Muscle is classified into three types:
skeletal, cardiac, and smooth muscle
Muscle cells possess contractile
filaments containing actin and
myosin
• Structure of Skeletal Muscle
• 1. Types of skeletal muscle cells include red
(slow), white (fast), and intermediate. All
three types may be present in a given muscle.
• 2. These three types vary in their content of
myoglobin, number of mitochondria,
concentration of various enzymes, and rate of
contraction
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Red (slow; type 1)
High - Myoglobin
Many -Mitochondria
High in oxidative enzymes; low in ATPase
Contraction -Slow but repetitive; not easily
fatigued
Intermediate (type 2A)
Intermediate-Myoglobin
Intermediate-Mitochondria
Intermediate in oxidative enzymes and ATPase
Contraction-Fast but not easily fatigued
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White (fast; type 2B)
Low -Myoglobin
Few -Mitochondria
Low in oxidative enzymes;high in ATPase and
phosphorylases
• Fast and easily fatigued
• Connective tissue
• 1. Epimysium surrounds an entire muscle.
• 2. Perimysium surrounds fascicles (small
bundles) of muscle cells.
• 3. Endomysium surrounds individual muscle
cells and is composed of reticular fibers and
an external lamina.
• Skeletal muscle cells are long, cylindrical,
multinucleated cells that are enveloped by an
external lamina and reticular fibers.
• Their cytoplasm is called sarcoplasm, and their
plasmalemma is called the sarcolemma
• and forms deep tubular invaginations, or T
(transverse) tubules, which extend into the
cells. The muscle cells possess cylindrical
• collections of myofibrils, 1-2 micrometers
(pm) in diameter, which extend the entire
length of the cell.
• Myofibrils are longitudinally arranged,
cylindrical bundles of thick and thin
myofilaments
• Skeletal muscle cross-striations
• A bands: They contain both thin and thick
filaments, which overlap.
• Six thin filaments surround each thick filament
• I bands : They contain only thin filaments.
• H bands are light regions transecting A bands;
they consist of thick filaments only.
• M lines are narrow, dark regions at the center
of H bands
• Z disks (lines) are dense regions bisecting each
I band
• Z disks contain a-actinin, which binds to thin
filaments
Molecular organization of
myofilaments
• Thin filaments are composed of F-actin,
tropomyosin, troponin, and associated
proteins.
• a. F-actin is a polymer of G-actin monomers
arranged in a double helix.
• (1) Each monomer possesses an active site that
can interact with myosin.
• (2) F-actin is present as filaments (with a
diameter of 5 nm) that exhibit
• polarity, having a (+) and (-) end.
• Tropomyosin molecules are 41 nm in length. They bind head-totail, forming filaments that are located in the grooves of the F-actin
helix.
• Troponin is associated with each tropomyosin molecule and is
composed
• of:
• (1) Troponin T (TnT), which forms the tail of the molecule and
functions in binding the troponin complex to tropomyosin.
• (2) Troponin C (TnC), which possesses four binding sites for calcium.
• (3) Troponin I (TnI), which binds to actin, inhibiting interaction of
• myosin and actin.
• Thick filaments each contain about 250 myosin
molecules arranged in an antiparallel fashion and three
associated proteins—myomesin, titin,and C protein.
• a. Myosin is composed of two identical heavy chains
and two pairs of light chains.
• (1) Myosin heavy chains consist of a long rod-like "tail"
and a globular "head." The tails of the heavy chains
wind around each other in an a-helical configuration.
• (a) Tails function in the self-assembly of myosin
molecules into bipolar thick filaments.
• (b) Actin-binding sites of the heads function in
contraction.
• (2) Myosin light chains are of two types; one molecule
of each type is associated with the globular head of
each heavy chain.
• The sarcomere is the regular repeating region
between successive Z disks and constitutes
the functional unit of contraction in skeletal
muscle.
• The sarcoplasmic reticulum (SR) is a modified
smooth endoplasmic reticulum (SER) that
surrounds myofilaments and forms a
meshwork around each myofibril.
• The SR forms a pair of dilated terminal
cisternae, which encircle the myofibrils at the
junction of each A and I band.
• It regulates muscle contraction by
sequestering calcium ions (leading to
relaxation) or releasing calcium ions (leading
to contraction).
• Triads are specialized complexes consisting of
a narrow central T tubule flanked on each side
by terminal cisternae of the SR.
They are located at the A-I junction in
mammalian skeletal muscle cells and help
provide uniform contraction throughout the
muscle cell.
Cardiac Muscle
• General features
• 1. Contract spontaneously and display a
rhythmic beat.
• 2. Contain one (occasionally two) centrally
located nuclei.
• 3. Contain glycogen granules, especially at
either pole of the nucleus.
• 4. Possess thick and thin filaments arranged in
poorly defined myofibrils.
Structural components of cardiac
muscle cells
• 1. T tubules are larger than those in skeletal
muscle and are lined by external lamina. They
invaginate at Z disks, not at A-I junctions as in
skeletal muscle.
• 2. SR is poorly defined and contributes to the
formation of dyads, each of which consists of one
T tubule and one profile of SR.
• 3. Mitochondria are more abundant than in
skeletal muscle; they lie parallel to the I bands
and often are adjacent to lipids.
• 4.Atrial granules are present in the atrial
cardiac muscle cells and contain the precursor
of atrial natriuretic peptide, which acts to
decrease resorption of sodium and water in
the kidneys, reducing body fluid volume and
blood pressure.
• 5. Intercalated disks are complex step-like
junctions forming end-to-end attachments
between adjacent cardiac muscle cells.