Skeletal Muscle Contraction

Muse

Bone

Perimysium

Endomysium(between individualmuscle fibers)

Muscle fiber

Fascicle(wrapped by perimysium)

Epimysium

Tendon

Epimysium

Muscle fiberin middle ofa fascicle

Blood vessel

Perimysium

Endomysium

Fascicle(a)

(b)

Muscle cells are composed of many long fibers

NucleusLight I bandDark A band

Sarcolemma

Mitochondrion

(b) Diagram of part of a muscle fiber showing the myofibrils. Onemyofibril is extended from the cut end of the fiber.

Myofibril

Cells are polynucleate and surrounded by sarcolemma

Skeletal Muscle Fibers

The overlap of fiber elements causes the striated

appearance

Z disc Z disc

The Sarcomere

• Smallest contractile unit (functional unit) of a muscle fiber

• The region of a myofibril between two successive Z discs

• Composed of thick and thin myofilaments made of contractile proteins

I band I bandA bandSarcomere

H zoneThin (actin)filament

Thick (myosin)filament

Z disc Z disc

M line

(c) Small part of one myofibril enlarged to show the myofilamentsresponsible for the banding pattern. Each sarcomere extends fromone Z disc to the next.

Z disc Z discM line

Sarcomere

Thin (actin)filament

Thick(myosin)filament

Elastic (titin)filaments

(d) Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments.

Positions of the fibers relative to each other

Thin filaments = actin Thick filaments = myosin

Actin is partially covered by tropomysosin

Actin

Myosin thick filament

Myosin is a tree of heads

10_10The signal to contract

Action potential arrives at neuromuscular junction.

acetocholine released

ligand gated sodium channels open and action potential continues along sarcolemma

Figure 9.11, step 3

Steps inE-C Coupling:

Terminal cisterna of SR

Voltage-sensitivetubule protein

T tubule

Ca2+

releasechannel

Ca2+

Sarcolemma

Action potential ispropagated along thesarcolemma and downthe T tubules.

1

Myofibril

Myofibrils

Triad:

Tubules ofthe SR

Sarcolemma

Sarcolemma

Mitochondria

I band I bandA band

H zone Z discZ disc

Part of a skeletalmuscle fiber (cell)

• T tubule• Terminal

cisternaeof the SR (2)

M line

Sarcoplasmic reticulum releases Ca2+

The Contraction Cycle

• Five Steps of the Contraction Cycle

– Exposure of active sites on actin

– Formation of cross-bridges

– Pivoting of myosin heads

– Detachment of cross-bridges

– Reactivation of myosin

The length of the sarcomere shrinks as the filaments undergo the contraction cycle

Cross bridge detachment requires ATP hydrolysis

Once detached, residual energy from detachment re-cocks myosin head

Tension builds quickly

10_11

review

Repeated action potentials lead to maximum tension

Different fiber types can be fast or slower to develop full

tension

.

10_12

Metabolic sources of energy for muscle contraction