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David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova
biologia.blu
Anatomia e fisiologia dei viventi S
Musculoskeletal Systems
How Do Muscles Contract?
Three types of vertebrate muscle:
• Skeletal—voluntary movement, breathing,
maintaining posture
• Cardiac—beating of heart
• Smooth—involuntary, movement of internal organs
How Do Muscles Contract?
Skeletal muscle (striated):
•Cells are called muscle fibers—large and
multinucleate
•One muscle consists of many muscle fibers bundled
together by connective tissue
How Do Muscles Contract?
Contractile proteins:
Actin—thin filaments
Myosin—thick filaments
Each muscle fiber has many myofibrils—bundles of
actin and myosin filaments.
Each myofibril consists of sarcomeres—repeating
units of overlapping actin and myosin filaments.
Each sarcomere is bounded by Z lines which anchor
the actin.
How Do Muscles Contract?
Other parts of a sarcomere:
•A band in center—contains myosin
•H zone and I band—no overlap of actin and myosin in
relaxed muscle
• M band within
H zone—
contains
proteins that
anchor myosin
filaments
The Structure of Skeletal Muscle
Sliding Filaments
When a muscle contracts, sarcomeres shorten and
band pattern changes.
How Do Muscles Contract?
The sliding filament model of muscle contraction
depends on structure of actin and myosin molecules.
• Myosin: two polypeptide chains coiled together,
ending in a globular head
A myosin filament is many molecules in parallel with
heads projecting sideways.
• Actin filament: actin monomers in a long, twisted
molecule
Tropomyosin twists around actin with troponin
attached at intervals.
Actin and Myosin Filaments Overlap to Form Myofibrils
Contraction of the sarcomere involves many cycles of
interaction between actin and myosin molecules.
How Do Muscles Contract?
Muscle cells are excitable—the plasma membranes
can conduct action potentials.
Contraction is initiated by action potentials from a
motor neuron at the neuromuscular junction.
Motor unit—one
motor neuron and
all the muscle fibers
it synapses with.
How Do Muscles Contract?
At the neuromuscular junction, acetylcholine is the
transmitter.
It binds to receptors in the postsynaptic membrane,
ion channels in the motor end plate open, Na+ flows in
and the motor end
plate is depolarized.
Depolarization
spreads; when
threshold is reached,
the muscle fiber
membrane fires an
action potential.
How Do Muscles Contract?
Action potentials in muscle fiber also travel deep within
the cell.
The plasma membrane is continuous with T tubules
that run through the sarcoplasm (muscle fiber
cytoplasm).
T tubules run close to
the sarcoplasmic
reticulum (muscle
fiber ER) that
surrounds every
myofibril.
How Do Muscles Contract?
An action potential spreads through the T tubules;
when it reaches the receptor proteins they change
conformation.
This opens Ca2+ channels
and Ca2+ flows out of the
sarcoplasmic reticulum.
How Do Muscles Contract?
Ca2+ binds to troponin on the actin filaments—this
twists the tropomyosin so that actin binding sites are
exposed.
When Ca2+ pumps
in the SR remove
Ca2+ from
sarcoplasm,
contraction stops.
How Do Muscles Contract?
Smooth muscle—in most internal organs; under
autonomic nervous system control.
Smooth muscle cells are arranged in sheets and have
electrical contact via gap junctions.
Action potential in one cell can spread to all others in
the sheet.
The plasma membrane of smooth muscle cells is
sensitive to being stretched.
How Do Muscles Contract?
Cardiac muscle is also striated—cells are smaller than
skeletal muscle and have one nucleus.
This cells also branch and interdigitate; are resistant to
tearing and can withstand high pressures.
Intercalated discs
provide mechanical
adhesions between cells
and allow cytoplasmic
continuity and electrical
coupling.
How Do Skeletal Systems and Muscles Work Together?
The human skeleton has 206 bones.
The axial skeleton includes skull, vertebral column,
sternum, and ribs.
Appendicular skeleton includes pectoral and pelvic
girdles, and bones of the arms, legs, hands, and feet.
The Human Endoskeleton
How Do Skeletal Systems and Muscles Work Together?
Vertebrate endoskeleton consists of two kinds of
connective tissue:
• Cartilage is found on bone surfaces in joints; also in
ears, nose, and larynx.
• Bone has an extracellular matrix of insoluble calcium
phosphate crystals.
It is the reservoir of calcium for the body, and is in
equilibrium with soluble calcium in the extracellular
fluids.
How Do Skeletal Systems and Muscles Work Together?
Bone is constantly being replaced and remodeled by
living bone cells.
Osteoblasts make new bone matrix. When they
become enclosed in bone they are called osteocytes.
Osteoclasts are cells that reabsorb bone.
How Do Skeletal Systems and Muscles Work Together?
Two types of bone development:
Membranous bone forms on a
scaffold of connective tissue
(e.g., outer bones of skull).
Cartilage bone is first
cartilaginous, then ossifies or
hardens (e.g., limb bones).
Growth can occur throughout
the ossification process.
How Do Skeletal Systems and Muscles Work Together?
Bone structure:
• Compact—solid and hard
• Cancellous—with numerous cavities, appears
spongy, lightweight but strong.
Most bones have both types.
How Do Skeletal Systems and Muscles Work Together?
Compact bone in mammals is called Haversian
bone—the structural units are Haversian systems.
Each system has concentric bony cylinders with
osteocytes in between.
Center canal has
blood vessels and
nerves.
Adjacent systems are
separated by glue
lines.
How Do Skeletal Systems and Muscles Work Together?
Joints are where two or
more bones come
together.
Different types of joints
allow motion in different
directions.
How Do Skeletal Systems and Muscles Work Together?
Muscles can exert force in only one direction—they
create movement by working in antagonistic pairs.
• Flexor—the muscle
that bends or flexes the
joint
• Extensor—the muscle
that straightens or
extends the joint.
How Do Skeletal Systems and Muscles Work Together?
• Ligaments are bands of connective tissue that hold
bones together at joints
• Tendons are connective tissue straps that join
muscle to bone.
Bones are a system of levers
moved by muscles.
What Are the Different Types of Skeletal System?
Skeletal systems are the rigid supports against which
muscles can pull.
Three types of skeletal systems:
• Hydrostatic
• Exoskeletons
• Endoskeletons
What Are the Different Types of Skeletal System?
Hydrostatic skeletons consist of fluid enclosed in a
body cavity surrounded by muscle.
When muscles oriented
in one direction contract,
the fluid-filled body
cavity bulges out in the
opposite direction.
Examples: cnidarians,
annelids, and other
invertebrates.
What Are the Different Types of Skeletal System?
An exoskeleton consists of a hardened outer surface
to which muscles attach.
Contractions of the muscles cause segments of the
exoskeleton to move.
Examples: mollusks, arthropods.
The complex arthropod exoskeleton (or cuticle)
covers all outer surfaces and appendages.
It is made of chitin secreted by cells just below the
exoskeleton.
For growth to occur, the exoskeleton must be shed
(molting).
What Are the Different Types of Skeletal System?
The endoskeleton of vertebrates is an internal
scaffold.
An advantage is that growth can occur without
shedding the skeleton.
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved