A+P - Ch. 9 - A summarization of Chapter 9 of the Marieb A+P textbook 11th edition. PDF

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A summarization of Chapter 9 of the Marieb A+P textbook 11th edition....

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Chapter 9 Overview of Muscle Tissue Muscle makes up nearly half of the body’s mass Can transform ATP into mechanical energy Three types of muscle tissue Skeletal: attaches to bone and skin, striated, voluntary Cardiac: makes up bulk of heart wall, striated, involuntary Smooth: hollow walls of organs, not striated, involuntary Skeletal and smooth muscle cells are elongated, called muscle fibers All muscles share the same four characteristics Excitability: ability to respond and receive Contractibility: ability to shorten forcibly when stimulated Extensibility: ability to be stretched Elasticity: ability to recoil to resting length This includes functions like producing movement (locomotion), maintaining posture and body position, stabilizing joints, and generating heat as they contract. Skeletal Muscle Anatomy Skeletal muscle is an organ made up of three distinct features Nerve and blood supply: each muscle has nerve, artery, and vein; waste removed quickly Connective tissue sheaths: epimysium, perimysium, and endomysium Epimysium: dense irregular CT, surrounded entire muscle Perimysium: fibrous CT, surrounds fascicles Endomysium: fine areolar CT, surrounds individual fibers Attachments: insertion/origin, direct (fused to peri) or indirect (ropelike tendons) Muscle Fiber Microanatomy/Sliding Filament Model Muscular Fiber Anatomy Sarcolemma: muscle fiber plasma membrane Sarcoplasm: muscle fiber cytoplasm Contains many glycosomes for glycogen storage as well as myoglobin Modified organelles: Myofibrils, Sarcoplasmic reticulum (smooth ER), T tubules Myofibrils are densely packed, rodlike elements; single muscle fiber contains 1000s Features of the myofibrils include Striations: stripes formed by dark and light bands repeating ACTIVELY STUDY 9A, SLIDE 25 (BELOW) A band: dark regions w/ H zones (lighter) in between and M line I band: light regions w/ Z disc (sheet of protein in between I band)

Sarcomeres: smallest contractile unit of muscle fiber; A band w/ ½ I band each end Myofilaments: actin and myosin Components of myofilaments: Tropomyosin and troponin: regulatory proteins bound to actin

Sarcoplasmic reticulum: network of smooth ER tubules surrounding each myofibril; hold the Ca+ T tubules: tube formed by protrusion of sarcolemma deep into cell interior Triad: area formed from cistern of a sarcomere, T tubule, and neighboring sarcomere Contraction is the activation of cross bridges to generate force During contraction, actin slides over myosin, shortening the muscle This occurs due to cross bridges when muscle fibers are stimulated via nerves Decision to move is activated by the brain sent down the spinal cord to motor neurons Action potential crosses from neuron to muscle cell via ACh (acetylcholine) Muscle Fiber Contraction Starts at the precentral gyrus in the frontal lobe Ion Channels play a major role in changing of membrane potential Chemically gated channels: opened by chemical messengers, i.e. ACh Voltage gated channels: open or close due to voltage changes in membrane potential Four steps must occur for skeletal muscles to contract

1. 2. 3. 4.

Events at neuromuscular junction Muscle fiber excitation Excitation-contraction coupling Cross bridge cycling

Events at the neuromuscular junction 1. AP arrives at axon terminal 2. Voltage-gated calcium channels open, calcium enters motor neuron 3. Calcium entry causes release of ACh neurotransmitter into the synaptic cleft 4. ACh diffuses across to ACh receptors (Na+ chemical gates) on sarcolemma 5. ACh binding to receptors, opens gates, allowing Na+ to enter resulting in end plate potential 6. Acetylcholinesterase degrades ACh Action Potential is caused by changes in electrical charges in three steps Generation of end plate potential: Na+ comes into MUSCLE due to chem-gated channels, become + Depolarization: generation and propagation of an action potential If enough change in membrane voltage, voltage-gated channels open, Na+ enter Repolarization: restoration of resting conditions Na+ channels close, K+ channels open; Na-K pump restore conditions Excitation-contraction coupling is when AP travels across the sarcolemma (excitation) while sliding of myofilaments occur (contraction) Cross bridge cycling Low Ca++ concentration: tropomyosin blocks actin sites, myosin cannot bind High Ca++ concentration: Ca+ binds to troponin, moves tropomyosin away from actin Four steps of cross bridge cycle 1. Cross bridge formation: high energy myosin head attach to actin 2. Working (power) stroke: myosin head pivots and pulls actin to M line 3. Cross bridge detachment: ATP attaches to myosin head, cross bridge detach 4. Cocking of myosin head: energy from hydrolysis of ATP puts myosin into high energy Whole Muscle Contraction Contraction produces muscle tension Isometric contraction: no shortening; tension increases but does not exceed load Isotonic contraction: muscle shortens cause muscle tension exceeds load Motor unit is the nerve-muscle functional unit Consists of the motor neuron and all the muscle fibers (4 - 100+) Smaller the number of fibers, the finer the control Muscle twitch is the simplest contraction from a response of a single action potential Recorded as a myogram There are three phases Latent period: events of excitation-contraction coupling Period of contraction: cross bridge formation Period of relaxation: Ca+ reentry into SR Differences in strength and frequency of twitch based on enzymes and metabolic property Graded muscle responses vary in strength of contractions Required for proper control of skeletal muscles Changing frequency of stimulation Wave (temporal) summation: two stimuli received in rapid succession

If frequency increases, muscle tension reaches maximum Prolonged muscular contraction leads to muscular fatigue Changing strength of stimulation Recruitment: stimulus is sent to more fibers Motor units with smaller fibers fire first, larger second, largest rarely Muscle tone is the constant, slightly contracted state of all muscles Isotonic contractions: muscle changes in length and moves load Concentric: muscle shortens and does work Eccentric: muscle lengths and generates force Isometric: load is greater than the maximum tension muscle can generate Energy For Contraction and ATP ATP supplies the energy needed for the muscle fibers to Move and detach cross bridges Pump calcium back into the SR Pump Na+ out of and K+ back into cell ATP depletes in 4-6 seconds ATP is regenerated by three mechanisms Phosphorylation of ADP by creatine phosphate Creatine kinase is an enzyme that carries out the transfer Anaerobic pathways: glycolysis and lactic acid formation Glycolysis is the first step in glucose breakdown, does not need oxygen; 2 pyruvic, 2 ATP Lactic acid diffuses into the bloodstream, used by the liver, kidneys, and heart Byproduct of unoxidized pyruvic acid Aerobic pathway (Kreb’s cycle) Produces 95% of ATP during rest and light-to-moderate exercise

Fatigue is the physiological inability to contract despite continued stimulation Ionic imbalances Increased inorganic phosphate Decreased ATP and increased magnesium EPOC is Excessive Post exercise Oxygen Consumption For a muscle to return to its pre-exercise state Oxygen reserves are replenished Lactic acid is converted back to pyruvic acid Glycogen stores are replaced ATP and creatine phosphate reserves are resynthesized Factors of Muscular Contraction Force of contraction depends on number of cross bridges attached Number of muscle fibers stimulated Relative size of fibers Frequency of stimulation Degree of muscle stretch How fast a muscle contracts and how long is influenced by Muscle fiber type Slow or fast fibers

Metabolic pathways used (aerobic or anaerobic) Slow oxidative Fast oxidative Fast glycolytic Load Muscles contract fastest when no load is added ↑ load, ↓ duration and speed of contraction Recruitment The more motor units contracting, faster the contraction Adaptation to Exercise Aerobic exercise results in greater endurance, strength, resistance fatigue; convert fast glycolytic Muscle capillaries Number of mitochondria Myoglobin synthesis Resistance exercise results in Muscular hypertrophy Increased mitochondria, myofilaments, glycogen stores, and connective tissue Increased muscular size and strength Smooth Muscle Found in the walls of most hollow organs: respiratory, urinary, digestive, circulatory, and reproductive Contain two layers of sheets with fibers oriented at right angles Longitudinal layer: parallel to the long axis of organ Circular layer: fibers run along the circumference of organ Contraction of Smooth Muscle Tissue The contractions are slow and synchronized, cells electrically coupled by gap junctions Sliding filament is how they contract Ca+ binds to calmodulin (not troponin), activates myosin kinase Relaxation requires Ca+ detachment, active transport of Ca+, and dephosphorylation of myosin Slower to contract and relax, but with less cost (ATP) Regulation of contraction by nerves, hormones, or local chemical changes Controlled by neural regulation (neurotransmitter binding cause graded or action potential) Some smooth muscle tissue have no nerve supply, use hormones Responds to stretch only briefly, then adapts to new length Can contract between ½ and 2x its resting length Types of Smooth Muscle Smooth muscle varies by fiber arrangement/organization, innervation, and responsiveness to stimuli Unitary Called visceral muscle, found in all hollow organs besides heart Possesses all common characteristics of smooth muscle Multiunit Located in large airways in lungs, large arteries, arrector pili muscles, and iris of the eye Very few gap junctions Consists of individual fibers, innervated by autonomic nervous, graded contractions...