Introduction

The muscular system is one of the most vital systems of the human body. It enables movement, posture, stability, and heat production. There are over 600 muscles in the body, making up about 40–50% of total body weight. Muscles not only help in voluntary motion such as walking or lifting but also perform involuntary activities like heart beating and intestinal movements.

For NHPC PCL General Medicine students, understanding the structure and function of the muscular system is crucial to explain common conditions like cramps, paralysis, muscle fatigue, and neuromuscular disorders.

[Diagram Placeholder: Overview of Muscular System showing skeletal, smooth, and cardiac muscles]

1. Classification of Muscles

Muscles are classified into three main types based on structure, location, and control mechanism.

| Type of Muscle | Structure | Control | Example | Function ||-----------------|------------|----------|----------|-----------|| Skeletal | Striated, multinucleated | Voluntary | Biceps, Quadriceps | Movement, posture || Smooth | Non-striated, spindle-shaped | Involuntary | Intestine, Blood vessels | Peristalsis, vessel constriction || Cardiac | Striated, branched, single nucleus | Involuntary | Heart | Pumping blood |

Exam Focus: The presence of striations indicates skeletal or cardiac muscle, but only skeletal muscle is voluntary.

2. Structure of Skeletal Muscle

Skeletal muscles are attached to bones by tendons. Each muscle is made up of bundles of fibers surrounded by connective tissue.

Layers of Muscle Structure:1. Epimysium: Outer covering of the entire muscle.2. Perimysium: Surrounds bundles (fascicles) of muscle fibers.3. Endomysium: Encloses individual muscle fibers.

Each muscle fiber is a long, cylindrical cell containing multiple nuclei and specialized structures for contraction.

Key Components:- Sarcolemma: Cell membrane of muscle fiber.- Sarcoplasm: Cytoplasm containing glycogen and myoglobin.- Myofibrils: Contractile elements arranged in repeating units called sarcomeres.- Sarcoplasmic Reticulum: Stores calcium ions.- T-tubules: Transmit nerve impulses deep into the fiber.

[Diagram Placeholder: Structure of skeletal muscle fiber showing sarcomere, myofibrils, and SR]

3. Structure of the Sarcomere

The sarcomere is the functional unit of muscle contraction. It lies between two Z-lines and contains thick (myosin) and thin (actin) filaments.

Components of a Sarcomere:- Z-line: Boundary of sarcomere.- A-band: Dark band containing myosin.- I-band: Light band with actin only.- H-zone: Center of A-band, no overlap.- M-line: Middle of the sarcomere supporting myosin alignment.

During contraction, actin filaments slide over myosin, shortening the sarcomere — this is the Sliding Filament Theory.

4. Mechanism of Muscle Contraction (Sliding Filament Theory)

The sliding filament theory explains how muscles contract to produce force. It involves the interaction between actin and myosin filaments within the sarcomere.

Steps of Contraction:1. Nerve Impulse: A motor neuron sends an electrical signal (action potential) to the muscle fiber.2. Calcium Release: The sarcoplasmic reticulum releases Ca²⁺ ions.3. Cross-Bridge Formation: Calcium binds to troponin, exposing actin sites for myosin heads to attach.4. Power Stroke: Myosin heads pull actin filaments inward using ATP energy.5. Detachment: A new ATP binds to myosin, detaching it from actin.6. Relaxation: Calcium returns to SR; troponin-tropomyosin blocks actin again.

[Diagram Placeholder: Sliding filament theory showing cross-bridge cycle]

Did You Know? A single muscle contraction requires millions of ATP molecules!

5. Neuromuscular Junction (NMJ)

The neuromuscular junction is the site where a motor neuron communicates with a muscle fiber to initiate contraction.

Structure of NMJ:- Motor Neuron Terminal: Releases the neurotransmitter acetylcholine (ACh).- Synaptic Cleft: Small gap between neuron and muscle.- Motor End Plate: Region of muscle fiber with ACh receptors.

Process:1. Action potential reaches neuron terminal.2. ACh is released into the synaptic cleft.3. ACh binds to receptors, causing Na⁺ influx and depolarization.4. Action potential travels along sarcolemma → T-tubules → SR → Ca²⁺ release.5. Muscle contracts.

[Diagram Placeholder: Neuromuscular junction with labeled parts]

Clinical Insight: Diseases like Myasthenia Gravis block ACh receptors, leading to muscle weakness.

6. Energy Metabolism in Muscles

Muscle contraction requires continuous ATP supply. Muscles derive energy from three main sources:

1. Stored ATP: Immediate source, lasts only a few seconds.2. Creatine Phosphate: Regenerates ATP from ADP (short-term energy).3. Aerobic Respiration: Uses oxygen to produce ATP from glucose, fats, and amino acids.4. Anaerobic Glycolysis: Produces ATP without oxygen, forming lactic acid (used during intense activity).

Exam Focus: During vigorous exercise, lack of oxygen causes lactic acid buildup leading to muscle fatigue.

7. Muscle Tone and Fatigue

Muscle Tone: Continuous, partial contraction of muscles that maintains posture. It’s regulated by spinal reflexes.

Muscle Fatigue: Inability of a muscle to contract due to depletion of energy stores, accumulation of lactic acid, and disturbed ion balance.

Did You Know? Even during sleep, some skeletal muscles maintain tone to keep posture and breathing steady.

8. Types of Muscle Contraction

1. Isotonic Contraction: Muscle changes length (e.g., lifting a weight).2. Isometric Contraction: Muscle length remains same but tension increases (e.g., holding a book still).3. Concentric Contraction: Muscle shortens during contraction.4. Eccentric Contraction: Muscle lengthens under tension (important for control and balance).

9. Smooth and Cardiac Muscle Physiology

Smooth Muscle:- Found in walls of hollow organs (intestine, bladder, blood vessels).- Contraction is slow and sustained.- Controlled by autonomic nervous system and hormones.- Calcium comes from extracellular fluid.

Cardiac Muscle:- Found only in the heart.- Striated and involuntary.- Has intercalated discs that allow coordinated contraction.- Generates its own impulses (autorhythmic).

[Diagram Placeholder: Difference between skeletal, cardiac, and smooth muscle structure]

10. Clinical Correlations

1. Muscular Dystrophy: Genetic disorder causing muscle weakness.2. Cramps: Involuntary painful muscle contractions due to dehydration or electrolyte imbalance.3. Myasthenia Gravis: Autoimmune disease affecting NMJ.4. Tetanus: Continuous contraction caused by bacterial toxin.5. Poliomyelitis: Viral infection leading to paralysis.

11. Aging and the Muscular System

With age, muscle fibers decrease in size and number, leading to reduced strength and endurance.

Effects of Aging:- Loss of muscle mass (sarcopenia)- Slower reflexes- Decreased flexibility- Reduced blood flow to muscles

Summary Table

| Topic | Key Point ||--------|------------|| Muscle Types | Skeletal (voluntary), Smooth (involuntary), Cardiac (involuntary) || Energy Source | ATP via creatine phosphate, aerobic and anaerobic respiration || Control | Somatic (skeletal), Autonomic (smooth & cardiac) || Disorders | Myasthenia gravis, Dystrophy, Tetanus, Cramps |

Conclusion

The muscular system is essential for all body movements and life functions such as heartbeat and digestion. For NHPC PCL General Medicine students, mastering the structure and physiology of muscles provides the foundation to understand diseases, patient mobility, and rehabilitation techniques. A healthy muscular system depends on balanced nutrition, regular exercise, and proper nerve coordination.