Muscular System Reviewer

© t.p. MUSCULAR SYSTEM

MAJOR FUNCTIONS: 1. Movement of the body Ex. Contraction of skeletal muscle - overall movements of the body (walking, running) 2. Maintenance of Posture Ex. Skeletal Muscle -maintain tone 3. Respiration Ex. Muscles of thorax 4. Production of Body Heat Ex. Contraction of sk. muscles -heat =>Byproduct 5. Communication Ex. Speaking, writing, typing 6. Constrictions of organs and vessels Ex. Contraction of sm. muscles 7. Contraction of the heart Ex. Cardiac Muscle -propel blood to all parts of body SKELETAL MUSCLE With its associated connective tissues,  constitutes approximately 40% of body weight Most muscles are attached to the skeletal  Also called STRIATED MUSCLE =>transverse  bands, or striations is seen FOUR MAJOR FUNCTIONAL CHARACTERISTICS: CHARACTERISTICS: 1. CONTRACTILITY - Ability of skeletal muscle to SHORTEN with force - Either GRAVITY or CONTRACTION OF OPPOSING MUSCLE produces a force that pulls on the shortened muscle causing it to lengthen 2. EXCITABILITY - Capacity of sk. muscle to RESPOND to a stimulus 3. EXTENSIBILITY - Sk. muscle STRETCH - After a contraction, sk. muscle can be stretched to their normal resting length… 4. ELASTICITY - Ability of sk. muscle to RECOIL to their original resting length after being stretched.

SKELETAL MUSCLE STRUCTURES CONNECTIVE TISSUE COVERINGS OF MUSCLE EPIMYSIUM/ MUSCULAR FASCIA  – connective  tissue sheath that surrounds each sk. muscle MUSCLE FASCICULI – numerous visible bundles  that composes a muscle  PERIMYSIUM – loose connective tissue w/c surrounds muscle fasciculi MUSLE FIBERS – several muscles that composes  a fasciculus  ENDOMYSIUM – loose connective tissue w/c surrounds muscle fiber MUSCLE FIBER STRUCTURE  MUSCLE FIBER - A single cylindrical fiber with several nuclei located at its periphery - LARGEST HUMAN MUSCLE FIBERS => 30 cm long / 0.15 mm in diameter STRUCTURES:  SARCOLEMMA – cell membrane of muscle fiber TRANSVERSE TUBULES/ T TUBULES  – tubelike  invaginations along the surface of sarcolemma; occur at regular intervals along the muscle fiber & extend inward into it; connect sarcolemma to sarco. reticulum SARCOPLASMIC RETICULUM – highly organize  smooth e.r.; high conc. Of Ca →muscle contraction SARCOPLASM  – cytoplasm in each muscle fiber  o Myofibrils –threadlike structures that extend fr. one end to other 2 kinds of protein fibers :  Actin Myofilaments -THIN filaments; resembles two minute strands of pearls twisted together, have attachment sites for the myosin myofilaments - 3 components: 1. Actin 2. Troponin - attached at spec. intervals along the actin myofilaments -have binding sites for Ca 3. Tropomyosin -located along the groove bet. the twisted strands of actin myofilaments subunits -block the myosin myofilament binding sites ont the actin myofilaments

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Myosin Myofilaments -THICK myofilaments, resemble bundles of minute golf clubs. MYOSIN HEADS  –  parts of o myosin molecule that resemble golf club heads 3 IMPORTANT PROPERTIES: 1. Can bind to attachment sites on actin myofilaments 2. Can bend and straighten during contraction 3. Can break down ATP; releasing energy Actin and Myosin are arranged into highly ordered, repeating units called SARCOMERES – joined end to end to form the myofibrils.

SARCOMERES -Basic structural and functional unit of sk. muscle -smallest portion of sk. m. that is capable of contracting -extends from one Z disk to another Z disk Z Disk -network of protein fibers forming an attachment site for actin myofilaments 

Light I Band -consist only of actin myofilaments -span each Z disk and ends of myosin myofilaments 

RESTING MEMBRANE POTENTIAL -charge difference -occurs because there is an uneven distributions of ions across cell membrane. 3 REASON WHY IT DEVELOPS: 1. Conc. of K+ is inside the cell m embrane is higher than that of the outside of the cell memb. 2. Concentration NA+ outside the cell memb. is higher than that the inside cell memb. 3. Cell membrane is more permeable to K+ thn it is to Na+ (READ p. 154)

DEPOLARIZATION -Na+ channels are open. Na+ diffuses down  its concentration gradient through the open Na+ channels, making the inside of the cell memb. positively charged compared to the outside. REPOLARIZATION -The change back to the resting membrane potential - Na+ channels are closed and, Na+ movement into the cells stops. More K+ channels open. K+ movement out of the cell increases, making the inside cell memb. negatively charged compared to the outside ACTION POTENTIAL -Rapid depolarization and repolarization of the cell membrane -in a muscle fiber → muscle contraction

A Band -darker, central region in each sarcomere -extends the length of the myosin myofilaments

NERVE SUPPLY  Motor Neurons -specialized nerve cells that stimulate muscles to contract -generate action potential

H Zone -second light zone, center of sarcomere -consist only oy myosin myofilaments

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M Line -dark-staining band w/c anchors myosin myofilaments at center of sarcomere

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Neuromuscular Junction -junction with a muscle fiber -near center of muscle fiber

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EXCITABILITY OF MUSCLE FIBERS

POLARIZED -inside of most cell membranes is negatively charged compared to the outside of cell membrane

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Synapse -cell-to-cell junction between a nerve cell and either another nerve cell or an effector cell ( muscle, gland) Motor Unit -single motor neuron & all the skeletal m uscle fibers it innervates

© t.p. -small, precisely controlled muscle e.g. hand -may have one or few muscle fibers per units large thigh muscles- 1000 muscle fibers per motor unit ** the fewer fibers in motor unit, the greater control over that muscle. 

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2. Contraction Phase -time during w/c the muscle contracts -results from cross-bridge movement and cycling w/c increases the tension produced by the muscle fibers.

Presynaptic Terminal -enlarged axon terminal

3. Relaxation Phase -time during w/c the muscle relaxes 2+ -Ca  is actively transported back to sarcoplasmic reticulum -cross-bridge prevented; tension decreases

Synaptic Cleft -space between presynaptic terminal and muscle fiber membrane Postsynaptic Membrane -muscle fiber membrane Synaptic Vesicles -small vesicles in each presynaptic terminal -contains actylcholine (Ach) →neurotransmitter Neurotransmitter -molecule release by presynaptic nerve cell -stimulates or inhibits postsynaptic cell

** acetylcholine is broken down by an enzyme called acteylcholinesterase

MUSCLE CONTRACTION **occurs as actin and myosin slide past each other Sliding Filament Model -sliding of actin myofilaments past myosin myofilaments **CONTRACTION -neither actin nor myosin shortens -H zones and I bands shorten -A bands don’t change in length **RELAXATION -sarcomeres lengthen 

-action potentials are produced in one or more motor neurons

Muscle Twitch -contraction of muscle fiber in response to a stimulus -usually involves all the muscle fibersin a motor unit 3 PHASES: 1. Lag Phase (Latent Phase) -time between the application of a stimulus and the beginning of contraction

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Tetanus -sustained contraction that occurs when frequency of stimulation is so rapid that no relaxation occurs. 2+ - occurs because Ca  builds up in myofibrils Recruitment -number of muscle fibers contracting is incre ased by increasing the number of motor units stimulated, and the muscle contracts with more force.