium is pumped out of the sarcoplasm to the sarcoplasmic reticulum for storage. Types of Muscle Fibers – Skeletal muscles contain both Type I and Type II fibers. Type I (Red) – Type I fibers have high aerobic endurance and are suited to low-intensity endurance activities. Type II (White) – Type II fibers are better for anaerobic or explosive activities. » Type IIa » Type IIb Type I Muscle Fibers Type I - Oxidative Fiber – High aerobic capacity and fatigue resistance – Rich in mitochondria – Slow contractile speed (110 ms) – Plentiful in muscles which the main function is slow prolonged activitiy – 10–180 fibers per motor neuron Type IIa Muscle Fibers Type IIa – Are mixed oxidative-glycolytic fiber – Moderate aerobic (oxidative) capacity and fatigue resistance – High anaerobic (glycolytic) capacity – Fast contractile speed (50 ms) – Highly developed sarcoplasmic reticulum – 300–800 fibers per motor neuron Type IIb Muscle Fibers FTb (Type IIb) – Glycolytic Fiber – Low aerobic (oxidative) capacity and fatigue resistance – High anaerobic (glycolytic) capacity and motor unit strength – Fast contractile speed (50 ms) – Highly developed sarcoplasmic reticulum – 300–800 fibers per motor neuron