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MUSCLE TISSUE
 Muscle tissue facilitates movement of the animal
by contraction of individual muscle cells (referred
to as muscle fibers).
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Three types of muscle fibers occur in animals:
skeletal (striated),
smooth and
cardiac
TYPES OF MUSCLE TISSUE
Muscle occurs in three distinct types:
 Skeletal muscle tissue attaches to bones (via tendons)
for voluntary movement: This is the type of muscle
that you eat if you eat meat.
 Smooth muscle tissue contains spindle shaped cells
with a single nucleus: it lines the gut, blood vessels and
glands its operation is involuntary.
 Cardiac (heart) muscle is composed of short, striated
cells that can function in units due to the contracting signal
that passes from cell to cell by way of the gap junctions.
Skeletal muscle tissue
attaches to bones (via tendons) for voluntary
movement
Cellular organization of skeletal
muscle fibers
 Unlike most tissues, skeletal muscle does not consist of
individual cells. Rather it is formed from huge, striated
(actin and myosin) multinucleated, long cells (fibers),
which are often bundled together to form a “muscle”.
 The muscle fibers develop by fusion of many individual
embryonic cells called myoblasts.
 Each individual skeletal muscle fiber extends over much of
the length of the muscle in which it resides (up to many
centimeters), with a uniform diameter that is typically
around 50 µm.
Microscopic features of the muscle
fiber
 The unique contractile cytoplasm of muscle fibers is called
sarcoplasm (sarco = flesh).
 The contractile machinery is concentrated into myofibrils (myo =
muscle), long narrow structures (1-2 µm in diameter) which extend the
length of the fiber and form the bulk of the sarcoplasm.
 itochondria (once called sarcosomes) and
 sarcoplasmic reticulum (a highly specialized form of endoplasmic
reticulum) surround each myofibril.
 The plasma membrane of muscle fibers is sometimes called the
sarcolemma.
 The many nuclei within each fiber are displaced from the center of the
fiber by the specialized sarcoplasm and are located adjacent to the
sarcolemma (except in special cases, like the intrafusal fibers of
muscle spindles, and regenerated fibers).
Skeletal Muscle
Identification: Teased or l.s. section shows distinct,
very large, straight fibers. Looks more like hair than
any other tissue. Also has distinct striations.
 Features to Know: striations
(1) composed of dark Abands and light I-bands;
nuclei (2) pushed to edge of
fiber; sarcolemna (plasma
membrane surrounding
fiber).
Where Located: skeletal
muscles; under voluntary
control.
Skeletal Muscle - also called
volutnary or striated muscle.
The striations (stripes) are due to a very orginized pattern
of proteins found within the cells.
 The black arrow is
pointing to a
nucleus.
 the entire length of
the fiber may include
hundreds nuclei.
Striations (regularly spaced bands)
 Each myofibril is marked by striations (alternating light
and dark bands) within the fibers along its length. Dark
and light bands are composed of myosin and actin,
respectively.
 In relaxed muscle, a single repeat of this pattern is about 2
µm long (the length varies with degree of contraction) and
is called a sarcomere.
 Because the striations on adjacent myofibrils are usually
aligned, the entire muscle fiber appears uniformly striated.
Sarcomere
 In each sarcomere, the broad band which appears dark in standard
histological procedures is called the A-band. This band indicates the
location of thick filaments (myosin); it is darkest where thick and thin
filaments overlap.
 The broad light band between the dark bands is the I-band. The Iband indicates the location where thin filaments (actin) extend
beyond the thick filaments.
 A distinct dark line running down the middle of the I-band is the Z-line,
where thin filaments are attached end to end. Where thick filaments
are attached end-to-end in the center of the A-band is the M-line.
 When muscle is stretched, an H-band appears along the middle of the
A-band, between the free ends of the thin filaments.
From Blue Histology
(Copyright Lutz Slomianka
1998-2004) The image
should be animated, if you
watch patiently.)
Skeletal Muscle fibers, Cross
Section
 Any particular
cross section
of a muscle
fiber may
reveal one or a
few nuclei, or
none, but the
entire length of
the fiber may
include
hundreds.
The structural organization of skeletal
muscle cross section, slide at 100X
Individual muscle fibers (4) are bundled into fascicles (5). Surrounding
individual fibers, fascicles, and the entire muscle are layers of connective
tissue (1-3).
 Features to Know:
connective tissue sheaths:
epimysium (1), perimysium
(2), & endomysium (3);
muscle fiber with nuclei (6)
along edges; fascicle (5).
You may see
Skeletal Muscle cross section at 400x
Smooth Muscle - also called involuntary or visceral
muscle.
Smooth muscle is stronger than skeletal muscle. If you ever had
stomach cramps you know how strong smooth muscle can be - a better
example is labor (for those of you who have experienced or seen child
birth) - the uterus has a large quantity of smooth muscle that works to
"birth" a baby.
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Muscle cells are packed tightly
together (no gaps between cells)
and usually not distinct.
These cells function in involuntary
movements and/or autonomic
responses (such as breathing,
secretion, ejaculation, birth, and
certain reflexes). These fibers are
components of structures in the
digestive system, reproductive
tract, and blood vessels.
Cellular organization of Smooth muscle
fibers
Smooth muscle fibers lack the banding, although actin and
myosin still occur.
Smooth muscle fibers are spindle shaped cells that form
masses.
 Nuclei may or
may not be
visible. Note lack
of striations.
Smooth muscle fibers are components of
structures in the wall of the digestive system,
Inner Circular, Outer Longitudinal of smooth
muscle fibers
Smooth muscle fibers are components of
structures in the blood vessels.
 Smooth Muscle in Arterial Wall
Cardiac (heart) muscle is
composed of short, striated cells
 Note faint striations
across fibers. Fibers
distinct, typically with
numerous small
gaps between them.
 Features to Know:
nuclei (1),
intercalated disk (2).
Like skeletal muscle, cartiac muscle is
striated,
but cardiac muscle also contains structures called
intercalated disks (blue arrow) that work to produce the
rhythmic contractions of the heart. Intercalated disks are
found only in cardiac muscle.
 And fibers in
cardiac
muscle are
much smaller
& branched.