Excitation-Contraction Coupling Study Notes

Excitation-Contraction Coupling is the physiological process that bridges the gap between the electrical depolarization of the muscle fiber (sarcolemma) and the mechanical shortening of the sarcomere.
Key Components and Anatomy:
- Sarcolemma: The plasma membrane of the muscle cell which carries the action potential (AP).
- Transverse Tubules (T-tubules): Deep invaginations of the sarcolemma that ensure the action potential reaches the interior depths of the muscle fiber rapidly.
- Sarcoplasmic Reticulum (SR): A specialized endoplasmic reticulum that acts as the primary storage site for intracellular $Ca^{2+}$ .
- Terminal Cisternae: Enlarged areas of the SR found adjacent to T-tubules; a T-tubule and its two flanking terminal cisternae form a structure known as a Triad.
- Contractile Proteins: Actin (thin filament) and Myosin (thick filament).
- Regulatory Proteins: Troponin complex and Tropomyosin.

The process begins when an action potential reaches the motor neuron terminal, triggering the release of Acetylcholine (ACh) into the synaptic cleft.
ACh binds to nicotinic receptors on the motor end plate, causing a localized depolarization called an End-Plate Potential (EPP). If the EPP reaches threshold, it triggers a propagated action potential along the sarcolemma.

Action Potential Propagation: The AP spreads across the sarcolemma and descends into the T-tubules.
DHPR-RyR Interaction:
- The T-tubule membrane contains voltage-sensing Dihydropyridine Receptors (DHPR).
- In skeletal muscle, a physical conformational change in the DHPR directly opens the Ryanodine Receptors (RyR1) located on the adjacent SR membrane.
Calcium Flux: Once the RyR channels open, $Ca^{2+}$ ions flow down their electrochemical gradient from the high-concentration environment of the SR into the lower-concentration environment of the sarcoplasm (cytosol).

Calcium Binding: Released $Ca^{2+}$ binds specifically to Troponin C on the thin filament.
Conformational Shift: This binding causes the troponin complex to shift, pulling Tropomyosin away from the active binding sites on the actin molecules.
Cross-Bridge Cycle:
1. Binding: Myosin heads (loaded with $(ADP + P_i)$ ) bind to the newly exposed active sites on actin.
2. Power Stroke: The release of $P_i$ and $ADP$ triggers the myosin head to pivot, pulling the actin filament toward the M-line (center of the sarcomere).
3. Detachment: A new molecule of ATP binds to the myosin head, causing it to release from the actin.
4. Reactivation: Myosin ATPase hydrolyzes the ATP into $(ADP + P_i)$ , resetting the myosin head into the "cocked" or high-energy position, ready for another cycle.

Cessation of Signal: Acetylcholinesterase breaks down ACh in the synapse, and the sarcolemma repolarizes.
Calcium Sequestration: The SERCA (Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase) pump actively transports $Ca^{2+}$ back into the SR against its concentration gradient.
Regulatory Reset: As sarcoplasmic $Ca^{2+}$ levels drop, $Ca^{2+}$ dissociates from Troponin C. Tropomyosin moves back to block the actin-binding sites, and the muscle returns to its resting length through elastic recoil.

Malignant Hyperthermia: A genetic mutation in the RyR receptor that causes excessive calcium release in response to certain anesthetics, leading to sustained muscle contraction and dangerously high body temperatures.
Rigor Mortis: Occurs after death because ATP is no longer produced; without ATP, myosin heads cannot detach from actin, leaving muscles in a state of rigid contraction.
Energy Balance: Muscle contraction is an energy-intensive process requiring ATP for both the cross-bridge cycle (contraction) and the SERCA pump (relaxation).