Unit 6 Muscular System

Structure of a skeletal muscle  Skeletal muscle is an organ of the muscular system. Composed of : –Skeletal muscle tissue –Nervous tissue –Blood –Connective tissue

Functions of Muscular System  Produce movement  Maintaining Posture  Stabilizing joints  Generating heat

Characteristics of Muscle  Excitability: respond to stimulus  Contractibility: ability to shorten  Extensibility: ability to extend  Elasticity: recoil and resume resting length after strectched

Connective Tissue coverings:  Fascia: layers of fibrous tissue, separate individual skeletal muscle from other muscle hold it in position. Some extend out farther then other muscles and this forms the cordlike tendon.  Fibers in a tendon interwine with those in the bones periosteum, attaching the muscle to the bone.

Connective tissue cont:  Aponeuroses: connective tissue that forms a broad sheet that attach to the coverings of adjacent muscles.

Skeletal Muscle  Connective Tissue Wrapping –Epimysium: “outside muscle” dense irregular connective tissue –Perimysium: separate muscle tissue into small compartments. These compartments contain bundles of skeletal muscle fibers called fascicles. –Endomysium: surrounds each muscle fiber, fine sheath of connective tissue.

Anatomy of Skeletal Muscle fiber  Long, cylindrical cells  Have multiple oval nuclei  Sarcolemma = cell membrane  Sarcoplasm: cytoplasm, lots of stored glycogen  Myoglobin: red pigment that stores oxygen

Skeletal muscle fibers:  Single cell that contracts in response to stimulation and then relaxes when stimulations stops.  Each fiber is : –Thin –Elongated, with round ends, and may extend to the full length of the muscle. –Fibers contain many small, oval nuclei and mitrochondria.

Myofibrils  Play a fundemental role in muscle contraction.  Contain two types of protein filaments: –Thick one : composed of the protein Myosin. (central, extend entire length of the A band) –Thin one: composed of the protein Actin. (extends across the I band) The organization of these two filaments produce light and dark striations, or bands, of the skeletal muscle fiber.

Myofibrils cont:  Rod like, runs parallel, for entire length of cell, densely packed together  Hundreds of thousands per fiber, 80% of cell volume  Contractile elements of skeletal muscle cells

Striations: two main parts  I bands: light bands = thin actin filaments.  Attached directly to structure called the Z lines.  A Bands: dark bands = thick myosin filaments.  Overlap thin Actin filaments.  Also present in the H zone. Plus around the M line.

Striation cont:  H zone: lighter strip in midsection of A band only visible in relaxed muscles  M line: dark line bisects H zone  Z line: midline interruption of I band  Sarcomere: region of myofibril between two successive Z lines, smallest contractile unit of muscle fiber

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 Sarcoplasmic reticulum: elaborate Smooth ER, regulates intracellular levels of ionic calcium. Stores calcium and releases it on demand when stimulated  Transverse tubules(T-tubules): at each A band – I band junction the sarcolemma.  Opens to the outside of the muscle fiber contains extercellular fluid. Lie between enlarged SR called the Cisternae. –Conducts nerve impulses to the deepest regions of the cell

Neuromuscular Junction  Motor Neuron: is a connection from the skeletal muscle fiber to an axon from the nerve cell. –Muscle fiber only contracts when the motor neuron stimulates it.  Neuromuscular junction: the connection between the motor neuron and the muscle fiber.  Motor end plate: specialized muscle fiber membrane.

 Neurotransmitters (Messenger): chemicals that are rich with mitrochondria. This neutranmitter is released into the synaptic cleft between the neuron and the motor end plate of the muscle fiber.  This stimulates the muscle to contract.

Muscle Contraction Hx.  Sliding Filament Theory of Contraction –1954 Hugh Huxley  During contraction, thin filaments slide past thick ones so that actin and myosin overlap to a greater degree  Muscle fibers stimulated by nervous system  Myosin crossbridges attaches to actin to generate tension and propel actin toward center of sarcomere.

Muscle Contraction Cont’d  Requires calcium ions: low: muscle relaxed and high= muscle contracted  When relaxed binding sites on Actin are blocked by tropomyosin  Binding sites are located on troponin

Steps of Muscle Contraction When binding sites expressed  Cross bridge attachment on myosin heads to actin  Working power Stroke: myosin head binds, it pivots, to become bent with low energy, pulls on thin filament, sliding it toward center of sarcomere, release of ADP and phosphate released

Steps Cont’d 3. Cross bridge detachment – ATP binds to myosin head, hold on actin loosens and detaches 4. “Cocking” of myosin head, ATP to ADP provides energy to return myosin head to high energy position – upright position

Regulation of muscle contraction  Motor neurons attached to muscle –Synaptic cleft – space –Synaptic vesicles – contain neurotransmitters –Acetylcholine (Ach) – binds on sarcolemma  Ach released by neuron  Sarcolemma stimulated to T tubules  Calcium releases from SR  Calcium attaches to troponin after tropomysosin moves out of way

Regulation cont’d  Crossbridge attachment  Myosin head bends – pulls actin  ATP – myosin attaches  CA detaches  Myosin straightens  Muscle relaxes

Energy Sources for contraction  ATP is main energy source  Creatine phosphate: stores excess phosphates when there is plenty of ATP  As ATP decomposes, energy from creatine phosphates transfers to ADP to make more ATP

Oxygen Supply and Cellular Respiration  Cellular respiration needs oxygen in order to release energy  Oxygen carried in blood by hemoglobin  Myoglobin: gives muscle its reddish-brown color, combines loosely with oxygen  Muscles ability to temporarily store oxygen reduces a muscles requirement for continuous blood supply

Oxygen Debt  Aerobic respiration: uses oxygen, more efficient  Anaerobic: when run out of oxygen, during strenuous exercise, lactic acid produced when there is an oxygen debt  Oxygen debt: amount of oxygen liver cells require to convert accumulated lactic acid into glucose and the amount needed to restore ATP in muscle cells

Muscle fatigue  When a muscle loses its ability to contract  Usually arises from accumulation of lactic acid in the muscle  Rigor mortis: increase in membrane permeability to calcium ions and a decrease in ATP which prevents relaxation –Occurs several hours after death –Fixes joints –Actin and myosin remained linked until muscle decomposes

Muscular Responses  Threshold stimulus: minimal strength required to cause a contraction  All-or-none response: a muscle contracts fully or not at all, no partial contractions  Recording a muscle contraction: –Myogram: pattern of muscle contraction –Twitch: contraction lasting only a fraction of a second –Latent period: delay between stimulus and muscle response

Summation  Force of individual twitches combing  Tetanic contraction: when contraction lacks even partial relaxation

Muscle Tone  Muscle tone: slightly contracted state, keeps muscle firm, healthy and ready to respond to stimulation, helps to stabilize joints and maintain posture  Exercise: increase muscle size and strength, more efficient, fatigue resistance  Resistance exercise: increase strength and size, more mitochondria,  Aerobic exercise: increase capillaries, mitochondria

Muscle Types  Cardiac Muscle –Makes a net like structure –Nuclei hard to see –Only in heart

Muscle Types  Striated or Skeletal Muscle –Striped –Obvious nuclei –Straight row of cells –Moves the skeleton voluntarily

Smooth Muscle –Closely woven –Obvious nuclei –Individual cells can be seen –Internal organs –Involuntary movement

Skeletal Muscle Actions  Origin: immovable end  Insertion: movable end  When muscle contract, insertion end moves toward origin  Prime mover, also called an agonist: main movement  Synergists: assist the prime mover  Antagonists: resist prime movers