Muscle Tissue Mamoun Kremli Al-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 - Units Muscle Fascicle Fiber

Skeletal Muscle - Coverings 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 O 2 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 nerve signal: this initiates calcium ion release and muscle contraction

Neuromuscular Junction Motor Unit: a motor neuron and all the muscle fibers it innervates (these all contract together)

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

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

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 actswf/index.html plates/student_resources/shared_resources/a nimations/muscles/muscles.html