Arrange the steps of muscle fiber contraction in the correct order.
The muscle impulse reaches the sarcoplasmic reticulum and calcium is released.
Thin filaments are pulled over the thick filaments.
Calcium floods the sarcoplasm and binds to troponin molecules leaving active sites.
The impulse arrives at the synapse and travels through the transverse tubules.
The muscle fiber shortens and contracts
Myosin heads bind to exposed active sites on actin, forming cross-bridges.
The Correct Answer is D,A,C,F,B,E
Muscle contraction follows a highly coordinated sequence known as excitation–contraction coupling. This process begins with a nerve impulse at the neuromuscular junction and continues through membrane depolarization, calcium release, and interaction between actin and myosin filaments. Each step is essential for converting an electrical signal into mechanical force. The orderly progression ensures efficient contraction and relaxation of skeletal muscle fibers.
4. The impulse arrives at the synapse and travels through the transverse tubules: This is the first step because muscle contraction begins when an action potential reaches the neuromuscular junction. Acetylcholine is released, triggering depolarization of the sarcolemma. The electrical signal then propagates deep into the muscle fiber through the T-tubules. This ensures the signal rapidly reaches all parts of the muscle cell.
1. The muscle impulse reaches the sarcoplasmic reticulum and calcium is released: Once the action potential travels through the T-tubules, it activates voltage-sensitive receptors that stimulate the sarcoplasmic reticulum. This causes the release of stored calcium ions into the sarcoplasm. The sarcoplasmic reticulum serves as the primary intracellular calcium reservoir. This step is essential for initiating contraction at the molecular level.
3. Calcium floods the sarcoplasm and binds to troponin molecules leaving active sites: Calcium ions bind to troponin C on the thin filament, causing a conformational change in the troponin–tropomyosin complex. This shifts tropomyosin away from actin’s myosin-binding sites. As a result, previously blocked active sites on actin become exposed. This regulatory step enables cross-bridge formation.
6. Myosin heads bind to exposed active sites on actin, forming cross-bridges: With binding sites exposed, energized myosin heads attach to actin filaments, forming cross-bridges. This interaction is the key mechanical event in contraction. The myosin heads are primed by ATP hydrolysis and are capable of generating force. This step directly initiates filament sliding.
2. Thin filaments are pulled over the thick filaments: Once cross-bridges form, repeated cycles of attachment, power stroke, and detachment pull actin (thin filaments) toward the center of the sarcomere. This sliding of filaments past each other shortens the sarcomere. It is the fundamental mechanism of force generation in muscle contraction.
5. The muscle fiber shortens and contracts: As multiple sarcomeres shorten simultaneously, the entire muscle fiber contracts. This results in macroscopic muscle shortening and force production. The coordinated activity of many myofibrils produces observable movement at the tissue level. This is the final outcome of the excitation–contraction process.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is D
Explanation
Sensory receptors are specialized structures that detect different types of stimuli and transmit information to the central nervous system for processing. They are classified based on the type of stimulus they detect, including pressure, light, pain, and body position. In movement and coordination, certain receptors provide continuous feedback about body position in space. This is essential for balance, posture, and coordinated athletic performance such as in activities requiring mid-air awareness.
A. Baroreceptors: Baroreceptors are mechanoreceptors located primarily in the carotid sinus and aortic arch. They detect changes in blood pressure by sensing stretch in the arterial walls. When blood pressure rises or falls, they send signals to the medulla to regulate heart rate and vascular tone. They do not provide information about limb position or spatial awareness during movement, making them unrelated to mid-air body tracking.
B. Photoreceptors: Photoreceptors are specialized sensory cells located in the retina of the eye, consisting of rods and cones. Rods detect low-light conditions, while cones are responsible for color vision and visual acuity. Their function is to convert light energy into electrical signals for visual perception. Although they contribute to spatial awareness through vision, they do not directly provide internal feedback about body position in space.
C. Nociceptors: Nociceptors are pain receptors found in skin, muscles, joints, and internal organs. They respond to potentially damaging stimuli such as extreme temperature, mechanical injury, or chemical irritation. Their primary function is to initiate pain perception as a protective mechanism. While they help detect injury, they do not provide information about body position or movement coordination in space.
D. Proprioceptors: Proprioceptors are specialized mechanoreceptors located in muscles, tendons, and joint capsules, including muscle spindles and Golgi tendon organs. They continuously monitor muscle length, tension, and joint position, sending this information to the central nervous system. This allows the brain to maintain awareness of body position, coordination, and balance without visual input. In activities like pole vaulting, proprioceptors enable precise mid-air spatial orientation and controlled landing.
Correct Answer is A
Explanation
The nucleus is a membrane-bound organelle found in eukaryotic cells that serves as the control center of the cell. It contains the cell’s genetic material in the form of DNA organized into chromosomes. Through gene expression and regulation, the nucleus directs cellular growth, metabolism, protein synthesis, and division. It is essential for maintaining cellular identity and coordinating all major cellular functions.
A. To store genetic information and control cellular activities: the nucleus houses DNA, which contains the genetic instructions required for all cellular functions. It regulates gene expression through transcription, producing messenger RNA that guides protein synthesis. The nucleus also controls the cell cycle and coordinates replication during cell division. As the central regulatory organelle, it determines overall cellular activity and inheritance.
B. To produce energy for the cell: energy production occurs primarily in the mitochondria, not the nucleus. Mitochondria generate ATP through oxidative phosphorylation and the electron transport chain. The nucleus does not participate directly in metabolic energy production.
C. To synthesize proteins for the cell: protein synthesis occurs in ribosomes, either free in the cytoplasm or attached to the rough endoplasmic reticulum. While the nucleus contains the genetic instructions for protein synthesis, it does not directly assemble proteins. Instead, it produces mRNA that is transported to ribosomes for translation.
D. To break down waste materials: waste degradation is primarily the function of lysosomes. Lysosomes contain hydrolytic enzymes that digest cellular debris, damaged organelles, and foreign material. The nucleus does not participate in catabolic processes or cellular digestion.
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