Muscular System - Kinesiology 1P99 Fall 2024 PDF

THE MUSCULAR SYSTEM  Muscle Muscles make up 40-50% of body weight Three types of muscle – Skeletal muscle Found throughout the body – Cardiac muscle Found only in the heart – Smooth muscle Appears throughout the body systems as components...

THE MUSCULAR SYSTEM  Muscle Muscles make up 40-50% of body weight Three types of muscle – Skeletal muscle Found throughout the body – Cardiac muscle Found only in the heart – Smooth muscle Appears throughout the body systems as components of hollow organs and tubes (e.g. in intestines, veins and arteries) Classified in two different ways – Striated or unstriated – Voluntary or involuntary  Categorization of Muscle Tissue Classified in two different ways 1. Striated or unstriated striated: alternation in light and dark bands called striations (stripes) 2. Voluntary or involuntary voluntary: consciously controlled by CNS & PNS (somatic nervous system) involuntary: subconsciously controlled by PNS (autonomic nervous system) Categorization of Muscle Tissue 4  Functions of Muscle Locomotion – obvious Stability – Posture/resist gravity Communication – Facial expressions, writing, speaking, etc. Control of body openings – Sphincter muscles Heat production – Shivering Store Glucose  Gross Anatomy of Skeletal Muscle SKELETAL MUSCLE Surrounded by: Epimysium Connective tissue Contains: Muscle fascicles (epimysium, perimysium, endomysium) Muscle MUSCLE FASCICLE Surrounded by: Muscle fascicle Perimysium Contains: Muscle fibers Muscle fiber Myofibrils MUSCLE FIBER Surrounded by: Endomysium Contains: Myofibrils  Connective Tissue Muscle is composed of muscle tissue and connective tissue Multiple layers of connective tissue – Fascia thick connective tissue around entire muscle Extension of tendonous tissue – Separates muscle from skin – Hold muscles of similar function together  Connective Tissue Each muscle has 3 layers: 1. Epimysium: Surrounds entire muscle 2. Perimysium: Divides muscle into sections (bundles) called fasciculi 3. Endomysium: Surrounds individual muscle Fibres  Connective Tissue 3 types of connective tissue – extensions of deep fascia 1. Epimysium Layer of connective tissue around entire muscle – deep to superficial fascia “Epi-” means on top of  Connective Tissue 2. Perimysium Connective tissue covering that surrounds a “bundle of muscle fibers” (muscle cells), called fascicle. “Peri-”means around  Macroscopic Structure: Connective Tissue 3. Endomysiu m Surrounds each muscle fiber (cell) Thin Capillaries and nerves run along surface Tortora & Grabowski, “Endo-”mean 2003  Skeletal Muscle Fiber Skeletal muscle consists of number of muscle fibers lying parallel to one another and held together by connective tissue Single skeletal muscle cell is known as a muscle fiber – Multinucleated – Large, elongated, and cylindrically shaped – Fibers usually extend entire length of muscle Microscopic Organization (inside muscle fiber)  Microscopic Organization Inside the muscle fiber Myofibrils – Make up the muscle fiber – Contractile elements of muscle fiber – Majority of the volume in a muscle fiber (cell) – Surrounded by sarcoplasmic reticulum  Microscopic Organization Myofibrils (cont’d) Regular arrangement of thick and thin filaments – Thick filaments: myosin (protein) – Thin filaments: actin (protein)  Microscopic Organization Myofibrils (cont’d) Viewed microscopically, display alternating dark bands (the A bands) and light bands (the I bands), giving appearance of striations Made up of sarcomeres joined end to end  Microscopic Organization (inside muscle fiber) Sarcolemma Cell membrane (electrically polarized) Sarcoplasm (nuclei, mitochondria, sarcoplasmic reticulum)  Microscopic Organization (inside muscle fiber) Mitochondria – either between myofibrils ‘intermyofibrill ar’ – or right under cell membrane ‘subsarcolem mal’  Microscopic Organization (inside muscle fiber) T-tubules – continuous with cell membrane or sarcolemma Sarcoplasmic reticulum – Membrane network – Ca2+ storage – Lateral sacs (Terminal cisternae)  Sarcomere smallest functional unit of skeletal muscle where actual contraction occurs made of myofilaments Actin - Thin protein filaments Myosin - Thick protein filaments Between two adjacent Z lines (discs)  Myofilaments ACTIN MYOSIN Make up the thin and thick filaments… the contractile machinery of the muscle cell  Actin and Myosin Actin and myosin are often called contractile proteins. Neither actually contracts. Actin and myosin are not unique to muscle cells, but are more abundant and more highly organized in muscle cells.  Myosin Component of thick filament Protein molecule consisting of two identical subunits shaped somewhat like a golf club – Tail ends are intertwined around each other – Globular heads project out at one end  Myosin (cont’d) Tails oriented toward center of filament and globular heads protrude outward at regular intervals – Heads form cross bridges between thick and thin filaments and have two important sites critical to contractile process: – An actin-binding site – ATP binding site Structure and Arrangement of Myosin Molecules Within Thick Filament 26  Actin Primary structural component of thin filaments Spherical in shape Each actin molecule has special binding site for attachment with myosin cross bridge – Binding results in contraction of muscle fiber Thin filament also has two other proteins – Tropomyosin and troponin  Tropomyosin and Troponin Often called regulatory proteins Tropomyosin – Thread-like molecules that lie end to end alongside of actin spiral – In this position, covers actin sites and blocks interaction with myosin that leads to muscle contraction  Tropomyosin and Troponin Troponin – Made of three polypeptide units One binds to tropomyosin (Troponin T) One binds to actin (Troponin I) One can bind with Ca2+ (Troponin C) Composition of a Thin Filament  Actin, Tropomyosin, and Troponin When not bound to Ca2+, troponin stabilizes tropomyosin in blocking position over actin’s cross-bridge binding sites – When Ca2+ binds to troponin, tropomyosin moves away from blocking position – With tropomyosin out of way, actin and myosin bind, interact at cross bridges – Muscle contraction results Role of Calcium in Turning on Cross Bridges  fig 17.4 Brent E. Faught Cross-sectional Arrangement of Thick and Thin Filaments  Sarcomere & Bands Sarcomere – Z-disc (or line) to Z-disc – Contractile unit of muscle A-Band – Dark band – Made up of thick filaments along with portions of thin filaments that overlap on both ends of thick filaments 35  Sarcomere & Bands (cont’d) H-Band (zone) – Lighter area in middle of A-band (where the thin filaments don’t reach) – M line extends vertically down middle of A band within centre of H zone I-Band – Light band – Consists of remaining portion of thin filaments that do not project into A band 36  Blood vessels in skeletal muscle Arteries Veins Capillaries Structure of Skeletal Tendon Muscle Connective tissue Figure: Levels of organization in a skeletal muscle Summary SKELETAL MUSCLE Surrounded by: Epimysium Contains: Muscle fascicles Skeletal muscles consist of muscle MUSCLE FASCICLE fasciculi Surrounded by: Perimysium Contains: Muscle fibers Muscle fascicle consist of muscle fibers MUSCLE FIBER Surrounded by: Endomysium Contains: Myofibrils Muscle fiber consist of myofibrils MYOFIBRIL Myofibrils consist of sarcomeres Surrounded by: Sarcoplasmic reticulum Consists of: Sarcomeres (Z line to Z line) Sarcomere consist of myofilaments SARCOMERE I band A band Contains: Myofilaments are made of actin and Thick filaments Thin filaments myosin Z line M line H band Titin Z line  Action of a Muscle Fiber Excitation-Contraction Coupling: 1. Excitation phase 2. Contraction phase  Contraction Phase muscle shortens in length caused by interactions between thick & thin filaments in the sarcomere triggered by the presence of calcium ions requires ATP When a muscle contracts, actin filaments slide toward each other The sarcomere is the “functional” unit of a muscle fibre. Z - line Z- line  Muscle contracts (shortens) when each sarcomere contracts (shortens)  Changes in the Sarcomere during Contraction – z-lines pull closer together – I - bands (thin filaments) and H-zones get smaller – myosin heads are pulling themselves closer to z-line – A-band (thick and thin filaments) does not change in size So, how do sarcomeres contract (shorten)?  Cross-Bridge Formation  Cross-bridge interaction between actin and myosin brings about muscle contraction by means of the sliding filament mechanism.  Sliding Filament Mechanism Sarcomeres contract when myosin filaments make contact with actin filaments (cross-bridge) and pull them towards the centre of the sarcomere. The length of the thick and thin filaments do not actually change (only the sarcomere length becomes shorter). Thousands of sarcomeres contract at the same time which causes the muscle as a whole to contract and generate force.  Sliding Filament Mechanism Increase in Ca2+ starts filament sliding Thin filaments on each side of sarcomere slide inward over stationary thick filaments toward center of A band during contraction As thin filaments slide inward, they pull Z lines closer together, Decrease in Ca2+ turns off sliding process sarcomere shortens  Sliding Filament Mechanism All sarcomeres throughout a muscle fiber’s length shorten simultaneously – Contraction is accomplished by thin filaments from opposite sides of each sarcomere sliding closer together between thick filaments  Power Stroke Sarcoplasmic reticulum releases Ca2+ into sarcoplasm Myosin heads bind to actin Myosin heads swivel toward center of sarcomere (power stroke) pulling the actin myofilament ATP binds to myosin head and detaches it from actin, and myosin head returns to its original position  Power Stroke Hydrolysis of ATP transfers energy to myosin head and reorients it – ATP  ADP + Pi Contraction continues if ATP is available and Ca2+ level in sarcoplasm is high  Cross Bridge Activity 1 2 3 1 During each cross-bridge cycle, the cross-bridge binds with the actin molecule, bend to pull the thin filament inwards during the power stroke, then detaches and returns to its resting conformation, ready to repeat the cycle.  Cross Bridge Activity Each thick filament is surrounded on each side by 6 thin filaments, which are all pulled inward simultaneously through the cross-bridging cycle during a muscle contraction  The Cross-bridge Cycle A single Power Stroke pulls the thin filament inward only a small percentage of the total shortening distance Repeated cycles of cross-bridge complete the shortening At the end of one cross-bridge cycle, the link between the myosin cross- bridge and the actin molecule breaks and the cross bridge returns to its original shape in order to bind to the  Contraction Cycle As long as Ca2+ is present myosin will be able to bind Allows a series of cycles to cause large contractions So, where does the calcium come from?  Action of a Muscle Fiber Excitation-Contraction Coupling: 1. Excitation phase 2. Contraction phase  Excitation Phase 1. An impulse (action potential) travels down the axon of a nerve 2. Neurotransmitter, acetylcholine (ACh) is released from the end of the axon into the neuromuscular junction (synapse) 3. This causes an action potential and ultimately causes the sarcoplasmic reticulum to release its stored calcium ions 4. Calcium binds to active site on actin 5. Begins the contraction of the muscle  Neuromuscular Junction (synapse) Fig. 9- 8 SR and T-Tubules  STEPS IN INITIATING MUSCLE CONTRACTION STEPS IN MUSCLE RELAXATION Neuromusc Synaptic Motor ular terminal end plate T tubule Sarcolemma junction 2 Action 1 ACh released, binding potential 6 ACh removed by AChE to receptors reaches T tubule 3 Sarcoplasmic 7 Sarcoplasmic reticulum reticulum releases Ca2+ recaptures Ca2+ Ca2+ 4 Active-site Actin 8 Active sites exposure, cross-bridge covered, no formation Myosin cross-bridge interaction 9 Contraction ends 5 Contraction begins 10 Relaxation occurs, passive return to resting length  Excitation-Contraction Coupling The excitation of the sarcolemma and T-tubules leads to release of Ca2+ from SR Thus, excitation and contraction are “coupled” Neural signal conveyed to the contractile machinery The anatomy of a nerve Cell body Houses the nucleus and organelles Dendrites (rootlike extensions) - Project from cell body and increase surface area available for receiving signals from other nerve cells - Move the signal toward the cell body - Dendrites and cell bodies serve as the neuron’s input zone Axon - Single, elongated tubular For a muscle to contract, it needs a neural impulse. Neural impulses are “electrical” currents that pass along nerve fibres. Electrical Action Impulse down membrane potential = Cell to cell communication (e.g. between nerve cell and muscle cell)  Excitable Cells e.g., nerve to nerve communication  Excitable cells e.g., nerve cell to muscle cells Each “motor” (to muscle) nerve innervates many muscle fibers & is called a motor unit. From Martini  Motor Units - Each muscle fiber is innervated by only one motor neuron. - Each motor neuron may innervate up to several thousand muscle fibers. - All muscle fibers within a single motor unit are of the same fiber type. Schematic Representation of Motor Units in Skeletal Muscle Muscle fibers belonging to a given motor unit are intermixed with fibers from other motor units. Motor units often differ in size, with some having more fibers than others Copyright © 2010 by Nelson Education Ltd.  Motor Unit Characteristics Number of muscle fibers varies among different motor units Number of muscle fibers per motor unit and number of motor units per muscle vary widely – Muscles that produce precise, delicate movements contain fewer fibers per motor unit (e.g. finger muscles) – Muscles performing powerful, coarsely controlled movement have larger number of fibers per motor unit (e.g. leg muscles)  Pathology Contracture: a condition in which a muscle shortens its length while at rest. Cramps: spastic and painful contractions of muscles. Myalgia: muscle pain. Fibromyalgia: a form of rheumatism characterized by long-term tendon and muscle pain accompanied by stiffness, occasional muscle spasms, and fatigue. Myositis: inflammation of muscle tissue. Tendinitis: inflammation of a tendon.  Pathology Atrophy: decrease in muscle mass due to a lack of activity, as when a limb is in a cast for a prolonged period. Muscular dystrophy: inherited muscular disorder (most often in males) in which the muscle tissue degenerates over time. Plantar fasciitis: inflammation of the connective tissue (fascia) of the arches of the foot. It is painful and caused by continuous stretching of the muscles and ligaments of the foot.