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SKELETAL MUSCLE

SKELETAL MUSCLE Smoking is injurious to health Rakesh kumar1

Introduction ClassificationMicroscopic structurePhysiologyApplied AnatomyReferences Protocol2

Introduction

MUSCULUS = LITTLE MOUSE

Muscle tissue is the one of the basic tissues of the body.

Responsible for movement .

Composed of differentiated cells containing contractile proteins(actin, myosin etc.)

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Muscles machines - chemical energy - mechanical energy.

40-50% of the body mass.

Attaches to bone, skin or fascia

Developed from MESODERM except a)Arrector pili b)Muscles of iris c)Myoepithelial cells of sweat ,salivary and lacrimal glands - ECTODERM

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Types of Muscle tissueSkeletal muscle

Smooth muscle

Cardiac muscle

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Skeletal MusclesAttached to skeleton

Voluntary control of contraction & relaxation

Each muscle cell = muscle Fiber

Muscle fibers develop from the fusion of small, individual muscle cells called myoblast.

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Microscopic structure shows light & dark bands Striations

Straight & Un-branched

Multinucleated Nuclei are located in the cytoplasm just beneath the plasma membrane(sarcolemma)

LENGTH :- varies from few mm (stapedius) to many cm (sartorius)

SHAPE :- long and cylindrical

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Development of Skeletal muscleMyotomes of paraxial mesoderm

5Th week spindle shape myoblast- single nucleus- repeated mitosis.

join to form myotubes- single row nuclei in centre

Cross striations seen in 2nd month myofibrils-periphery

Increase in no. of myofibril nuclei pushed- periphery

Myotubes converted into muscle fibres.

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Parts of Voluntary Muscle

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Two parts of a muscle1. Belly part:- Contractile part

2. Fibrous part:- Non contractile and non elastic

a) Tendon

b) Aponeurosis

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ClassificationAccording to colour

According to Insertion of muscle

According to direction of muscle fibre - Parallel - Pennate - Spiral - Cruciate11

1. Slow (or Red) Oxidative Fibers Type I

Slow but continuous contraction for extended periods Smaller diameter contain myoglobin more capillaries more mitochondriaDoes not fatigue as fast due to aerobic production of ATP Eg: Soleus.

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2. Fast (Red) Oxidative Fibers Type IIaHave attributes in between fast and slow types

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3. Fast Glycolytic (white)Fibers Type IIb

Fast contraction after nervous stimulationLarge diameterLarge glycogen reserveFewer mitochondriaDensely packed myofibrils Fatigue fast due to mainly anaerobic respiration Eg. Biceps brachii, Gastrocnemius.

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According to insertion of muscleInsertion near the proximal end of bone Eg biceps brachii, psoas major

Insertion towards the distal end of bone, away from the joint Eg- brachioradialis

Insertion at the middle of shaft of bone Eg- Coracobrachialis, pronator teres

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According to direction of muscle fiber

a) Parallel muscles

The muscle fibers are parallel to line of pullThe fibers are long, but their numbers are few

Functions: Range of movement . total force of contraction

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Sub divisions of parallel musclesStrap muscle- Sartorius, Rectus abdominis

Quadrate muscle- Quadratus lumborum

Fusiform muscle- Biceps brachii

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b) Pennate musclesFibers are oblique to line of pull

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CircumpennateThe muscle is cylindricalOblique fibers converge into central tendon from all sides Eg- Tibialis anterior

c) Spiral muscleMuscles are twisted in arrangement close to their insertion Eg- Pectoralis major & latissimus dorsi

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d) Cruciate muscle Fasciculi in the muscle are crossed Eg: Sternocliedomastoid, Masseter

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22Circular: surrounds a bodyorifice, constricting it whencontracted Eg: Orbicularis occuli (closes the eyelids)

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Connective TisssueEpimysium = On top of the muscle

Perimysium = Around a bundle of myofibers

Endomysium = Surrounds each myofiber

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Vessels and nerves run in epimysium and perimysium

Branch in to arterioles and give off capillaries which are carried by endomysium

Each muscle fiber is accompanied by a set of parallel capillaries, which give off side branches at right angles to the fiber

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Epimysium consists mainly type I collagen fibers

Perimysium contain type I and III collagen fibers

Endomysium contain type III and IV collagen fibers

Collagen IV is associated with the basal lamina that invests each muscle fiber

All these connective tissue layers extend beyond the muscle belly to form the tendons, aponeurosis and fascia.

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Attachment of tendon to skeletal muscle

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Fascicles Myofibrils MyofilamentsActinMyosinIt is important to remember the Heirarchy29

Microscopic Structure

Some vocabulary:Skeletal muscle cell = fiber or myofiber Sarcolemma Sarcoplasm Sarcoplasmic reticulumMyofibrilMyofilamentsT-tubules

MuscleEpimysiumMyofibrilsSarcomere

MyofilamentMyofibrils30

Muscle and Muscle fibreEach muscle divides into several muscle bundles or fasciculi by perimysium

Each fasciculus is composed of variable number of muscle fibers

Individual muscles are separated by fascia, which also forms tendons and aponeurosis

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Muscle fibre(Cont..)

Each muscle fiber is elongated,

multinucleated.

Is essential for force transduction

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Muscle fibre(Cont..) sarcolemma- Plasma membrane sarcoplasm - Cytoplasmsarcoplasmic reticulum- Smooth endoplasmic reticulum

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Light and dark bandsThe cytoplasm or sarcoplasm shows alternate dark and light bandsDark band is called A-bandLight band is called I-band

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Light and dark bands

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SarcomereThe portion of myofibrils between every two Z-linesZ disc: filamentous network of protein. Serves as attachment for actin myofilamentsTitin filaments: elastic chains of amino acids; make muscles extensible and elastic

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MyofibrilMuscle fibers are filled with threads called myofibrils, separated by SR (sarcoplasmic reticulum)Myofilaments (thick & thin filaments) are the contractile proteins of muscle

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38 Transverse TubulesT (transverse) tubules are invaginations of the sarcolemma into the center of the cellfilled with extracellular fluidcarry muscle action potentials down into cellMitochondria lie in rows throughout the cellnear the muscle proteins that use ATP during contraction

Sarcoplasmic Reticulum (SR)System of tubular sacs similar to smooth ER Placed horizontally between T-tubulesStores Ca2+ in a relaxed muscleRelease of Ca2+ triggers muscle contraction

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Proteins of MuscleMyofibrils are built of 3 kinds of proteinContractile proteins myosin and actin

Regulatory proteins which turn contraction on & off troponin and tropomyosin

Structural proteins which provide proper alignment, elasticity and extensibilitytitin, myomesin, nebulin and dystrophin

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Neuromuscular Junction SYNAPSE: axon terminal resting in an invagination of the sarcolemma

Neuromuscular junction (NMJ):Presynaptic terminal: axon terminal with synaptic vesiclesSynaptic cleft: spacePostsynaptic membrane or motor end-plate

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Sliding Filament ModelActin myofilaments sliding over myosin myofilaments result shorten sarcomeresActin and myosin do not change length during slidingShortening sarcomeres responsible for skeletal muscle contraction

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During relaxation:-Sarcomeres lengthen because of some external force, like forces produced by other muscles (contraction of antagonistic muscles) or by gravity.

Agonist = muscle that accomplishes a certain movement, such as flexion.Antagonist = muscle acting in opposition to agonist Synergists= they prevent undesirable movement

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Energy SourcesATP provides immediate energy for muscle contractions. Produced from three sourcesCreatine phosphateDuring resting conditions stores energy to synthesize ATPADP + Creatine phosphate------------------ Creatine + 1ATP

Anaerobic respirationOccurs in absence of oxygen and results in breakdown of glucose to yield ATP and lactic acidAerobic respirationRequires oxygen and breaks down glucose to produce ATP, carbon dioxide and waterMore efficient than anaerobic

(Creatine Kinase)47

Skeletal muscle: Nerve supplySupplied by motor and sensory nervesMotor fibers: Two typesAlpha fibresThickly myelinated axons supply extrafusal muscle fibresProduces movements(Contraction)Gamma fibresThinly myelinated fibres whic