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