Download Chapter 47

Effectors: Making Animals
Move
47
Microtubules, Microfilaments, and Cell Movement
• Microfilaments are proteins that generate
contractile forces by changing conformation.
• Microfilaments reach their highest level of
organization in muscle cells.
• Myosin and actin are the proteins responsible for
the contraction and relaxation of muscle.
Figure 47.1 Types of Vertebrate Muscle Tissue
The three
types of
vertebrate
muscle are
smooth,
cardiac,
and
skeletal.
47
Muscle Contraction
• Smooth muscle provides
contraction for internal organs,
which are under the control of the
autonomic nervous system.
• Smooth muscle moves food
through the digestive tract, controls
the flow of blood, and empties the
urinary bladder.
• Smooth muscle cells are the
simplest muscle cells structurally;
they have a single nucleus and are
usually long and spindle-shaped.
Smooth muscle in
human uterus.
47
Muscle Contraction
• Cardiac muscles are branched and appear
striated because of the regular arrangement of
their actin and myosin filaments.
• Branching creates a meshwork that resists
tearing and allows the heart to withstand the
high pressures of blood pumping without
leaking.
• Intercalated discs provide strong mechanical
adhesions between adjacent cells.
• Cardiac muscle cells are also in electrical
contact with one another, and depolarization
begun at one point in the heart rapidly spreads
through the muscle mass.
Human cardiac muscle
47
Muscle Contraction
• All voluntary movements are controlled
by skeletal muscle.
• Skeletal muscle is also called striated
muscle because of its striped
appearance.
Human skeletal muscle
• Skeletal muscle cells are called muscle
fibers. They are large and have many
nuclei because they are a fusion of
many individual cells.
• Each muscle fiber is packed with
bundles of myofibrils, each made up
of thin actin units surrounding thick
myosin units.
Myofibrils; bands of
actin and myosin
together appear darkest
Figure 47.3 The Structure of Skeletal Muscle (Part 1)
47
Muscle Contraction
• Myofibrils consist of repeating units called sarcomeres.
• Each sarcomere is bounded by Z lines, which anchor the thin
actin filaments.
• At the center is the A band, housing all the myosin filaments.
• The H zone and I band are areas where actin and myosin do
not overlap and appear light.
• The M band contains proteins that support the myosin
filaments.
Figure 47.3 The Structure of Skeletal Muscle (Part 2)
47
Muscle Contraction
• When a muscle contracts, the sarcomere
shortens, the H zone and the I band become
much narrower, and the Z lines move toward the A
band as if the actin filaments were sliding into the
region occupied by the myosin filaments.
• Actin and myosin slide past each other as the
muscle contracts.
47
Muscle Contraction
• Each myosin molecule consists of two long polypeptide chains
coiled together, each ending in a large globular head.
• A myosin filament is made of many such molecules arranged
in parallel.
• An actin filament consists of two chains of actin molecules
twisted together.
47
Muscle Contraction
• A myosin head binds to actin and its orientation
changes. This exerts a force that causes the actin
to slide.
• The myosin head then binds ATP and releases
the actin. The myosin head returns to its original
formation and can bind to actin again.
• Contraction of the sarcomere involves many
cycles of interaction between many myosin heads
and actin.
• Backsliding of actin does not occur because the
many surrounding filaments create a system of
interacting cycles.
Figure 47.6 The Release of Ca2+ from the Sarcoplasmic Reticulum Triggers Muscle Contraction
47
Muscle Contraction
• The ATP is needed to break the actin–myosin
bonds, not to form them.
• The energy is actually used to stop muscles from
contracting.
• This accounts for the stiffening of muscles (rigor
mortis) after death. With no ATP being made, the
actin–myosin bonds can’t be broken.
47
Muscle Contraction
• When the muscle is at rest, two proteins,
tropomyosin and troponin, block the myosin
binding sites on the actin filament.
• When Ca2+ is released to the sarcoplasm, it binds
to troponin. Troponin and tropomyosin change
shape, exposing the actin–myosin binding sites.
• With the binding sites exposed, the actin–myosin
bonds are made, and the filaments are pulled past
each other, resulting in muscle fiber contraction.
• If Ca2+ remains available, the cycle repeats and
muscle contraction continues.
Figure 47.6 The Release of Ca2+ from the Sarcoplasmic Reticulum Triggers Muscle Contraction
47
Muscle Strength and Performance
• Slow-twitch fibers (red muscle) have many mitochondria
and a lot of the oxygen-binding molecule myoglobin to
provide steady, prolonged ATP production.
• Red muscle is also well supplied with blood vessels and fuel
reserves (glycogen and fat).
• Long-term aerobic work such as running and swimming
depend on this type of fiber.
• Fast-twitch fibers (white
muscle) have fewer mitochondria
and very little myoglobin.
• They develop maximum tension
more rapidly and with greater
tension, but fatigue rapidly.
47
Skeletal Systems
• The vertebrate skeleton provides support and
protection for the body, and is capable of
movement with the help of joints.
• Bones are connected by joints that allow a range
of movements.
• The human body has 206 bones which work
together with more than 600 muscles to provide
movement for the body.
47
Skeletal Systems
• The appendicular skeleton is one of the two
main anatomical categories of bones, consisting
of the bones of the shoulder and pelvic girdles,
the bones of the upper extremities, and the bones
of the lower extremities.
• The axial skeleton is the other main categories of
bones, consisting of the bones that form the
body’s upright axis – the skull, the vertebral
column, the ribs, and the sternum.
Figure 47.12 The Human Endoskeleton
47
Skeletal Systems
• Cartilage is connective tissue with an extracellular
matrix of a rubbery mix of collagen and
polysaccharide, which gives strength and
resiliency.
• It is found in joints and in stiff but flexible structures
such as the nose and ear.
• The embryonic skeleton of vertebrates is primarily
cartilage, which is gradually replaced by bone
during development.
• Bone is mostly extracellular matrix material of
collagen fibers and crystals of calcium phosphate,
which makes bone hard and rigid.
Figure 47.14 The Growth of Long Bones
47
Skeletal Systems
• A joint is where two bones meet. The human
skeleton has several types of joints.
• Movement around joints is accomplished by
antagonistic muscle pairs—one contracting, the
other relaxing.
• One muscle is the flexor (bends the joint) and the
other is the extensor (straightens the joint).
• Bones at a joint are held together by ligaments,
flexible bands of tough, fibrous tissue that
connects and supports bones.
• Straps of connective tissue called tendons attach
the muscles to bones.
Figure 47.16 Joints, Ligaments, and Tendons
47
Skeletal Systems
• The vertebra is any of the 33 bones of the spinal
column
• Includes the 7 cervical, 12 thoracic, 5 lumbar, 5
sacral, and 4 coccygeal vertebrae.
• The sacral and coccygeal vertebrae normally fuse
to become 2 bones, the coccyx and the sacrum.
• Thus, in an adult, the spine usually consists of 26
vertebrae.