Muscle Tissue – Vocabulary Review

Types of Muscle Tissue

• Skeletal Muscle
  • Location: attached to bones (and skin for facial expression).
  • Striated; voluntary; most powerful of the three types.
  • Cells: long (up to 30\,\text{cm}), cylindrical, multinucleated.
• Cardiac Muscle
  • Location: heart only.
  • Striated; involuntary.
  • Cells: branching, uni- or binucleated, intercalated discs (desmosome-gap-junction complexes).
  • Detailed study in A&P II, Chapter 18.
• Smooth Muscle
  • Location: walls of hollow organs (digestive, vessels, urinary, reproductive, respiratory).
  • No striations; involuntary.
  • Cells: fusiform (spindle-shaped), uninucleated.

Special Characteristics of All Muscle Tissue

• Excitability (irritability / responsiveness) – receive & respond to stimulus.
• Contractility – shorten & generate force.
• Extensibility – stretch.
• Elasticity – recoil to resting length.

General Functions of Muscle

• Movement of bones, blood & other fluids.
• Maintaining posture & body position.
• Stabilizing joints (e.g., rotator-cuff).
• Heat generation through shivering.
• Each muscle supplied by 1 nerve, 1 artery & ≥ 1 vein.

Connective-Tissue Organization & Attachments

• Sheaths (dense irregular CT)
  • Epimysium – surrounds whole muscle.
  • Perimysium – surrounds fascicle (bundle of fibers).
  • Endomysium – surrounds individual muscle fiber (cell).
• Attachment types
  • Direct: epimysium fused to periosteum (bone) or perichondrium (cartilage).
  • Indirect: CT extends beyond muscle → rope-like tendon or sheet-like aponeurosis.

Skeletal Muscle Fiber (Cell) Ultrastructure

• Sarcolemma – plasma membrane.
• Sarcoplasm – cytoplasm containing:
  • Many mitochondria.
  • Glycosomes (glycogen stores).
  • Myoglobin (O₂-binding pigment).
• Organelles present: myofibrils, sarcoplasmic reticulum (SR), T-tubules.

Myofibrils, Striations & Sarcomere

• Myofibrils: rod-like elements, ~80\% of cell volume.
• Striations = repeating dark A bands & light I bands.
• Sarcomere (smallest contractile unit)
  • Region between two Z discs.
  • Components:
  • Thick filaments (myosin) – span entire A band.
  • Thin filaments (actin + tropomyosin + troponin) – span I band & part of A band.
  • H zone – central area of A band with thick but no thin filaments.
  • M line – protein (myomesin) holding thick filaments in place.

Thick Filament (Myosin)

• Tail + two heads.
• Each head has binding sites for actin and ATP.

Thin Filament (Actin)

• Actin subunits possess active sites for myosin.
• Tropomyosin – rod-shaped protein blocking active sites at rest.
• Troponin – Ca²⁺-sensitive complex that anchors tropomyosin.

Sarcoplasmic Reticulum (SR) & T-Tubules

• SR: modified smooth ER encircling myofibril; forms terminal cisternae (Ca²⁺ reservoirs).
• T-tubules: invaginations of sarcolemma penetrating cell at every A-I junction.
• Triad = 1 T-tubule + 2 terminal cisternae; conducts AP deep into fiber & triggers Ca²⁺ release.

Sliding Filament Model of Contraction

• Rest: thick & thin filaments overlap slightly.
• Contraction: myosin heads cyclically bind actin → pull thin filaments toward M line.
• Result:
  • Z discs drawn closer; sarcomere length shortens.
  • I bands shorten; H zone disappears; A band length constant.

Requirements for Skeletal Muscle Contraction

1. Activation (neuromuscular junction) – motor neuron stimulates muscle fiber.
2. Excitation–contraction (E-C) coupling – AP along sarcolemma/T-tubules leads to Ca²⁺-mediated cross-bridge cycling.
3. ATP & Ca²⁺ availability are final triggers.

Neuromuscular Junction (NMJ) Events

• Axon terminal → synaptic vesicles with acetylcholine (ACh).
• Synaptic cleft – gel-filled gap.
• Sarcolemma at NMJ forms junctional folds with ACh receptors.
• Sequence:
  1. AP arrives → voltage-gated Ca²⁺ channels open in axon terminal.
  2. Ca²⁺ influx → exocytosis of ACh.
  3. ACh diffuses, binds receptors → opens Na⁺/K⁺ ligand-gated channels.
  4. End-plate potential (local depolarization).
  5. If threshold (≈ -50\text{ to }-55\,\text{mV}) reached → AP propagates.
  6. Acetylcholinesterase rapidly degrades ACh to terminate signal.

Action Potential Phases in Muscle

• Resting potential ≈ -70\,\text{mV}.
• Depolarization: Na⁺ influx → spike to +30\,\text{mV}.
• Repolarization: Na⁺ channels close; K⁺ channels open; return to rest.
• Refractory period – fiber cannot be restimulated until repolarization complete.

Excitation–Contraction Coupling Steps

1. AP travels along sarcolemma → down T-tubules.
2. Voltage-sensitive proteins open SR Ca²⁺ channels.
3. Ca²⁺ released → binds troponin.
4. Troponin shifts → tropomyosin moves off active sites.
5. Cross-bridge cycle begins:
   • Attach (myosin-ADP-Pi binds actin).
   • Power stroke (ADP + Pi released).
   • Detach (new ATP binds).
   • Cock (ATP → ADP + Pi, head re-energized).
6. Cycle continues while Ca²⁺ & ATP adequate.
7. When stimulation ceases: Ca²⁺ pumped back to SR → tropomyosin re-blocks → relaxation.

Cross-Bridge Cycling & Rigor Mortis

• ATP required for detachment.
• Post-mortem lack of ATP causes persistent cross-bridges → rigor mortis.

Contraction Mechanics

• Tension = force from cross-bridges.
• Load = opposing weight.
• Isometric (same length): tension < load → no shortening.
• Isotonic (same tone): tension ≥ load → shortening occurs.
  • Concentric – muscle shortens/does work.
  • Eccentric – muscle lengthens while generating force (e.g., downhill walking).

Motor Unit & Recruitment

• Motor unit = one motor neuron + all fibers it innervates (4 – several hundred).
• Small units → fine control; large units → gross power.
• Units contract asynchronously to delay fatigue.
• Recruitment (multiple-motor-unit summation)
  • Sub-threshold: no observable contraction.
  • Threshold stimulus → first contraction.
  • Increasing stimulus strength recruits more, larger units → up to maximal stimulus (all units active).

Muscle Twitch & Frequency Summation

• Twitch (single stimulus) phases: latent → contraction → relaxation.
• Temporal (wave) summation: 2+ stimuli before full relaxation → increased tension.
  • Unfused (incomplete) tetanus – wavering contraction.
  • Fused (complete) tetanus – sustained plateau when stimuli are very frequent.

Muscle Tone

• Constant, low-level activation of fibers by spinal reflexes; keeps muscles firm & ready.

Energy Sources for Contraction

1. Stored ATP – ≈ 4\text{–}6\,\text{s} supply.
2. Direct phosphorylation
   • Creatine phosphate + ADP → ATP + creatine.
   • \approx15\,\text{s} of energy (e.g., starting a sprint).
3. Anaerobic glycolysis
   • Glucose → 2 ATP + pyruvic acid → lactic acid.
   • Provides \approx60\,\text{s} at 70\% max effort.
4. Aerobic respiration
   • Glucose, glycogen, fatty acids + O₂ → \approx32\,\text{ATP}.
   • Supports prolonged, light-to-moderate activity (≈ 95\% of ATP at rest).

Muscle Fatigue & Oxygen Debt

• Fatigue: ionic (K⁺, Ca²⁺, Pi) imbalances that halt E-C coupling.
• EPOC / Oxygen debt: extra O₂ needed post-exercise to restore:
  • ATP & creatine phosphate reserves.
  • Glycogen stores.
  • Convert lactic → pyruvic acid.
  • Re-oxygenate myoglobin.

Heat Production

• \approx40\% of energy → work; \approx60\% lost as heat; homeostatic mechanisms (sweating, vasodilation) dissipate excess.

Force, Velocity & Length–Tension Relationship

• Force ↑ with:
  • Recruitment (number of fibers).
  • Fiber size (hypertrophy).
  • Frequency of stimulation.
  • Optimal resting length: 80\text{–}120\% of sarcomere length → maximal cross-bridge formation.
• Velocity & duration influenced by: fiber type, load (greater load → slower & briefer), and recruitment.

Skeletal Muscle Fiber Types

Effects of Exercise

• Aerobic (endurance) → ↑ capillaries, mitochondria, myoglobin; may convert FG → FO.
• Resistance (anaerobic/weights) → hypertrophy (fiber size ↑), stronger CT sheaths & glycogen stores.
• Principle of overload: continual challenge required for further gains.

Smooth Muscle Structure

• Two layers in most organs:
  • Longitudinal – organ shortens & dilates when contracts.
  • Circular – organ constricts & lengthens when contracts.
• Peristalsis = alternating longitudinal/circular contractions propelling contents.
• Cell features:
  • Spindle-shaped; uninucleate; no sarcomeres/myofibrils/T-tubules.
  • Caveolae (sarcolemma invaginations) store Ca²⁺.
  • Only endomysium present.
• Innervation: autonomic fibers form varicosities that release several neurotransmitters into diffuse junctions.

Filament Organization in Smooth Muscle

• Thick : thin ratio 1:13 (less thick).
• Myosin heads along entire filament length.
• No troponin; Ca²⁺ binds calmodulin.
• Dense bodies & intermediate filaments anchor thin filaments → corkscrew contraction.

Smooth Muscle Types

• Single-unit (visceral): walls of hollow organs; gap junctions; pacemaker cells; stress-relaxation response.
• Multi-unit: large airways, large arteries, arrector pili, iris; few/no gap junctions; independently innervated fibers.

Smooth Muscle Contraction Mechanism

1. Ca²⁺ enters from ECF & SR.
2. Ca²⁺ binds calmodulin → activates myosin light-chain kinase (MLCK).
3. MLCK phosphorylates myosin → cross-bridge with actin.
4. Slow, synchronized contractions; can enter latch state (prolonged tension with minimal ATP).
5. Relaxation: Ca²⁺ detaches from calmodulin, returned to SR/ECF; myosin dephosphorylated.

• Stress–relaxation response: smooth muscle adapts to stretch → temporary storage (e.g., bladder filling).
• Hyperplasia: smooth muscle cells can divide (e.g., uterine growth in pregnancy).

Development, Aging & Clinical Correlations

• All muscle derives from embryonic myoblasts.
• Skeletal & cardiac are amitotic after birth; smooth can regenerate.
• Sex differences: muscle mass ≈ 36\% (female) vs 42\% (male); strength per unit mass equal.
• Aging: loss of muscle mass (sarcopenia); reversible with exercise.
• Muscular dystrophies: inherited, progressive muscle-destroying diseases.
  • Duchenne MD: X-linked; absence of dystrophin; onset 2-7 yrs; clumsiness, falls; death (~20s) due to respiratory failure.