Skeletal Muscles

Front 1

Skeletal muscles are attached to the skeleton by the ____.

Back 1

Tendon

Front 2

Muscle fiber

Back 2

- Multinucleate cell type + elongated shape
- Each fiber is formed during development by the fusion of many undifferentiated, mononucleated cells (MYOBLASTS)
- Large compared to other cell types

**In some muscles, individual fibers extend the entire length of the muscle, but in most, the fibers are shorter, often oriented at an angle to the longitudinal axis of the muscle

Front 3

What happens if muscle fibers are damaged or destroyed?

Back 3

They can't be replaced by existing fibers, but undifferentiated stem cells (SATELLITE CELLS) can become active and undergo mitotic proliferation

Front 4

What is the function of each nucleus?

Back 4

Each nucleus participates in regulation of gene expression and protein synthesis within its local domain

Front 5

The A band of the myofibril is made up of:

Back 5

Actin AND myosin

Front 6

The I band of the myofibril is made up of:

Back 6

Actin

**They contain portions of thin filament that DON'T overlap with the thick filament

Front 7

The H zone of the myofibril is made up of:

Back 7

Myosin

Front 8

Thin (actin) filaments contain:

Back 8

- 2 proteins (TROPONIN AND TROPOMYOSIN) that play an important role in regulating contraction

Front 9

A sarcomere spans between:

Back 9

- Z lines/discs

**Each contains 2 sets of thin filaments--one end anchored to the Z-line and the other overlaps a portion of the thick filament)

Front 10

Titin filaments

Back 10

Extend from the Z-line to the M-line and are linked to both the M-line proteins and the thick filaments

*Anchor myosin

Front 11

M line

Back 11

Proteins that link together the central region of adjacent thick filaments

Front 12

Nebulin

Back 12

- Stabilizes actin (which is anchored to Z-lines)
- Regulates formation of M line

Front 13

Each thick filament is surrounded by ____, and each thin filament is surrounded by ____.

Back 13

6 thin; 3 thick

Front 14

Contraction

Back 14

- Activation of cross-bridges
- Myosin's cross-bridges bind to actin
- Cross-bridges flex to slide actin (POWER STROKE) toward the center of the sarcomere
- This pulls the Z-lines closer together, and reduces the width of the I-bands

Front 15

Relaxation

Back 15

Mechanisms generating forces are turned off and tension declines

Front 16

Interaction of filaments changes:

Back 16

The length of the muscle (shortening the sarcomere), but NOT the lengths of the myosin and actin filaments--they just overlap

**The I band shortens and the H zone essentially disappears

Front 17

Cross-bridge

Back 17

Projection that bridges the space between overlapping thick and thin filaments (exerting force on the THIN filaments during contraction

Front 18

____ ____ on each side of myosin forms a cross bridge

Back 18

Actin polymers

Front 19

Common pattern of muscle shortening involves:

Back 19

One end of the muscle remaining at a fixed position while the other end shortens towards it

**The ability to generate FORCE and MOVEMENT depends on the interaction of actin and myosin

Front 20

Actin

Back 20

Made up of a single polypeptide that polymerizes with other actin monomers to form a polymer made up of 2 intertwined, helical chains

Front 21

Myosin

Back 21

- Made of 2 large polypeptide heavy chains and 4 smaller light chains
- Polypeptides combine to form a molecule with 2 globular heads and a long tail formed by the 2 intertwined heavy chains

Front 22

Each globular head of myosin contains:

Back 22

2 binding sites--one for actin and one for ATP

**An enzyme hydrolyzes the bound ATP (harnessing energy for contraction)

Front 23

Cross-bridge binding sites are covered by ____

Back 23

Tropomyosin

Front 24

Resting muscle conditions

Back 24

- Low cytoplasmic [calcium]
- Myosin cross-bridges can't bind to actin (but are energized)
- Hydrolysis products (ADP and inorganic Pi) are still bound to myosin

Front 25

What is needed to initiate the cross-bridge cycle?

Back 25

An increase in intracellular calcium

Front 26

Cross-bridge cycle

Back 26

1) The myosin-binding site on actin becomes available, so the energized cross-bridge binds
2) The full hydrolysis and departure of ADP + Pi causes the flexing of the bound cross-bridge (producing tension in the thin filament)
3) Binding of a "new" ATP to the cross-bridge uncouples the bridge (ALLOSTERIC REGULATION)
4) Partial hydrolysis of the bound ATP energizes or "re-cocks" the bridge

**Hydrolysis of ATP and the movement of the cross-bridge are NOT simultaneous
**In step 2, energy is stored by ATP hydrolysis and released after binding to actin

Front 27

Rigor Mortis

Back 27

- Essential cross-bridging process stops when the head is bound to actin
- no ATP so cross-bridges are immobilized
- After a period of time (48-60 hours) the muscle breaks down

Front 28

2 roles of ATP

Back 28

1) Energy released from hydrolysis provides energy for movement
2) Binding to myosin breaks the actin-myosin link, allowing for repetition

Front 29

In relaxed skeletal muscle, ____ blocks the cross-bridge binding site on action.

Back 29

Tropomyosin

Front 30

Contraction occurs when:

Back 30

Calcium ions bind to troponin

**This complex then pulls tropomyosin away from the cross-bridge binding site

Front 31

Cytosolic [calcium] determines:

Back 31

The number of troponin sites occupied by calcium--determining the number of actin sites available for cross-bridging

Front 32

Calcium stored in lateral sacs is:

Back 32

Released following membrane excitation

Front 33

Sarcoplasmic reticulum holds ____.

Back 33

Calcium, which is released when an action potential goes through the muscle cells