Muscle Tissue

Functions of Muscle Tissue zMovement zHeat production zMaintenance of posture zMuscle contraction produces 85% of body heat

Characteristics of Muscle Tissue zExcitability - able to be stimulated zContractibility - able to shorten and thicken zExtensibility - stretchable and extendable zElasticity - the ability to return to its original shape

Muscle Tissue Types zSkeletal - found attached to bone, striated, voluntary zCardiac - forms the walls of heart, striated, involuntary zSmooth - found in viscera, non-striated, involuntary

Gross Anatomy of Skeletal Muscle zEach muscle is an organ, containing muscle, blood vessels, nerves, & connective tissue

Connective Tissue Components zFascia - found under the skin, covering organs, and muscles zEpimysium – surrounds entire muscle zPerimysium – surrounds groups of muscle fibers called “fascicles” zEndomysium – surrounds individual muscle fibers zTendons; Aponeurosis – connect muscle to bones

Cell Structure zMuscle Cell = Muscle Fiber yElongated, multi-nucleate, striated cells containing parallel bundles of myofibrils ySarcolemma – plasma membrane ySarcoplasm – cytoplasm containing: xMyoglobin – stores oxygen xGlycosomes – store starch yPeripheral nuclei ySarcoplasmic reticulum – smooth E.R. – maintains calcium levels yTransverse tubules – network of membranes connected to sarcolemma; penetrates deep into each contractile unit.

Specialized contractile organelles zMyofibrils - thread-like structures y100’s to 1000’s in each muscle fiber (cell) y Actin - thin filaments containing actin protein, 2 strands of tropomyosin, & troponin yMyosin - thick protein filaments composed of myosin molecules

Myofibrils zActin zMyosin

SARCOMERE – the basic contractile unit zZ discs (lines) - separate sarcomeres; anchor thin filaments zA band - overlapping area of thick and thin filaments zI band - contains only actin zH zone - part of A band containing only myosin zM line - center of H zone; anchors myosin

Sliding Filament Mechanism

Skeletal Muscle Contraction zMuscle contraction occurs when actin and myosin are allowed to interact with each other and form crossbridges zThe binding sites on actin are blocked by the troponin/tropomyosin complex zCalcium ions in the sarcoplasm will bind to troponin

Muscle Contraction zThis binding will cause the troponin / tropomyosin complex to pull away from the active binding site on actin, thus allowing myosin to bind zThe myosin head pivots, pulling the thin filaments toward the center of the sarcomere thus shortening the sarcomere zRepeated cycles of attachment, pivoting, detach and release occurs

Muscle Contraction zSuccessive interaction causes “sliding” of the filament, shortening of the sarcomere, thus shortening of the entire muscle zCalcium is removed from the troponin molecule and returned to the S.R. zRelaxation occurs

What Role Does Calcium Play? What triggers the release of calcium?

ACTION POTENTIALS The sudden change in the transmembrane potential

Action Potentials zResting Membrane Potential zPolarized - positive charge outside, negative charge inside zDepolarized - positive charge inside, negative charge outside zRepolarized - positive charge reestablished outside, negative charge inside

Resting Potential/ Polarized zWhen muscle is relaxed, the sarcolemma is polarized having a charge difference between the inside /outside of the cell zWhen a stimulus is received opening a channel gate, Na+ ions will flow into the cell changing the polarity of the cell

Depolarized zThe net charge of the sarcolemma becomes negative in regards to the inside of the cell which is now positive. zThe cell is said to be depolarized and the muscle contracted

Repolarized zMembrane pumps quickly restore the original status or condition zThe positive charge outside is reestablished once again and resting membrane potential is restored

Neuromuscular Junction zEach fiber is controlled by a motor neuron at a neuromuscular junction zMotor neurons stimulate muscle fibers zAcetylcholine (ACh ) is released into the synaptic cleft with the arrival of an action potential zACh diffuses across the cleft, binding to receptors on the motor end plate, initiating a muscle action potential

Once initiated, the action potential is unstoppable and self-propagating

RELAXATION zResting membrane potential is restored by: yAcetylcholinesterase yactive transport pumps that pump Ca +2 ions back into the sarcoplasmic reticulum yCalsequestrin – binds calcium

Role of ATP zUsed to activate the myosin head in order to bind to actin zAfter power stroke, ATP used to break the bond between actin and myosin zATP used to pump calcium back into the SR

Production of ATP for muscles zDirect phosphorylation yCreatine phosphate couples with ADP to form ATP yProvides about 15 sec of energy zGlycolysis yGlucose broken down anaerobically yProduces lactic acid as waste product yProvides about sec of energy zAerobic Respiration yGlucose broken down with oxygen yHours of energy

Muscle Fatigue zInsufficient oxygen zBuild-up of lactic acid zDepletion of glycogen zRECOVERY OXYGEN CONSUMPTION yOXYGEN DEBT

ALL - or -None Principle zMuscle fibers will contract fully OR not at all once they are stimulated zThreshold stimulus yminimal level of stimulation needed to cause the muscle to contract

Motor Units z Motor units - motor neuron and all the muscle fibers it controls zNumber of muscle fibers in motor unit will vary z The fewer the number of fibers per motor unit, the more precise the contraction z The number of motor units being stimulated will determine the strength of contraction of the entire muscle

Muscle Contraction zTwitch contraction yrapid, jerky contraction to a single stimuli yphases: latent, contraction, relaxation,refractory zWave summation yincrease in the strength of muscle contraction due to rapid successive stimulation zTetany ycontinuous, smooth, sustained contraction

Muscle Contraction zTreppe y repeated stimulation following stimulation causing a staircase effect zIsotonic ytone or tension remains constant-muscle shortens zIsometric ytension increases - muscle length remains same

Muscle Fiber Types zRed oxidative fibers ymore myoglobin ymore capillaries ymore mitochondria ylong, slow contraction ysustained energy yaerobic respiration ynon-fatiguable fibers z White glycolytic fibers yless myoglobin yless capillaries yfewer mitochondria yrapid,powerful contraction yquick energy yanaerobic respiration yfatigue easily

Benefits of Exercise zIncrease the size of size and strength of each fiber zIncrease muscle tone zIncreases the blood supply, thus increasing the number of red blood cells zIncreased respiratory and cardiovascular function zLowers blood pressure

Cardiac Muscle zInvoluntary zIntercalated discs zForms syncytium zLong refractory period zLong contraction rate zMore mitochondria than skeletal muscle

Smooth Muscle zInvoluntary - neural & hormonal stimulation zNo sarcomeres - no striations zVery, very long contraction rate zCalmodulin - regulatory protein zNo tendons or aponeuroses

Muscle / Bone Interaction zOrigin – attachment of a muscle to a stationary bone zInsertion – attachment of a muscle to a movable bone zPrime mover – provides major force for specific movement zAntagonist – opposes prime mover zSynergist – assists the prime mover (secondary muscle)

Muscle / Bone Interactions zLevers – rigid bar (bones) moving on fixed point zFulcrum = fixed point (joints) zEffort = applied force zResistance = load

Levers zFirst class yFulcrum in center = seesaw yLifting head off chest zSecond class yLoad (resistance) in center = wheelbarrow yLeast common yStanding on tiptoes zThird class yEffort in center = tweezers yBiceps brachii yMost common