Action and support: the muscles and skeleton
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Transcript of Action and support: the muscles and skeleton
ACTION AND SUPPORT:THE MUSCLES AND SKELETON
Chapter 40, pages 774-791
Muscles and Skeleton Work Together Animals—jellyfish, earthworms, crabs, horses,
and people—move using the same fundamental mechanism:
Contracting muscles exert forces on the skeleton and cause the body to change shape
A body with muscles but no skeleton would not have coordinated movement
A skeleton without muscles remains in one position
Types of Skeletons
Types of skeletons
Hydrostatic skeletons Exoskeletons Endoskeletons
Coordinated movement is produced by alternating contractions of antagonistic muscles
Antagonistic muscles act on each type of skeleton to provide movement
Hydrostatic Skeleton
Worms, cnidarians and many mollusks (snails, octopuses)
A hydrostatic skeleton is a sac or tube filled with a liquid
“Hydrostatic” means “to stand with water,” which is how hydrostatic skeletons function
A water-filled balloon “stands up” because it contains water, but if punctured it collapses
The volume of the balloon is fixed, but you can change its shape by squeezing
Earthworm Movement
An animal with a hydrostatic skeleton controls the overall shape of its body using two sets of antagonistic muscles -circular and longitudinal
For an earthworm to move forward it uses wavelike, alternating contractions of longitudinal and circular muscles
Setae hold the front of the worm in place The worm first contracts its longitudinal muscles
in its front end, so it becomes shorter and fatter Other longitudinal muscles in the middle and tail
contract, making it shorter
Earthworm Locomotion
The worm contracts circular muscles in its front half, making that half longer and thinner
When the worm is fully extended, longitudinal muscles in the head contract again, fattening and anchoring the head
As the wave of circular muscle contraction moves down the worm, the tail gets thin
Longitudinal muscle contraction in the back half of the worm pulls the tail up toward the head
This cycle is repeated over and over as the worm crawls through the soil
Hydrostatic Skeleton
(a) Hydrostatic skeleton
Circular musclescontract
Longitudinalmuscles relax
Circular musclesrelax
liquidliquid
Longitudinalmuscles contract
Exoskeleton
Arthropods (spiders, crustaceans) and insects, have rigid exoskeletons – outside skeletons
Movement occurs at joints of the legs, mouthparts, antennae, base of the wings, and body segments
Thin, flexible tissue joins stiff sections of exoskeleton
Antagonistic muscles attach to opposite sides of the inside of a joint, contraction causes movement
Contraction of a flexor muscle bends a joint; contraction of an extensor muscle straightens a joint
Alternating contraction of antagonistic muscles moves the joints
Exoskeleton
(b) Exoskeleton
Flexor musclecontracts
Extensor musclecontracts
Extensor musclerelaxes
Flexor musclerelaxes
Molting
The exoskeleton cannot expand, an arthropod molt its exoskeleton so that it can grow (27 times in up to 3 yrs)
Endoskeleton
Rigid structures found inside the bodies of echinoderms and chordates
Movement also occurs primarily at joints, where two parts of the skeleton are attached to one another
Biceps - a flexor and triceps – extensor attach on opposite sides of the outside of a joint and move the joint back and forth, or rotate them in one direction or the other
Endoskeleton
(c) Endoskeleton
Flexor muscle(biceps) contracts
elbow
Extensor muscle(triceps) relaxes
Flexor musclerelaxes
Extensor musclecontracts
Functions of Vertebrate Skeleton
Provides a rigid framework that supports the body and protects its internal organs
Allows locomotion
Participates in sensory function
Bones produce red blood cells, white blood cells, and platelets in red bone marrow
Store calcium and phosphorus
Skeletal Categories
The axial skeleton, which includes the bones of head, vertebral column, and rib cage
The appendicular skeleton – pectoral, pelvic girdles and the appendages attached to them
Structure of Vertebrate Skeletons Three types of connective tissue—cartilage,
bone, and ligament—make up the skeleton All are living cells embedded in a matrix of collage
protein, with various other substances included in the matrix
Bone - larger amounts of minerals composed mostly of calcium and phosphate, and is hard and rigid
Cartilage contains large amounts of glycoproteins and includes elastic fibers, which make some cartilages flexible
Ligaments hold bones together at joints and have small amounts of elastic fibers
Cartilage plays many roles
Provides flexible support and connections In some fishes (sharks and rays)
the entire skeleton is composed of cartilage
During embryonic development, the skeleton (except for skull and collarbone) is first formed as cartilage, and later replaced by bone
Skeletal Development
More Cartilage Functions
Covers the ends of bones at joints
Supports the flexible portions of the nose and external ear
Provides the framework for the larynx, trachea, and bronchi of the respiratory system
Forms the tough, shock-absorbing intervertebral discs between the vertebrae of the backbone
Cartilage Structure
Chondrocytes are the living cells Secrete the glycoproteins and
collagen that make up the matrix No blood vessels penetrate
cartilage To exchange wastes and nutrients,
chondrocytes rely on diffusion of materials through the collage matrix
Cartilage cells have low metabolic rate, damaged cartilage repairs itself slowly, if at all
Bone
Hard outer shell of compact bone encloses spongy bone
Compact bone is dense and strong, provides an muscle attachment site Develops as small tubes called osteons with collagen and
calcium phosphate surrounding a central canal containing blood vessels
Spongy bone is an open network of bony fibers Porous, lightweight, rich in blood vessels Red bone marrow is found in the cavities of spongy bone
Cartilage and Bone
compactbone
spongybone(containsmarrow)
cartilage
chondrocytes
collagenmatrix
osteon
osteocytes
centralcanal
blood vessels
Bone Cells
There are three types of bone cells: Osteoblasts—bone-forming cells Osteocytes—mature bone cells Osteoclasts—bone-dissolving cells
Early in development, when bone replaces cartilage in the skeleton, osteoclasts invade and dissolve the cartilage
Ostoblasts secrete a hardened matrix of bone and gradually become entrapped within it
As bones mature, the trapped osteoblasts mature into osteocytes Not capable of enlarging a bone Essential to bone health because they
rework the calcium phosphate deposits, preventing excessive crystallization that would make the bone brittle
Bone Remodeling
Allows skeletal repair and adaptation to stress Each year, 5% -10% of your bone is dissolved.
Replaced by the coordinated activity of osteoclasts that secrete acid and dissolve small amounts of bone, and osteoblasts that secrete new bone
This process allows the skeleton to alter its shape in response to demands placed on it Bones that carry heavy loads or are subjected to extra
stress become thicker, providing more strength and support
Bone remodeling varies with age Early in life, the activity of osteoblasts
outpaces that of osteoclasts, allowing bones to become larger and thicker as a child grows
In the aging body, however, the balance shifts to favor osteoclasts, and bones become more fragile as a result Although both sexes lose bone mass with age,
this is typically more pronounced in women
Broken Bone Repair
The ultimate bone remodeling occurs after a fracture – healing takes about 6 weeks Typically, the ends of the broken bone are put
back into proper alignment
1. A clot is formed surrounding the broken ends2. Cartilage replaces the clot3. Bone replaces the cartilage4. Completed when mature bone completely
replaces cartilage, etc.
Bone Repair
Blood fromruptured bloodvessels forms aclot surroundingthe ends of thebroken bone
1 Healing beginswhen a callus ofcartilage replacesthe clot
2 Bone gradually replaces thecartilage in thecallus
3 When maturebone completelyreplaces the callusand the originalshape of the bonehas been mostlyrestored, thefracture is healed
4
largebloodclot
compact bone
spongy bone
Muscles produce force by contracting
A muscle can only contract or not contract
Muscle lengthening is passive, occurring when muscles relax and are stretched by other forces such as: contractions of other muscles weight of a limb pressure from food
Coordinated movement is produced by alternating contractions of muscles with opposing actions by antagonistic muscles
Structures of Vertebrate Muscles The muscles of all animals have striking
similarities in both the cellular components that produce contractions and in the structural arrangement of these components The details of muscle structure and function,
however, show a tremendous range of adaptations For example, clams possess a special type of
muscle that holds their shells tightly closed for hours using very little energy
Some flies have flight muscles that can contract 1,000 times per second
Types Vertebrate Muscle
Skeletal, cardiac, and smooth
All work on the same basic principles but differ in function, appearance, and control
Skeletal muscle
Moves the skeleton Cells are striated Multinucleate Voluntary or conscious
control Contractions range from
quick twitches to powerful, sustained tension
Many nuclei located just beneath the cell’s plasma membrane; largest fibers have several thousand nuclei
Cardiac muscle
Striated One nucleus per cell Branched cells Located only in the heart
Initiates its own contractions, but is influenced by nervous system and hormones
Biofeedback training allows some people to regulate their heartbeat
Smooth muscle
Not striated
Spindle shaped
One nucleus per cell
Surrounds large blood vessels and most hollow organs, producing slow, sustained contractions
Involuntary Control
Skeletal Muscle Cell Structure
Highly organized, repeating structures Skeletal muscle consists of a series of nested,
repeating parts
Skeletal muscles are encased in connective tissue sheaths and attached to the skeleton by tendons
Within the muscle’s outer sheath, individual muscle cells called muscle fibers are grouped into bundles by further coverings of connective tissue
More Details…
Blood vessels and nerves pass through the muscle in the spaces between the bundles
Each individual muscle fiber has its own thin connective tissue wrapping These multiple connective tissue coverings,
each connected to the others, provide the strength needed to keep the muscle from bursting apart during contraction
Muscle cells are among the largest cells in the human body, ranging from 10 - 100 micrometers in diameter and some run the entire length of a muscle, so they can be over 30 centimeters long
Skeletal Muscle Structure
tendon (connectsto bone)
skeletal muscle
connective tissue
nerves andblood vessels
bundle of muscle cells
muscle fiber(muscle cell)
myofibril
Individual muscle fibers contain many parallel cylinders called myofibrils
Each myofibril is surrounded by a specialized type of endoplasmic reticulum called a sarcoplasmic reticulum
SR is flattened, membrane-enclosed compartments filled with fluid containing a high concentration of calcium ions
The plasma membrane that surrounds each muscle fiber tunnels deep into the inside of the cell at regular intervals, producing tubes called T tubules T tubules encircle the myofibrils, running between and
closely attached to segments of the SR Each myofibril has repeating subunits called
sarcomeres that are aligned end to end along the length of the myofibril, connected to one another by protein discs or Z lines Within each sarcomere is a precise arrangement of thin and
thick protein filaments Each thin filament is anchored to a Z line at one end Suspended between the thin filaments are thick
filaments
Animation: Muscle Structure
Thin and thick filaments of myofibrils are composed of actin and myosin, they interact to contract the muscle fiber
A myofibril also contains smaller amounts of other proteins hold the fibril together, attach the thin filaments to the Z lines, and regulate contraction Dystrophin binds thin filaments to proteins in
the plasma membrane, which is are attached to extracellular proteins that surround the muscle fiber
Dystrophin helps to distribute the forces generated during muscle contraction so the fiber doesn’t tear itself apart
Individual actin proteins are nearly spherical A thin filament consists of two strands of actin
proteins wound about each other Accessory proteins that regulate contraction
called troponin and tropomyosin lie atop the actin
A myosin protein is shaped like a hockey stick—a head attached at an angle to a long shaft The myosin head is hinged to the shaft and
can move back and forth
A thick filament consists of a bundle of myosin proteins with a shaft in the middle of the bundle and the heads protruding out
The heads of the two ends of the thick filament are oriented in opposite directions
(b) A sarcomere
(c) Thick and thin filaments
(a) Cross-section of a musclefiber
myofibril
T tubules
plasmamembrane
sarcoplasmicreticulum
sarcomere
myofibril
thick filamentthin filament
Z lines
myosin
thin filament
thick filament
troponin
tropomyosin
myosin heads
actin
accessoryproteins
mu
scle
fib
er
A Skeletal Muscle Fiber
Skeletal Muscle Contraction
Contraction happens through interaction between thin and thick filaments The molecular architecture of thin and thick
filaments allows them both to grip and to slide past one another, shortening the sarcomeres and producing muscle contraction by what is called the sliding filament mechanism Each spherical actin protein has a binding site
for the myosin head In a relaxed muscle cell, however, these binding
sites on actin are covered by tropomyosin, which prevents the myosin heads from attaching
When a muscle contracts, tropomyosin moves aside, exposing the binding sites on the active proteins
The myosin head binds to these sites, temporarily linking the thick and thin filaments
The myosin heads flex, pulling on the thin filaments and causing them to slide a tiny distance along the thick filament
The myosin heads on the two ends of each thick filament pull the thin filaments toward the middle of the sarcomere
Because thin filaments are attached to the Z lines at the ends of the sarcomere, this movement shortens the sarcomere
All of the sarcomeres of the entire muscle fiber shorten simultaneously, so the whole muscle fiber contracts a little
The myosin heads release the thin filament, extend, reattach farther along the thin filament, and flex again, shortening the muscle fiber a little more, much like a sailor hauling in a long anchor line a little at a time, hand over hand
The cycle repeats as long as the muscle is contracting
Author Animation: Fiber Structure
ATP
ADP
thin filament
myosin (part of a thick filament)
myosinhead
binding sites
myosinhead
actintroponin
tropomyosin
Tropomyosin coversthe binding sites, so themyosin head cannotattach
1
When the bindingsites of actin are exposed, the myosinhead attaches to abinding site
2
The myosin head flexes,pulling the thin filament pastthe thick filament andshortening the sarcomere
3
Using energyfrom ATP, themyosin headdetaches from theactin, extends, andthen attaches toanother actinbinding site fartheralong on the thinfilament
4
The Sliding Filament Mechanism of Muscle Contraction
Filament Sliding Shortens Sarcomeres
Muscle contraction requires ATP Contracting muscles require a lot of energy One might think that the energy is used to flex
the myosin head and pull the thin filament along
The energy of ATP is used not to flex the myosin head, but to extend it and store the energy in this “stretched” position
When the head binds to actin, the stored energy flexes the myosin head and pulls the thin filament toward the center of the sarcomere
There is another crucial role for ATP in muscle contraction
Picture a sailor hauling in an anchor line When he has pulled the line as far as he can
with one arm, he must release the rope before he can move this arm further down and grasp the rope again for another pull
Similarly, when a myosin head has flexed and pulled on the thin filament, the head must release the actin before the head can extend and bind again at a second location a little further along on the thin filament
When ATP binds to a myosin head, it causes the head to release actin
Only then can the energy of ATP be used to extend the head, storing that energy to use during the next pull on the thin filament
A skeletal muscle’s reserves of ATP are used up after only a few seconds of high-intensity exercise Skeletal muscles also stock a supply of creatine
phosphate, an energy-storage molecule that can donate a high-energy phosphate to ADP, thus regenerating ATP However, creatine phosphate is also depleted
rapidly During brief, high-intensity exertion, muscle cells
generate a bit more ATP using glycolysis, which does not require oxygen but is also not very efficient
For prolonged or low-intensity exercise, muscle cells produce ATP from glucose and fatty acids using cellular respiration, which requires a continuous supply of oxygen delivered to the muscles by the cardiovascular system
Nervous System
The nervous system controls contraction of skeletal muscles Skeletal muscle contraction is voluntary We have already seen that moving the
accessory proteins away from the binding sites on actin begins the cycle of myosin head movements that cause skeletal muscle fibers to contract
What links activity in the nervous system and the position of the accessory proteins?
Muscle fibers can fire action potentials, much like neurons Action potentials in muscle fibers cause the
fibers to contract The role of the nervous system is to trigger
action potentials in muscle fibers Motor neurons, mostly in the spinal cord, send
axons out to the skeletal muscles These axons innervate muscle fibers at
specialized synapses called neuromuscular junctions
All vertebrate neuromuscular junctions use the neurotransmitter acetylcholine Each action potential in a motor neuron
releases enough acetylcholine to produce a huge excitatory postsynaptic potential in the muscle fiber, bringing its membrane potential above threshold and triggering an action potential
The muscle fiber’s action potential moves down the T tubules to the SR, where it causes calcium ions (Ca2+) to be released from the SR into the cytoplasmic fluid surrounding the thin and thick filaments
Ca2+ binds to the smaller accessory protein, troponin, causing it to pull the larger accessory protein, tropomyosin, off the actin binding sites
With tropomyosin out of the way, myosin heads can bind to actin
The myosin heads repeatedly attach, flex, extend, and reattach to actin, pulling the thin filament toward the center of each sarcomere
Animation: Fiber Function
Activity in a Motor Neuron Stimulates Contraction of a Skeletal Muscle Fiber
Acetylcholine release bya motor neuron triggers anaction potential in a musclefiber
1
3 In response to the actionpotential, the sarcoplasmicreticulum releases Ca2+ into thecytoplasmic fluid surrounding thethin and thick filaments
4 Ca2+ bindsto troponin, whichthen pullstropomyosin awayfrom the bindingsites on actin
5 The myosin heads bind toactin and flex, shortening thesarcomere; the myosin headscontinue to attach, flex, release,extend, and reattach as longas Ca2+ is present
The muscle fiberaction potential travelsdown the T tubules tothe sarcoplasmicreticulum
2
T tubule
thin filament
actionpotential
plasmamembrane
acetyl-choline
Ca2+
neuro-muscularjunction
axon of amotor neuron
(cytoplasm)sarcoplasmicreticulum
myosinhead
myosin (part of a thick filament)
binding sites on actin
troponintropomyosin
A single action potential in a muscle fiber causes all of its sarcomeres to shorten simultaneously, slightly shortening the fiber
What makes the fiber stop contracting? When the action potential in the muscle fiber
is over, the SR stops releasing Ca2+ Active transport proteins in the membrane of
the sarcoplasmic reticulum pump Ca2+ back into the SR
Ca2+ leaves the accessory proteins, which move back over the active binding sites
Therefore, the myosin head can no longer attach to actin, and contraction stops within a few hundredths of a second
Regulating the intensity of contraction
To control the force, distance, and duration of muscle contraction, you must be able to control how many muscle fibers in a single muscle contract, how they contract, and how long they contract
How does this work? First, a single motor neuron typically synapses
with several muscle fibers in a single muscle A motor neuron and all the muscle fibers that it
innervates are called a motor unit
Motor units vary in size
In muscles used for fine control, such as those that move the eyes or fingers, motor units are small A single motor neuron may synapse on just
a few muscle fibers In muscles used for large-scale
movements, such as those of the thigh and buttocks, motor units are large A single motor neuron may synapse on
dozens or even hundreds of muscle fibers
Second, the nervous system controls the strength of muscle contraction by varying both the number of muscle fibers stimulated and the frequency of action potentials in each fiber Because motor neurons synapse on
multiple muscle fibers in a given muscle, and because the muscle fibers are attached to one another and to the muscle’s tendons, a single action potential in a single motor neuron will cause some contraction of the entire muscle
The contractions caused by a single motor neuron firing multiple action potentials in rapid succession add up to a larger contraction
Firing multiple motor neurons that innervate fibers in the same muscle will also cause a larger contraction of the muscle
Finally, rapid firing of all the motor neurons that innervate all of the fibers in the muscle will cause a maximal contraction
Muscle Fibers are Specialized Specialized for different types of activity
Two basic types, slow twitch and fast twitch
Slow-twitch and fast-twitch fibers have different forms of myosin, causing them to contract slowly or more rapidly
Slow-twitch Fibers
Contract with less power, but can keep on contracting for a very long time Have lots of mitochondria and a plentiful blood supply
that provides oxygen for cellular respiration in the mitochondria
Slow-twitch fibers are thin Thin fibers packed with mitochondria have fewer
myofibrils, but they trade the resulting decreased power for rapid diffusion of oxygen in and wastes out
Thus, slow-twitch fibers produce abundant ATP and have fewer filaments to use it up, so they resist fatigue
40.3 How Do Skeletal Muscles Contract? Fast-twitch fibers, on the other hand, contract
more powerfully They have a smaller blood supply, fewer
mitochondria, and a larger diameter Thick fibers with relatively few mitochondria
have more myofibrils and are therefore more powerful
The extreme versions of fast-twitch fibers use mostly glycolysis for energy production, which does not require oxygen but supplies a lot less ATP than cellular respiration does
Fast-twitch fibers fatigue more rapidly than do slow-twitch fibers
40.4 How Do Cardiac and Smooth Muscles Differ From Skeletal Muscle? Although all muscle cells are built on the
same general principles—filaments of actin and myosin attaching and sliding past one another—cardiac and smooth muscles differ significantly from skeletal muscles
40.4 How Do Cardiac and Smooth Muscles Differ From Skeletal Muscle? Cardiac muscle powers the heart
Cardiac muscle, like skeletal muscle, is striated due to its regular arrangement of sarcomeres with their alternating thick and thin filaments
The fibers of cardiac muscle are branched, smaller than most skeletal muscles cells, and possess only a single nucleus
40.4 How Do Cardiac and Smooth Muscles Differ From Skeletal Muscle? Cardiac muscle powers the heart (continued)
Because cardiac muscles must contract around 70 times each minute, and sometimes much faster, for your whole life, cardiac muscle fibers have enormous numbers of mitochondria, which occupy as much as 25% of the volume of the fibers
Unlike skeletal muscle fibers, cardiac muscle fibers can initiate their own contractions This ability is particularly well developed in the
specialized cardiac muscle fibers of the heart’s pacemaker
40.4 How Do Cardiac and Smooth Muscles Differ From Skeletal Muscle? Cardiac muscle powers the heart
(continued) Action potentials from the pacemaker
spread rapidly through gap junctions in the intercalated discs that interconnect cardiac muscle fibers
Strong cell-to-cell attachments in the intercalated discs, called desmosomes, hold cardiac muscle fibers firmly to one another, preventing the forces of contraction from pulling them apart
40.4 How Do Cardiac and Smooth Muscles Differ From Skeletal Muscle? Smooth muscle produces slow,
involuntary contractions Smooth muscle surrounds blood vessels
and most hollow organs, including the uterus, bladder, and digestive tract
Smooth muscle cells are not striated because the thin and thick filaments are scattered throughout the cells
Like cardiac muscle fibers, smooth muscle fibers each contain a single nucleus
Smooth muscle fibers are directly connected to one another by gap junctions, allowing the cells to contract in synchrony
Smooth muscle contraction is either slow and sustained (such as the constriction of arteries that elevates blood pressure during times of stress) or slow and wavelike (such as the waves that move food through the digestive tract)
Smooth muscle stretches easily, as can be observed in the bladder, the stomach, and the uterus
Smooth muscle contraction is involuntary and can be initiated by stretching, hormones, signals from the autonomic nervous system, or by a combination of stimuli
Almost all animals move by the action of pairs of antagonistic muscles working on a skeleton Not all joints are movable; for example, immobile
joints called sutures join the bones of the skull In movable joints, however, the portion of each
bone that forms the joint is coated with a layer of cartilage; its smooth, resilient surface allows the bone surfaces to slide past one another with relatively little friction Joints are held together by ligaments that are
strong and flexible but usually not very elastic Tendons attach muscles to the bones
tendon: insertionof quadriceps
femur
kneecap
cartilage
ligament: kneecapto tibia
tibia
Biceps femoris(flexor): bendsthe leg
tendon: insertionof biceps femoris
ligament: femurto fibula
fibula
Quadriceps(extensor):straightensthe leg
The Human Knee
How Do Muscles Move the Skeleton? When one of a pair of antagonistic
muscle contracts, it moves the bone around its joint and simultaneously stretches the relaxed opposing muscle Antagonistic muscles can cause a
remarkable range of motions depending on the configuration of a joint, including moving bones back and forth, moving them side to side, or rotating them
Hinge Joints
Elbows, knees, and fingers
These joints move in only two dimensions The antagonistic muscle pair—the flexor and
extensor muscles—lies in roughly the same plane as the joint
The tendon at one end of each muscle, called the origin, is fixed to a bone that remains stationary while the other end, the insertion, is attached to the bone on the far side of the joint, which is moved by the muscle
When the flexor muscle contracts, it bends the joint; when the extensor muscle contracts, it straightens the joint Contraction of the biceps femoris (the flexor)
bends the leg at the knee, while contraction of the quadriceps (the extensor) straightens it
Alternating contractions of flexor and extensor muscles cause the lower leg bones to swing back and forth at the knee joint
A Hinge Joint
humerus
radius
ulna
hinge joint(elbow)
(a) A hinge joint
Ball-and-socket Joints
Hip and shoulder The round end of one bone fits into a hollow
depression of another Ball-and-socket joints allow movement in
several directions The range of motion in ball-and-socket joints
is made possible by at least two pairs of antagonistic muscles oriented at angles to each other to move the joint in three dimensions
A Ball-and-Socket Joint
pelvis
ball-and-socket joint (hip)
femur
(b) A ball-and-socket joint