Muscle Tissue

Mamoun KremliAl-Maarefa College

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

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

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

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

Muscle Tissue Types• Skeletal:– attached to bones

• Cardiac:– muscle of the heart

• Smooth:– muscles associated with tubular structures and

with the skin

Muscle Tissue Types

Skeletal Muscle - UnitsMuscle

FascicleFiber

Skeletal Muscle - CoveringsEpimysium

surrounds whole muscle

Epimysium surrounds whole

muscle

Skeletal Muscle - Coverings

Perimysium is around fascicle

Perimysium

Skeletal Muscle - Coverings

Endomysium is around each muscle fiber

Endomysium

Skeletal Muscle - Coverings• Epimysium, Perimysium, Endomysium

Skeletal Muscle - Coverings

Skeletal Muscle – Blood Supply

Vessels injected with plastic material

• Muscles must have plenty of blood supply• High demand for O2 and nutrients

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

nuclear(multinucleated cells – embryonic cells fuse)

• 40% of body weight

Skeletal Muscle

Figure 4.14a

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

membrane

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

A band

I band

Z line

Giemsa stain

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

A bands

I bands

Z line

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

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

creatine kinase)

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.

Skeletal Muscle Structure

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

Skeletal Muscle – Sarcomere structure

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

The Thin Filament - Actin

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

Myofibrils• Made of three types of filaments (or

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

______actin_____________myosin

titin_____

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

Sliding Filament Model

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

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

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

muscle fibers it innervates (these all contract together)

Motor Unit

Motor Unit

Motor End-plate

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.

• 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)

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

• Heart muscle

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

Cardiac Muscle

Figure 4.14b

Cardiac Muscle

Cardiac Muscle

Cardiac Muscle

Cardiac Muscle

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

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

Smooth Muscle

Smooth Muscle• Centrally located nucleii

Smooth Muscle• Cells are surrounded by a net of reticular

fibers

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.

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.

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.

Summary

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