Cytoskeleton and Muscle Contraction Mind Map PDF

Actin filaments are long helical polymers of filamentous...

Actin filaments are long helical polymers of filamentous (F) actin, with a diameter of approximately 7 nm Formed through the polymerization of globular (G) actin, which requires ATP They are polarized with a + (plus) end for rapid Structure and Organization growth and a − (minus) end for slower disassembly contains actin and myosin filaments but not ordered (not organized into sarcomeres). The filaments are composed of two protofilaments wrapped around each other in a helix, repeating every 37 nm No troponin. Regulation is done by phosphorylation of myosin light chains Actin filaments are flexible and can form bundles or Flexibility and Arrangements: networks, depending on the cell's needs The contraction mechanism is also dependent on the level of Ca2+. Smooth Muscle Nucleation: Formation of a "nucleus" made of actin trimers (3 G-actin molecules with ATP bound) Contraction involves phosphorylation of myosin light chains by the enzyme MLCK (myosin light chain kinase) Elongation: Addition of ATP-bound G-actin at Stages of Polymerization: both ends, predominantly at the + end The enzyme is activated by binding calmodulin bound Contraction mechanism: to Ca2+ Cc at (+) End = 0.1 µM Steady State: Equilibrium between assembly and Polymerization and Dynamics **Critical Concentration (Cc):** disassembly, characterized by critical concentration (Cc) 3.When the light chain is phosphorylated, the Cc at (-) End = 0.8 µM myosin head can interact with the actin filaments and cause muscle contraction At steady state, net assembly occurs at the + end and **Treadmilling:** disassembly at the − end, creating a dynamic equilibrium maintening the actin lenght steady Striated, actin and myosin sliding, tropomyosin- Similar to skeletal muscle: Toxins can stabilize or destabilize actin filaments, Influence polymerization and depolymerization troponin regulation and Ca2+ influencing their function and dynamics rates Actin Toxins unite cells through desmosomes Cardiac Muscle Examples: Stabilizing or shortening actin filaments contain "gap" junctions which allow the rapid propagation of action potentials between cells, Functions of discs: Cells joined by **intercalated discs** Differences: **Tropomyosin:** Stabilizes actin filaments enabling synchronization of contraction **Fimbrin & Filamin:** Organize actin into **Z-line Connections:** Continuity across cells  bundles or networks **Gelsolin:** Severs actin filaments and binds to the (+) end to prevent elongation Striated appearance due to sarcomeres They regulate various aspects of actin filament Actin-Binding Proteins (ABPs) behavior: Myosin I: Vesicle transport **Skeletal Muscle:** connected to skeleton Responsible for **Myosin I & II:** voluntary movements Cytoskeleton Microfilaments (Actin Filaments) Myosin II: Actin sliding Similar to skeletal but functions involuntarily Cytoskeleton and **Spectrin:** Links actin to membranes for continuous heart contractions **Cardiac Muscle:** Types of Muscle Muscle Contraction **Thymosin:** Binds actin subunits, preventing intercalated discs for synchronization and branched Mind Map polymerization Microvilli: Projections that increase cell surface area Non-striated; involved in involuntary, slower **Smooth Muscle:** for absorption, particularly in epithelial intestinal cells contractions like in the intestines **I Band:** Thin actin filaments partially overlapping A band Structural units between Z-discs, containing: **Sarcomeres:** Basic units of contraction **A Band:** Dark Thick myosin filaments Cytokinesis: Actin forms a contractile ring to split cells during division formed by actin and myosin **Titin:** a very long elastic protein extending from the Z line to Functions Actin filaments are crucial in: the M line, passing through the center of the thick filament. Serves as a spring to keep the sarcomere in the center during contraction Skeletal Muscle Structure **Nebulin:** large protein that binds to actin contributing to the structure and stability of **Accessory Proteins:** Stress fibers: Involved in adhesion, migration, and filaments, regulating their length the sarcomere mechanical sensing **Cap Z & Tropomodulin:** cap the ends of the Pseudopodia and lamellipodia: Cell membrane actin to keep the length of the filament constant protrusions with role in cell migration like phagocytosis or Invasion of tissues by leukocytes during an infection **Actin Filaments:** Polarized, helical structure Thick and Thin Filaments Muscle cells: Integral to contraction and structure Bipolar arrangement **Myosin II:** Heads project outward, bare zone in the center Protrusion: Actin polymerization drives Actin slides over myosin, shortening the the leading edge (lamellipod) forward sarcomere Muscle Contraction Lamellipodia and cell migration involves three Adhesion: Formation of focal contacts Cell Movement 1. ATP binding to myosin, detaching it from actin coordinated steps: linking the lamellipod to the support Muscle contraction follows Mechanism of Contraction the Sliding Filament Model: Retraction: The cell’s rear detaches to move forward. 2. ATP hydrolysis and repositioning of the myosin head (ADP and Pi remain bound to the head) 3. This binding promotes the dissociation of Pi and the energy thus Steps involved: released promotes the movement of the myosin head propelling the actin filament towards the center of the sarcomere; this is accompanied by the release of ADP 4. ADP released → allows the return to the rigor state Tropomyosin (filamentous protein spirally wound around the actin filament) blocks myosin-binding sites **Troponin and Tropomyosin:** Regulation of muscle contraction by troponin Troponin: A tripeptide (T, C and I) which is linked to tropomyosin and both mask the binding site of the myosin head in the absence of Ca2+ When Ca2+ ions are released from the SR, they bind to troponin C. This displaces the tropomyosin/troponin complex. This unmasks the binding site of the myosin head and, therefore, induces muscle contraction Released from sarcoplasmic reticulum (SR) **Calcium Role:** Binds troponin → Initiates contraction Reabsorbed by SR → Muscle relaxation 1. The muscle fiber receives the action potential that activates a Ca2+ channel in the membrane of the T tubules Regulation of Contraction 2. this triggers the opening of a Ca2+ release channel at the membrane of the SR Excitation-Contraction Coupling 3. intracellular Ca2+ increases and promotes contraction by sliding actin and myosin filament; 4. Intracellular Ca2+ is restored by the Ca2+ pump of  the SR membrane, allowing the myofibrils to relax 1. Arrival of the action potential at the presynaptic motor neuron and opening of voltage-gated Ca2+ channels; 2. Release of acetylcholine that binds to the ligand- gated Na+ channels of the muscle cell causing depolarization of its membrane; Calcium release mechanism 3. Opening of voltage-gated Na+ channels and action potential generation; 4. Depolarization reaches the T tubules, causing the voltage-gated channels of Ca2+ to open and Ca2+ to exit from the SR to the cytosol