Muscle Physiology

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Rapid summary for last-minute revision before your exam.

Muscle Physiology — Key Facts for FMGE Core concept: Muscle contraction occurs via the sliding filament mechanism; excitation-contraction coupling links neural input to mechanical output High-yield point: The sarcomere is the functional unit; actin slides over myosin using ATP; troponin and tropomyosin regulate this process ⚡ Exam tip: Tetanus occurs when stimuli arrive faster than the refractory period; unfused (incomplete) tetanus vs fused (complete) tetanus

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Muscle Physiology — FMGE Study Guide

Muscle Types

Skeletal muscle:

• Striated, voluntary
• Attached to bones
• Multiple nuclei, peripheral
• Innervated by somatic motor neurons
• Can be isotonic or isometric

Cardiac muscle:

• Striated, involuntary
• Found only in heart
• Single nucleus, central
• Intercalated disks with gap junctions
• Autorhythmic (pacemaker cells)
• Long refractory period (prevents tetanus)

Smooth muscle:

• Non-striated (no sarcomeres)
• Involuntary
• Found in hollow organs (blood vessels, gut, bladder)
• Single nucleus, central
• Two types: Unitary (visceral) and Multiunit

Skeletal Muscle Structure

Organization

• Muscle → Fascicles → Muscle fibers → Myofibrils → Sarcomeres

Sarcomere Structure

• Z lines: Define boundaries of each sarcomere
• I band: Light band; actin only; Z line in middle
• A band: Dark band; myosin + actin overlap
• H zone: Center of A band; myosin only (no actin)
• M line: Center of H zone; structural proteins
• I band narrows with contraction; A band stays same length

Contractile Proteins

• Myosin (thick filament): Has heads that bind actin and hydrolyze ATP; arranged in staggered array with central M line
• Actin (thin filament): Has binding sites for myosin (blocked by tropomyosin in relaxed state); has troponin at regular intervals
• Tropomyosin: Blocks myosin-binding sites on actin when muscle is relaxed
• Troponin: Complex of 3 subunits (TnC binds Ca, TnI inhibits actin-myosin interaction, TnT attaches to tropomyosin)

Excitation-Contraction Coupling

Steps

1. Motor neuron AP travels down axon to NMJ
2. ACh released from motor neuron terminal
3. ACh binds nicotinic receptors on motor end plate
4. End plate potential (EPP) triggers muscle AP
5. Muscle AP propagates along sarcolemma and into T-tubules
6. Depolarization of T-tubule triggers DHP receptor conformational change
7. DHP receptor mechanically opens ryanodine receptor (RyR) on sarcoplasmic reticulum
8. Ca²⁺ released from SR into cytoplasm (100x increase from resting ~0.1 μM to ~10 μM)
9. Ca²⁺ binds troponin C → troponin conformational change
10. Tropomyosin moves away from myosin-binding sites on actin
11. Myosin heads bind actin → cross-bridge formation
12. Power stroke: Myosin head pivots, actin slides
13. ATP binds myosin → myosin detaches from actin
14. ATP hydrolysis energizes myosin head for next cycle

Relaxation

• Ca²⁺-ATPase (SERCA) pumps Ca²⁺ back into SR (uses ATP)
• Ca²⁺ binds to calsequestrin in SR (storage)
• Without Ca²⁺, troponin-tropomyosin complex returns to blocking position
• Actin and myosin slide apart → muscle relaxes

Role of T-tubules

• Carry depolarization deep into muscle fiber
• Located at A-I junction (where SR closely surrounds)
• DHP receptor (voltage sensor) in T-tubule membrane
• RyR (Ca release channel) in SR membrane
• Together they couple depolarization to Ca release

Cross-Bridge Cycle

1. Rigor state: Myosin bound to actin (no ATP)
2. ATP binding: Myosin dissociates from actin
3. ATP hydrolysis: Myosin head cocks (energized state)
4. Cross-bridge formation: Myosin binds to new actin site (weaker binding in cock state)
5. Power stroke: Myosin pivots, actin slides toward M line
6. ADP release: Prepares for next cycle

Energy supply:

• Phosphocreatine (immediate energy, stores high-energy phosphate)
• Anaerobic glycolysis (fast, produces lactate)
• Aerobic metabolism (most efficient, requires oxygen)

Mechanics of Contraction

Twitch

• Single stimulus → single contraction
• Latent period: 5-10 ms (time for AP and Ca release)
• Contraction time: 50-100 ms
• Relaxation time: 50-100 ms

Summation

• Wave summation: Second stimulus arrives before relaxation complete → sum of contractions
• Tetani: High frequency stimulation → no relaxation between contractions
  • Incomplete tetanus: Oscillations between partial relaxation
  • Complete tetanus: Smooth, sustained maximal contraction

Length-Tension Relationship

• Optimal length: Maximum overlap of actin and myosin → maximum tension
• Shortened sarcomere: Less overlap → less tension
• Lengthened sarcomere: Less overlap → less tension
• Starling’s law of the heart: Increased preload → increased force (same principle)

Types of Contraction

• Isotonic: Tension constant, muscle changes length
  • Concentric: Muscle shortens (lifting weight)
  • Eccentric: Muscle lengthens while contracting (lowering weight)
• Isometric: Length constant, tension changes (holding position)
• Isoinertial: Load constant

Muscle Fiber Types

Type I (Slow-twitch, red):

• Oxidative (aerobic) metabolism
• Many mitochondria, myoglobin
• Resistant to fatigue
• Postural muscles (soleus)
• Small diameter, slow contraction

Type IIA (Fast-twitch, red):

• Oxidative + glycolytic
• Intermediate fatigue resistance
• Mixed functions

Type IIB/IIX (Fast-twitch, white):

• Glycolytic metabolism
• Few mitochondria, less myoglobin
• Fatigues quickly
• Large diameter, fast contraction
• Powerful movements

Smooth Muscle

Structure

• Spindle-shaped cells, single nucleus
• No sarcomeres; actin and myosin arranged diagonally
• Dense bodies (like Z lines) anchor actin
• Caveolae (T-tubule equivalent)

Contraction Mechanism

• Ca²⁺ enters via voltage-gated channels or receptor-operated channels
• Calmodulin binds Ca²⁺ (instead of troponin)
• Ca²⁺-calmodulin activates MLCK (myosin light chain kinase)
• MLCK phosphorylates myosin light chain → enables cross-bridge cycling
• MLCP (phosphatase) dephosphorylates → relaxation

Regulation

• Neural: Autonomic (sympathetic/parasympathetic)
• Hormonal: Epinephrine (via β2 → relaxes), oxytocin (contracts uterus)
• Local: Stretch, hypoxia, H⁺, histamine

Types

Unitary (visceral):

• Gap junctions (syncytium)
• Spontaneous activity (pacemaker cells)
• Found in: intestine, uterus, blood vessels
• Respond as a unit

Multiunit:

• No gap junctions
• Independent contraction
• Found in: eye, large blood vessels, airway

Cardiac Muscle

• Striated like skeletal
• Single nucleus (usually)
• Intercalated disks (gap junctions + desmosomes)
• Long refractory period (no tetanus - important for pumping)
• All-or-none contraction of entire heart (functional syncytium)
• Autorhythmic: Pacemaker cells generate AP spontaneously
• Frank-Starling mechanism: Increased preload → increased force

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