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Muscle Tissue

Mamoun KremliAl-Maarefa College

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Objectives• Identify basic structure of Muscles• Recognize types of muscular tissues and the

difference between them• Recognize the relation between structure and

function of various muscular tissues

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Tissues • Four fundamental tissues are recognized:– Epithelial tissue– Connective tissue– Muscular tissue– Nervous tissue

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Muscle Tissue - Characteristics• Cells are referred to as fibers because they are

elongated• Contracts or shortens with force when

stimulated• Contraction depends on myofilaments– Actin– Myosin

• Plasma membrane is called sarcolemma– Sarcos = flesh– Lemma = sheath

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Special functional characteristics• Contractility– Only one action: to shorten– Shortening generates pulling force

• Excitability– Nerve fibers cause electrical impulse to travel

• Extensibility– Stretch with contraction of an opposing muscle

• Elasticity– Recoils passively after being stretched

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Muscle Tissue Types• Skeletal:– attached to bones

• Cardiac:– muscle of the heart

• Smooth:– muscles associated with tubular structures and

with the skin

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Muscle Tissue Types

Figure 10–1. Structure of the 3 muscle types. The drawings at right show these muscles in cross section. Skeletal muscle is composed of large, elongated, multinucleated fibers. Cardiac muscle is composed of irregular branched cells bound together longitudinally by intercalated disks. Smooth muscle is an agglomerate of fusiform cells. The density of the packing between the cells depends on the amount of extracellular connective tissue present.

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Skeletal Muscle - UnitsMuscle

FascicleFiber

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Skeletal Muscle - CoveringsEpimysium

surrounds whole muscle

Epimysium surrounds whole

muscle

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Skeletal Muscle - Coverings

Perimysium is around fascicle

Perimysium

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Skeletal Muscle - Coverings

Endomysium is around each muscle fiber

Endomysium

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Skeletal Muscle - Coverings• Epimysium, Perimysium, Endomysium

Figure 10–2. Structure and function of skeletal muscle. The drawing at right shows the area of muscle detailed in the enlarged segment. Color highlights endomysium, perimysium, and epimysium.

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Skeletal Muscle - Coverings

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Skeletal Muscle – Blood Supply

Vessels injected with plastic material

• Muscles must have plenty of blood supply• High demand for O2 and nutrientsFigure 10–5. Longitudinal section of striated muscle fibers. The blood vessels were injected with a plastic material before the animal was killed. Note the extremely rich network of blood capillaries around the muscle fibers. Giemsa stain. Photomicrograph of low magnification made under polarized light.

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Skeletal Muscle• Voluntary movement• Long and cylindrical• Transverse striations• Each fiber is multi-

nuclear(multinucleated cells – embryonic cells fuse)

• 40% of body weight

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Skeletal Muscle

Figure 4.14a

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Skeletal Muscle• Large, elongated, multinucleated fibers. • Nucleii are in periphery of cells, just under cell membrane

Figure 10–6. Striated skeletal

muscle in longitudinal section

(lower) and in cross section

(upper). The nuclei can be seen

in the periphery of the cell, just

under the cell membrane,

particularly in the cross

sections of these striated

fibers. H&E stain. Medium

magnification.

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Skeletal Muscle• A band (dark-stained)• I band (light-stained)• Z line

A band

I band

Z line

Giemsa stain

Figure 10–8. Longitudinal section of skeletal muscle fibers. Note the dark-stained A bands and the light-stained I bands, which are crossed by Z lines. Giemsa stain. High magnification.

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Skeletal Muscle• A bands (dark-stained)• I bands (light-stained)• Z lines

A bands

I bands

Z line Figure 10–8. Longitudinal section of skeletal muscle fibers. Note the dark-stained A bands and the light-stained I bands, which are crossed by Z lines. Giemsa stain. High magnification.

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Skeletal Muscle• A-band (actin & myosin )• I-band( actin only)• Z lines(attachment of

actin)• H-band(myosin only)• M-line (Myomesin,

creatine kinase)

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Skeletal Muscle Structure• Invaginations of the T system at transition

between A and I bands (twice in every sarcomere)

• They associate with terminal cisternae of the sarcoplasmic reticulum (SR)(which is the specialized calcium-storing smooth endoplasmic reticulum).

• Abundant mitochondria is present between myofibrils.

Figure 10–17. Segment of mammalian skeletal muscle. The sarcolemma and muscle fibrils are partially cut, showing the following components: The invaginations of the T system occur at the level of transition between the A and I bands twice in every sarcomere. They associate with terminal cisternae of the sarcoplasmic reticulum (SR), forming triads. Abundant mitochondria lie between the myofibrils. The cut surface of the myofibrils shows the thin and thick filaments. Surrounding the sarcolemma are a basal lamina and reticular fibers. (Reproduced, with permission, from Krstíc RV: Ultrastructure of the Mammalian Cell. Springer-Verlag, 1979.)

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Skeletal Muscle Structure

Bloom W, Fawcett DW: A Textbook of Histology, 9th ed, Saunders

Figure 10–11. Structure and position of the thick and thin filaments in the sarcomere. The molecular structure of these components is shown at right. (Drawing by Sylvia Colard Keene. Reproduced, with permission, from Bloom W, Fawcett DW: A Textbook of Histology, 9th ed, Saunders, 1968.)

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Skeletal Muscle – Sarcomere structure

Bloom W, Fawcett DW: A Textbook of Histology, 9th ed, Saunders

Figure 10–11. Structure and position of the thick and thin filaments in the sarcomere. The molecular structure of these components is shown at right. (Drawing by Sylvia Colard Keene. Reproduced, with permission, from Bloom W, Fawcett DW: A Textbook of Histology, 9th ed, Saunders, 1968.)

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The Thin Filament - Actin

Figure 10–13. Schematic representation of the thin filament, showing the spatial configuration of 3 major protein components—actin, tropomyosin, and troponin. The individual components in the upper part of the drawing are shown in polymerized form in the lower part. The globular actin molecules are polarized and polymerize in one direction. Note that each tropomyosin molecule extends over 7 actin molecules. TnI, TnC, and TnT are troponin subunits.

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Skeletal muscle• Fibers have striations• Myofibrils are

organelles of the cell: these are made up of filaments

• Sarcomere– Basic unit of contraction– Myofibrils are long rows

of repeating sarcomeres– Boundaries: Z discs (or

lines)

-an organelle

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Myofibrils• Made of three types of filaments (or

myofilaments):– Thick (myosin)– Thin (actin)– Elastic (titin)

______actin_____________myosin

titin_____

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Sliding Filament Model__relaxed sarcomere__ _partly contracted_

fully contracted

“A” band constant because it is caused by myosin, which doesn’t change length

Sarcomere shortens because actin pulled towards its middle by myosin cross bridges

Titin resists overstretching

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Sliding Filament Model

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Sarcoplasmic reticulum is smooth ER• Tubules surround myofibrils• Cross-channels called “terminal

cisternae”• Store Ca++ and release when

muscle stimulated to contract• Two thin filaments triggering

sliding filament mechanism of contraction

• T tubules are continuous with sarcolemma, therefore whole muscle (deep parts as well) contracts simultaneously

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Neuromuscular Junction• Motor neurons innervate muscle fibers• Motor end plate is where they meet• Neurotransmitters are

released by nervesignal: this initiatescalcium ion releaseand muscle contraction

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Neuromuscular Junction• Motor Unit: a motor neuron and all the

muscle fibers it innervates (these all contract together)

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Motor Unit

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Motor Unit

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Motor End-plate

Figure 10–18. Ultrastructure of the motor end-plate and the mechanism of muscle contraction. The drawing at the upper right shows branching of a small nerve with a motor end-plate for each muscle fiber. The structure of one of the bulbs of an end-plate is highly enlarged in the center drawing. Note that the axon terminal bud contains synaptic vesicles. The region of the muscle cell membrane covered by the terminal bud has clefts and ridges called junctional folds. The axon loses its myelin sheath and dilates, establishing close, irregular contact with the muscle fiber. Muscle contraction begins with the release of acetylcholine from the synaptic vesicles of the end-plate. This neurotransmitter causes a local increase in the permeability of the sarcolemma. The process is propagated to the rest of the sarcolemma, including its invaginations (all of which constitute the T system), and is transferred to the sarcoplasmic reticulum (SR). The increase of permeability in this organelle liberates calcium ions (drawing at upper left) that trigger the sliding filament mechanism of muscle contraction. Thin filaments slide between the thick filaments and reduce the distance between the Z lines, thereby reducing the size of all bands except the A band. H, H band; S, sarcomere.

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Types of Skeletal Muscle:Type I fibres (red fibres). • Red muscles (large amounts of myoglobin and

mitochondria). Type II fibres(white fibers). • White muscles (less amounts of myoglobin and

mitochondria). Type III Fibres (Intermediate).• Have characteristics between type I & II In

humans, skeletal muscles are composed of mixtures of these 3 types of fibres.

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• Red muscles are used when sustained production of force is necessary, e.g. in the control of posture.

• White muscles are for rapid accelerations and short lasting maximal contraction e.g. extraocular muscles of the human eye)

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Cardiac Muscle• Striations• Involuntary• One nucleus– Deep center

• Heart muscle

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Cardiac muscle• Bundles form thick

myocardium• Cardiac muscle cells are

single cells (not called fibers)• Cells branch• Cells join at intercalated discs• 1-2 nuclei in center• Here “fiber” = long row of

joined cardiac muscle cells• Rhythmicity: • More T-Tubules

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Cardiac Muscle

Figure 4.14b

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Cardiac Muscle

Figure 10–22. Drawing of a section of heart muscle, showing central nuclei, cross-striation, and intercalated disks.

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Cardiac Muscle

Figure 10–23. Photomicrograph of cardiac muscle. Note the cross-striation and the intercalated disks (arrowheads). Pararosaniline–toluidine blue (PT) stain. High magnification.

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Cardiac Muscle

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Cardiac Muscle

Figure 10–26. Junctional specializations making up the intercalated disk. Fasciae (or zonulae) adherentes (A) in the transverse portions of the disk anchor actin filaments of the terminal sarcomeres to the plasmalemma. Maculae adherentes, or desmosomes (B), found primarily in the transverse portions of the disk, bind cells together, preventing their separation during contraction cycles. Gap junctions (C), restricted to longitudinal portions of the disk—the area subjected to the least stress—ionically couple cells and provide for the spread of contractile depolarization.

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Smooth muscle

•Muscles are spindle-shaped cells•One central nucleus•Grouped into sheets: often running perpendicular to each other•Peristalsis•No striations (no sarcomeres)•Contractions are slow, sustained and resistant to fatigue•Does not always require a nervous signal: can be stimulated by stretching or hormones

6 major locations: 1. inside the eye 2. walls of vessels 3. respiratory tubes 4. digestive tubes 5. urinary organs 6. reproductive organs

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Smooth Muscle• Spindle shaped• Not striated• Single nucleus• Involuntary movement• Internal organs

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Smooth Muscle

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Smooth Muscle• Centrally located nucleii

Figure 10–29. Photomicrographs of smooth muscle cells in cross section (upper) and in longitudinal section (lower). Note the centrally located nuclei. In many cells the nuclei were not included in the section. PT stain. Medium magnification.

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Smooth Muscle• Cells are surrounded by a net of reticular

fibers

Figure 10–30. Drawing of a segment of smooth muscle. All cells are surrounded by a net of reticular fibers. In cross section, these cells show various diameters.

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Smooth Muscle• Cytoplasmic filaments insert on

dense bodies located in the cell membrane and deep in the cytoplasm.

• Contraction of these filaments decreases the size of the cell and promotes the contraction of the whole muscle.

• During the contraction the cell nucleus is deformed.

Figure 10–33. Smooth muscle cells relaxed and contracted. Cytoplasmic filaments insert on dense bodies located in the cell membrane and deep in the cytoplasm. Contraction of these filaments decreases the size of the cell and promotes the contraction of the whole muscle. During the contraction the cell nucleus is deformed.

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Smooth Muscle• A rudimentary sarcoplasmic reticulum is present • T tubules are not present in smooth muscle cells.• Caveoli(look like pinocytotic or endocytotic vesicles)

function like T tubules.• Contains thin filaments made of actin and

tropomyosin and thick filaments made of myosin• Intermediate filaments (Desmin and Vimentin)• Dense bodies are 2 types • - Membranous(membrane associated) -Cytoplasmic• Both thin and intermediate filaments insert into

dense bodies.

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Regeneration of Muscle Tissue• Injured cardiac fibers after childhood are replaced

by fibrous tissue.

• Injured skeletal fibers have limited potential for

regeneration. Satellite cells (Undifferentiated

myoblasts) within the basal lamina of skeletal

fibers become activated and proliferate and fuse

together to give new muscle fibers.

• Injured smooth fibers have active regenerative

activity.

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Summary

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Some sites showing animations of muscle contraction

• http://entochem.tamu.edu/MuscleStrucContractswf/index.html

• http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/muscles/muscles.html