MEAT SCIENCE

Hmali Samaraweera

PGIA

University of Peradeniya

Peradeniya

Applied Muscle Biology & Meat Science

Edited

by

Min du

Richard J McCormick

2009

Handbook of Meat and Meat Processing, Second Edition

Y . H . Hui

CRC Press 2012

Muscle structure & Muscle composition

Introduction • Two types of muscles

• Striated

• Skeletal muscles (connected to the skeleton)

• Cardiac muscle - involuntary

• Smooth

• Found in the walls of hollow organs

• Involuntary movements

• Muscle cell • Enriched in proteins – Myosin and actin – to produce

force /shorten the cells

• Skeletal muscle – contraction for movements • Most important in Animal Agriculture

Skeletal muscle Smooth muscle

Cardiac muscles

Skeletal muscle

Light microscope

Electron microscope

Cardiac muscle • Striated

• Highly resistant to fatigue

• large number of mitochondria

• Branched • Increased cell membrane

surface area to contact with each other – provide efficient transmit of signals

• Mononucleated

• Smaller than skeletal muscle

• Intercalated disk • Provide super tight junctions

• Individual cells connect with neighboring cells

Histology and gross anatomy – Skeletal

muscle • Attached to the skeleton

• Contains other tissue types • Nerves

• Epithelia

• Connective tissues

• Super complex organization

• Cells of striated muscle • Multi-neucleated

• Very large

• Elongated ( mm – cm in lengths) and cylindrical – called fiber (muscle cell,

Myofiber)

• Cell membrane and all other intracellular organelles

• No propensity to divide – renewal by satellite cells

Organizational

Units of the

Skeletal Muscle

80-90% of muscle is filled with

muscle fiber

Connective tissue network of muscle

• Entire muscle - Epimysium

• Muscle bundle (Fasciculi/grain) - Perimysium

• Muscle fiber (Muscle cell/Myofiber) - Endomysium

• Whole connective tissue flow:

Interconnected – Epimysium is essentially

continuous with perimysium and

endomysium

Skeletal muscle fibers, colored scanning electron micrograph (SEM). Endomysial

connective tissue is yellow. Magnification: x300 when printed at 10 centimetres

wide. http://www.visualphotos.com/image/2x4138964/skeletal_muscle_fibres_coloured_scanning_electron

• Allow efficient transmit of signals - Key for the

contraction

• Provide support and organization

• Aids in conducting contraction event from muscle

cells to the attached limb or organ

• Structural stability

• Conduct vascular and neural

supply to and from muscle

Perimysium

• Intramuscular fat ( Marbling) associated with

• Larger blood vessels and nerves

Connective tissues

• Muscle for locomotion

– thick perimysium

Provide cushion – withstand the forces – need protection

• Powerful movements – Large bundles and coarse texture

• Fine movements – Small bundles and fine texture

•

Muscle fiber

• Multinucleated

• Satellite cells

• Nucleus is located in the periphery of the cell (out side of the

cytoplasm)

• Small cytoplasm

• Mitochondria can be located through out the cytoplasm

• Contains myofibrils which are bundles of myofilaments

• Each myofibril is surrounded by the sarcoplasmic reticulum

• Sarcoplasmic reticulum

• To transfer or sequester Ca+2

• T- tubular –highly specialized

Muscle organization • Muscle fiber = Muscle cell

• Contains myofibrils

• Sarcolemma (muscle cell membrane) - found beneath the

endomysium

• T-tubules (Transverse tubes)

- Perpendicular extension/

invagination of sarcolemma

- Regular interval

- Very efficient way of conduct

the signals/transmits

Sarcoplasmic Reticulum

• Membranous tubes – network of tubes surrounding

individual myofibrills

• Sequesters (hold) calcium – release of calcium from SR is

the key step for muscle

contraction

• Plays an important aspects in meat quality too

Myofibrils

• Highly specialized contacting organelle

• 1000/adult muscle fiber

• 80-87% of the cell volume

• Myofilaments • Thick filaments – Myosin

• Thin filaments – Actin

Myofibril

• Contains the contractile element of the muscle fiber

• Striated pattern of the skeletal muscle

• Repeating units – Sarcomeres

sarcomeres – has all the structures requires for

the contraction

Myofibril • Has striation – alternative light and dark bands

• A-band (An-isotropic) • Dark band

• Mainly thick filaments – Myosin

• Both thick and thin filaments are overlapped

• I-band • Light band

• Thin filaments – Actin

• Bisected by z-line

• Sarcomere • Area between two z-lines

(one A band and two ½ I band)

Z

Sarcomere

Sarcomere

Thin filaments

Are in Hexagonal

pattern

M line – a protein

network

holding thick

filaments together

Zigzag

structure

of Z line

Each of the

thick filament

is surrounded

by 6 thin

filaments

Each sarcomere is bounded

by Z line consists of overlapping

thin and thick filaments

Each thin filament is attached to the Z- line

Each thick filament is attached to the Z-line by elastic filament of titin

Titin

Thick filament

• Tails form the very strong thick structure

• molecular motor - convert chemical energy into mechanical

energy

• Capable of interacting with thin filaments

• Myosin heavy and light chains -separated subjected to a

specific proteolytic enzyme activity

• Myosin heavy chains - myosin’s heads

has ability to split the ATP molecule

into adenosine diphosphate and

phosphate (ADP and PO4)

• Very long portion - Tail

• Globular portion – Head

• Molecules face opposite directions

• C terminal rods pointing towards the center

Structure of thin filament

Thin filament

Made up of a few proteins

Predominant protein – Actin

Two types of actin

F-actin – Filamentous actin (found in thin filaments)

G-actin – Globular actin ( single active monomers which have been

not incorporated to the thin filaments

• Many proteins are

associated

• 2 major proteins

involves and form a

double standard

helix

Troponin and tropomyosin

primarily responsible for

muscle contraction

Tropomyosin

• Stiff and elongated molecule

• Involves in regulating the contraction & stabilize actin

filamenets

• Tropomyosin regulates the contraction either by covering or

un-covering the reactive sites of actin

• Troponin – 3 types

• Troponin C

• Troponin I

• Troponin T

• At rest troponin molecules come and cover the active sites

(behave like a gate)

• During contraction Ca+2 is dumped into the sarcoplasm and

will bind to TnC

• Binding Ca+2 to TnC will cause conformational change and

it moves troponin molecules allowing myosin head to bind

with active site of thin filament

• Troponin-I which inhibits ATP

• Troponin-T which binds tropomyosin

Nebulin • Running with tropomyosin

• Attached to the Z-line

• Very tightly bound with thin

filament

• Provide strength

• During the muscle development Nebulin provides scaffolding

for actin

• Large molecule – same as the total length of actin

Titin • The largest protein

• Spring like structure

• Extends from the Z-line

to the center of thick

filaments

• Very elastic in the I-band

region in A-band region titin very tightly binds with myosin

• Keeps myosin molecule proper place during the contractions/regulate length

• Provides strength to thick filament

Z-line structure

• Multiple highly ordered protein complexes

• Responsible for transmitting force between sarcomeres

during contraction

• Signaling

• Also it connects myofibrils to the sarcolemmal membrane

and ultimately the extracellular matrix

Alpha actin • Bind actin together at z- disk

• Anti parallel fusion – provide very tight binding ability

• Two alpha actin molecule form dumbbell like structure

•

Actin

Alpha-actin

Costameres

• The myofibrils are linked to the sarcolemma by filamentous structures called costameres

• The protein constituents of the costameres :

Desmin

Flamin

Synemin

Dystrophin

Talin

Vinculin

• Extend into the muscle cell where they encircle the myofibrils at the Z-disk and run from myofibril to myofibril and from myofibril to sarcolemma

Muscle contraction • Chemical energy, which is stored as a high-energy bond

within the adenosine triphosphate (ATP) molecule, is

converted into physical movement

• Myosin heads form cross bridges with the actin molecules using that energy

• Different theories of contraction

Sliding-filament theory • Thick myosin filaments are sliding in between the thin actin

filaments toward the Z-lines

http://www.youtube.com/watch?v=gJ309LfHQ3M

• Trigger for starting the contraction process comes from the

brain and is transferred via the nervous system.

• Signal travels through the nerve by depolarizing the

membrane and changing the inside electrical potential from

about 80 mV to 20 mV

• When the signal arrives to the nerve’s ending (motor end

plates), the message is transferred to the muscle by

Acetylcholine

• Electrical depolarization in the muscle cell’s membrane and

is transferred to the myofibrils via a special arrangement of

T-tubules within the sarcoplasmic reticulum

• Calcium release from the sarcoplasmic reticulum’s terminal

cisternae into the sarcoplasm

• Free calcium is quickly bound by troponin-C

• Causes the tropomyosin to shift from the actin binding sites

• Actin and myosin molecules form cross bridges –

Actomyosin complex

• The repeated formation and breaking of the cross bridges

results in sliding of the thick filaments toward the Z-line and,

resulting , sarcomere shortening

• During the relaxation phase, the signal coming from the

nerve is stopped

• The sarcolemma and the T-tubules are re-polarized, making

them ready for the next signal

• The calcium pump, within the sarcoplasmic reticulum, is re-

sequestering the calcium

• The actomyosin bridges are broken

• Tropomyosin molecules return to the actin binding site

passive sliding of the filaments is observed, and the

sarcomeres return to their resting state.

• Calcium concentration in the sarcoplasm

During rest - free calcium is below 10 -8 mole/liter.

When calcium is released - around 10-5 mole/liter.

• This causes the troponin-C to bind calcium - triggers

movement

• During relaxation, free calcium is resequestered,

concentration goes back to around 10-8 mole/liter

Resting state-contraction

Motor nerve action potential arrives at motor end plate

Acetylcholine released, sarcolemma and membranes depolarized (Na+ flux into fiber)

Action potential transmitted via T-tubules to SR

Ca++ released from SR terminal cisternae into sarcoplasm

Ca++ bound by troponin

Myosin ATPase activated and ATP hydrolyzed

Tropomyosin shift from actin binding site

Actin-myosin crossbridge formation

Repeated formation & breaking of crossbridges resulting in sliding

of filaments and sarcomere shortening

Relaxation Phase

Cholinesterase released and acetylcholine breakdown

Sarcolemma & T-tubules repolarized

SR Ca++ pump activated & Ca++ returned to SR terminal cisternae

Actin-myosin crossbridge formation terminated

Return of tropomyosin to actin binding site

Mg++ complex formed with ATP

Passive sliding of filaments

Sarcomeres return to resting state

http://www.youtube.com/watch?v=e3Nq-P1ww5E