Actin and Myosin

Question 1

Muscle structural hierarchy

Answer 1

Muscle-Fascicle-Myofiber-Myofibril-Sarcomere-Myofilament-Protein-Thick and thin filament proteins

Question 2

Components thin filament

Answer 2

F-actin (Actin helix composed of G-actin), Tropomyosin and Troponin wrapping around.

Question 3

Thick filament structure

Answer 3

Overlapping myosin molecules. Cross-bridges on the edges with a head with myosin ATPase site and actin binding site.

Question 4

Components of the sarcomere

Answer 4

Sarcomere extends from Z-line to Z-line. A-band is thick filaments length including overlapping thin filaments (only unchanged length). I band is thin filaments only extending on either side of the Z-line. H-zone: thick filament section that doesn’t overlap thin filaments (shrinks during contraction). M-line: center of sarcomere, middle of H-band where they are crosslinked.

Question 5

Cross-sectional alignment of thin and thick filaments

Answer 5

Each thin filament is surrounded by 3 thick filaments in a triangle. Each thick filament is surrounded by 6 thin filaments in a hexagon.

Question 6

What in the sarcomere shortens during contraction?

Answer 6

H zone and I band

Question 7

What is force at resting muscle length?

Answer 7

Maximum active force.

Question 8

When is the maximum force produced in cardiac muscle?

Answer 8

At about 120-140%, when the chamber is more filled with blood.

Question 9

What is temporal stimulation (multiple twitches)?

Answer 9

Multiple twitches are applied to the motor unit without time for full relaxation and it creates a more forceful contraction until there is maximum passive force. When even more stimulation is appled: fused tetanus.

Question 10

What represents the active and passive elements?

Answer 10

Active: sarcomere (muscle contracting). Passive: elastic elements (spring).

Question 11

What happens in the muscle in isotonic contraction?

Answer 11

Muscle tone remains the same while muscle shortens. Sarcomere contracts.

Question 12

What happens to the muscle in isometric contraction?

Answer 12

Muscle tension may change but muscle doesn’t shorten and object does not move. Spring (elastic fibers) stretched but force (sarcomere) contracted so total length doesn’t change.

Question 13

What are the series elastic elements of the muscle?

Answer 13

Connective tissue and cytoskeletal components.

Question 14

What does the rise in tension developed in the muscle over time represent?

Answer 14

Sliding of actin and myosin over each other.

Question 15

Myosin crossbridge cycle

Answer 15

If there is no Ca2+ present, muscle will relax, if there is no ATP: rigor mortis.

Question 16

How does calcium act in the muscle contraction

Answer 16

Binds to troponin to allow release of tropomyosin off binding sites. Causes the delay in contraction after the AP.

Question 17

How does the contractile response protect against erratic fibrillation?

Answer 17

Contraction happens during the refractory period so chambers have ample time to fill.

Question 18

What is the transverse tubular system (TTS)?

Answer 18

Storage of caclium network in close contact with the plasma membrane. Can activate myiofibrils inside.

Question 19

How do the SR compare in skeletal vs cardiac muscle?

Answer 19

Triad in skeletal muscle, less abundant in cardiac so dyads: 1 T-tubule and 1 terminal cisternae