Skeletal Muscle Tissue | BIO 110

Professor Lindboom-Broberg (LB) Skeletal Muscle Tissue Muscle Tissue Anatomical Structures Muscular System  A system of ~700 muscles Two Divisions 1. Axial division Axial muscles support and position axial skeleton 2. Appendicular division...

Professor Lindboom-Broberg (LB) Skeletal Muscle Tissue Muscle Tissue Anatomical Structures Muscular System  A system of ~700 muscles Two Divisions 1. Axial division Axial muscles support and position axial skeleton 2. Appendicular division Appendicular muscles support, move, and brace the limbs Muscular System Functions 1. Produce body movement Muscle tendons pull and move bones 2. Maintain posture and body Stabilize joints position 3. Support soft tissues Surround, support, and shield internal structures, such as tissues and organs 4. Guard body Sphincters encircle openings; provide entrances/exits voluntary control of swallowing, defecation, and urination 5. Maintain body temperature Contraction uses energy; energy use generates heat 6. Store nutrients Muscle proteins can break down; release amino acids—can be used to synthesize glucose or provide energy Muscle Tissue Muscle tissue  Contractile cells  3 types of tissue skeletal muscle cardiac muscle smooth muscle  Myocyte = Muscle cell / fiber Skeletal muscle  An organ made of mostly skeletal muscle tissue Plus connective tissue, nerves, blood vessels  Directly/indirectly attached to bones Muscle Tissue Comparison of muscle tissues Skeletal muscle tissue Voluntary Multinucleated Long, cylindrical cells Striated Cardiac muscle tissue Involuntary Uninucleated Branching cells Striated Smooth muscle tissue Involuntary Uninucleated Fusiform cells (tapered) Skeletal Muscle Skeletal muscle = organ At each end of a muscle, connective tissues merge to form a tendon or aponeurosis  Tendon: Attaches muscle to specific point on a bone  Aponeurosis: Broad sheet with broad attachment to bone(s)  Contracting muscle pulls on tendon or aponeurosis, which pulls on and moves the bone Skeletal Muscle Building a muscle Skeletal muscle fibers  Individual muscle cells  Contain myofibrils = bundles of protein filaments Endomysium  Thin layer of areolar connective tissue around each muscle fiber  Collagen & elastic fibers, blood vessels, nerves Skeletal Muscle Building a muscle Muscle fascicle  Bundle of muscle fibers/cells & surrounding endomysium Perimysium  Fibrous layer dividing fascicles Epimysium  Dense sheath of collagen fibers around muscle  Separates muscle from other tissues/organs  Connected to deep fascia Skeletal Muscle Inside a skeletal muscle fiber  Sarcolemma: Plasma membrane  Sarcoplasm: Cytoplasm  Myofibril: Cylindrical structures arranged parallel inside muscle fiber; run length of muscle fiber  Myofibril arrangement gives skeletal muscle stripes (striations)  Many mitochondria along myofibrils  Nuclei are pushed to the edge of the cell Skeletal Muscle Inside a skeletal muscle fiber  Sarcolemma: Plasma membrane Selective permeability allows uneven distribution of +/– charges Reversal of charge is 1st step leading to muscle contraction  Transverse tubules (T tubules) Continuous with sarcolemma and extend into sarcoplasm Form passageways through muscle fiber and encircle sarcomere Allows events at cell surface to penetrate “into” the cell Skeletal Muscle Inside a skeletal muscle fiber  Sarcoplasmic reticulum (SR) Similar to smooth endoplasmic reticulum of other cells Makes contact with T-tubule Surrounds each muscle cell Stores calcium ions (actively pumped in from cytosol) – Major role in muscle contraction Skeletal Muscle Inside a skeletal muscle fiber  Myofilaments: Bundles of protein filaments inside myofibrils Thin filaments mostly composed of the protein actin Thick filaments mostly composed of the protein myosin  Arrangement creates striations  Professor Lindboom-Broberg (LB) Sliding Filament Theory How are muscles contractile? Sliding Filament Theory Inside a skeletal muscle fiber  Sarcomeres: Repeating functional units of skeletal muscle fiber Overlapping sections of thick & thin filaments ~10,000 sarcomeres/myofibril, each ~2 µm resting length Sliding Filament Theory Inside a skeletal muscle fiber  Sarcomeres Z lines: Junction of adjacent sarcomeres I band: Lighter band with only thin filaments (l-I-ght) A band: Dark/dense region containing thick filaments (d-A-rk) Zone of overlap: Within A band; overlapping thick/thin filaments M line: Center of A band where adjacent thick filaments connect H band: Region on each side of M line with only thick filaments Sliding Filament Theory Sliding filament theory of muscle contraction  When muscles contract, thin filaments slide over thick filaments H and I bands get smaller; zones of overlap get larger Z lines move closer together, but A bandwidth is unchanged  Sliding occurs in all sarcomeres in a myofibril  As myofibrils shorten, so does the muscle fiber (contraction) Sliding Filament Theory Sliding filament theory of muscle contraction  Muscle contraction cycle Nerve signals muscle to contract (electrical) Electrical signal travels through Transverse Tubules Sarcoplasmic reticulum releases calcium Calcium activates muscle contraction Myosin and actin bind Z-lines are pulled closer to one another, sarcomere shortens Muscle shortens  Professor Lindboom-Broberg (LB) Excitable Membranes How membranes create electrical changes Excitable Membranes Membrane potentials  Electrical charge difference across a cell membrane All cells unequally distribute charged ions across their membrane Inside of cell slightly more negative  Measured in millivolts (mV)  Neurons have resting membrane potentials of about –70 mV  Skeletal muscle fibers resting potentials of about –85 mV Excitable Membranes Unequal ion distribution  Cytosol and extracellular fluid (ECF) have different compositions  Membrane is selectively permeable  Pumps maintain distribution Positive charges: – More Na+ outside (ECF) – More K+ inside (cytosol) Negative charges: – Mostly proteins inside cell; cannot cross plasma membrane – More Cl– in ECF but not much diffuses across (small impact) Excitable Membranes Movement of sodium and potassium  Integral transmembrane proteins act as channels  Sodium–Potassium Pump: Export 3 Na+ and import 2 K+ Uneven distribution maintains resting membrane potential Excitable Membranes 5 steps in an action potential  Action Potential: Change in membrane potential due to ion movement 1. Small increase in membrane permeability to Na+ Na+ entering cell moves membrane potential in positive direction to threshold (–55 mV) Excitable Membranes 5 steps in an action potential  Action Potential: Change in membrane potential due to ion movement 2. Voltage-gated Na+ channels open Rush of positive Na+ ions into cell Depolarization: Change of membrane potential to positive Excitable Membranes 5 steps in an action potential  Action Potential: Change in membrane potential due to ion movement 3. Membrane potential reaches +30 mV Voltage-gated Na+ channels close Voltage-gated K+ channels open and K+ leaves cell Repolarization: Membrane potential returns to polarized state Excitable Membranes 5 steps in an action potential  Action Potential: Change in membrane potential due to ion movement 4. Repolarization continues to resting membrane potential Excitable Membranes 5 steps in an action potential  Action Potential: Change in membrane potential due to ion movement 5. Membrane potential stabilizes Voltage-gated K+ channels close at resting potential Na+/K+ pump restores original distribution of Na and K+ Excitable Membranes 5 steps in an action potential  Action Potential: Change in membrane potential due to ion movement  Refractory period: Time when firing an AP is impossible or difficult Excitable Membranes Action potential (AP) propagation  Neurons and skeletal muscle fibers have excitable membranes APs propagate along plasma membrane Generated in less than 2 ms Travels in only one direction due to refractory period Allows rapid communication  Professor Lindboom-Broberg (LB) Neuromuscular Junction Nervous system control of muscular tissue Neuromuscular Junction Neuromuscular junction (NMJ)  Location where motor neuron controls a skeletal muscle fiber  One NMJ per muscle fiber, but each motor neuron may branch and control multiple muscle fibers Neuromuscular Junction Neuron  Nervous tissue Neuromuscular Junction Components 1. ​ xon terminal (synaptic terminal) of motor neuron A Has vesicles with acetylcholine (ACh), a neurotransmitter Neuromuscular Junction Components 2. ​ otor end plate: Portion of muscle fiber that receives nerve signal M Has ACh receptors able to bind ACh Junctional folds (creases) increase # of ACh receptors Neuromuscular Junction Components 3. ​ ynaptic cleft = space between axon terminal and motor end plate S Contains acetylcholinesterase (AChE) – Breaks down ACh Neuromuscular Junction Activities 1. Action potential arrives at motor neuron axon terminal ACh vesicles fuse with neuron plasma membrane ACh released into synaptic cleft (exocytosis) Neuromuscular Junction Activities 2. ACh diffuses across synaptic cleft Binds ACh-receptor on motor end plate Changes sarcolemma Na+ permeability Na+ enters muscle fiber sarcoplasm Neuromuscular Junction Activities 3. Na+ influx generates action potential in sarcolemma ACh diffuses away or broken down by AChE ACh-receptor membrane channels close Neuromuscular Junction Activities 5. Action potential moves down T tubules between terminal cisternae of sarcoplasmic reticulum (SR) Ca2+ permeability of SR changes Neuromuscular Junction Activities 6. SR releases stored Ca2+ into sarcoplasm Contraction begins  Professor Lindboom-Broberg (LB) Muscle Tension & Contraction Twitching, Tetanus, and Myofibril Recruitment How do muscles move? Muscle Tension Tension produced by a skeletal muscle determined by: 1. Amount of tension produced by each muscle fiber On vs. Off 2. Number of muscle fibers stimulated Depends on stimulation frequency Motor Unit  A motor neuron and all of the muscle fibers it innervates  One motor neuron for multiple muscle fibers Muscle Contraction Contraction: A muscle activation usually generating movement  Non-resting contractions (2 categories) Isotonic Isometric Defined by the amount of tension and length of muscle Muscle Contraction Isotonic contraction (iso-, equal + tonos, tension) Tension is stable and skeletal muscle length changes Examples: lifting an object, walking, running  Two types of isotonic contractions 1. ​Concentric contraction 2. ​Eccentric contraction Defined by the direction of movement Muscle Contraction Concentric contraction  Muscle tension rises until it exceeds load  As muscle shortens, tension remains constant (isotonic)  Example: flexing elbow while holding a dumbbell Muscle Contraction Eccentric contraction  Peak tension produced is less than the load  Muscle lengthens (elongates)  Example: returning dumbbell from flexed position to extended  When contraction ends, load stretches muscle until… Muscle tears Tendon breaks Elastic recoil opposes load Muscle Contraction Isometric Contraction (iso-, equal + metric, length)  Muscle length does not change  Tension generated, but never exceeds load  Contracting muscle bulges but not as much as during isotonic contraction  Example: postural muscle contractions Muscle Contraction Contractions Occur in pairs Muscle Activity Origin: Where fixed end of a skeletal muscle attaches  Most are bones  Some are connective tissue sheaths or bands  Typically proximal to insertion in anatomical position Insertion: Where the movable end of a skeletal muscle attaches Action: Specific movement produced by a skeletal muscle  Professor Lindboom-Broberg (LB) Muscle Disorders We know how things go right. How can they go wrong? Muscular Disorders Atrophy: Decreased muscle size, tone, and power  Due to decreased stimulation Reduced use (cast after fracture, extended hospital stay) Normal aging (decreased activity) Paralysis/nervous system damage  Initially reversible; if prolonged muscle fibers can die and not be replaced Muscular Disorders Hypertrophy: Muscle enlargement  High stress in short time scale  The # of muscle fibers does not change  Size increase due to: More/wider myofibrils More myofilaments More mitochondria More glycogen/glycolytic enzyme Muscular Disorders Muscular dystrophy  A group of inherited diseases; several versions  Produce progressive muscular weakness/deterioration  Most common/understood are Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) Childhood-onset, male only Death usually from respiratory paralysis Sex-linked gene, carried by female Muscular Disorders Polio  Virus  Attacks CNS motor neurons  Causes atrophy and paralysis (loss of voluntary movement)  President Franklin D. Roosevelt  Vaccine available since 1955 Muscular Disorders Tetanus  Toxin from bacteria (Clostridium tetani)  Suppresses mechanism that inhibits motor neuron activity  Causes sustained, powerful muscle contractions  Thrives in low-oxygen areas (puncture wounds)  If severe, 40–60% mortality  Vaccines came in 1940s Muscular Disorders Myasthenia Gravis  Autoimmune disease  Loss of ACh receptors at neuromuscular junctions No receptor = no effect  Results in progressive muscular weakness Muscular Disorders Rigor mortis  Generalized muscle contraction shortly after death  Depletion of ATP leaves calcium in sarcoplasm triggering sustained contraction  Myosin cross-bridges cannot detach from active sites  Ends 1–6 days later as muscle tissue decomposes