Contractile apparatus, control of smooth and skeletal muscle contraction.

Introduction

Muscle contraction is a highly regulated process essential for movement, circulation, digestion, and respiration. The contractile apparatus, composed mainly of actin and myosin filaments, functions in both skeletal and smooth muscle. Although the basic mechanism involves cross-bridge cycling, the regulation of contraction differs significantly between these muscle types.

Structure and Function of the Contractile Apparatus

Skeletal Muscle

- Striated, multinucleated fibers under voluntary control.

- Composed of repeating sarcomeres containing actin (thin) and myosin (thick) filaments.

- Contraction occurs via the **sliding filament theory**: myosin heads bind to actin and pull, powered by ATP hydrolysis.

Smooth Muscle

- Non-striated, single-nucleus cells under involuntary control.

- Actin and myosin filaments are present but not organized into sarcomeres.

- Contraction is slower and more energy-efficient, allowing sustained tone.

Control of Muscle Contraction

Skeletal Muscle Contraction

- Initiated by motor neuron stimulation (neuromuscular junction).

- Action potential triggers release of Ca²⁺ from sarcoplasmic reticulum.

- Ca²⁺ binds to troponin, shifting tropomyosin to expose actin binding sites.

- Myosin heads bind actin → power stroke → ATP-dependent detachment and re-cocking.

Smooth Muscle Contraction

- Triggered by various stimuli: neural, hormonal, or mechanical.

- Ca²⁺ enters cytoplasm from extracellular space and sarcoplasmic reticulum.

- Ca²⁺ binds **calmodulin** → activates **myosin light chain kinase (MLCK)**.

- MLCK phosphorylates myosin light chains → enables actin binding and contraction.

- Relaxation requires dephosphorylation via myosin light chain phosphatase.

Clinical Relevance

- **Myasthenia gravis**: Autoimmune attack on acetylcholine receptors → skeletal muscle weakness.

- **Smooth muscle disorders**: Asthma (airway smooth muscle), hypertension (vascular tone).

- **Muscle relaxants**: Drugs that inhibit neuromuscular transmission or calcium signaling.

- **Calcium channel blockers**: Used to reduce smooth muscle contraction in cardiovascular disease.

Conclusion

The contractile apparatus is fundamental to muscle function across the body. While skeletal and smooth muscle share core components, their regulation reflects their functional roles. Understanding these mechanisms aids in diagnosing and treating neuromuscular and vascular conditions.

References

1. Guyton and Hall Textbook of Medical Physiology, 14th Edition

2. Lehninger Principles of Biochemistry, 7th Edition

3. Lippincott Illustrated Reviews: Biochemistry, 7th Edition