???
− Considered as a connective tissue because they give form and maintains the shape of the body
- Optimized function of contractility
- _______ origin
- Differentiate by a gradual process of cell lengthening with
abundant synthesis of the myofibrillar proteins actin and
myosin
MUSCLE TISSUE
- Mesodermal origin
Types of Muscle Tissue (3)
A. _______
B. _______
C. _______
Types of Muscle Tissue
A. Skeletal muscle
B. Cardiac muscle
C. Smooth muscle
Type of Muscle Tissue
• Bundles of very long, multinucleated cells with cross-
striations.
• Contraction is quick, forceful
• Under voluntary control.
Skeletal muscle
• Also has cross-striations
• Composed of elongated, often branched cells bound to
one another at structures called ________
• Contraction is involuntary, vigorous, and rhythmic.
Cardiac muscle
one another at structures called intercalated discs
• Collections of fusiform cells that lack striations
• Slow, involuntary contractions.
Smooth muscle
- _________: (Gr.sarkos, flesh + plasma, thing formed):
Cytoplasm
- _________: Smooth endoplasmic reticulum
- _________: (sarkos + Gr.lemma, husk): Smooth muscle
membrane
- Sarcoplasm (Gr.sarkos, flesh + plasma, thing formed):
- Sarcoplasmic reticulum: Smooth endoplasmic reticulum
- Sarcolemma (sarkos + Gr.lemma, husk): Smooth muscle
______________ MUSCLE
− “Striated muscle”
− Consists of muscle fibers - long, cylindrical multinucleated cells with diameters of 10 to 100 μm.
− During embryonic muscle development, mesenchymal
myoblasts (L. myo, muscle) fuse, forming myotubes with
many nuclei.
− Myotubes then further differentiate to form striated muscle fibers
− Elongated nuclei are found peripherally just under the
sarcolemma, a characteristic nuclear location unique to
skeletal muscle fibers/cells.
− A small population of reserve progenitor cells called _____________ remains adjacent to most fibers of
differentiated skeletal muscle.
SKELETAL MUSCLE
− A small population of reserve progenitor cells called muscle satellite cells remains adjacent to most fibers of
ORGANIZATION OF SKELETAL MUSCLE:
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• External sheath of dense connective tissue
• Surrounds the entire muscle.
• Extend inward, carrying the larger nerves, blood vessels,
and lymphatics of the muscle.
− Epimysium is continuous with the dense regular connective
tissue of a tendon at myotendinous junctions
• Collagen fibers from the tendon insert among
muscle fibers and associate directly with infoldings
of sarcolemma
Epimysium
ORGANIZATION OF SKELETAL MUSCLE
• Thin connective tissue layer that immediately surrounds
each bundle of muscle fibers termed a _______.
• Fascicle of muscle fibers: functional unit in which the fibers work together.
• Nerves, blood vessels, and lymphatics penetrate the
perimysium to supply each fascicle.
Perimysium
each bundle of muscle fibers termed a fascicle.
• Within fascicles a very thin, delicate layer of reticular fibers and scattered fibroblasts
• Surrounds the external lamina of individual muscle fibers.
• In addition to nerve fibers, capillaries form a rich network
in the endomysium bringing O2 to the muscle fibers
− Collagen in these layers serve to transmit the mechanical
forces generated by the contracting muscle cells/fibers
− Individual muscle fibers seldom extend from one end of a
muscle to the other.
Endomysium
ORGANIZATION WITHIN THE MUSCLE FIBER
− Longitudinally sectioned skeletal muscle fibers show cross striations of alternating light and dark bands.
• The dark bands are called ___ bands (_______ or
______ in polarized light microscopy)
• The light bands are called ___ bands (_______, do not
alter polarized light) - bisected by a dark transverse
line
• _______ (Ger. zwischen, between) is a dark line that
bisects each I band
• The dark bands are called A bands (anisotropic or
birefringent in polarized light microscopy)
• The light bands are called I bands (isotropic, do not
• Z disc (Ger. zwischen, between) is a dark line that
ORGANIZATION WITHIN THE MUSCLE FIBER (continuation)
− The sarcoplasm has little RER and contains primarily long
cylindrical filament bundles, called myofibrils, running
parallel to the long axis of the fiber.
− The repetitive functional subunit of the contractile apparatus, the ________, extends from ____ disc to ____ disc and is about 2.5 μm long in resting muscle.
− Mitochondria and sarcoplasmic reticulum are found between the myofibrils (1 to 2 μm).
− Myofibrils consist of an end-to end repetitive arrangement of sarcomeres; the lateral registration of sarcomeres in adjacent myofibrils causes the entire muscle fiber to exhibit a characteristic pattern of transverse striations.
− The A and I banding pattern - due mainly to the regular
arrangement of thick and thin myofilaments organized within each myofibril in a symmetric pattern containing thousands
of each filament type.
− The repetitive functional subunit of the contractile apparatus, the sarcomere, extends from Z disc to Z disc and is about 2.5 μm long in resting muscle.
THICK FILAMENTS
− 1.6 μm long and 15 nm wide
− They occupy the ____ band at the middle region of the sarcomere.
− Composed of _____: Large complex (~500 kDa) with two identical heavy chains and two pairs of light chains.
− Two ___ chains are thin, rodlike motor proteins (150 nm long and 2-3 nm thick) twisted together as ___ tails.
− Four myosin light chains form a head at one end of each heavy chain.
− The _______ bind both actins, forming transient cross bridges between the thick and thin filaments, and ATP, catalyzing energy release (actomyosin ATPase activity).
− Several hundred myosin molecules are arranged within each thick filament with overlapping rodlike portions and the globular heads directed toward either end.
− They occupy the A band at the middle region of the sarcomere.
− Composed of myosin: Large complex (~500 kDa) with two identical heavy chains and two pairs of light chains.
− Two heavy chains are thin, rodlike motor proteins (150 nm long and 2-3 nm thick) twisted together as myosin tails.
− The myosin heads bind both actins, forming transient cross bridges between the thick and thin filaments, and ATP, catalyzing energy release (actomyosin ATPase activity).
THIN FILAMENTS
− Composed of ________
− Helical
− 1.0 μm long and 8 nm wide
− Run between the thick filaments
− Anchored perpendicularly on the Z disc by actin-binding
protein ________
− Exhibit opposite polarity on each side of the Z disc
− Composed of F-actin
protein α-actinin
THIN FILAMENTS (continuation)
− Thin filaments are also tightly associated with two regulatory proteins:
• ______: a 40-nm-long coil of two polypeptide chains located in the groove between the two twisted actin
strands.
• ______: a complex of three subunits:
o ______ – attaches to tropomyosin
o ______ – binds Ca2+
o ______ – regulates the actin-myosin interaction
− Troponin complexes attach at specific sites regularly spaced along each tropomyosin molecule.
• Tropomyosin: a 40-nm-long coil of two polypeptide chains located in the groove between the two twisted actin
• Troponin: a complex of three subunits:
o TnT – attaches to tropomyosin
o TnC – binds Ca2+
o TnI – regulates the actin-myosin interaction
− _______
• Each bisected by a Z disc, consist of the portions of the thin filaments that do not overlap the thick filaments (which is why I bands stain more lightly
− ______
• Contain both thick filaments and the overlapping portions
of thin filaments.
• A lighter zone at the center of the A band
• Corresponding to a region with only the rodlike portions
of the myosin molecule and no thin filaments.
− ______ (Ger. Mitte, middle),
• Bisects the H zone
• Containing a myosin-binding protein ______ that holds
the thick filaments in place. This enzyme catalyzes transfer
of phosphate groups from phosphocreatine, a storage
form of high-energy phosphate groups, to ADP, helping to
supply ATP for muscle contraction.
− I bands
− A bands
− H zone
− M line (Ger. Mitte, middle),
• Containing a myosin-binding protein myomesin that holds
− Other proteins found within sarcomere:
• _______ (3700 kDa)
o Accessory protein in the I band
o The largest protein in the body
o With scaffolding and elastic properties
o Supports the thick myofilaments and connects them
to the Z disc
• ______ (600-900 kDa)
o Binds each thin myofilament laterally,
o Helps anchor them to α-actinin
o Specifies the length of the actin polymers during
myogenesis.
• Titin (3700 kDa)
• Nebulin(600-900 kDa)
(check later if i can get something from this)
SARCOPLASMIC RETICULUM & TRANSVERSE TUBULE
SYSTEM
− In skeletal muscle fibers the membranous smooth ER, called here sarcoplasmic reticulum, contains pumps and other proteins for Ca2+ sequestration and surrounds the myofibrils.
− Specialized for Ca2+ sequestration.
− Depolarization of the sarcoplasmic reticulum membrane, which causes release of calcium, is initiated at specialized motor nerve synapses on the sarcolemma.
− Sarcolemma is folded into a system of transverse or T tubules - penetrate deeply into the sarcoplasm and encircle every myofibril near the aligned A- and I-band boundaries of sarcomeres - to trigger Ca2+ release from sarcoplasmic reticulum throughout the fiber simultaneously and cause uniform contraction of all myofibrils
????
• Complex of a T tubule with two closely associated small
cisterns of sarcoplasmic reticulum on each side
– After depolarization, calcium ions concentrated
within these cisternae are released through Ca2+
channels in the membrane into cytoplasm - Ca2+
binds troponin and allows bridging between actin
and myosin molecules.
– When depolarization ends, the sarcoplasmic
reticulum pumps Ca2+ back into the cisternae,
ending contractile activity.
• Together, the triad components make up a signaling
apparatus for converting repeated cell membrane
depolarizations into spikes of free, cytoplasmic Ca2+ that
trigger contraction.
Triad
(just read)
MECHANISM OF CONTRACTION
− Filaments do not change their length.
− Results as the overlapping thin and thick filaments of each
sarcomere slide past one another
− Induced when an action potential arrives at a synapse, the
neuromuscular junction (NMJ), and is transmitted along the T
tubules to the sarcoplasmic reticulum to trigger Ca2+ release.
1. A nerve impulse triggers release of ACh from the synaptic knob
into the synaptic cleft. ACh binds to Ach receptors in the motor
end plate of the neuromuscular junction, initiating a muscle
impulse in the sarcolemma of the muscle fiber.
2. As the muscle impulse spreads quickly from the sarcolemma
along T tubules, calcium ions are released from terminal
cisternae into the sarcoplasm.
3. Calcium ions bind to troponin. Troponin changes shape,
moving tropomyosin on the actin to expose active sites on
actin molecules of thin filaments. Myosin heads of thick
filaments attach to exposed active sites to form
crossbridges.
4.Myosin heads pivot, moving thin filaments toward the
sarcomere center. ATP binds myosin heads and is broken
down into ADP and P. Myosin heads detach from thin
filaments and return to their prepivot position. The
repeating cycle of attach- pivot-detach-return slides thick
and thin filaments past one another. The sarcomere
shortens and the muscle contracts. The cycle continues as
long as calcium ions remain bound to troponin to keep
active sites exposed.
5. When the impulse stops, calcium ions are actively
transported into the sarcoplasmic reticulum, tropomyosin
re-covers active sites, and filaments passively slide back to
their relaxed state
INNERVATION
− Myelinated motor nerves branch out within the perimysium connective tissue - gives rise to several unmyelinated terminal twigs that pass-through endomysium and form synapses with individual muscle fibers. Schwann cells enclose the small axon branches and cover their points of contact with the muscle cells; the external lamina of the Schwann cell fuses with that of the sarcolemma
− Motor end plate (MEP), or NMJ- a dilated termination of each axonal branch
− Within the axon terminal are mitochondria and numerous
synaptic vesicles containing acetylcholine.
− Between the axon and the muscle is a space, the synaptic cleft.
− Adjacent to the synaptic cleft, the sarcolemma is thrown into numerous deep junctional folds, to provide greater
postsynaptic surface area and more transmembrane
acetylcholine receptors.
− Nerve action potential reaches the MEP – acetylcholine is
liberated from the axon terminal - diffuses across the cleft - binds to its receptors in the folded sarcolemma.
− Acetylcholine receptor contains a nonselective cation channel opens upon neurotransmitter binding – influx of Na+ - depolarizing the sarcolemma - producing the muscle action potential.
− Acetylcholine quickly dissociates from its receptors, and all free neurotransmitter is removed from the synaptic cleft by the extracellular enzyme acetylcholinesterase, preventing prolonged contact of the transmitter with its receptors.
− Motor unit -single axon and all the muscle fibers in contact with its branches
− “All or nothing” contraction
− To vary the force of contraction, the fibers within a muscle fascicle do not all contract at the same time.
− An axon from a single motor neuron can form MEPs with one or many muscle fibers.
− The lesser the number of fibers innervated by one axon the greater the precision of its control and movement
SKELETAL MUSCLE FIBER TYPES
A. ________
• Slow oxidative
• Adapted for slow contractions over long periods without
fatigue, having many mitochondria, many surrounding
capillaries, and much myoglobin, all features that make
fresh tissue rich in these fibers dark or red in color.
B. ________
• Fast, Oxidative-Glycolytic Fibers
• Have physiological and histological features intermediate
between those of the other two types.
C. ________
• Fast, Glycolytic Fibers
• Specialized for rapid, short-term contraction, having few
mitochondria or capillaries and depending largely on
anaerobic metabolism of glucose derived from the stored
glycogen, features that make such fibers appear white.
Rapid contractions lead to rapid fatigue as lactic acid
produced by glycolysis accumulates.
A. Type I
B. Type IIa
C. Type IIb
SMOOTH MUSCLE
− Also called ___________
− Smooth muscle is specialized for slow, steady contraction under the influence of autonomic nerves and various hormones, and is controlled by a variety of involuntary mechanisms.
− This type of muscle is a major component of blood vessels and of the digestive, respiratory, urinary, and reproductive tracts and their associated organs.
− Fibers are elongated, tapering, and nonstriated cells, each of which is enclosed by a thin basal lamina and a fine network of reticular fibers, the endomysium.
− Connective tissues serve to combine the forces generated by each smooth muscle fiber into a concerted action.
− May range in length from 20 μm in small blood vessels to 500 μm in the pregnant uterus.
− Each cell has a single long nucleus in the center of the cell’s central, broadest part.
− The cells stain uniformly along their lengths
− The narrow part of one cell lies adjacent to the broad parts of neighboring cells.
− All cells are linked by numerous gap junctions.
− The borders of the cell become scalloped when smooth muscle contracts and the nucleus becomes distorted.
− Concentrated near the nucleus are mitochondria,
polyribosomes, RER, and the Golgi apparatus.
− The short membrane invaginations, called caveolae, are often frequent at the smooth muscle cell surface.
− Also called visceral muscle
SMOOTH MUSCLE (continue)
− Smooth muscle cells also have an elaborate array of 10-nm intermediate filaments, usually composed of desmin.
− The intermediate filaments and F-actin filaments both insert into cytoplasmic and plasmalemma- associated dense bodies.
− Dense bodies contain α-actin in and are functionally similar to the Z discs of striated and cardiac muscle.
− The attachments of thin and intermediate filaments to the dense bodies helps transmit contractile force to adjacent smooth muscle cells and their surrounding network of reticular fibers
− Not under voluntary control, and its
− Fibers lack MEPs.
− Control can involve autonomic nerves, a variety of hormones and similar substances, and local physiologic conditions such as the degree of stretch.
− Whether smooth muscle fibers contract as small groups or throughout an entire muscle to produce waves of contraction is determined largely by the degree of autonomic innervation and the density of the gap junctions; both conditions vary considerably in different organs.
simply lie in close contact with the sarcolemma with little or no specialized structure to the junctions.
− Smooth muscle is most often spontaneously active without nervous stimuli, its nerve supply serves primarily to modify activity rather than to initiate it.
− Receives both adrenergic and cholinergic nerve endings that act antagonistically, stimulating or depressing its activity.
− Supplement fibroblast activity, synthesizing collagen, elastin, and proteoglycans, with a major influence on the extracellular matrix (ECM) in tissues
− Active synthesis of ECM by the small cells/fibers of smooth muscle may reflect less specialization for strong contractions than in skeletal and cardiac muscle
− And is similar to this synthetic function in other contractile cells, such as myofibroblasts and pericytes.
− Swellings of autonomic nerve axons with synaptic vesicles simply lie in close contact with the sarcolemma with little or no specialized structure to the junctions.
REGENERATION OF MUSCLE TISSUE
− Repair and regeneration can occur in skeletal muscle because of a population of reserve muscle satellite cells that can proliferate, fuse, and form new muscle fibers.
− Cardiac muscle lacks satellite cells and has little capacity for regeneration. Defects or damage (e.g., infarcts) to heart muscle are generally replaced by proliferating fibroblasts and growth of connective tissue, forming only myocardial scars
− Regeneration is rapid in smooth muscle because the cells/fibers are small and relatively less differentiated, which allow renewed mitotic activity after injury.
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