14.9 Muscle Types, Sarcomere Anatomy & Excitation-Contraction Coupling

14.9 Muscle Types, Sarcomere Anatomy & Excitation-Contraction Coupling

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Histological and Physiological Classification of Muscle Types

Vertebrates possess three distinct classes of muscle tissue specialized for distinct mechanical demands:

1. Skeletal Muscle: Composed of elongated, cylindrical, unbranched fibers formed by the embryonic fusion of multiple myoblasts, resulting in multinucleated cells with nuclei located peripherally beneath the sarcolemma. Display alternating light and dark transverse bands (striations) reflecting the crystalline alignment of sarcomeres. Under voluntary control via the somatic nervous system releasing Acetylcholine (ACh) onto nicotinic acetylcholine receptors ($N_M$) at the neuromuscular junction (NMJ). Features an extensive sarcoplasmic reticulum (SR) forming triads (one central transverse [T] tubule flanked by two SR terminal cisternae) located at the A-I band junctions.
2. Cardiac Muscle: Striated, involuntary muscle tissue composed of shorter, branching cardiomyocytes possessing one or two centrally located oval nuclei. Demonstrates myogenic rhythmicity, initiated by specialized pacemaker cells in the sinoatrial (SA) node and modulated by the autonomic nervous system. Cardiocytes are physically and electrically tethered end-to-end by specialized junctional complexes called intercalated discs:
   • Fascia Adherens and Desmosomes (Maculae Adherentes): Provide mechanical anchoring, transmitting tensile force from cell to cell during contraction and preventing separation under high systolic wall stress.
   • Gap Junctions (Nexus): Provide low-resistance hexameric connexon channels permitting rapid intercellular diffusion of ions ($Ca^{2+}, Na^+$), allowing action potentials to spread across the myocardium as a synchronized functional syncytium.
   • T-tubules are broader and form diads (one T-tubule paired with one SR terminal cisterna) located at the Z-lines. Extracellular $Ca^{2+}$ entry through L-type calcium channels (Cav1.2 / DHPR) triggers Calcium-Induced Calcium Release (CICR) from the SR via ryanodine receptors (RyR2).
3. Smooth Muscle: Involuntary, non-striated muscle tissue lining the walls of hollow visceral organs (gastrointestinal tract, vascular tree, bronchioles, bladder, uterus). Cells are fusiform (spindle-shaped) with a single centrally located elongated nucleus. Lacks sarcomeres, T-tubules, and troponin:
   • Thin (actin) and thick (myosin) filaments are arranged obliquely in a lattice network anchored to proteinaceous cytoplasmic and membrane-associated dense bodies (composed of $\alpha$-actinin, functionally analogous to Z-lines).
   • Contraction mechanism: Rise in cytosolic $[Ca^{2+}]$ (from extracellular entry through Cav channels and $IP_3$-mediated SR release) binds the regulatory protein Calmodulin. The $Ca^{2+}$-calmodulin complex binds and activates Myosin Light Chain Kinase (MLCK), which phosphorylates the regulatory light chain on myosin heads, activating myosin ATPase to permit cross-bridge cycling on actin.
   • Relaxation strictly requires dephosphorylation of myosin light chains by Myosin Light Chain Phosphatase (MLCP).
   • Can enter a prolonged low-energy contracted state known as the "latch state", maintaining tonic vascular or sphincter tone with minimal ATP consumption.

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Muscle Tissue Comparison

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Sarcomere Microanatomy & The Sliding Filament Model

The sarcomere is the fundamental contractile unit of striated muscle, bounded by two successive Z-lines (Z-discs):

Sarcomere Architectural Landmarks:
|--- Z-Line --- I-Band ---|-------- A-Band --------|--- I-Band --- Z-Line ---|
                          |   Overlap   |  H-Zone  |   Overlap   |
                                        |  M-Line  |

• Z-Lines (Z-Discs): Protein boundaries flanking each sarcomere; anchor thin actin filaments via the structural protein $\alpha$-actinin.
• M-Line: Central structural meshwork running down the middle of the sarcomere within the bare zone of thick filaments; anchors myosin filaments via myomesin and C-protein.
• I-Band (Isotropic): Light band containing thin actin filaments only; bisected by the Z-line. Does not contain any thick filaments.
• A-Band (Anisotropic): Dark band encompassing the entire physical length of thick myosin filaments; contains both thick filaments and regions where thin filaments overlap, as well as the central H-zone.
• H-Zone: Central pale region within the A-band containing thick myosin filaments only; lacks overlapping thin filaments; bisected by the M-line.
• Titin: Giant elastic protein extending from Z-line to M-line, stabilizing myosin centering and providing passive recoil tension during stretch.

The Sliding Filament Model

During muscle contraction, thin actin filaments slide past stationary thick myosin filaments toward the central M-line, pulling the Z-lines closer together. As a result:

• Sarcomere shortens (distance between adjacent Z-lines decreases).
• I-band shortens (narrows as thin filaments are drawn into the A-band).
• H-zone shortens (narrows and can disappear completely during maximal contraction).
• A-band length remains strictly constant (never changes because the physical length of the thick myosin filaments is invariant).

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Excitation-Contraction Coupling & The Cross-Bridge Cycle

Excitation-Contraction Coupling Sequence:
1. Somatic Action Potential -> Depolarization of presynaptic motor axon terminal.
2. Voltage-Gated Ca2+ Channels Open -> Influx of Ca2+ triggers exocytosis of Acetylcholine (ACh).
3. ACh binds Nicotinic Receptors (NM) at Motor Endplate -> Influx of Na+ produces Endplate Potential (EPP).
4. Sarcolemmal Action Potential propagates across fiber and plunges deep into Transverse (T) Tubules.
5. Conformational shift of Dihydropyridine Receptors (DHPR / Cav1.1) mechanically uncorks
   adjacent Ryanodine Receptors (RyR1) in Sarcoplasmic Reticulum terminal cisternae.
6. Sarcoplasmic Ca2+ floods the cytosol (concentration surges from ~0.1 microM to ~10 microM).
7. Ca2+ binds Troponin C -> Conformational change pulls Tropomyosin out of the myosin-binding groove on actin.

The Molecular Cross-Bridge Cycle (5 Core Steps):
Step 1: Resting / Cocked State
  Myosin head is energized in high-energy perpendicular (90-degree) orientation with bound ADP and inorganic phosphate (Pi)
  derived from previous ATP hydrolysis. Binding sites on actin are exposed.
Step 2: Cross-Bridge Attachment
  Cocked myosin head binds to exposed active site on actin, forming an actin-myosin cross-bridge.
Step 3: Power Stroke
  Dissociation of inorganic phosphate (Pi) triggers the structural power stroke: the myosin head pivots from 90 to 45 degrees,
  pulling the thin actin filament ~10 nm toward the M-line. ADP is subsequently released from the nucleotide pocket.
Step 4: Cross-Bridge Detachment
  A NEW molecule of ATP binds to the nucleotide-binding pocket on the myosin head. ATP binding induces an allosteric
  conformational shift that drastically lowers myosin's affinity for actin, causing immediate cross-bridge detachment.
Step 5: ATP Hydrolysis & Recocking
  Myosin ATPase hydrolyzes bound ATP into ADP and Pi. The released free energy re-cocks the myosin head back into its
  high-energy 90-degree orientation. If sarcoplasmic Ca2+ remains elevated, the cycle repeats immediately.

The Molecular Pathology of Rigor Mortis

Following biological death, cellular respiration halts and glycogen reserves are depleted, causing intracellular $[\text{ATP}]$ to plunge toward zero. In the absence of ATP:

1. Cross-bridge detachment (Step 4) cannot occur because detachment strictly requires the binding of a fresh ATP molecule to the myosin head.
2. Active transport of $Ca^{2+}$ back into the SR via SERCA pumps fails, allowing $Ca^{2+}$ to leak from the SR into the sarcoplasm, keeping troponin binding sites continuously exposed.
3. Consequently, myosin heads remain irreversibly locked to actin thin filaments in the rigid, contracted post-power-stroke state, producing rigor mortis (peaking at ~12 hours post-mortem and persisting until autolytic lysosomal enzymes degrade myofibrillar proteins ~24–48 hours later).

Relaxation Phase

Relaxation requires active pumping of sarcoplasmic $Ca^{2+}$ back into the SR lumen against a thousand-fold gradient by the Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase (SERCA), consuming ATP. Within the SR, $Ca^{2+}$ is buffered by the low-affinity, high-capacity binding protein calsequestrin. As cytosolic $[Ca^{2+}]$ drops below $0.1\;\mu\text{M}$, $Ca^{2+}$ dissociates from Troponin C, allowing tropomyosin to slip back into the myosin-binding groove on actin, preventing cross-bridge formation.

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