BMED 2801 Lecture 17 – Histology of Muscle Tissue
Muscle classification
Striated – Skeletal and Cardiac
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Cross-striations
Due to arrangement of actin and myosin myofilaments
Smooth
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Actin and myosin not as precisely arranged
Muscle
Function = contraction
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Movement of body
Changes in size and shape of organs
Muscle cell = composed of muscle fibre
muscle fibre are
o Elongated
o Parallel to each other
Some definitions
Sarcolemma = plasma membrane
Sarcoplasm = cytoplasm
Sarcoplasmic reticulum = sER
Muscle fibre cells
Cross-section view
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Polygonal shape
10-100µ diameter
2mm up to 1metre long
Nucleus directly below plasma membrane
Structural hierarchy:
Actin + myosin
= myofilaments
Bundles of myofilaments = myofibrils
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Surrounded by sER, mitochondria and glycogen
Bundles of myofibrils = fill sarcoplasm of muscle fibres
Bundles of fibres = muscle
Myofilaments
2 types associated with contraction
Thin (actin) myofilaments
o 6-8nm diameter
o 1μ long
o Actin, troponin (Ca2+), tropomyosin
Thick (myosin) filaments
o 15nm diameter
o 1.5μ long
o Myosin, ATP binding sites
Smooth muscle fibres
o Occurs as bundles or sheets of elongated fusiform cells with finely tapered ends.
o The cells are also called fibers and range in length from 20 microns in the walls of smooth blood vessels
to about 200 microns in the wall of the intestine; they may be as large as 500 microns in the wall of the uterus
during pregnancy.
o It is specialised for slow prolonged contraction.
o There are no direct neural endings on smooth muscle cells; nerve terminals in smooth muscles are in
immediately adjacent connective tissue, and the neurotransmitter diffuses to the muscle cells.
o Gap junctions + communication junctions between smooth muscle cells allow propagation of small cells
and ions of the contraction within the muscle.
Ultrastructure of smooth muscle:
o Elongated cells = fibres  Arranged in bundles and sheets
o The nucleus appears as an elongated structure with tapering ends, lying in the centre axis of the cell. In a
cross section, the nucleus appears as a circular or round profile whether the cell is contracted or relaxed.corkscrew appearance in longitudinal sections - a characteristic that is a result of contraction of the cell
during fixation.
o Tapered cytoplasmic ends
o Thin myofilaments = actin, Thick myofilaments = myosin
o Sarcolemma invaginates to form vesicles
o Sarcoplasmic reticulum
o Does not form a T-system
o Actin and myosin myofilaments in no specific arrangement
o Organelles concentrated at each end of the nucleus
Myofilament structure in smooth muscle cells
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Possess a contractile apparatus of thin & thick filaments & a cytoskeleton of desmin and vimentin
intermediate filaments.
Thick myosin filaments are scattered throughout the sarcoplasm of a smooth muscle cell. They are extremely
labile & tend to be lost during tissue preparation.
The thin filaments of smooth muscle cells are attached to cytoplasmic densities or dense bodies that are
visible among the filaments.
The components of the contractile apparatus in smooth muscle are:
- Thin filaments containing actin, the smooth muscle isoform of tropomyosin, and two smooth muscle-specific
proteins caldesmon and the calponin.
- Thick filaments containing myosin II- the myosin II are orientated in one direction on one side of the filament
and in an opposite direction on the other side of the filament --> staggered in parallel between two immediate
neighbours & are also bound to an anti-parallel partner via an overlap at the very tip of their tails.
- Myosin light chain kinase (MLCK), α-actinin and calmodulin are other smooth muscle proteins associated with
the contractile apparatus.
Skeletal muscle fibers
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A multinucleated syncytium- The multiple nuclei are at the periphery of the cell, just under sarcolemma
Arranged in parallel with their neighbours.
When viewed in cross section, the mature multinucleated muscle fiber reveals a polygonal shape with a
diameter of 10 to 100 microns.
Their length varies from almost a meter, as in the sartorius muscle of the lower limb, to as little as a few
millimeters, as in the stapedius muscle of the middle ear.
They are held together by connective tissue.
Contractile elements fill the rest of the cell.
The highly ordered arrangement of the contractile filaments of actin & myosin accounts for the crossstriations characteristics of a longitudinal section of skeletal muscle fibers (thus they are also called
striated muscle).
Skeletal muscle has a rich blood supply, and each fiber is usually close to several capillaries.
Ultrastructure of skeletal muscle:
o multinucleated, nuclei located at the periphery immediately under the sacrolemma
o Defined cross striations – due to the highly arrange myofilaments – into the sacromere
o Triad configuration present in the scarolemma – 2 adjacent cisternae and a transverse-tubule at the A-I
junction
o Sacroplasmic reticulum arranged as repeating series of networks around myofibrils
Functional unit of Muscle = the Sarcomere
The Sarcomere
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Extends from Z-line to Z-line
2-3µ long when relaxed
4µ long at maximum stretch
1µ long when contracted
The Anatomy of a Sacromere:
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The thick filament produce the dark A band.
The thin filament extends in each direction from the Z line.
Where they do not over lap the thick filaments, this creates the light I band.
The A band is bisected by the H zone.
The I bands are bisected by the A line.
The H zone is that portion of the A band where the thick and thin filaments do not overlap.
The entire arrangement of thick and thin filaments between the Z lies is what makes up one sacromere.
Shortening of the sarcomeres in a myofibril produces the shortening of the myofibril and, in turn, of the
muscle fiber of which it is a part.
During contraction  H and I bands are shortened and A bands stay the same and overlap.
Sacroplasmic reticulum
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Is arranged as a repeating series of networks around the myofibrils.
Each network of the reticulum extends from one A-I junction to the next A-I junction with a sarcomere.
Thus one network of sarcoplasmic reticulum surrounds the A band, and the adjacent network surrounds the I
band. Where the two networks meet, at the junction between A and I bands, the sarcoplasmic reticulum forms
a slightly more regular ringlike channel called the terminal cisternae.
This serves as reservoirs for Ca2+. To release Ca2+ into the sarcoplasm, the plasma membrane of the
terminal cisternae contains an abundance of gated Ca2+ -release channels.
Also located around the myofibrils in association with the sarcoplasmic reticulum are large numbers of
mitochondria and glycogen granules, both of which are involved on providing the energy necessary for the
reactions involved on contraction.
T-tubules
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Transverse tubular system, or T system consists of numerous tubular invaginations of the plasma
membrane; each one is called a T tubule.
o T tubules penetrate to all levels of the muscle fiber and are located between adjacent terminal cisternae
at the A-I junctions.
o They contain voltage sensor proteins, depolarization-sensitive transmembrane channels that are
activated when the plasma membrane depolarizes.
o Conformational changes of these proteins directly affect the gated Ca2+ -release channels located in the
adjacent plasma membrane of the terminal cisternae.
o The complex of T tubule and the two adjacent cisternae is called a triad.
Regulation of contraction involves Ca2+, sarcoplasmic reticulum and the transverse tubular system.
SUMMARY
• Invaginations of the sarcolemma
• Located at the A-I junction
• Depolarization of T-Tubule
• Ca2+ release from sarcoplasmic reticulum
• Initiates muscle contraction
The triad
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2 adjacent cisternae of the scaroplasmic reticulum and a complex of a transverse tubule = triad
In skeletal muscles – triad is located in the A-I band junction
Terminal cisternae – are regular ring like channels formed by the scaroplasmic reticulum at the junction of
the A-I bands  it serves as a reservoir for Ca2+ and releases Ca2+ though gated Ca2+ release channels.
Physiological significance of the triad configuration
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Depolarisation of the T-tubule membrane, triggering the release of Ca2+ from the terminal cisternae
needed for the reaction between myosin and actin = muscle contraction.
The sliding filament mechanism of contraction
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Myofilaments are the individual filamentous polymers of myosin II (thick filaments) and actin and its
associated proteins (thin filaments).
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The contraction cycle: shortening of a muscle involves the rapid contraction cycles that move the thin
filaments along the thick filament.
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Each contraction cycle consists of five stages: attachment, release, bending, force generation and
reattachment.
1. Attachment is the initial stage of the contraction cycle, in which the myosin head is tightly bound to the
actin molecule of the thin filament.
2. Release is the second stage of the cycle, in which the myosin head is uncoupled from the thin filament.
3. Bending is the third stage of the cycle, in which the myosin head, as a result of hydrolysis of ATP,
advances a short distance in relation to the thin filament.
4. Force generation is the fourth stage of the cycle, in which the myosin head releases inorganic phosphate
and the power stroke occurs.
5. Reattachment is the fifth & last stage if the cycle in which the myosin head binds tightly to a new actin
molecule.
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Cardiac muscle fibres
Cardiac muscle fibers consist of numerous cylindrical cells arranged end to end – with a single centrally
nucleus
Some cardiac muscle cells in a fiber may join with two or more cells through intercalated discs, thus leading
to a branched fiber.
Has the same types of arrangement of contractile filaments as skeletal muscle, - exhibit cross-striations
evident in sections.
They also exhibit densely staining cross-bands, called intercalated discs --> these cross the fibers in a linear
fashion or frequently in a way that resembles the risers of a stairway.
The intercalated discs represent highly specialised attachment sites between adjacent cells.
This linear cell-to-cell attachment of the cardiac muscle cells results in ‘fibers’ of variable length.
Ultrastructure of Cardiac muscle
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Centrally located nucleus
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Microfibrils of cardiac muscle separate to pass around the nucleus, thus outlining a biconical juxtanuclear
region in which the cell organelles are concentrated.
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Cytoplasmic granules (hormones)
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Large mitochondria, glycogen granules located between the myofibrils
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longer T-tubles
• Sarcoplasmic reticulum
Glycogen = stores energy
Mitochondria = releases energy
Myofibrils = use energy
= Intercalated Discs
1. Attachment sites between muscle fibres (cells), allowing for cell-to-cell communication
2. Fibres of variable length
3. Fibres can branch
Light microscope appearance
- Dense staining cross-bands
Electron microscope appearance
- Fascia adherence
- Macula adherens – desmisomes
- Gap junctions
- Possibly – zonula adherence
The locations of the three types of muscle
Smooth muscle: Vessels, organs and viscera
Skeletal muscle: Muscles of skeleton visceral striated (e.g. tongue, esophagus, diaphragm)
Cardiac muscle: Heart, superior & inferior vena cava, pulmonary veins