Chapter 10 Module 1 Muscle Tissue

Chapter 10 Module 1 Muscle Tissue


This is Chapter 10,
Module 1, Muscle Tissue. The learning objectives
of this module are, one, describe the
characteristics and functions of muscular tissue, and,
two, explain the organization of muscle at the tissue level. Muscle is one of the
primary tissue types. Skeletal muscle perform
six major functions. Skeletal muscle produces
skeletal movement by contracting and pulling
on the bones of the skeleton. Skeletal muscles maintain
posture and body position by maintaining tension,
thereby allowing you to hold your head while
sitting and reading a book or balancing your body
when you walk or stand. Skeletal muscles
support soft tissues, such as abdominal muscles
and pelvic floor muscles, supporting the organs of
the abdominal pelvic cavity. Skeletal muscles guard
entrances and exits by surrounding the openings
of the digestive and urinary tracts. Skeletal muscles
maintain body temperature by releasing heat
when they are working. And finally, skeletal muscles
can store nutrient reserves because the protein in
muscles can be broken down into amino acids, which can
then be used to produce energy. Each muscle is composed
of muscle cells called muscle fibers. These muscle fibers
are contained in bundles called fascicles. Muscles have connective
tissue that’s associated with
the entire muscle, with the fascicle
or muscle bundle, and with the individual
muscle fiber. Epimysium is a dense
layer of collagen fibers that surrounds
the entire muscle. It separates the muscle from
nearby tissues and organs. The perimysium divides
the skeletal muscle into a series of compartments. Each compartment contains
a muscle fascicle. The perimysium contains
blood vessels and nerves. Within each fascicle, the
endomysium is more delicate and surrounds each
individual muscle fiber. The endomysium contains
capillary blood vessels, nerve fibers, and
myosatellite cells, which are stem cells, that
help to repair damaged muscle tissue. The collagen fibers of the
epimysium, the perimysium, and the endomysium come
together to form either a bundle, known as a tendon,
or a broad sheet called aponeurosis. Tendons and aponeuroses
usually attach skeletal muscles to bones. Skeletal muscle
cells or fibers are very different from the typical
cells we’ve seen so far. One obvious difference is
that these skeletal muscle fibers are much larger
than other cells. A muscle fiber from
the thigh muscle could have a length
up to 12 inches. A second difference is
that skeletal muscle contains hundreds of nuclei just
internal to the cell membrane. These nuclei are needed
to produce enzymes and structural proteins that
are required for normal muscle contractions. The cell membrane
of a muscle fiber is known as the sarcolemma,
and the cytoplasm is known as the sarcoplasm. Inside each muscle
fiber are hundreds to thousands of structures
called myofibrils. These structures are
cylindrical in shape. They can actively
shorten in shape and are responsible for skeletal
muscle fiber contraction. Myofibrils consist of protein
filaments called myofilaments. These myofilaments are either
a thin filaments composed primarily of actin, or thick
filaments composed primarily of myosin. The myofibrils are
anchored to each end of the muscle fiber, which
is connected to its tendon. As a result, when the
myofibrils contract, the entire cell shortens
and pulls on the tendon. Transverse tubules,
or T-tubules, are narrow tubes that are
continuous with the sarcolemma and extend deep
into the sarcoplasm. They are filled with
extracellular fluid and form passageways through the muscle
fiber like a network of tunnels through a mountain. Electrical impulses
called action potentials travel along the T-tubules
into the cell interior. These action potentials trigger
muscle fiber contraction. Branches of the T-tubules
surround each myofibril. The sarcoplasma
reticulum is similar to the smooth endoplasmic
reticulum of other cells. The sarcoplasmic reticulum fits
over each individual myofibril like a lacy shirt sleeve. Where a T-tubule
encircles the myofibril, the sarcoplasmic tubule expands. And these chambers are
called terminal cisternae. The terminal cisternae
contains stored calcium, and sometimes the concentration
of calcium inside the cisternae is 1,000 times higher than the
levels inside the sarcoplasm. The thick and thin
filaments of the myofibril are organized into repeating
functional units called sarcomeres. Sarcomeres are the
smallest functional units of the muscle fiber. Interactions between the
thick and thin filaments of sarcomeres as are responsible
for muscle contraction. One myofibril consists
of approximately 10,000 sarcomeres, end-to-end. A sarcomere contains thick
filaments, thin filaments, proteins that stabilize the
positions of the thick and thin filaments, and
proteins that regulate the interactions between the
thick and thin filaments. The thick and thin filaments
are different in size, density, and distribution. These differences account
for the banded appearance of each myofibril. The thick filaments are at
the center of each sarcomere. Proteins of the M line
connect to the central portion of each thick filament to
neighboring thick filaments. M stands for middle. The thin filaments are located
between the thick filaments in an area called
the zone of overlap. A single thin filament
contains two rows of 300 to 400 individual
globular molecules. Each of these molecules
contains an active site that combine to myosin. Under resting
conditions, a complex called the
tropomyosin-troponin molecule covers the active sites and
prevents the binding of myosin to the active site. Tropomyosin is a double-stranded
rope-like structure that covers the active sites
on the actin molecules. Troponin is a molecule that
locks the tropomyosin molecule to the actin molecule,
thereby preventing the exposure of the actin
molecule’s active site. A thick filament contains
about 300 myosin molecules. The myosin molecules are
twisted around each other. Each myosin molecule has
a long tail and a head, which projects outward toward
the nearest thin filament. When the myosin heads
interact with thin filaments during a contraction, they
are known as cross-bridges. The connection between the
head and the tail of the myosin acts as a hinge that
lets the head pivot. When the head pivots, it
swings toward the M line, or the center of the sarcomere. All of the myosin molecules
are arranged with their tails pointing toward the M line. When the myosin head forms
cross-bridge with the actin molecules active site
and the head pivots, causing the thin
filaments to slide toward the center
of each sarcomere, this is known as the
sliding filament theory. During a contraction, sliding
occurs in every sarcomere along the myofibril. As a result, the
myofibril gets shorter. When myofibrils get shorter,
so does the muscle fiber. This ends Chapter 10,
Module 1, Muscle Tissue.

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