Muscle contraction Knowledge-Based Study

Study notes

1. The Functional Unit of Contraction

   Skeletal muscle tissue is composed of elongated cells termed muscle fibers. Within each muscle fiber lie thousands of cylindrical organelles known as myofibrils, which exhibit alternating light and dark bands under microscopic examination. The functional contractile unit of a myofibril is the sarcomere, bounded on both ends by dense protein sheets called Z lines (or Z discs).

   Key Components of the Sarcomere

   • Thin Filaments: Primarily composed of the globular protein actin arranged in a double helix. Associated with actin are two critical regulatory proteins: tropomyosin, a filamentous protein that covers active binding sites in resting muscle, and troponin, a globular protein complex capable of binding calcium ions.
   • Thick Filaments: Formed predominantly of bundled myosin molecules. Each myosin protein features a tail region and two flexible, protruding globular heads equipped with actin-binding sites and intrinsic ATPase activity.

   Bands and Zones

   The arrangement of these overlapping filaments creates distinct microscopic regions:

   • A Band: The dark region spanning the entire length of the thick (myosin) filaments. Its length remains constant during contraction.
   • I Band: The light region containing only thin (actin) filaments, bisected by the Z line.
   • H Zone: The central, paler area within the A band where thick filaments do not overlap with thin filaments at rest.
   • M Line: A structural protein meshwork in the exact center of the sarcomere that anchors thick filaments.

   Bridging Electrical Signal to Chemical Trigger

   Excitation-contraction coupling is the sequence of events by which an electrical nerve impulse leads to mechanical filament sliding.

   Sequence of Events

   1. Neuromuscular Junction: A motor neuron transmits an action potential down its axon, arriving at the synaptic terminal. This stimulates the exocytosis of acetylcholine (ACh) into the synaptic cleft.
   2. Sarcolemma Depolarization: Acetylcholine binds to ligand-gated receptors on the motor end plate of the muscle fiber, initiating local depolarization and triggering a muscle action potential along the sarcolemma (plasma membrane).
   3. Propagation via T-Tubules: The action potential travels across the surface and penetrates deep into the muscle fiber interior through invaginations known as transverse tubules (T-tubules).
   4. Calcium Release: Depolarization reaches the terminal cisternae of the sarcoplasmic reticulum (SR), opening voltage-gated calcium channels and causing a massive influx of stored calcium ions into the sarcoplasm surrounding the myofilaments.

   Molecular Mechanics of Contraction

   According to the sliding filament model, contraction occurs when thin actin filaments slide past thick myosin filaments toward the center of the sarcomere. The filaments do not change length; rather, their extent of overlap increases, causing the I band and H zone to narrow and the Z lines to move closer together.

   Steps of the Cross-Bridge Cycle

   1. Exposure of Binding Sites: Calcium ions bind to troponin, inducing a conformational change that pulls tropomyosin away from the myosin-binding sites on actin.
   2. Cross-Bridge Formation: An energized (cocked) myosin head carrying ADP and inorganic phosphate binds firmly to the exposed site on actin, forming a cross-bridge.
   3. The Power Stroke: The release of inorganic phosphate and ADP triggers the pivoting of the myosin head. The head bends toward the center of the sarcomere, pulling the attached thin filament along with it.
   4. Cross-Bridge Detachment: A new molecule of ATP binds to the myosin head. This binding destabilizes the actin-myosin interaction, causing the myosin head to detach from actin.
   5. Re-energizing the Myosin Head: The enzyme ATPase within the myosin head hydrolyzes the newly bound ATP into ADP and inorganic phosphate. The energy released returns the myosin head to its high-energy, cocked orientation, ready to initiate another cycle if calcium remains available.

   Termination of Contraction and Role of ATP

   Contraction continues as long as calcium ions remain elevated in the sarcoplasm and ATP supplies persist. When neural stimulation ceases, relaxation occurs through active restoration of resting conditions.

   The Mechanism of Relaxation

   • Acetylcholine Degradation: The enzyme acetylcholinesterase rapidly breaks down ACh remaining in the synaptic cleft, stopping new action potentials.
   • Calcium Reuptake: Primary active transport pumps, known as $Ca^{2+}$-ATPase pumps, expend ATP to actively sequester calcium ions back into the lumen of the sarcoplasmic reticulum against their concentration gradient.
   • Steric Blockade Restored: As sarcoplasmic calcium levels plummet, calcium dissociates from troponin. Tropomyosin moves back into its resting position, sterically blocking the myosin-binding sites on actin and preventing further cross-bridge interaction.

   Essential Functions of ATP in Muscle Physiology

   ATP is directly required for three vital processes in muscle function:

   1. Hydrolysis to provide the mechanical energy needed to re-cock the myosin head prior to the power stroke.
   2. Binding to the myosin head to break the cross-bridge linkage and permit filament detachment.
   3. Powering the active transport pumps that return calcium into the sarcoplasmic reticulum to enable relaxation.

   Physiological Note: When cellular respiration halts and ATP stores are completely depleted after death, myosin heads remain irreversibly locked to actin filaments, resulting in the temporary state of muscular stiffness termed rigor mortis.