Excitation–Contraction Coupling Comprehensive Notes

Excitation–Contraction Coupling (ECC)

• ECC = process by which an action potential (AP) triggers mechanical shortening of a muscle fibre.

• Present in all muscle types, but molecular details vary:

  • Skeletal muscle

  • Cardiac (heart) muscle

  • Smooth muscle

• Core sequence (shared logic):

  1. Electrical excitation → depolarisation of surface membrane/T-tubule.

  2. Rise in cytosolic $[Ca^{2+}]_{i}$ via release from internal stores and/or influx from extracellular fluid (ECF).

  3. Ca$^{2+}$ binds to specific regulatory proteins → exposes actin-binding sites.

  4. Actin–myosin cross-bridge cycling → tension/force generation.

  5. Active removal of Ca$^{2+}$ and dissociation of cross-bridges → relaxation.

ECC in Skeletal Muscle

• FOUR canonical steps were highlighted.

  1. Ca$^{2+}$ Release from Terminal Cisternae (T.C.) of Sarcoplasmic Reticulum (SR)

  • AP travels along sarcolemma and dives into T-tubules.

  • Voltage-sensor = dihydropyridine (DHP) receptor in T-tubule membrane.

  • DHP mechanically couples to Ryanodine Receptor (RyR1) Ca$^{2+}$-release channel on SR terminal cisternae.

  • Conformational change → RyR1 opens → massive Ca$^{2+}$ flood into cytosol.

  1. Activation of Contractile (Regulatory) Proteins

  • Cytosolic Ca$^{2+}$ binds to Troponin-C (TnC) of the troponin complex (TnC–TnI–TnT).

  • Ca$^{2+}$-bound TnC pulls Tropomyosin away from actin’s myosin-binding sites.

  1. Generation of Tension (Cross-bridge Cycle)

  • Myosin head (pre-energised with bound $ADP+P_i$) attaches to exposed actin.

  • Power stroke ejects $P_i$ then $ADP$ → filament sliding → muscle shortening.

  • New ATP binds to myosin → detachment; ATP hydrolysis re-cocking head.

  1. Relaxation

  • Requires energy: SR Ca$^{2+}$-ATPase (SERCA) pumps Ca$^{2+}$ back into SR terminal cisternae.

  • Decline in $[Ca^{2+}]_{i}$ → Ca$^{2+}$ dissociates from TnC → tropomyosin re-blocks sites → cross-bridge cycling stops.

  • Rigor state

    • If ATP unavailable (e.g., exhaustion or post-mortem) myosin remains latched to actin → fixed rigidity (rigor mortis when after death).

Mechanical Manifestations in Skeletal Muscle

• Isotonic Contraction

  • Muscle shortens appreciably while tension remains ~constant.

  • Does mechanical work: $Work = Force \times Distance$.

• Isometric Contraction

  • Overall muscle length unchanged; tension rises sharply.

  • No external work done despite high energy consumption.

Muscle Fatigue (Skeletal)

• Failure to produce expected force despite continued stimulation.

• Multifactorial biochemical causes:

  1. Accumulation of $ADP$ and $P_i$: slows cross-bridge detachment & may inhibit SR Ca$^{2+}$ release.

  2. Lactic acid build-up: lowers pH → impairs metabolic enzymes.

  3. Extracellular $K^{+}$ accumulation: depolarises membrane → reduces AP amplitude.

  4. Depletion of glycogen & phosphoryl-creatine → energy deficit.

ECC in Cardiac Muscle

• Electrical–mechanical relationship displayed as simultaneous intracellular AP and mechanical twitch (~0.5 g tension)

  • Absolute Refractory Period (ARP) ≈ duration of AP; relative refractory period (RRP) follows.

• Key distinctions from skeletal muscle

  1. Ca$^{2+}$ source

  • Depolarisation opens voltage-gated L-type Ca$^{2+}$ channels (DHP) in T-tubules.

  • Influx of extracellular Ca$^{2+}$ during AP plateau (~phase 2).

  1. Ca$^{2+}$-induced Ca$^{2+}$ Release (CICR)

  • Entering Ca$^{2+}$ binds RyR2 on SR → amplifies Ca$^{2+}$ liberation.

  1. Ca$^{2+}$ binds TnC → activation identical to skeletal muscle.

  2. Relaxation mechanisms

  • SERCA reuptake into SR.

  • Sarcolemmal $Na^{+}/Ca^{2+}$ exchanger (NCX) extrudes Ca$^{2+}$ to ECF (secondary active via $Na^{+}$ gradient).

  • Sarcolemmal Ca$^{2+}$-ATPase (minor).

ECC in Smooth Muscle

• Lacks troponin; regulation occurs at myosin rather than actin.

• Sequence

  1. Depolarisation OR hormone/ligand binding opens voltage-gated or receptor-operated Ca$^{2+}$ channels.

  2. Ca$^{2+}$ influx (and some SR release) binds Calmodulin → Ca$^{2+}$_4-Calmodulin complex.

  3. Complex activates Myosin Light-Chain Kinase (MLCK).

  4. MLCK phosphorylates myosin regulatory light chains (RLC) (uses ATP) → increases myosin ATPase activity.

  5. Phosphorylated myosin forms cross-bridges with actin → contraction.

  6. Termination via Myosin Phosphatase

  • Dephosphorylates RLC → cross-bridges disengage; muscle relaxes even if Ca$^{2+}$ still somewhat elevated (latch state phenomenon allows sustained tone with little ATP).

Energy Requirements & Clinical Correlates

• Contraction costs: ATP consumption by myosin ATPase; Ca$^{2+}$ re-uptake by SERCA; RLC phosphorylation (smooth).

• Phosphoryl-Creatine (PCr) buffer: $PCr + ADP \leftrightarrow ATP + Creatine$ (creatine kinase) supplies rapid ATP regeneration.

• Pathophysiology

  • Heart failure: impaired Ca$^{2+}$ handling → weak contraction.

  • Malignant hyperthermia (skeletal): RyR1 mutation → excessive Ca$^{2+}$ release, sustained contraction & heat.

  • Calcium channel blockers: inhibit L-type channels → reduce cardiac contractility & vascular smooth muscle tone.

Numerical & Temporal References

• Cardiac AP amplitude (trans-sarcolemmal): $\approx 150\,\text{mV}$ (surface) vs 0 mV baseline inside cell.

• Cardiac mechanical twitch onset lags AP by ~$5–10\,\text{ms}$; peak tension ~$200–250\,\text{ms}$ post-stimulus.

• Skeletal SERCA turnover ≈ $30–40\,\text{Ca}^{2+}\,/\,s$ per pump molecule (temperature-dependent).

• Time to full rigor mortis post-mortem: $\approx 3–4\,\text{h}$ (varies with temperature/conditioning).

Conceptual Connections & Implications

• Cross-bridge theory unified across muscle types; regulation point differs (thin-filament vs thick-filament control).

• ECC couples electrophysiology (ion channels) and biomechanics (sarcomere), illustrating systems integration.

• Pharmacological manipulation of any ECC step (e.g., RyR blockers, SERCA inhibitors, MLCK inhibitors) yields therapeutic or toxic effects.

• Ethical relevance: understanding rigor mortis assists forensic time-of-death estimates; cardiac ECC knowledge underlies life-saving anti-arrhythmic drug design.