6.3 Excitation-Contraction Coupling in Skeletal, Cardiac, and Smooth Muscle

6.3 Comparative Muscle Physiology and Excitation-Contraction Coupling

Skeletal Muscle Excitation-Contraction Coupling

Skeletal muscle contraction is under voluntary somatic motor control, initiated at the neuromuscular junction (NMJ):

1. Neuromuscular Junction Transmission: An action potential reaching the alpha-motor neuron terminal releases acetylcholine (ACh) into the synaptic cleft. ACh binds nicotinic acetylcholine receptors (nAChR) on the muscle motor endplate, inducing localized $Na^+$ influx and an Endplate Potential (EPP) that triggers a propagating sarcolemmal action potential.
2. T-Tubule Propagation: The action potential travels across the sarcolemma and plunges deep into the muscle fiber interior via Transverse Tubules (T-tubules), which are invaginations of the plasma membrane.
3. Triad Architecture and Mechanical Coupling:
   • In skeletal muscle, a triad consists of one central T-tubule flanked by two terminal cisternae of the Sarcoplasmic Reticulum (SR) located at the A-I band junction.
   • Dihydropyridine Receptor (DHPR): Located in the T-tubule membrane, DHPR acts primarily as a voltage sensor.
   • Ryanodine Receptor 1 (RyR1): Located in the terminal cisternae membrane of the SR.
   • Mechanical Coupling Mechanism: Depolarization of the T-tubule membrane causes a conformational shift in DHPR that is physically/mechanically linked to RyR1. This physical tug opens RyR1, releasing stored $Ca^{2+}$ from the SR lumen into the myoplasm down a $10,000$-fold concentration gradient.

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Sarcomere Architecture and Sliding Filament Theory

The functional contractile unit of striated muscle (skeletal and cardiac) is the sarcomere, delimited by adjacent Z-lines:

• Sarcomere Structural Zones:
  • Z-line (Z-disc): Anchors thin actin filaments via $\alpha$-actinin.
  • I-band: Contains thin actin filaments only (bisected by Z-line; narrows during contraction).
  • A-band: Spans the full length of thick myosin filaments (length remains constant during contraction).
  • H-zone: Central region of A-band containing thick myosin filaments only (narrows/disappears during contraction).
  • M-line: Structural center of sarcomere anchoring thick filaments.
• Steric Regulation by Troponin and Tropomyosin:
  • In resting muscle, Tropomyosin covers the myosin-binding sites on actin filaments.
  • Troponin Complex:
    • Troponin C (TnC): Calcium-binding subunit ($4\text{ }Ca^{2+}$ binding sites).
    • Troponin I (TnI): Inhibitory subunit that anchors troponin complex to actin.
    • Troponin T (TnT): Binds the troponin complex to tropomyosin.
  • Upon SR $Ca^{2+}$ release, cytosolic $[Ca^{2+}]$ rises from $100\text{ nM}$ to $>10\text{ }\mu\text{M}$. $Ca^{2+}$ binds TnC, inducing a conformational shift in TnI and TnT that pulls tropomyosin $30^\circ$ deeper into the actin groove, exposing the myosin-binding sites on actin.

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Molecular Mechanics of the Cross-Bridge Cycle

Cross-bridge cycling converts chemical energy (ATP) into mechanical force:

1. Attachment (Cross-Bridge Formation): ADP and inorganic phosphate ($P_i$) remain bound to the cocked myosin head from the previous cycle. The myosin head binds strongly to the exposed binding site on actin.
2. Power Stroke: Release of $P_i$ (followed by ADP) triggers a conformational pivot of the myosin lever arm ("power stroke"). The myosin head rotates $\sim 45^\circ$, pulling the thin actin filament $\sim 10\text{ nm}$ toward the center of the sarcomere (M-line).
3. Detachment (ATP Binding): A new molecule of ATP binds to the nucleotide-binding pocket on the myosin head. ATP binding induces an immediate conformational change that causes the myosin head to detach from actin.
   • Clinical Correlate - Rigor Mortis: Following death, cellular ATP depletion prevents ATP from binding myosin. Myosin heads remain permanently locked to actin filaments, causing rigid muscle stiffening (rigor mortis).
4. Re-cocking (ATP Hydrolysis): Myosin ATPase hydrolyzes bound ATP into ADP and $P_i$. The released energy re-cocks the myosin head into its high-energy resting conformation ($90^\circ$ angle), ready to reattach to actin if $Ca^{2+}$ remains elevated.
5. Relaxation: SERCA (Sarcoplasmic/Endoplasmic Reticulum $Ca^{2+}$ ATPase) pumps cytosolic $Ca^{2+}$ back into the SR against its concentration gradient (where it is sequestered by calsequestrin). Cytosolic $[Ca^{2+}]$ drops, $Ca^{2+}$ dissociates from TnC, and tropomyosin slides back to cover actin binding sites.

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Cardiac Muscle Physiology: Calcium-Induced Calcium Release (CICR)

Cardiac myocytes are striated, branched cells connected end-to-end by intercalated discs containing desmosomes (mechanical adhesion) and gap junctions (low-resistance electrical coupling creating a functional syncytium).

• Dyad Architecture: Cardiac muscle features dyads (one T-tubule paired with one SR terminal cisterna) located at Z-lines.
• Calcium-Induced Calcium Release (CICR):
  • Unlike skeletal muscle, cardiac DHPR (L-type $Ca^{2+}$ channel) is NOT mechanically coupled to RyR2.
  • During Phase 2 (plateau) of the cardiac action potential, depolarization opens L-type $Ca^{2+}$ channels (DHPR), allowing a small influx of extracellular $Ca^{2+}$ ("trigger calcium") into the dyadic cleft.
  • This trigger $Ca^{2+}$ binds to Ryanodine Receptor 2 (RyR2) on the SR membrane, opening RyR2 and releasing a massive wave of stored $Ca^{2+}$ into the cytosol (CICR).
• Prevention of Tetanus:
  • The cardiac action potential features a prolonged plateau phase ($\sim 200-300\text{ ms}$) caused by inward $Ca^{2+}$ current balancing outward $K^+$ current.
  • This results in a long absolute refractory period that lasts almost as long as the mechanical twitch contraction.
  • Physiological Advantage: Cardiac muscle cannot undergo tetanic summation, ensuring mandatory alternating contraction (systole) and relaxation (diastolic filling) phases.
• Regulation of Cardiac Relaxation:
  • SERCA2a returns $Ca^{2+}$ to SR, regulated by Phospholamban (PLB).
  • Unphosphorylated PLB inhibits SERCA2a. $\beta_1$-adrenergic stimulation (epinephrine/norepinephrine) activates Protein Kinase A (PKA), which phosphorylates PLB, relieving SERCA2a inhibition and accelerating cardiac relaxation (lusitropy).
  • Remaining cytosolic $Ca^{2+}$ is extruded across the sarcolemma by the $Na^+/Ca^{2+}$ exchanger (NCX) and plasma membrane $Ca^{2+}$ ATPase (PMCA).

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Smooth Muscle Excitation-Contraction and Relaxation Pathways

Smooth muscle lines hollow visceral organs, blood vessels, and airways. It is non-striated and lacks sarcomeres, T-tubules, and troponin.

• Structural Architecture:
  • Thin actin and thick myosin filaments are arranged in side-polar lattice bundles traversing the cell.
  • Thin filaments anchor to Dense Bodies (containing $\alpha$-actinin) in the cytoplasm and dense bands along the cell membrane (analogous to Z-discs).
• Excitation-Contraction Pathway:
  1. $Ca^{2+}$ Elevation: Stimulated by electrical APs, neurotransmitters ($\alpha_1, M_3$ GPCRs via $IP_3$ pathway), or hormones, cytosolic $[Ca^{2+}]$ increases via voltage-gated $Ca^{2+}$ channels, receptor-operated channels, or $IP_3$-mediated SR release ($IP_3R$).
  2. $Ca^{2+}$-Calmodulin Complex: $4\text{ }Ca^{2+}$ ions bind to the protein Calmodulin ($CaM$).
  3. MLCK Activation: The $Ca^{2+}-CaM$ complex binds to and activates Myosin Light Chain Kinase (MLCK).
  4. Myosin Phosphorylation: MLCK hydrolyzes ATP to phosphorylate the 20-kDa regulatory light chain on the myosin head.
  5. Cross-Bridge Cycling: Phosphorylation of the myosin light chain enables myosin ATPase activity and binding to actin filaments to initiate cross-bridge cycling and contraction.
• Smooth Muscle Relaxation:
  • Cytosolic $[Ca^{2+}]$ decreases via SERCA and PMCA/NCX.
  • Myosin Light Chain Phosphatase (MLCP) dephosphorylates the myosin light chain, returning myosin to an inactive state and inducing muscle relaxation.
• Nitric Oxide (NO) / cGMP Pathway:
  • Endothelial cells produce NO (via eNOS). NO diffuses into vascular smooth muscle cells and activates soluble Guanylyl Cyclase (sGC).
  • sGC converts GTP into cyclic GMP (cGMP), which activates Protein Kinase G (PKG).
  • PKG stimulates MLCP activity, inhibits voltage-gated $Ca^{2+}$ channels, and activates $K^+$ channels (causing hyperpolarization), leading to profound smooth muscle relaxation (vasodilation).
  • Pharmacological Target: Nitroglycerin (NO donor) and Sildenafil (PDE-5 inhibitor preventing cGMP breakdown).
• The Latch State:
  • Dephosphorylated myosin heads can remain attached to actin for prolonged periods without consuming additional ATP.
  • Allows smooth muscle (e.g., vascular walls) to maintain tonic isometric tension (vascular tone) continuously with minimal metabolic energy expenditure.