Lesson 11_Skeletal Muscle Tissue PDF

Human Anatomy Skeletal Muscle Tissue Course: BL3420 Date: 02/08/24 Instructor: Benjamin T. Enslow, M.D. Email: [email protected] Phone: (830) 391-4298 Office: Moody 214 Text: Chapter 10; Sections 10.1-10.4 Slides Adapted From: PowerPoint® Lecture Presentations prepared by Leslie Hendon University...

Human Anatomy Skeletal Muscle Tissue Course: BL3420 Date: 02/08/24 Instructor: Benjamin T. Enslow, M.D. Email: [email protected] Phone: (830) 391-4298 Office: Moody 214 Text: Chapter 10; Sections 10.1-10.4 Slides Adapted From: PowerPoint® Lecture Presentations prepared by Leslie Hendon University of Alabama, Birmingham Molly Selba University of Florida Copyright © 2020 Pearson Education, Inc. All Rights Reserved Learning Objectives: List four functional properties that distinguish muscle tissue from other tissues. Name the layers of connective tissue that occur in and around the skeletal muscle, and briefly describe a muscle’s blood supply and nerve supply. Describe the various ways in which muscles attach at their origins and insertions. Explain some of the symptoms of muscular dystrophy, myofascial pain syndrome, fibromyalgia, and delayed-onset muscle soreness. Muscle Muscle—a Latin word for “little mouse” Muscle is the primary tissue in the Heart (cardiac muscle tissue) Walls of hollow organs (smooth muscle tissue) Skeletal muscle Makes up nearly half the body’s mass 10.1a Properties of Muscle Tissue Contractility Myofilaments are responsible for shortening of muscles cells They are comprised of either actin or myosin proteins Excitability Nerve signals excite muscle cells, causing electrical impulses to travel along the cell’s plasma membrane Extensibility Contraction of a skeletal muscle stretches the opposing muscle Smooth muscle is stretched by substances within that hollow organ Food in stomach; urine in urinary bladder Elasticity Recoils after being stretched 10.1a Terminology Specific to Muscle Tissue Myo and mys—prefixes meaning “muscle” Sarco—prefix meaning “flesh” Sarcolemma—plasma membrane of muscle cells Sarcoplasm—cytoplasm of muscle cells Sarcoplasmic reticulum- endoplasmic reticulum of muscle cells 10.1b Functions of Muscle Tissue Produce movement Skeletal muscle—attached to skeleton Moves body by moving the bones Smooth muscle—squeezes fluids and other substances through hollow organs Open and close body passageways Sphincter muscles function as valves Open to allow passage of a substance Contract to close the passageway Maintain posture and stabilize joints Enables the body to remain sitting or standing Muscle tone helps stabilize many joints Heat generation Muscle contractions produce heat Helps maintain normal body temperature 10.1c Types of Muscle Tissue Cardiac muscle tissue Occurs only in the walls of the heart Cells are striated; branching appearance Contraction is involuntary Smooth muscle tissue Occupies the walls of hollow organs Cells are NOT striated; “fusiform” appearance Contraction is involuntary Cardiac Muscle Smooth Muscle 10.1c Types of Muscle Tissue Skeletal muscle Attach to and move the skeleton 40% of our body weight Cells are striated; long and cylindrical Multinucleated Contraction is voluntary Skeletal muscle also contains Connective tissue Blood vessels Nerves Skeletal Muscle 10.2a Gross Anatomy of a Skeletal Muscle Skeletal Muscle Organization Smallest Fiber Largest Fascicle (cell) Muscle Belly (organ) Sheaths of connective tissue bind a skeletal muscle and its fibers together Endomysium—a fine sheath of connective tissue wrapping each muscle fiber (cell) Perimysium—fibrous connective tissue that surrounds each fascicle (group of muscle fibers) Epimysium—dense regular connective tissue surrounding entire muscle belly Connective tissue sheaths are continuous with tendons When muscle fibers contract, pull is exerted on all layers of connective tissue Sheaths provide elasticity and carry blood vessels and nerves 10.2a Gross Anatomy of a Skeletal Muscle Figure 10.1 Epimysium Bone Epimysium Perimysium Endomysium Tendon Muscle fiber in middle of a fascicle Blood vessel Fascicle (wrapped by perimysium) Endomysium (between individual muscle fibers) Epimysium Perimysium Fascicle Endomysium Muscle fiber 10.2a Gross Anatomy of a Skeletal Muscle Nerves and blood vessels Each skeletal muscle supplied by branches of One nerve One artery One or more veins Nerves and vessels branch repeatedly Smallest branches serve individual muscle fibers Spinal Cord Motor neuron The arterial supply was injected with dark red gelatin to demonstrate the capillary bed. The muscle fibers, which run horizontally across the photograph, are stained orange. 10.2a Gross Anatomy of a Skeletal Muscle Muscle attachments Most skeletal muscles run from one bone to another One bone will move; other bone remains fixed Origin—less movable attachment Insertion—more movable attachment When muscle contracts, its insertion is pulled toward its origin. 10.2a Gross Anatomy of a Skeletal Muscle Muscles attach to origins and insertions by connective tissue (C T) Direct attachments—C T fibers are short Indirect attachments—C T forms a tendon or aponeurosis Bone markings present where tendons meet bones Tubercles, trochanters, and crests 10.2b Microscopic Structure of Skeletal Muscle Tissue The skeletal muscle fiber Fibers are long and cylindrical Huge cells—diameter is 10–100 µm Length—several centimeters to dozens of centimeters Each cell formed by fusion of embryonic cells Cells are multinucleate Nuclei are peripherally located Striations result from internal structure of myofibrils Are long rods within sarcoplasm Make up 80% of the sarcoplasm Are a specialized contractile organelle found in muscle tissue 10.2b Microscopic Structure of Skeletal Muscle Tissue Myofibrils contain myofilaments, which are the contractile proteins of muscle Myofilaments are arranged in repeating segments called sarcomeres The sarcomere is the basic contractile unit of skeletal muscle. Sarcomere Organization Z disc (Z line)— Boundaries of each sarcomere Thin (actin) filaments— Extend from Z disc toward the center of the sarcomere Thick (myosin) filaments— Located in the center of the sarcomere Overlap inner ends of the thin filaments Muscle contraction occurs when the thin filaments (actin) interact with the thick filaments (myosin)! 10.2b Microscopic Structure of Skeletal Muscle Tissue A bands—full length of the thick filament Overlaps thin filaments Anisotropic (DARK) I band—region with only thin filaments Is part of two adjacent sarcomeres Isotropic (LIGHT) H zone—center part of A band where no thin filaments (actin) occur M line—in center of H zone Contains tiny rods that hold thick filaments together Remembering the Sarcomere “Zee Intelligent Animal Has Muscles” Z is the final alphabet: Z lines represents the end of sarcomere I is a thin letter: I band has only thin filaments A is a hybrid of “I” and “H”: A band has both thin and thick filaments H is a thick letter: H zone has only thick filaments M for middle: M line represents the midline of sarcomere 10.2b Microscopic Structure of Skeletal Muscle Tissue What changes during a sarcomere contraction? Remember: “A Always stAys the sAme” “HIZ muscles shrank” 10.2b Microscopic Structure of Skeletal Muscle Tissue Sarcoplasmic reticulum (SR) A specialized smooth endoplasmic reticulum Interconnecting tubules surround each myofibril Some tubules form cross-channels called terminal cisterns Cisterns occur in pairs on either side of a t tubule Contains calcium ions—released when muscle is stimulated to contract Calcium ions diffuse through sarcoplasm Triggers the sliding filament mechanism T tubules—deep invaginations of sarcolemma Triad—T tubule flanked by two terminal cisterns Terminal cisterns of SR Myofibril Sarcolemma Sarcolemma T tubule Triad 10.2b Microscopic Structure of Skeletal Muscle Tissue Contraction in skeletal muscle is ultimately controlled by nervegenerated impulses that travel along the sarcolemma of the muscle fiber. Because the T tubules are continuations of the sarcolemma, they conduct each impulse to the deepest regions of the muscle fiber. Impulses traveling down the T tubules stimulate the release of calcium from the terminal cistern of the SR. Now that Ca2+ concentrations have risen in the sarcoplasm, Ca2+ can interact with the elements of the myofibril to induce sarcomere contraction. 10.2c Functional Anatomy of Skeletal Muscle Tissue To generate a contraction of the sarcomere, the thick filament myosin needs to interact with the thin filament actin. However, when the muscle fiber is at rest, actin needs to be protected from myosin so that the muscle can relax. This is accomplished by an assortment of proteins that block myosin from interacting with actin while at rest. When the muscle is at rest, a protein called tropomyosin blocks the myosin binding sites on actin. Another protein, called troponin, senses rises in the calcium concentration in the sarcoplasm. Calcium binds to troponin, and troponin forces tropomyosin to change its shape, exposing the myosin binding sites on actin. 10.2c Functional Anatomy of Skeletal Muscle Tissue 10.2c Functional Anatomy of Skeletal Muscle Tissue 1 2 Fully relaxed sarcomere of a muscle fiber Fully contracted sarcomere of a muscle fiber Z H Z Z I A I I Z A I 10.2c Functional Anatomy of Skeletal Muscle Tissue Two major types of contraction Concentric contraction—muscle shortens to do work Eccentric contraction—muscle generates force as it lengthens Muscle acts as a “brake” to resist gravity “Down” portion of a pushup is an example 10.2c Functional Anatomy of Skeletal Muscle Tissue Motor neurons innervate skeletal muscle tissue Neuromuscular junction is the point where nerve ending and muscle fiber meet Terminal boutons (axon terminals) Located at ends of axons Store neurotransmitters (acetylcholine) Synaptic cleft Space between axon terminal and sarcolemma Nerve impulse Myelinated axon of motor neuron Terminal bouton of neuromuscular junction Nucleus Sarcolemma of the muscle fiber 1 Nerve impulse stimulates the release of the neurotransmitter acetylcholine (ACh) into the synaptic cleft. Terminal bouton of motor neuron Synaptic cleft 2 ACh stimulates changes in the sarcolemma that excite the muscle fiber. This stimulus is carried down the T tubules to initiate fiber contraction. Synaptic vesicle containing ACh Sarcolemma Terminal cistern of SR Triad Muscle fiber Ca2+ 3 Enzymes in the synaptic cleft break down ACh and thus limit its action to a single muscle twitch. 10.2c Functional Anatomy of Skeletal Muscle Tissue The axon of a motor neuron branches to innervate a number of fibers in a skeletal muscle. A motor neuron and all the muscle fibers it innervates are called a motor unit When a motor neuron fires, all the skeletal muscle fibers in the motor unit contract together. Muscles that require very fine control have few muscle fibers per motor unit, whereas bulky, weight-bearing muscles, whose movements are less precise have many muscle fibers per motor unit. 10.2c Functional Anatomy of Skeletal Muscle Tissue In addition to variation in the size of motor units in different muscles, each individual muscle contains many motor units. The addition of motor units to accomplish a movement is called recruitment. If a small force is required, a small number of motor units are stimulated. As more force is needed, additional motor units are recruited. The uncontrollable contraction of a small portion of a muscle, usually a single motor unit, is a muscle twitch, also called a muscle fasciculation. 10.3a Muscular Dystrophy Muscular dystrophy is a group of inherited muscle-destroying diseases that generally appear in childhood. The affected muscles enlarge with fat and connective tissue while the muscle fibers degenerate. The most common form is Duchenne muscular dystrophy (DMD) X-linked recessive (affects males; females carry) Mutation affects a submembrane protein called dystrophin, which links the cytoskeleton of a muscle fiber to its extracellular matrix Without this “anchor”, the sarcolemma weakens and tears, damaging the cell. Muscle weakness begins between 2 and 7 years of age; begins in pelvic muscles and ascends Can eventually affect the diaphragm Patients rarely live past 30 years of age. Healthy DMD Myofascial Pain Syndrome vs. Fibromyalgia Myofascial Pain Syndrome Pain is caused by tightened bands of muscle fibers that twitch when the skin over them is touched. The sensitive areas of skin are called trigger points. Associated with overused or strained postural muscles Very common; up to half of all people Treated with NSAIDS; massage; stretching Fibromyalgia Mysterious chronic-pain syndrome of unknown cause To be identified as fibromyalgia, pain must be present in at least 11 of 18 standardized points that are spread widely over the body Treated with antidepressants; exercise; OTC pain relievers Delayed-Onset Muscle Soreness Soreness that begins 8–24 hours after an activity is called delayed-onset muscle soreness or post-exercise muscle soreness. Soreness is caused by microscopic tears in the muscle fibers and is most common after eccentric exercise. Inflammatory response to the tears → swelling → compression of the sensory nerves of the muscle → Ouch! Low-level aerobic activity increases blood supply to the muscle and speeds recovery. Microscopic tears stimulate increased production of myofibrils and myofilaments, resulting in increased muscle strength.