NURS 207 (N01) Skeletal Muscle - Basic Structure & Contractile Unit - PDF

NURS 207 (N01) Skeletal muscle Basic structure of skeletal muscle, ultrastructure of muscle contractile unit October 10, 2024 Dr. P. Lee  Objectives: 1) Explain the structural differences among the three types of muscular...

NURS 207 (N01) Skeletal muscle Basic structure of skeletal muscle, ultrastructure of muscle contractile unit October 10, 2024 Dr. P. Lee  Objectives: 1) Explain the structural differences among the three types of muscular tissue 2) Describe the microscopic anatomy of a skeletal muscle fiber 3) Distinguish thick filaments from thin filaments 4) Describe the functions of skeletal muscle proteins 5) Describe the basic unit of the skeletal muscle fiber 6) Know the ultrastructure of muscle contractile unit responsible for muscle contraction for skeletal muscle  Muscular system ❖ Occupies about 40 – 50% of total adult body weight Primary function: Transform chemical energy into mechanical energy ✓ Generate force → muscle shortening ✓ Perform work → movement ✓ Maintain equilibrium → posture (tension generation) ✓ Transport substances → circulation & digestion ✓ Heat production (by-product) → maintain body temperature (e.g. Shivering reflex)  Various types of muscular tissue Three types of muscle tissues in human: → Skeletal muscle, cardiac muscle, & smooth muscle Nucleus Skeletal muscle Muscle fiber (cell) Striations Intercalated disk Nucleus Muscle fiber Intercalated Cardiac muscle disk Striations Nucleus Smooth muscle Muscle fiber  General properties of the muscular system Striation Repeating light and dark protein bands when observed with a microscope Intercalated disk Muscle fiber (cell) Striations Striations Skeletal muscle Cardiac muscle Smooth muscle lacks the striation  General properties of the muscular system Nucleated cells Both the smooth & cardiac muscles are uninucleated cells (one nucleus per muscle cell) Skeletal muscle fiber is mutinucleated ✓ During embryonic development, many myoblasts fuse to form one skeletal muscle fiber → With each myoblast has its own nucleus, skeletal muscle fiber is then mutinucleated Muscle fiber and muscle cell are two terms for the same structure  General properties of the muscular system Contraction can be involuntary or voluntary Action for both the smooth muscle & cardiac muscle are involuntary ✓ Under the regulation of: ▪ Autonomic nervous system, and ▪ Endocrine system Skeletal muscle is mainly in voluntary system ✓ Belongs to the somatic nervous system ✓ Can also be controlled subconsciously e.g. Control of posture; contraction and relaxation of diaphragm during respiration  General properties of the muscular system Function as pacemaker cell Both the smooth muscle & cardiac muscle have the pace-making capacity i.e. can initiate their own contraction ✓ Pacemaker activities are modulated by the autonomic nervous system Skeletal muscle lacks pace-making capacity ▪ Contract only in response to a signal from a somatic motor neuron ▪ Autonomic nervous system normally has no influence on the contraction of skeletal muscle → With exception e.g.) diaphragm for respiration  Skeletal muscle Skeletal muscle Tendon Nerve and blood vessels Connective tissue Muscle fascicle: bundle of fibers Connective tissue Nucleus Muscle fiber  Skeletal muscle Skeletal muscles are: ✓ Striated muscle ✓ Mutinucleated ✓ Usually under the voluntary control ✓ Innervated by the somatic nervous system ✓ Acetylcholine as the neurotransmitter in the neuromuscular junction  Transmission of signals from the brain to the skeletal muscle Electrical signals (impulses/action potential) from the brain Through the motor neurons (nerve) To the junction between motor neuron and skeletal muscle (neuromuscular junction) where electrical is transformed into chemical (neurotransmitter) Neurotransmitter is transformed into electrical in the outer membrane of the muscle fibers Electrical is then transformed into mechanical (muscle contraction)  Functional UNIT of a skeletal muscle Skeletal muscle is a collection of muscle fibers Each fiber, sheathed in connective tissue (endomysium), are arranged with their long axes in parallel A group of fibers are bundled together into units called fascicles Skeletal muscle Tendon Nerve and blood vessels Connective tissue Muscle fascicle: bundle of fibers Connective tissue Endomysium Nucleus Muscle fiber  Functional unit of a skeletal muscle Each fascicle is enclosed by a layer of dense connective tissue (perimysium) with 10 to 100 more muscle fibers ❖ The entire muscle is then covered by another layer of connective tissue called epimysium Skeletal muscle The connective Tendon Nerve and Connective blood vessels tissue tissue layers may (epimysium) extend beyond the Muscle fascicle: bundle of fibers Connective muscle fibers to tissue (perimysium) form a tendon that Nucleus attaches to a bone Muscle fiber  Microscopic structures of a skeletal muscle fiber Each skeletal muscle fiber is a fusion of a hundred or more myoblasts during embryonic development ✓ Then each of the mature skeletal muscle fiber should has hundred or more nuclei Mitochondria Sarcoplasmic reticulum Nucleus Thick Thin filament filament T-tubules Myofibril Sarcolemma  Microscopic structures of a skeletal muscle fiber Nuclei of a skeletal muscle fiber is located just beneath the plasma membrane (sarcolemma) of the fiber ❖ Thousands of tiny invaginations (infolded membrane) of the sarcolemma called transverse (T) tubules tunnel toward the center of each fiber Mitochondria Sarcoplasmic reticulum Nucleus Thick Thin filament filament T-tubules Myofibril Sarcolemma  Microscopic structures of a skeletal muscle fiber T tubules allow fast conduction of muscle action potentials (electrical signals) into the fibers ✓ To ensure an action potential can excite all parts of the muscle fiber at essentially the same instant Myoglobin (red-colored protein) is found in the cytoplasm (sarcoplasm) of the skeletal muscle Mitochondria Sarcoplasmic reticulum Nucleus Thick Thin filament filament T-tubules Myofibril Sarcolemma  Microscopic structures of a skeletal muscle fiber Myoglobin has the ability to bind oxygen molecules ✓ It releases the oxygen molecules when it is needed (e.g. during exercise) which enhance the mitochondria’s ability to produce ATP Mitochondria Sarcoplasmic reticulum Nucleus Thick Thin filament filament T-tubules Myofibril Sarcolemma  Microscopic structures of a skeletal muscle fiber A fluid-filled system of membranous sacs called the sarcoplasmic reticulum (SR) encircles each myofibril ✓ SR with dilated end sacs known as terminal cisternae sitting against the T tubule from both sides form a triad (T tubule sandwiched by 2 terminal cisterns) ✓ Release of Ca2+ by SR triggers muscle contraction Sarcoplasmic Mitochondria reticulum Nucleus Thick Thin filament filament T-tubules Myofibril Sarcolemma terminal cisterns  Sarcoplasmic reticulum & terminal cisterna T-tubule brings action potentials into interior Sarcoplasmic of muscle fiber. reticulum stores Ca2+. Sarcolemma T-tubule Triad Thick Thin Terminal filament filament cisterna Two terminal cisternae  Contractile structures of a skeletal muscle fiber Within the muscle fibers (myofibrils) are smaller structures called myofilaments containing 3 kinds of proteins Ultrastructure of Muscle Mitochondria Sarcoplasmic reticulum Nucleus Thick Thin filament filament T-tubules Sarcolemma Myofibril A band Sarcomere Z disk Z disk Myofibril M line I band H zone  Contractile structures of a skeletal muscle fiber 3 kinds of important proteins in skeletal muscle → Contractile proteins, regulatory proteins, and structural proteins 1) The contractile proteins are actin and myosin → Actin and myosin are the thin filaments and thick filaments, respectively in muscle fibrils A band Sarcomere Z disk Z disk Myofibril M line I band H zone Titin Z disk Z disk M line Myosin M line crossbridges Thick filaments Thin filaments Nebulin Titin Troponin Myosin heads Hinge Myosin tail region G-actin molecule Myosin molecule Tropomyosin Actin chain  Contractile structures of a skeletal muscle fiber The 3 kinds of proteins are: 2) The regulatory proteins are tropomyosin & troponin (both are located in thin filaments) A band Sarcomere Z disk Z disk Myofibril M line I band H zone Titin Z disk Z disk M line Myosin M line crossbridges Thick filaments Thin filaments Nebulin Titin Troponin Myosin heads Hinge Myosin tail region G-actin molecule Myosin molecule Tropomyosin Actin chain  Contractile structures of a skeletal muscle fiber The 3 kinds of proteins are: 3) The structural proteins are titin (also known as connectin) and nebulin A band Sarcomere Z disk Z disk Myofibril M line I band H zone Titin Z disk Z disk M line Myosin M line crossbridges Thick filaments Thin filaments Nebulin Titin Troponin Myosin heads Hinge Myosin tail region G-actin molecule Myosin molecule Tropomyosin Actin chain Contractile structures of a skeletal muscle fiber → Titin function as a spring which is responsible for the passive elasticity of the muscle → Nebulin regulates the length of thin filament during assembly A band Sarcomere Z disk Z disk Myofibril M line I band H zone Titin Z disk Z disk M line Myosin M line crossbridges Thick filaments Thin filaments Nebulin Titin Troponin Myosin heads Hinge Myosin tail region G-actin molecule Myosin molecule Tropomyosin Actin chain TITIN AND NEBULIN Titin and nebulin are giant accessory proteins. Titin spans the distance from one Z disk to the neighboring M line. Nebulin, lying along the thin filaments, attaches to a Z disk but does not extend to the M line. Z disk Nebulin helps Actin Z disk align actin M line Titin provides Myosin elasticity and stabilizes myosin  Organization of a sarcomere (basic unit of the skeletal muscle) Mitochondria Sarcoplasmic reticulum Nucleus Thick Thin filament filament T-tubules Myofibril Sarcolemma A band Sarcomere Z disk Z disk Myofibril M line I band H zone Titin Z disk Z disk M line Myosin crossbridges  Organization of a sarcomere A band Sarcomere Z disk Z disk Myofibril M line I band H zone Sarcomere Basic unit of the skeletal muscle between two Z disks Z disks ✓ Are zigzag protein structures that serve as the attachment site for thin filaments  Organization of a sarcomere A band Sarcomere Z disk Z disk Myofibril M line H zone I band I bands ✓ The lightest color of band of the sarcomere which occupied only by thin filaments with a Z disk in the middle of every I band  Organization of a sarcomere A band Sarcomere Z disk Z disk Myofibril M line I band H zone A bands ✓ The darkest of the sarcomere bands and occupied the entire length of a thick filament ✓ The thick and thin filaments overlap at the outer edges ✓ The center of the A band is occupied by thick filaments only  Organization of a sarcomere A band Sarcomere Z disk Z disk Myofibril M line I band H zone H zone ✓ The center region of the A band is lighter than the outer edges of the A band ✓ The H zone is occupied by thick filaments only  Organization of a sarcomere A band Sarcomere Z disk Z disk Myofibril I band H zone M line M line ✓ This band represents proteins that form the attachment site for thick filaments ✓ Each M line divides an A band in half Ultrastructure of muscle contractile unit responsible for muscle contraction  Contractile proteins of the muscle fiber ❖ The 2 contractile proteins in muscle are myosin & actin, correspond to thick & thin filaments, respectively Sarcomere Titin Z disk Z disk M line Myosin crossbridges M line Thick filaments Thin filaments Titin Troponin Nebulin Myosin heads Hinge Myosin tail region Tropomyosin G-actin molecule Myosin molecule Actin chain  Contractile proteins of the muscle fiber Myosin The main components of thick filaments In skeletal muscle, about 250 - 300 molecules of myosin molecules in a single thick filament Each myosin molecule is shaped like 2 golf clubs M line twisted together Thick filaments ✓ With projection of two globular heads for each myosin Myosin heads molecule and they Myosin tail Hinge region are called myosin heads Myosin molecule  Contractile proteins of the muscle fiber Myosin (thick filaments) Myosin functions as a motor protein in all 3 types of the muscle tissues The protruding myosin heads are attached to the stiff tail with an elastic hinge region, allowing the heads to swivel around the point of attachment and to create movement Myosin heads Hinge Myosin tail region  Contractile proteins of the muscle fiber Myosin head (thick filaments) With site for binding actin and hydrolyzing ATP i.e., ATP ADP + Pi Myosin head contains ATPase ✓ ATPase hydrolyzes ATP to release energy which will then be utilized by the myosin head to create movement ▪ ATP is hydrolyzed to form ADP as well as inorganic phosphate (Pi) and energy is released during this process  Contractile proteins of the muscle fiber Actin (thin filaments) Actin molecule is a globular protein (G-actin) that represents the major components of the thin filaments within the muscle fiber Polymerization of multiple G-actin molecules forms a long filament called F-actin (actin chain or filamentous actin) Thin filaments Titin Troponin Nebulin Tropomyosin G-actin molecule Actin chain  Contractile proteins of the muscle fiber Actin (thin filaments) Two F-actin polymers twisted together, with the regulatory proteins (tropomyosin & troponin), into a helix and forms the thin filament of the myofibril Each G-actin molecule has a single myosin- binding site (allows myosin head attaches to this site) Thin filaments Titin myosin-binding TroponinNebulin site Tropomyosin Actin chain G-actin molecule  Regulatory proteins of the skeletal muscle fiber Regulatory proteins: tropomyosin & troponin They both are part of the thin filament Troponin Nebulin G-actin molecule Tropomyosin Actin chain In a relaxed muscle, tropomyosin strands cover the myosin-binding sites located within the G-actin molecule on actin ✓ Preventing myosin heads in the thick filaments from binding to actin molecules ✓ The role of tropomyosin is to regulate the binding of myosin head to actin  Regulatory proteins of the skeletal muscle fiber Regulatory proteins: tropomyosin & troponin  Regulatory proteins of the skeletal muscle fiber Regulatory proteins: tropomyosin & troponin  Regulatory proteins of the skeletal muscle fiber Regulatory proteins: tropomyosin & troponin Troponin is a heterotrimer consisting of: 1) Troponin T (TnT) ✓ Binds to a single molecule of tropomyosin 2) Troponin C (TnC) ✓ Binds Ca2+ 3) Troponin I (TnI) ✓ Covers the myosin head binding site on G-actin molecule and inhibits muscle contraction The interaction among troponin, tropomyosin, and actin allows actin-myosin interaction to be regulated through the changes in sarcoplasmic [Ca2+]  Regulatory proteins of the skeletal muscle fiber Regulatory proteins: tropomyosin & troponin With TnI covering the myosin binding site (white) Troponin located on the attached to G-actin of the tropomyosin thin filament TnI is pulled away from the Myosin head myosin binding of the thick site (white), filament allowing the detached binding of actin from the thin and myosin filament  Sample questions 1) Which muscle type(s) is(are) uninucleated? a) Cardiac muscle only. b) Skeletal muscle only. c) Smooth muscle only. d) Cardiac and skeletal muscles. e) Cardiac and smooth muscles. 2) Which muscle type(s) has(have) the pace-making capacity? a) Cardiac muscle only. b) Skeletal muscle only. c) Smooth muscle only. d) Cardiac and skeletal muscles. e) Cardiac and smooth muscles.  Sample questions 3) Which of the following forms the attachment site for thick filaments? a) Z disk. b) M line. c) A band. d) H zone. 4) Which of the following is the function of titin? a) Provides elasticity and stabilizes myosin. b) Regulates the length of thin filament during assembly. c) Serves as the regulatory proteins. d) Serves as the contractile proteins.  Sample questions 5) Sarcoplasmic reticulum: a) Release K+ that causes muscle relaxation. b) Release Ca++ that initiates muscle contraction. c) Is the contractile protein that regulate muscle contraction. d) None of the above. 6) A band normally contains: a) Thick elements only. b) Thin elements only. c) Both thick and thin elements.  Answer to sample questions 1) e 2) e 3) b 4) a 5) b 6) c