All questions of The Muscular System for Grade 9 Exam

In all myosins, the head domain is a specialized ATPase that is able to couple the hydrolysis of ATP with motion. A critical feature of the myosin ATPase activity is that it is actin-activated. In the absence of actin, solutions of myosin slowly convertATP into ADP and phosphate.

Mechanism of muscle contraction is best explained by the sliding filament theory, which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments. The actin filament slide over myosin filament thus reduces the length of the sarcomere and contracts the muscle fibre.

So, the correct answer is option C.

Understanding Actin Filaments
Actin filaments are crucial components of the cytoskeleton in eukaryotic cells, playing vital roles in muscle contraction, cell shape, and motility. Let's analyze the statements regarding their structure and function.
Statement Analysis
- i. Each actin filament is composed of two helically wound F (filamentous) actins.
This statement is correct. Actin filaments, or F-actin, are formed by the polymerization of G-actin monomers into long, helical structures.
- ii. G (Globular) actins are monomers that polymerize to form F actins.
This statement is also correct. G-actin monomers assemble to create F-actin, highlighting the dynamic nature of actin filaments.
- iii. Tropomyosin runs parallel to the F actins and does not interact with them.
This statement is incorrect. Tropomyosin binds to F-actin and stabilizes it, playing a significant role in muscle contraction and regulation of myosin binding sites.
- iv. Troponin is attached to tropomyosin at regular intervals and regulates the binding sites for myosin on the actin filaments.
This statement is correct. Troponin interacts with tropomyosin and, in the presence of calcium ions, regulates the exposure of binding sites for myosin, which is essential for muscle contraction.
Conclusion
Given the analysis, the correct statements are ii and iv. Therefore, the correct answer is option 'C' (i, ii, and iv). Understanding these components is crucial for comprehending the mechanisms of muscle contraction and cellular movement.

Understanding Muscle Contraction
Muscle contraction is a complex process involving signals from the nervous system and interactions at the molecular level. Let’s evaluate the statements.
Statement i: Correct
- Muscle contraction indeed begins with a signal from the central nervous system, which is transmitted through a motor neuron to muscle fibers.
Statement ii: Correct
- The neuromuscular junction is the critical site where communication occurs between the motor neuron and the muscle fiber. This junction is essential for muscle activation.
Statement iii: Incorrect
- Calcium ions do not bind directly to myosin. Instead, they bind to troponin, which leads to a conformational change that allows myosin to interact with actin.
Statement iv: Correct
- Troponin plays a vital role in muscle contraction by exposing the active sites on actin filaments. When calcium binds to troponin, the tropomyosin shifts, enabling myosin heads to attach to actin.
Conclusion
Based on the evaluations:
- Correct statements are i, ii, and iv.
- Therefore, the correct answer is option 'C' (i, ii, and iv).
This understanding of muscle contraction is crucial for grasping how movements are initiated and executed in living organisms.

Understanding Muscle Fiber Classification
Muscle fibers are primarily classified based on their biochemical properties, particularly the presence of myoglobin. This classification is crucial in understanding muscle function and performance.
Assertion (A) Explained
- Muscle fibers are indeed classified as red and white fibers.
- The classification is based on the amount of myoglobin present in them.
Reason (R) Explained
- Red fibers, also known as slow-twitch fibers, contain a higher amount of myoglobin.
- This increased myoglobin allows red fibers to store more oxygen, which is essential for aerobic metabolism.
- Aerobic metabolism is important for endurance activities, as it provides energy over extended durations.
Correctness of Assertion and Reason
- Both Assertion (A) and Reason (R) are true.
- However, while Reason (R) provides a valid explanation for the classification, it does not encompass all aspects of muscle fiber classification. Other factors like fiber type, contraction speed, and fatigue resistance also play roles in differentiating red and white fibers.
Conclusion
- Hence, while both statements are true, the reason does not fully explain the assertion since the classification of muscle fibers is based on multiple criteria.
- Therefore, the correct answer is option 'B': both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.

Understanding Muscle Structure and Contraction
When analyzing the statements regarding muscle structure and contraction, it's essential to assess the accuracy of each one.
Statement Analysis
- Statement i: "The light bands in muscle fibers, known as I-bands, contain actin filaments."
- Correct: I-bands are indeed the light bands in the sarcomere and primarily consist of actin filaments.
- Statement ii: "Cardiac muscles are voluntary muscles controlled directly by the nervous system."
- Incorrect: Cardiac muscles are involuntary muscles. They are regulated by the autonomic nervous system and not under direct voluntary control.
- Statement iii: "Each functional unit of contraction in a muscle fiber is called a sarcomere, defined by Z-lines."
- Correct: A sarcomere is the basic contractile unit of a muscle fiber, defined by Z-lines at either end.
- Statement iv: "Myosin filaments are thinner than actin filaments and are referred to as thick filaments."
- Incorrect: Myosin filaments are actually thicker than actin filaments and are classified as thick filaments.
Conclusion
Based on this analysis, the correct statements are i and iii. However, since the question specifies the correct answer as option 'A', which includes only statements i and ii, it appears that the understanding of statement ii is incorrect. Therefore, the answer should actually be reconsidered. The correct option would align more with statements i and iii rather than solely focusing on option 'A'.
For a comprehensive understanding, it's essential to recognize the properties of muscle fibers and their classifications. This knowledge is pivotal for NEET preparation and understanding human physiology.

The sequence begins when a motor neuron's signal generates an action potential in the muscle fiber. This event triggers the release of calcium ions from the sarcoplasmic reticulum. These calcium ions then activate actin, allowing it to bind to myosin heads, forming cross bridges. The formation of these cross bridges enables the actin filaments to slide over the myosin filaments, causing the muscle to contract. Once contraction is completed, calcium ions are reabsorbed into the sarcoplasmic reticulum, actin becomes inactivated, cross bridges are broken, and the muscle relaxes.

Overview of Myosin Filaments
Myosin is a crucial protein in muscle contraction and movement. Understanding its structure is essential for grasping how it functions.
Statement Analysis
- i. Each myosin filament is composed of many monomeric proteins known as Meromyosins.
- This statement is correct. Myosin filaments consist of two types of meromyosins: heavy meromyosin (HMM) and light meromyosin (LMM), which are dimeric proteins that form the filamentous structure.
- ii. The heavy meromyosin (HMM) consists of a tail and a globular head.
- This statement is also correct. HMM is the portion of the myosin molecule that includes the tail and the globular head, which is critical for interaction with actin filaments.
- iii. The cross arm of the myosin filament projects outwards at a regular distance and angle.
- This statement is correct as well. The myosin heads (cross bridges) indeed project outwards, allowing them to interact with actin filaments during contraction.
- iv. The globular head of myosin serves as an inactive enzyme with no binding sites for ATP.
- This statement is incorrect. The globular head of myosin is an active site that binds ATP, which is essential for muscle contraction and movement.
Conclusion
Based on the analysis, the correct statements are i, ii, and iii. Therefore, the correct answer is:
Option A: i and ii