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Muscles - Anatomy of the Musculoskeletal System

MBBS First Year - Exam Notes

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1. CLASSIFICATION OF MUSCLE TYPES

Exam tip: Skeletal muscle = somatic/voluntary = striated. Cardiac = visceral + striated (unique combo). Smooth = visceral + non-striated.

• Gray's Anatomy for Students, p. 41

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2. NAMING OF SKELETAL MUSCLES

• Shape - e.g., rhomboid major, deltoid
• Attachments - e.g., sternohyoid (sternum + hyoid), brachioradialis
• Function - e.g., flexor pollicis longus, extensor carpi radialis
• Position - e.g., palmar interosseous, lateral pterygoid
• Fiber orientation - e.g., external oblique (fibers run obliquely), transversus abdominis
• Number of heads - e.g., biceps (2 heads), triceps (3), quadriceps (4)
• Size - e.g., gluteus maximus/medius/minimus

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3. GROSS ANATOMY OF SKELETAL MUSCLE

A. Connective Tissue Coverings (High-yield!)

Memory trick: EPA - Epi wraps the whole (E = External), Peri wraps the packet (fascicle), Endo wraps each (individual fiber)

B. Tendons and Aponeuroses

• Tendon: Cord of dense fibrous CT connecting muscle to bone. Formed when epimysium/perimysium fuse at the muscle ends.
• Aponeurosis: Flat, sheet-like tendon (e.g., palmar aponeurosis, epicranial aponeurosis)
• At each end, the sarcolemma's outer coat fuses with tendon fibers; tendon fibers collect into bundles that connect to bone.

C. Attachments

• Origin: The proximal/fixed attachment - usually the less mobile bone
• Insertion: The distal/mobile attachment - the bone that moves
• Note: These are functionally defined and can reverse during movement (e.g., when doing pull-ups, the humerus is fixed and the body moves)

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4. MICROSCOPIC ANATOMY OF SKELETAL MUSCLE FIBER

A. The Muscle Fiber (Cell)

• Diameter: 10-80 micrometers
• In most muscles, each fiber runs the entire length of the muscle
• Each fiber has only one nerve ending, located near the middle
• Cell membrane = sarcolemma (true plasma membrane + outer polysaccharide coat with collagen fibrils)
• Cytoplasm = sarcoplasm

B. Organizational Hierarchy (Smallest to Largest)

Myosin/Actin filaments → Sarcomere → Myofibril → Muscle fiber → Fascicle → Muscle

C. The Myofibril

• ~1,500 myosin filaments (thick, ~10-15 nm)
• ~3,000 actin filaments (thin, ~5-7 nm)

Memory trick for bands: "I like icing (I = actin only, pale), A band has All (both actin and myosin)"

D. The Sarcomere

• Definition: The functional unit of muscle contraction
• The segment of myofibril between two successive Z disks
• At rest: ~2.0-2.5 micrometers long
• At maximum contraction: actin filaments completely overlap myosin, ~2.0 µm
• Z disks connect myofibrils to each other across the fiber

E. Important Proteins (High-yield for biochemistry overlap!)

F. T-Tubule and Sarcoplasmic Reticulum

• T-tubules = extensions of the sarcolemma; penetrate deep into the fiber around each myofibril
• Sarcoplasmic reticulum (SR) = modified ER; surrounds myofibrils; stores Ca2+
• Terminal cisternae = enlarged ends of SR; closely associated with T-tubules
• Together: form the triad (one T-tubule flanked by two terminal cisternae)
• Function: T-tubules conduct action potentials to the interior; SR releases Ca2+ to trigger contraction

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5. SLIDING FILAMENT MECHANISM OF MUSCLE CONTRACTION

Steps (must know in order):

1. Motor nerve action potential arrives at neuromuscular junction (NMJ)
2. Acetylcholine (ACh) is released from nerve terminal
3. ACh opens nicotinic ACh-gated cation channels (Na+ rushes in)
4. Local depolarization → opens voltage-gated Na+ channels → muscle action potential
5. Action potential travels along sarcolemma and down T-tubules
6. T-tubule depolarization → Ca2+ release from sarcoplasmic reticulum
7. Ca2+ binds Troponin C → conformational change → tropomyosin shifts, exposing actin's myosin-binding sites
8. Myosin cross-bridges attach to actin → power stroke (myosin head bends, pulls actin toward M line)
9. ATP binds myosin → cross-bridge detaches
10. ATP hydrolysis → myosin re-cocks for next stroke
11. Actin filaments slide inward → Z disks pulled together → sarcomere shortens → muscle contracts
12. Ca2+ pumped back into SR by Ca2+-ATPase pump → troponin-tropomyosin re-blocks actin → muscle relaxes

What changes and what doesn't during contraction (classic MCQ):

• A band length = constant (myosin length unchanged)
• I band = shortens (actin moves deeper into A band)
• H zone = disappears (actin fully overlaps myosin)
• Sarcomere length = shortens

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6. MUSCLE FIBER TYPES (Type I vs Type II)

Key principle: The fiber type of a motor unit is determined by the innervating neuron - all fibers in one motor unit are the same type.

Plasticity: Muscle fiber type is genetically determined but can be influenced by training (exercise shifts type IIB toward IIA and even type I characteristics).

• THIEME Atlas of General Anatomy and Musculoskeletal System, p. 61

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7. ARCHITECTURE / SHAPES OF SKELETAL MUSCLES

Rule: More pennation = more fibers per unit length = more force but less range of motion

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8. MUSCLE ACTIONS AND ROLES

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9. NERVE SUPPLY OF MUSCLES

• Each skeletal muscle is innervated by a spinal nerve or its branch
• Contains motor (efferent) and sensory (afferent) fibers
• Motor nerve fibers: alpha motor neurons → innervate extrafusal fibers (main contraction); gamma motor neurons → innervate intrafusal fibers (muscle spindle)
• Sensory fibers: from muscle spindles (Ia, II afferents) and Golgi tendon organs (Ib afferents)

Neuromuscular Junction (NMJ)

• Located near the middle of each muscle fiber
• Motor end plate: specialized region of sarcolemma
• Neurotransmitter: Acetylcholine (ACh)
• Receptor: Nicotinic ACh receptor (ligand-gated Na+/K+ channel)
• ACh broken down by acetylcholinesterase in synaptic cleft

Clinical MCQ: Myasthenia gravis = autoantibodies against nicotinic ACh receptors → progressive muscle weakness. Suxamethonium (succinylcholine) = depolarizing neuromuscular blocker acts at NMJ.

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10. MUSCLE ATTACHMENTS TO BONE

• Muscles attach to the periosteum of bone via tendon fibers
• Sharpey's fibers = collagen fibers of the tendon that penetrate into bone
• Where tendons cross joints, they are often protected by tendon sheaths (synovial sheaths) that reduce friction
• Sesamoid bones form within tendons where they cross bony prominences (e.g., patella in quadriceps tendon)

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11. MUSCLE BLOOD SUPPLY

• Skeletal muscle is highly vascularized (especially type I fibers)
• Blood vessels enter via the perimysium and branch into capillaries within the endomysium
• Capillary beds open and close depending on metabolic demand
• Exercising muscle receives up to 80-90% of cardiac output (compared to 15-20% at rest)

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12. CLINICAL CORRELATES (High-yield for exams!)

A. Muscle Paralysis

• Inability to move a muscle or group; may be associated with loss of sensation
• Causes: stroke (upper motor neuron), spinal cord injury, poliomyelitis (lower motor neuron), iatrogenic
• Long-term paralysis → secondary muscle wasting and atrophy
• Upper motor neuron (UMN) lesion: spastic paralysis, hyperreflexia, Babinski sign
• Lower motor neuron (LMN) lesion: flaccid paralysis, hyporeflexia, fasciculations, muscle wasting

B. Muscle Atrophy

• Wasting of muscle due to:
  • Neurogenic: nerve damage (denervation atrophy - occurs rapidly)
  • Disuse atrophy: prolonged immobilization; reversible with physiotherapy
  • Myopathic: primary muscle disease (e.g., muscular dystrophy)

C. Muscle Hypertrophy

• Increase in muscle fiber diameter (not number - cells can't multiply)
• Testosterone causes hypertrophy of limb muscles
• Response to resistance training: myofibrils increase; protein synthesis up

D. Muscle Tears / Strains

• Range from small interstitial injury to complete muscle disruption
• Commonly occur in adductor longus, hamstrings, quadriceps, gastrocnemius
• Seen in athletes with sudden exertion
• MRI (inversion recovery sequence): shows edema in and around the muscle as high signal
• Treatment: RICE (Rest, Ice, Compression, Elevation); physiotherapy; surgical repair for complete tears

E. Rhabdomyolysis

• Massive breakdown of skeletal muscle → release of myoglobin into blood
• Myoglobin in urine → myoglobinuria (tea-colored urine)
• Can cause acute kidney injury (AKI)
• Causes: crush injury, extreme exercise, drugs (statins), seizures

F. Compartment Syndrome

• Increased pressure within a fascial compartment compresses muscle blood supply
• Results in ischemia → necrosis if untreated
• Classic signs: Pain (especially on passive stretch), Pressure increase, Paresthesias, Paralysis, Pallor
• Treatment: Emergency fasciotomy

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13. MUSCLE STABILIZATION OF JOINTS

• Ankle joint (sagittal plane): stabilized mainly by triceps surae + tibialis anterior
• Knee joint: stabilized by quadriceps femoris (anterior), hamstrings (posterior)
• Hip joint (sagittal plane): stabilized by gluteal muscles (especially gluteus maximus)
• Hip joint (frontal plane): stabilized by gluteus medius and minimus - loss causes Trendelenburg sign
• Spine: stabilized by intrinsic back muscles (erector spinae group)

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14. SUMMARY TABLE: QUICK REVISION

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SOURCES

• Gray's Anatomy for Students - Muscular System chapter
• THIEME Atlas of General Anatomy and Musculoskeletal System - Section 5: The Muscles; Muscle fiber types
• Guyton and Hall Textbook of Medical Physiology - Chapters 6-7: Physiological Anatomy of Skeletal Muscle; Sliding Filament Mechanism