Lecture 30 Muscle

Question 1

Features of skeletal muscle x4:

Answer 1

• Under voluntary control
• Striated
• Single long cylindrical cells
• Multiple peripheral nuclei

Question 2

Features of cardiac muscle x5

Answer 2

• Located only in the heart,
• Striated
• Branched cells with 1-3 central nuclei
• Connected via intercalated discs
• Involuntary control

Question 3

What is excitotoxicity

Answer 3

neurons are damaged and killed by the overactivations of receptors for the excitatory neurotransmitter glutamate, such as the NMDA receptor and AMPA receptor.

Question 4

Features of smooth muscle

Answer 4

• Involuntary
• Found in the wall of internal organs (gut, blood vessels)
• Spindle shaped, uninucleated cells
• Not striated

Question 5

Skeletal muscle cells are composed of __

Answer 5

fibrils containing highly organised contractile filaments

Question 6

Thick filaments:

Answer 6

run the entire length of an A band

Question 7

Thin filaments:

Answer 7

run the length of the I band and partway into the A band

Question 8

Z disc:

Answer 8

coin shaped sheet of proteins that anchor the thin filaments and connect myofibrils to one another

Question 9

H zone:

Answer 9

lighter mid region where filaments do not overlap

Question 10

M line:

Answer 10

Line of protein myomesin that holds adjacent thick filaments together

Question 11

T-tubules

Answer 11

deep invagination continuous with the sarcolemme (cell membrane) and circle each sarcoemer at each of the junctions of the A and I bands. Allows action potentials to be carried deep within the muscle cell.

Question 12

Sarcoplasmic reticulum:

Answer 12

The calcium storage site. The terminal cisternae of the SR lies close to the T-tubules

Question 13

The thick filaments are:

Answer 13

composed of myosin.

Question 14

Myosin structure:

Answer 14

Each myosin has 2 subunits each with a globular head and a tail, the two tails intertwine to form a helix. The heads have a binding site for actin.

Question 15

Myosin head:

Answer 15

an enzyme that hydrolyzes ATP (an ATPase).

Arranged in a polarised fashion i.e. with the myosin heads projection away from the M-line.

Titin anchors the thick filament to the Z-line

Question 16

Thin filaments are:

Answer 16

composed primarily of globular actin proteins.

The filaments are composed of double stranded helical actin chain )polymers)

-Troponin and tropomyosin are regulatory proteins associated with actin in skeletal and cardiac muscle

Question 17

Sliding filament theory of muscle contraction:

Answer 17

• The sarcomere shortens as the thin filaments are pulled over the thick filaments.
• The Z-line is pulled over the M line
• The I band and H zone become narrower

Question 18

Troponin is regulated by:

Answer 18

Ca2+

Question 19

Tropomyosin interacts with:

Answer 19

the myosin binding sites

Question 20

4 steps of the cross bridge cycle:

Answer 20

1. Cross bridge formation
2. Power stroke
3. Detachment
4. Energization of myosin head

Question 21

1. Cross bridge formation:

Answer 21

• Myosin binds to the actin binding site to form a cross-bridge
• Note: cross bridges can only occur in the presence of calcium when the myosin binding site on actin is exposed.

Question 22

2. The power stroke:

Answer 22

-ADP is released, myosin head rotates, pulling it with the thin filament sarcomere is shortened.

Question 23

3. Detachment:

Answer 23

A new ATP molecule binds to the myosin, actin-myosin bind is weakened and the myosin detaches.

-Note: No ATP = no detachment

Question 24

4. Energization of the myosin head

Answer 24

Myosin head hydrolyzes the ATP to ADP + Pi. The myosin head moves back to its high energy (cocked formation) about 90 degrees to the actin

Question 25

Importance of calcium

Answer 25

Calcium ions provide the on switch for the cross bridge cycle to begin.

When the calcium binds with troponin the tropomyosin moves to expose the myosin binding sites on actin.

Question 26

The cross-bridge cycle as long as ____

Answer 26

calcium levels remain above the critical threshold.

Question 27

;Isotonic

Answer 27

• Shortening
• Tension constant
• Velocity variable

Question 28

Isometric:

Answer 28

• No shortening
• Length constant
• Tension variable

Question 29

Length-tension relationship

Answer 29

During isometric contraction, at the level of the sarcomere the maximum active force (tension developed) is dependent on the degree of actin and myosin overlap.

Question 30

At lengths <2.0 um:

Answer 30

-Filaments collide and interfere with each other reducing force developed.

Question 31

At lengths > 2.2 um:

Answer 31

Active forces decline as the extent of overlap between filaments reduces, reducing the number of cross-bridges

Question 32

Maximal force between:

Answer 32

2.0-2.2

Question 33

Total tensions =

Answer 33

active + passive force