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UPSC MainsMEDICAL-SCIENCE-PAPER-I201115 Marks
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Q10.

Name the contractile proteins in skeletal muscle. What is the electron microscopic appearance of muscle ? Write sequence of events in muscular contraction.

How to Approach

This question requires a detailed understanding of muscle physiology and histology. The approach should be structured as follows: First, identify the contractile proteins – actin and myosin – and briefly describe their structure. Second, detail the electron microscopic appearance of different muscle components (sarcomere, myofibrils, etc.). Finally, explain the sliding filament theory of muscular contraction, outlining the sequence of events from nerve impulse to muscle shortening. Diagrams, though not explicitly requested, would enhance the answer.

Model Answer

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Introduction

Skeletal muscle, responsible for voluntary movement, is a highly organized tissue exhibiting remarkable contractile properties. These properties stem from the intricate arrangement of proteins within muscle fibers. Understanding the contractile proteins, the ultrastructure of muscle as revealed by electron microscopy, and the sequence of events leading to contraction is fundamental to comprehending musculoskeletal function. Muscular contraction is a complex process involving electrical and chemical signals, ultimately converting chemical energy into mechanical work.

Contractile Proteins in Skeletal Muscle

The primary contractile proteins in skeletal muscle are actin and myosin.

  • Actin: A globular protein that forms thin filaments. It exists in two forms: G-actin (globular) and F-actin (filamentous). Tropomyosin and troponin are regulatory proteins associated with actin, controlling myosin binding.
  • Myosin: A larger protein forming thick filaments. It has a head region (containing ATPase activity) and a tail region. The myosin head binds to actin, initiating the contraction cycle.

These proteins interact in a highly regulated manner to generate force and shorten the muscle fiber.

Electron Microscopic Appearance of Muscle

Electron microscopy reveals the highly organized structure of skeletal muscle:

  • Muscle Fiber: The basic unit, a long cylindrical cell.
  • Myofibrils: Long, cylindrical structures within muscle fibers, composed of repeating units called sarcomeres.
  • Sarcomere: The functional unit of muscle contraction, delineated by Z-lines. It contains:
    • A-band: Contains the entire length of the thick (myosin) filaments.
    • I-band: Contains only thin (actin) filaments.
    • H-zone: The central region of the A-band, containing only thick filaments.
    • M-line: Located in the center of the H-zone, anchoring the thick filaments.
  • Sarcoplasmic Reticulum (SR): A network of tubules surrounding each myofibril, storing and releasing calcium ions.
  • T-tubules: Inward extensions of the sarcolemma (muscle cell membrane) that transmit action potentials.
  • Triad: The region where a T-tubule intersects with two terminal cisternae of the SR.

Sequence of Events in Muscular Contraction (Sliding Filament Theory)

Muscular contraction occurs via the sliding filament theory, a process involving the following steps:

  1. Nerve Impulse: A motor neuron releases acetylcholine (ACh) at the neuromuscular junction.
  2. Depolarization: ACh binds to receptors on the sarcolemma, causing depolarization and generating an action potential.
  3. Action Potential Propagation: The action potential travels along the sarcolemma and down the T-tubules.
  4. Calcium Release: The action potential triggers the release of calcium ions (Ca2+) from the SR.
  5. Calcium Binding: Ca2+ binds to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin.
  6. Cross-Bridge Formation: Myosin heads bind to actin, forming cross-bridges.
  7. Power Stroke: The myosin head pivots, pulling the actin filament towards the M-line, shortening the sarcomere. This requires ATP hydrolysis.
  8. Cross-Bridge Detachment: ATP binds to the myosin head, causing it to detach from actin.
  9. Myosin Reactivation: ATP is hydrolyzed to ADP and inorganic phosphate, re-energizing the myosin head.
  10. Cycle Repetition: The cycle of cross-bridge formation, power stroke, detachment, and reactivation continues as long as Ca2+ is present and ATP is available.
  11. Muscle Relaxation: When nerve stimulation ceases, Ca2+ is actively transported back into the SR, tropomyosin blocks the myosin-binding sites on actin, and the muscle relaxes.

Conclusion

In conclusion, skeletal muscle contraction is a meticulously orchestrated process reliant on the interplay of contractile proteins, a highly organized ultrastructure, and a precise sequence of events triggered by nerve impulses and calcium signaling. The sliding filament theory provides a robust explanation for how these components work together to generate force and movement. Understanding these mechanisms is crucial for comprehending both normal muscle function and the pathophysiology of neuromuscular disorders.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Sarcomere
The basic contractile unit of a muscle fiber, extending from one Z-line to the next. It is the functional unit responsible for muscle shortening.
Neuromuscular Junction
The synapse between a motor neuron and a muscle fiber, where acetylcholine is released to initiate muscle contraction.

Key Statistics

Approximately 40-60% of the human body weight is skeletal muscle.

Source: Guyton and Hall Textbook of Medical Physiology (Knowledge cutoff 2023)

Over 750 skeletal muscles exist in the human body.

Source: Gray's Anatomy (Knowledge cutoff 2023)

Examples

Rigor Mortis

Rigor mortis, the stiffening of muscles after death, occurs because ATP production ceases, preventing myosin from detaching from actin, resulting in sustained muscle contraction.

Frequently Asked Questions

▶What is the role of ATP in muscle contraction?

ATP provides the energy for the power stroke (myosin head pivoting) and for detaching myosin from actin. It's essential for both contraction and relaxation.

Topics Covered

PhysiologyAnatomyMuscular SystemMuscle ContractionMicroscopy
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