Muscular system Knowledge-Based Study

Study notes

1. Overview of Muscle Tissue

   The muscular system produces movement, maintains posture, and stabilizes joints. Muscle tissue is classified into three distinct histological and functional types:

   • Skeletal Muscle: Composed of long, cylindrical, multinucleated fibers with prominent cross-striations. It is under voluntary control of the somatic nervous system and is attached to bones via tendons.
   • Cardiac Muscle: Found exclusively in the heart wall (myocardium). Its cells are branched, typically possess one or two central nuclei, and contain intercalated discs with gap junctions that allow coordinated, involuntary contractions initiated by intrinsic pacemaker cells.
   • Smooth Muscle: Made of spindle-shaped (fusiform), uninucleated cells that lack striations because their contractile proteins are not organized into regular sarcomeres. Located in the walls of hollow organs, blood vessels, and the digestive tract, it operates involuntarily under autonomic nervous control.

   The Sarcomere and Myofilaments

   A skeletal muscle fiber contains numerous bundled cylindrical structures called myofibrils. Each myofibril is divided into repeating functional units called sarcomeres, bounded at each end by a Z-disc (or Z-line).

   • Actin (Thin Filaments): Composed of globular actin molecules arranged in double helical strands, associated with regulatory proteins tropomyosin and troponin. They anchor directly to the Z-disc.
   • Myosin (Thick Filaments): Composed of multiple myosin molecules with protruding globular heads that possess ATP-binding sites and actin-binding sites. Thick filaments are anchored in the center of the sarcomere along the M-line.
   • Sarcomere Banding: The A-band spans the full length of the thick filaments. The I-band contains only thin filaments and is bisected by the Z-disc. The H-zone lies in the center of the A-band, containing only thick filaments when the muscle is at rest.

   Sliding Filament Theory and Cross-Bridge Cycling

   Muscle contraction occurs without shortening the individual filaments; instead, thin filaments slide past thick filaments, pulling Z-discs closer together and shortening the sarcomere.

   1. Neuromuscular Junction Transmission: A nerve impulse reaches the axon terminal of a motor neuron, triggering the release of acetylcholine (ACh) into the synaptic cleft. ACh binds to receptors on the motor end plate, depolarizing the sarcolemma.
   2. Calcium Ion Release: Action potentials travel along transverse tubules (T-tubules) into the cell interior, causing voltage-gated channels on the sarcoplasmic reticulum to release $Ca^{2+}$ into the sarcoplasm.
   3. Uncovering Active Sites: Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin.
   4. Cross-Bridge Formation and Power Stroke: Energized myosin heads bind to actin. The release of inorganic phosphate triggers the power stroke, pulling the thin filament toward the M-line. ADP is then released.
   5. Cross-Bridge Detachment: A new ATP molecule binds to the myosin head, causing it to detach from actin. Hydrolysis of ATP into ADP and $P_i$ recocks the myosin head into its high-energy state.

   Energy Production and Antagonistic Pairs

   Muscle fibers require a continuous supply of ATP to sustain contraction and pump $Ca^{2+}$ back into the sarcoplasmic reticulum for relaxation.

   • Direct Phosphorylation: Creatine phosphate transfers a high-energy phosphate group to ADP, generating ATP rapidly for the first several seconds of intense exertion.
   • Anaerobic Glycolysis: Glucose from the bloodstream or muscle glycogen is converted into pyruvate and then lactic acid, yielding a net of 2 ATP per glucose molecule when oxygen availability is limited.
   • Aerobic Respiration: Occurs in mitochondria during rest or moderate exercise, breaking down glucose, fatty acids, and amino acids to produce high yields of ATP using oxygen.

   Because muscles can only pull and cannot actively push, skeletal movement relies on antagonistic pairs. An agonist (or prime mover) contracts to produce a specific movement, while the antagonist relaxes or yields. For example, during forearm flexion, the biceps brachii acts as the agonist while the triceps brachii acts as the antagonist.