MUSCLE & MOVEMENT

C H A P T E R 3 3

KEY CONCEPTS

• 33.1 Muscle Cells Develop Forces by Means of Cycles of Protein–Protein Interaction

• 33.2 Skeletal Muscles Pull on Skeletal Elements to Produce Useful Movements

• 33.3 Skeletal Muscle Performance Depends on ATP Supply, Cell Type, and Training

• 33.4 Many Distinctive Types of Muscle Have Evolved

• Intro Crash Course Video on Muscular System

INTRODUCTION

• Muscle tissue makes up a large portion of body mass, almost half of our body mass

• Muscles are important functionally as they are anatomically.

• Muscles are the basis for virtually all behavior, movement, breathing, swimming, flying, eating

• Many other physiological actions also

depend on muscle contraction, such as

blood circulation by the beating of the

heart.

INTRODUCTION

• There are several types of muscle tissue

in the animal kingdom

• The contractile mechanism is virtually

the same in most muscles, however we

will be focusing on the contractile

mechanism in vertebrate skeletal muscle

• This is the type of muscle that is

attached to the bones of skeleton and

that provide power for walking and

other forms of locomotion

• Crash Course Video

CONTRACTION OCCURS BY A SLIDING-FILAMENT MECHANISM• Muscle contraction is the development of mechanical force.

• Sliding filament theory—current theory of how muscle contracts

– This name describes what the process looks like and is useful in that way

– The motions of the filaments is not really sliding as the filaments are using ATP to develop mechanical forces to pull on each other

– The ‘sliding’ of the filaments is a consequence of this energy-using, force-generating process

Sliding filament video

ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION• Muscle cells are called muscle fibers – these are synonymous

– they are large and multinucleate.

• They form from fusion of embryonic muscle cells called myoblasts.

• One muscle consists of many muscle fibers bundled together by connective tissue.

A skeletal muscle is made up of

many bundles of muscle fibres

Each muscle fiber is a

multinucleate cell containing

numerous myofilbrils, which are

highly ordered assemblages of

thick myosin and thin actin

filaments

ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION

• Muscle contraction is due to interaction of two contractile proteins:

• Actin—thin filaments (actin filaments)

• Myosin—thick filaments (myosin filaments)

• Each muscle fiber is made of several myofibrils—bundles of actin and myosin filaments

arranged in repeating units called sarcomeres.

One sarcomere,

bounded by a Z

line at each end.

Sarcomeres are

the units of

contraction

Cross section of

one myofibril

near the middle

of the

sarcomere. This

cross section

shows how actin

and myosin

filaments are

organized in

three dimension

ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION

• These structures give skeletal muscle a banded appearance.

THE STRUCTURE OF SKELETAL MUSCLE

Where there are only

actin filaments and titan

molecules, the myofibril

appears light; where there

are both actin and myosin

filaments, the myofibril

appears dark.

Bozeman – Muscular System

ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION

• Each sarcomere starts and ends with a Z line, to which actin filaments attach.

• Myosin filaments extend out from the M band in the center.

• H zone and I band—no overlap of actin and myosin, so they appear light in color.

ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION

• Titin—the largest protein in the body, runs the full length of the sarcomere.

• Bundles of myosin filaments are held in the center of the sarcomeres by titin.

• Each titin molecule runs between the M band and a Z line, directly through a myosin filament.

ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION• When muscle contracts and shortens, sarcomeres shorten and band pattern changes.

• The actin and myosin filaments slide past each other, increasing the degree of overlap and

shortening the muscle fiber.

ATP – REQUIRING ACTIN – MYOSIN INTERACTIONS ARE RESPONSIBLE FOR CONTRACTION

• Myosin molecule: two polypeptide chains coiled together, ending in a

globular head

– A myosin filament is made of many molecules with heads projecting at the

sides.

• Actin filaments are twisted chains of actin monomers.

– The protein tropomyosin is twisted around each actin filament, with

molecules of troponin attached at intervals.

Tropinin has three subunits: one

binds actin, one binds

tropomyosin, and one binds Ca2+

Myosin heads have ATPase

activity

Animated tutorial

ATP – REQUIRING ACTIN – MYOSIN INTERACTIONS ARE RESPONSIBLE FOR CONTRACTION• So how to myosin and actin interact?

Myosin and actin interact when the

globular heads of myosin bind to actin,

forming cross-bridges.

• When the myosin head binds, it changes

shape, which pulls the actin filament

towards the M band.

• It also hydrolyzes ATP, and the released

energy causes the myosin head to

disconnect and unbend, ready to bind to

actin again and repeat the process.

Cross Bridges Video

Cross Bride Video # 2

EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS

• Muscle cells are excitable—the membranes

can conduct action potentials (electrical

impulses).

• Ordinarily, the outside of the cell membrane

is more positive than the inside.

• An impulse, or action potential, is a

region of reversed polarity, also called

depolarization.

EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS

• In an excitable cell, if depolarization is initiated at one point, it travels the full length of the cell

membrane:

EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS

• Muscle contraction is initiated by

nerve impulses.

• Each muscle fiber is in contact with an

axon of a nerve cell. The point of

contact is a neuromuscular

junction.

• Excitation: when a nerve impulse

arrives at the neuromuscular junction;

an action potential is initiated in the

muscle fiber membrane

EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS

• Excitation–contraction coupling: process

by which excitation of a muscle fiber leads to

contraction; Ca2+ plays an important role

• Muscle fiber cell membranes extend inward to

form T tubules (transverse tubules). Action

potentials also travel along the T tubules.

• T tubules run close to the sarcoplasmic

reticulum (endoplasmic reticulum), which

surrounds every muscle fiber.

1. Black arrows symbolize an impulse of action potential. When an

action potential is an axon arrives at a neuromuscular junction…

2. …It initiates an action potential in the muscle fiber

cell membrane. The action potential spreads along the

entire length of the cell membrane, and, as it does so, it

spreads down the T tubules…

3. …Which causes the sarcoplasmic reticulum to release

Ca2+ from its internal stores of Ca2+

4. Release Ca2+ diffuses into sarcoplasm bathing the

myofibrils, stimulating muscle contraction.

5. After stimulation by the nerve cell ends, Ca2+

is taken up from the sarcoplasm by the

sarcoplasmic reticulum, terminating muscle

contraction.

EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS

• Ca2+ pumps in the sarcoplasmic reticulum take up Ca2+ from the sarcoplasm (cytoplasm of

muscle cell) and store it inside the sarcoplasmic reticulum.

EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS

• Two proteins span the space between T tubules and sarcoplasmic reticulum and are physically connected.

• The dihydropyridine (DHP) receptor on the T tubule membrane is voltage-sensitive.

• The ryanodine receptor in the sarcoplasmic reticulum membrane is a Ca2+ channel.

EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS

• When an action potential reaches the DHP receptor it changes conformation.

• Ryanodine receptor then allows Ca2+ to leave the sarcoplasmic reticulum.

• Ca2+ ions diffuse into the sarcoplasm and trigger interaction of actin and myosin and sliding of

filaments.

1. Ca2+ is released from the

sarcoplasmic reticulum

2. Ca2+ in the SR binds troponin and

exposes myosin-binding sites on the

actin filaments

3. Myosin heads bind to actin; release

of Pi initiates power stroke

4. In the power stroke, the myosin head changes conformation,

bending so that the filaments are forced to slide past one

another

5. ADP is released; ATP binds to

myosin, causing it to release actin

6. ATP is hydrolyzed.

The myosin head

returns to its unbent,

extended conformation

7. As long as Ca2+

remains available, the

cycle repeats and

muscle contraction

continues

8. When excitation stops, Ca2+ is

returned to the SR by Ca2+ pumps

using ATP, and the muscle relaxes.

EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS

• When Ca2+ binds to troponin, a conformation

change exposes the myosin binding site on the

actin.

• As long as Ca2+ remains available, the cycle of

myosin binding and release repeats.

• When excitation ends, calcium pumps remove Ca2+

from the sarcoplasm. Troponin and tropomyosin

return to their original state, tropomyosin blocks

binding of myosin heads to actin, and contraction

stops.

S K E L E T A L M U S C L E S P U L L O N S K E L E T A L E L E M E N T S T O P R O D U C E

U S E F U L M O V E M E N T S

3 3 . 2

INTRODUCTION• Skeletal systems are the rigid

supports against which muscles can pull.

• Two types of skeletal systems in animals

– exoskeletons and endoskeletons.

• Vertebrates – deep inside bodies –

endoskeleton

• Arthropods – skeleton encases the rest

of the body - exoskeleton

IN VERTEBRATES, MUSCLES PULL ON THE BONES OF THE ENDOSKELETON

• Vertebrate endoskeletons are mostly composed of bone—an extracellular matrix of collagen fibers with insoluble calcium phosphate crystals.

– Also has living cells that remodel and repair bones throughout the life of an animal.

• Bones also serve as a reservoir for calcium.

• Exchange of calcium with the rest of the body is under control of hormones such as parathyroid hormone and vitamin D.

IN VERTEBRATES, MUSCLES PULL ON THE BONES OF THE ENDOSKELETON

• Cartilage is flexible skeletal tissue.

• Joints: where two or more bones come

together

• Muscles attach to bones by bands of flexible

connective tissue called tendons, which

often extend across joints.

• In the tuna, forces developed by the

swimming muscles are transmitted to the

tail by tendons.

The swimming

muscles are in the

middle of the body

Tendons run from the

swimming muscles to

the tail

The tail beats back and

forth with enormous

strength while the rest of

the body is stiff

IN VERTEBRATES, MUSCLES PULL ON THE BONES OF THE ENDOSKELETON

• Muscles can exert force in only

one direction, so they must work

in antagonistic pairs—when

one contracts, the other relaxes.

• Different sets of muscles work

together to control complex

movements. Movement generated

by contraction of a muscle

depends on the state of

contraction or relaxation of other

muscles.

IN ARTHROPODS, MUSCLES PULL ON INTERIOR EXTENSIONS OF THE EXOSKELETON

• Arthropods have an exoskeleton composed of chitin that covers the entire body.

• It is hardened by calcium minerals in crabs and lobsters.

• The exoskeleton protects the soft tissues, but the animal cannot grow after it is formed. It

must be shed periodically and replaced with a larger one.

IN ARTHROPODS, MUSCLES PULL ON INTERIOR EXTENSIONS OF THE EXOSKELETON

• The hard exoskeleton provides the structure

against which muscles can pull.

• Muscles are attached to inward projections of

the exoskeleton called apodemes.

HYDROSTATIC SKELETONS HAVE IMPORTANT RELATIONSHIPS WITH MUSCLE

• Hydrostatic skeleton: body or a

part of the body becomes stiff and

skeleton-like because of high fluid

pressure inside

• Earthworms use muscles in the

body wall to create a hydrostatic

skeleton.

– When muscles oriented in one

direction contract, the fluid-filled

body cavity bulges out in the

opposite direction.

S K E L E T A L M U S C L E P E R F O R M A N C E D E P E N D S

O N AT P S U P P LY, C E L L T Y P E , A N D T R A I N I N G

3 3 . 3

INTRODUCTION

• Skeletal muscles vary in performance.

• Examples:

– Postural muscles sustain loads steadily over long periods

of time; back and gravity

– Finger muscles typically contract quickly for brief periods.

• The rate of doing work (power output) can be only as

high as the rate at which ATP is supplied.

MUSCLE POWER OUTPUT DEPENDS ON A MUSCLE’S CURRENT RATE OF ATP SUPPLY

• Muscles have three ways to supply ATP:

• Immediate system uses preformed ATP and creatine phosphate

• Glycolytic system synthesizes ATP by anaerobic glycolysis, producing lactic acid

• Oxidative system synthesizes ATP from food molecules by aerobic respiration

Immediate system: Preformed ATP

is immediately available but

quickly exhausted

Glycolytic System: Anaeorbic

glycolysis accelerates its synthesis of

ATP to its peak rate within seconds

but is self-limiting

Oxidative System: Production of

ATP by aerobic metabolism ramps

up in the first minute and can be

sustained indefinitely

MUSCLE POWER OUTPUT DEPENDS ON A MUSCLE’S CURRENT RATE OF ATP SUPPLY

• Immediate system supplies ATP quickly when muscles begin to do work. But only a small

amount of preformed ATP is present, so this system works for a brief time.

• The glycolytic system can supply ATP at a high rate, but it is self-limiting, probably due to lactic

acid build-up.

• Oxidative system produces ATP at a slower rate, but it can be sustained for long periods.

MUSCLE POWER OUTPUT DEPENDS ON A MUSCLE’S CURRENT RATE OF ATP SUPPLY

• In terms of power output:

– The immediate system permits a muscle cell to reach highest power output

– The oxidative system permits lowest power output

– The glycolytic system permits intermediate output

• In terms of endurance, the three systems rank in the opposite order.

MUSCLE POWER OUTPUT DEPENDS ON A MUSCLE’S CURRENT RATE OF ATP SUPPLY

• Athletics

• Sports such as sprints require high power output for a short time—depend on the immediate

system.

• Long-endurance sports such as marathons must depend on the oxidative system. Jogging and

swimming are thus called aerobic exercise.

MUSCLE CELL TYPES AFFECT POWER OUTPUT AND ENDURANCE

• Different muscles use different systems:

– Slow oxidative cells (slow-twitch, or “red”)

– Fast glycolytic cells (fast-twitch, or “white”)

• The cell types are usually mixed in one muscle in mammals, but in other animals a muscle may

be nearly all red or all white.

MUSCLE CELL TYPES AFFECT POWER OUTPUT AND ENDURANCE

• Slow oxidative cells use the oxidative system of ATP synthesis. They have high levels of the

hemoglobin-like compound myoglobin, which makes them red.

• Myoglobin speeds entry of O2 into cells.

• They also have many mitochondria and the enzymes required for oxidative metabolism.

• They contract and develop tension slowly, but sustain low power output for long periods.

MUSCLE CELL TYPES AFFECT POWER OUTPUT AND ENDURANCE• Fast glycolytic cells have high levels of the enzymes for anaerobic glycolytic ATP synthesis.

– ATP can be produced rapidly.

– They contract and develop tension rapidly but also fatigue rapidly.

– They have relatively few mitochondria and lack myoglobin.

MUSCLE CELL TYPES AFFECT POWER OUTPUT AND ENDURANCE

• Individuals vary in the proportion of red and white muscle cells.

• Champion athletes in endurance sports (e.g., long-distance running) have more red cells.

• Champion sprinters, wrestlers, etc. have more white cells.

Competitors in sustained aerobic

events have high proportions of slow

oxidative cells

TRAINING MODIFIES MUSCLE PERFORMANCE

• Skeletal muscle also shows phenotypic plasticity, for instance, muscles become larger with

training such as lifting weights.

• The muscle cells increase the amount of actin and myosin present.

TRAINING MODIFIES MUSCLE PERFORMANCE

• Endurance training—long distance running or cycling

• Cells increase number of mitochondria.

• Some cells may transform from fast glycolytic to slow oxidative.

• Growth of capillaries in muscles is stimulated.

• Investigators are searching for transcription factors that are upregulated by endurance

exercise.

TRAINING MODIFIES MUSCLE PERFORMANCE

• Resistance exercise—generates large forces, such as weight lifting

• Amount of actin and myosin increases

• Causes some muscle cells to transform from slow oxidative to fast glycolytic

M A N Y D I S T I N C T I V E T Y P E S O F M U S C L E

H AV E E V O LV E D

3 3 . 4

VERTEBRATE CARDIAC MUSCLE IS BOTH SIMILAR TO AND DIFFERENT FROM SKELETAL MUSCLE

• Cardiac muscle is also striated, but cells are smaller than skeletal muscle and have one nucleus.

• Adjacent cells are electrically connected by gap junctions in intercalated discs.

• At gap junctions, the sarcoplasms of the two cells are continuous with each other.

Two adjacent cells are shown here pulled

apart. When together, the pores connect their

sarcoplasms, and the overall structure joining

them is called an intercalated disc

VERTEBRATE CARDIAC MUSCLE IS BOTH SIMILAR TO AND DIFFERENT FROM SKELETAL MUSCLE • Gap junctions allow electrical impulses to spread rapidly, so all cells are excited at about the

same time and contract at the same time.

• Some cardiac cells are modified to generate the heartbeat rhythm.

• Be Still My Beating Stem Cell Heart

VERTEBRATE SMOOTH MUSCLE POWERS SLOW CONTRACTIONS OF MANY INTERNAL ORGANS• Smooth muscle does not appear striated; the filaments are not regularly arranged.

VERTEBRATE SMOOTH MUSCLE POWERS SLOW CONTRACTIONS OF MANY INTERNAL ORGANS

• Smooth muscle is in most internal organs and in the walls of blood vessels.

• 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.

SOME INSECT FLIGHT MUSCLE HAS EVOLVED UNIQUE EXCITATION – CONTRACTION COUPLING

• Some insects have evolved asynchronous flight muscles—each excitation results in many

contractions.

• At high frequencies of contraction, asynchronous muscle maintains higher efficiency and

greater power output than synchronous muscle.

• It has evolved independently several times and occurs in about 75% of insect species.

MANY DISTINCTIVE TYPES OF MUSCLE HAVE EVOLVED

• Scallops and clams have powerful adductor muscles to hold the two halves of the shell

together.

CATCH MUSCLE IN CLAMS AND SCALLOPS STAY CONTRACTED WITH LITTLE ATP USE

• The adductor muscles sometimes need to remain contracted for long periods of time when

predators are nearby.

• The muscle enters a specialized state called catch in which high contractile force is maintained

with almost no use of ATP.

• The mechanism of catch is not well understood.

FISH ELECTRIC ORGANS ARE COMPOSED OF MODIFIED MUSCLE

• Electric fish can produce external high voltage pulses to stun their prey.

• The electric organs consist of modified muscle cells with little actin or myosin.

• The tiny voltage differences across the cell membranes of many cells all add together when the

cells are excited during an electric pulse.