Muscle Contraction Mechanisms and Cellular Energy

Essential Concepts in Muscle Physiology

Skeletal Muscle Anatomy: The Sarcomere

8. A sarcomere (The basic contractile unit)
9. Myofibril (A bundle of myofilaments)
10. T-tubule; terminal cistern (Components of the Triad)

Sarcomere Components Defined

29. Thin filaments
30. I band: (Light band containing only thin filaments; spans from the Z line into the sarcomere.)
31. Z line: (Anchors thin filaments and defines the boundary of a sarcomere.)
32. H band: (Central part of the A band where only thick filaments are present—no overlap with thin filaments.)
33. M line: (The center of the sarcomere, within the H zone, where thick filaments are linked.)

Excitation-Contraction Mechanisms

6. None of these: The correct answer is the Nicotinic ACh receptor, which was not listed.
7. Sarcoplasmic reticulum (The primary source of intracellular calcium)
8. Depolarization (The electrical event initiating contraction)
9. Ryanodine receptors (Calcium release channels on the sarcoplasmic reticulum)

Missing Protein in E-C Coupling

The missing protein is the voltage-gated sodium channel. These channels are essential for starting and spreading an action potential. Without them, the signal cannot travel—it just fades out before triggering a full muscle contraction.

Myosin Cross-Bridge Cycling Steps

13. Troponin (The calcium binding protein in skeletal muscle)
14. Myosin (The motor protein that forms cross-bridges)
15. ADP and Pi (Required for the high-energy, cocked state of myosin)
16. Hydrolysis of ATP (The process that powers the cocking of the myosin head)
17. In rigor (The state of sustained binding when ATP is absent)
18. Sequence of Events (4, 3, 2, 1):
    1. Binding to actin
    2. Loss of Pi (Inorganic Phosphate)
    3. Loss of ADP
    4. Return of the myosin head (Requires new ATP binding)

Muscle Contraction Dynamics and Types

19. One twitch (The response to a single action potential)
20. 10 ms (Typical duration of a muscle action potential)
21. Fused tetanus (A sustained, maximal contraction with no relaxation)
22. Fewer muscle fibers in each motor unit (Characteristic of muscles requiring fine motor control)
23. Type IIx (Fast glycolytic muscle fibers)
24. False (Regarding a statement about muscle fiber characteristics or recruitment)
25. B and C (Factors influencing muscle force generation)

Cellular Energy Production

2. Linking step and Krebs cycle (Key stages in aerobic metabolism)
3. Thirty-two (Likely referring to the net ATP yield per glucose molecule)
4. Three (Likely related to ATP yield per specific metabolic intermediate)
5. About 112 (Likely related to the total ATP yield from fatty acid oxidation)

Smooth Muscle Regulation and Function

1. Smooth muscle does not rely on tropomyosin to stop contractions (Regulation is primarily via myosin light chain phosphorylation)
2. Cardiac muscle and Skeletal muscle (Muscle types that rely on the troponin-tropomyosin complex)
3. Cardiac muscle (Muscle type found in the heart)
4. Around tubes (digestive, blood, etc.) (Location of smooth muscle tissue)
5. Calmodulin (The calcium binding protein in smooth muscle)
6. Myosin light-chain phosphatase (The enzyme responsible for smooth muscle relaxation)
7. Single-unit smooth muscle (Muscle type characterized by electrical coupling)

Single-Unit vs. Multi-Unit Smooth Muscle

Single-unit smooth muscle acts like a team—the cells contract together because they are linked by gap junctions, which let electrical signals pass directly from one cell to the next.

In contrast, multi-unit smooth muscle cells work solo. They do not share signals, so each one needs its own nerve input to contract.