Muscles and Muscle Tissues (Chapter 9) Flashcards
What is the function of muscle tissue?
Transform chemical energy (ATP) into mechanical energy
Smooth Muscle Structure/Function/Location
Structure: no visible striations, spindle-shaped uninucleate cells; Function: propels substances along internal passageways, involuntary; Location: walls of hollow organs such as stomach, small/large intestines, blood vessels
Cardiac Muscle Structure/Function/Location
Structure: branching striated fibers with 1-2 nuclei per cell, intercalated discs at junction of adjacent cells; Function: propels blood throughout body, involuntary (ANS can speed up or slow down rate of contraction); Location: Walls of heart
Skeletal Muscle Structure/Function/Location
Structure: striated, multinucleated long cylindrical cells; Function: movement of body parts, heat generation, voluntary; Location: attached to bones and occasionally skin
Primary Functions of Muscles
Stabilize joints, muscle contractions oppose gravity -> maintain upright posture, muscle contractions produce movements within body and of body, muscle contractions generate heat via thermogenesis
First Characteristic of Muscle Tissue
Electrical Excitability: ability to produce electrical signals (action potentials) in response to stimuli
Second Characteristic of Muscle Tissue
Contractibility: ability to shorten when stimulated by AP
Third Characteristic of Muscle Tissue
Extensibility: ability to stretch without being damaged
Fourth Characteristic of Muscle Tissue
Elasticity: ability to return to original length and shape after stretching or contraction
Superficial Fascia
Separates muscle from skin, composition: areolar and adipose CT; contains blood vessels, lymphatics, and nerves
Deep Fascia
Lines body walls, holds muscles together with similar functions, allows free movement of muscle groups, composition: primarily dense irregular CT; contains: blood vessels, lymphatics, and nerves
3 Layers of Deep Fascia
Epimysium: outermost layer -> surrounds entire muscle, composition: dense, irregular CT; Perimysium: surrounds 10-100+ individual muscle fibers -> form fascicles, composition: dense, irregular CT; Endomysium: surrounds individual muscle fibers withing fascicle, composition: thin layer of areolar CT
Muscle Attachments
Tendons: attach muscles to periosteum, composition: bundles/cords of dense regular CT; Aponeuroses: sheet-like tendons, composition: sheets of dense regular CT
Nerve and Blood Supply
Each muscle -> a nerve, artery, and 1-2 veins; Nerve branches supply each fiber -> stimulates contraction; Each muscle cell is in loose contact with multiple capillaries -> highly vascularized; Contracting muscle fibers require large amounts of oxygen and nutrients; Also need carbon dioxide and waste products removed quickly
Motor Unit of NMJ
Somatic motor neuron, all muscle fibers innervated by motor neuron (4 to several hundred)
Neuromuscular Junction
Contact site between skeletal muscle fiber and axon branch of somatic motor neuron; Occurs about midway down skeletal muscle fiber; Axon branches many times; Each branch typically innervates one muscle fiber; Axon branch subdivides to form axon terminals; Axon terminals expand to form synaptic end bulbs; Location of vesicles filled with neurotransmitters
Embryonic Development
Myoblasts; Derived from embryonic mesodermal cells; 100+ fuse to form multinucleated skeletal muscle fiber; Lose mitotic ability once fused; At birth, number of muscle fibers/muscle is already determined; Muscle growth: hypertrophy vs. hyperplasia -> enlargement of fibers; Satellite Cells: myoblast that persist as stem cells for skeletal muscle in mature muscle
Sarcolemma
muscle cell plasma membrane
T-Tubules
Formed by tiny invagination of sarcolemma -> extend in center of fiber; Filled with tissue fluid; Thousands of t-tubules per fiber; Function: transmit muscle APs into muscle fiber, all parts of muscle fiber stimulated at same time
Sarcoplasm
Muscle cell cytoplasm; Abundant glycogen granules -> ATP production; Myoglobin: red-pigmented protein containing heme group, found only in muscle cells, function: binds O2 into cell -> releases O2 as needed by mitochondria for ATP production, MT lie in rows throughout muscle fiber -> close to proteins requiring ATP during muscle contraction
Sarcoplasmic Reticulum
Intracellular system of tubules, similar to SER surround contractile proteins within muscle fiber; Function: regulate intracellular Ca+2 levels -> stores Ca+2 when fiber is relaxed and releases Ca+2 when fibers are stimulated to contract
Myofibrils
Groups of contractile proteins within muscle fibers -> 80% cell volume, extend entire length of muscle fiber, organelles are squished around them
Myofilaments
Do not extend entire length of muscle fiber, arranged into functional units -> sarcomeres, Actin: main component of thin filaments, Myosin: main component of thick filaments
Sarcomere Structure
Z disc, A Band, I Band, H Zone, M Line
Z Disc
Region of dense material separating sarcomeres, site of actin attachment
A Band
Regions of myosin and actin overlap, forms “dark” band
I Band
Region of actin only, forms “light” band
H Zone
Regions of myosin only -> central, narrow region of A band
M Line
Site of support proteins anchoring myosin at H zone
Myosin
Function: motor protein, convert chemical energy (ATP) into mechanical energy (movement); Structure: 300 myosin proteins per thick filament, tails: pointed toward M line -> parallel, heads: extend toward thin filaments -> spirally arranged, form cross bridges during contraction, contain: ATP binding site, ATPase, Actin binding site
Actin and Regulatory Proteins
Actin: thin filaments -> numerous actin subunits, each subunit contains 1 myosin binding site, actin filament folds back on itself forming a helix; Tropomyosin: blocks myosin binding site on actin when muscle is relaxed; Troponin: attaches to actin (holds tropomyosin in place), TnL subunit - binds actin (inhibits actomyosin ATPase activity associated with myosin head, TnT subunit - binds tropomyosin, TnC subunit - binds up to 4 Ca+2
Sarcomere-Associated Structural Proteins
Titin: extends from Z disc to M line, anchors thick filaments and contributes to elasticity and extensibility; Myomesin: forms M line, binds titin and anchors adjacent thick filaments; Dystrophin: anchors thin filaments to TM proteins in sarcolemma -> TM proteins anchored to proteins in ECM
Sliding Filament Model of Contraction
Thin filaments slide past thick filaments, thin overlap thick when fully contracted, I band shortens and H zone disappears, A band width stays the same
Contraction Cycle: ATP Hydrolysis
ATP bound to myosin head, myosin heads break down ATP via ATPase -> ADP + Pi + energy, head reorients -> actin binding site faces actin subunit, head is energized
Contraction Cycle: Attachment of myosin to actin to form cross bridges
Energized myosin head attaches to myosin binding site on actin subunit, Pi group is released -> triggers power stroke
Contraction Cycle: Power/Working Stroke
Pocket containing ADP opens up, myosin head rotates toward center of sarcomere and releases ADP, thin filaments slides past thick filament towards M line
Contraction Cycle: Detachment of myosin from actin
ATP binds to myosin head, binding of ATP causes head to detach from actin -> breaks cross bridge
Excitation-Contraction Coupling: Generation of AP in sarcolemma
Nerve impulse triggers release of acetylcholine from synaptic end bulbs of somatic motor neuron, Ach binds to receptors in sarcolemma and stimulates AP in sarcolemma -> t-tubules
Excitation-Contraction Coupling: Release of Ca+2 from SR
AP stimulates release of Ca+2 from SR by opening Ca+2 channels in SR membrane
Excitation-Contraction Coupling: Binding of Ca+2 to TnC of Troponin
Ca+2 binds to TnC subunit which changes shape of troponin-tropomyosin complex, tropomyosin rolls away from myosin binding site on actin
Excitation-Contraction Coupling: Cross-Bridge formation leading to contraction
Myosin heads alternately attach to/detach from actin, actin filaments are pulled toward M line, ATP and Ca+2 required for cycle to continue
Excitation-Contraction Coupling: Removal of Ca+2 from Sarcoplasm
After AP ends, Ca+2 pumped back into SR via Ca+2 active transport pumps, Ca+2 release channels also close
Excitation-Contraction Coupling: Cross-Bridge inhibition by tropomyosin leading to relaxation
Occurs as Ca+2 levels drop, troponin-tropomyosin complex rolls back into place, myosin binding sites on actin blocked -> relaxation
Rigor Mortis
Dying cells can’t keep out Ca+2 in TF -> Ca+2 rushes into muscle cells, Ca+2 also leaks out of SR -> increased intracellular Ca+2 myosin binds to actin -> muscles contract, once breathing stops ATP synthesis stops due to lack of oxygen, without ATP myosin cannot detach from actin and myosin is now irreversibly bound to actin and muscles cannot relax (body appears stiff)
Neuromuscular Junction: Motor End Plate
Region of sarcolemma opposite to synaptic end bulb; Axon Terminals: branches of motor neuron, each terminates as a synaptic end bulb; Synaptic Cleft: space between axon PM and sarcolemma, filled with TF, Ach receptors are found in folds of sarcolemma -> motor end plate
Gated Ion Channels within Sarcolemma
Chemically-Gated Ion Channel: Na+/K+ channels (binding of chemical messengers); Voltage-Gated Ion Channel: Na+ channels and K+ channels (changes in membrane potential)
Generation of APs at NMJ: Release of Acetylcholine
1 nerve impulse -> Ca+2 channels open in axon PM, Ca+2 influx stimulates exocytosis of vesicles containing Ach
Generation of APs at NMJ: Activation of Ach Receptors
Ach diffuses across synaptic cleft and binds to Ach receptors in motor end plate, 2 Ach molecules open the Na+/K+ channel associated with receptor and Na+ diffuse into sarcoplasm and K+ out
Generation of APs at NMJ: Production of Muscle Action Potential
Generation of APs at NMJ: Termination of Ach Activity
Diffusion away from synaptic cleft, acetylcholinesterase (ACase) associated with sarcolemma, Ach -> acetic acid and choline
Depolarization of Sarcolemma
Outside sarcolemma [Na+] hi, [K+] lo / inside of sarcolemma [Na+] lo, [K+] hi, Na+/K+ channels open (chemically-gated ion channels), more Na+ rush in than K+ rush out and membrane polarity changes, adjacent voltage-gated Na+ channels open and initiates wave of depolarization in sarcolemma -> action potential
Repolarization of Sarcolemma
Na+ channels begin to close, K+ ion channels begin to open and K+ diffuse out of sarcoplasm (reestablishes polarity of sarcolemma), Na+/K+ active transport pumps are activated -> Na+ pumped out and K+ pumped in, reestablishes resting ion contractions
Factors that increase the force of skeletal muscle contraction
Frequency (rapid stimulation), Number of fibers, Size of fibers, Degree of stretch
Visualizing Muscle Contractions: Electromyography
Allows visualization of muscle contractions via myogram, uses isolated muscles attached to apparatus that measures muscle tension in response to a stimulus
Visualizing Muscle Contractions: Muscle Twitch
Latent Period: initiation of excitation-contraction coupling, Ca+2 released from SR and cross bridges begin to form, no force evident yet; Contraction Period: cross bridges actively form between actin and myosin, muscle contracts
Refractory Period
Refers to period during which a muscle fiber cannot be re-stimulated, represents time during which sarcolemma is repolarized, Na+ channels close and K+ channels open -> membrane polarity re-established, Na+/K+ pumps are activated -> correct ion concentrations re-established just inside/outside membrane
Muscle Twitch in Different Muscles
Enzyme variations, myofibril metabolism, ATP hydrolysis, pathway used for ATP production
Unfused Tetanus
Increased rate of stimulation (20-30 per second), decreased relaxation time: produces a wavering yet sustained contraction, 3-4x greater strength than single twitch, sufficient for everyday tasks
Fused Tetanus
Increased rate of stimulation (8-100x per second), no detectable relaxation time: not all Ca+2 returns to SR, allows increased strength of contraction, produces a smooth sustained contraction, typically used when maximum force required
Muscle Fatigue
Often occurs due to prolonged activities, ATP production cannot keep up with ATP usage, gradual decreases in ability of muscles to generate force/contractions, protective mechanisms to prevent injury, reversed with rest
Methods of ATP Production: Conversion of Creatine-Phosphate
Stored ATP: 4-6 sec of activity, at rest muscle cells produce more ATP than needed for resting metabolism, excess ATP converted to creatine-phosphate, 3-6x more CP stored than ATP, CP only found in muscle fibers, replenished during inactivity from excess ATP produced, no O2 required for CP conversion to ATP, 1 CP + ADP -> 1 ATP + creatine
Methods of ATP Production: Anaerobic Mechanisms
Glycolysis and lactic acid formation, occurs in cytoplasm, glucose -> 2 pyruvate + 2 ATP (net gain), insufficient O2 -> pyruvate converted into lactic acid, requires lots of glucose to produce sufficient amounts of ATP (1 glucose -> 2 ATP)
Methods of ATP Production: Aerobic Cellular Respiration
Occurs in mitochondria when O2 is present, converts glucose, fatty acids, and amino acids into ATP (1 glucose -> 32 ATP, 1 fatty acid -> 100 ATP), slower process, requires constant supply of O2 and nutrients
Oxygen Debt
Refers to temporary oxygen shortage in body tissues after exercise, increased intake of O2 required; Restores homeostasis, converts lactic acid back to glycogen in liver, resynthesizes Cp and ATP in muscle fibers, replaces O2 removed from hemoglobin
Cardiac Muscle
CT sheets lie between layers of cardiac muscle fibers, contains blood vessels, nerves, and heart conduction system fibers; intercalated discs: desmosomes hold adjacent cardiac muscle fibers together, gap junctions: TM connexons -> form passageways between adjacent cells which enables cardiac muscle APs to spread quickly
Cardiac Muscle: Response to APs
Cardiac muscle remains contracted 10-15x longer than skeletal muscle due to increased levels of Ca+2, due to prolonged Ca+2 into sarcoplasm
Cardiac Muscle: Stimulation by Autorhythmic Cardiac Muscle Fibers
Cardiac muscle contracts when stimulated by its own authorhythmic fibers
Smooth Muscle
Thick and thin filaments longer in length, ratio of thick and thin filaments differ, arranged diagonally and not longitudinally, no striations
Smooth Muscle: Dense Bodies and Intermediate Filaments
Bundles of intermediate filaments attach to dense bodies within sarcoplasm and sarcolemma, dense bodies attach to thin filaments
Smooth Muscle: Response to AP
Sliding of thick and thin filaments during contraction pulls on dense bodies and intermediate filaments, muscle fibers shorten and rotating as a corkscrew into a helix and untwists when in relaxes
ANS and Other Factors
Conduction of nerve impulses, stretching, hormones, changes in pH and O2/CO2 levels, temperature
Regeneration of Skeletal Muscle
Satellite Cells: divide slowly and fuse with existing fibers to assist in muscle growth and repair of damaged fibers, not enough to compensate for significant damage or degeneration, fibrosis - replacement of muscle fibers with fibrous scar tissue
Hypertrophy and Hyperplasia
Myasthenia Gravis
Duchenne Muscular Dystrophy
Myotonic Dystrophy
Myofascial Pain Syndrome
Fibromyalgia
Muscle Strain
Disuse Atrophy
Tetanus
Aerobic Training
Strength/Resistance Training
Interval Training
Anabolic Steroids
