Study Guide: Muscle Tissue PDF

Study Guide: Muscle Tissue 1. Muscle Types Skeletal Muscle o Structure: Striated appearance, multinucleated fibers. o Function: Voluntary control over body movements, posture maintenance, and stabilization of joints. o Location: Attached to bone...

Study Guide: Muscle Tissue 1. Muscle Types Skeletal Muscle o Structure: Striated appearance, multinucleated fibers. o Function: Voluntary control over body movements, posture maintenance, and stabilization of joints. o Location: Attached to bones via tendons. Cardiac Muscle o Structure: Striated, branched fibers, typically one to two nuclei per cell. o Function: Involuntary control, responsible for pumping blood throughout the body. o Location: Heart walls. o Unique Features: Intercalated discs that facilitate electrical signaling and contraction synchronization. Smooth Muscle o Structure: Non-striated, spindle-shaped fibers, usually with one nucleus. o Function: Involuntary control, regulates internal organ function, including digestion and blood vessel diameter. o Location: Walls of hollow organs (e.g., intestines, blood vessels). 2. Common Muscle Properties Contractility: Ability to shorten forcefully, enabling movement. Extensibility: Capacity to stretch without damage. Elasticity: Ability to return to resting length after stretching, due to elastic fibers in the tissue. Structural Organization of Skeletal Muscle 1. Skeletal Muscle as an Organ Components: o Muscle fibers (cells) o Blood vessels (supply oxygen and nutrients) o Nerve fibers (innervate muscle fibers for contraction) o Connective tissues (support and structure) 2. Connective Tissue Layers Epimysium: Dense connective tissue surrounding the entire muscle, allowing independent movement. Perimysium: Surrounds individual fascicles, providing pathways for blood vessels and nerves. Endomysium: Encloses individual muscle fibers, containing extracellular fluid and nutrients. 3. Muscle Fibers (Cells) Morphology: o Long and cylindrical, with diameters up to 100 μm and lengths up to 30 cm. Terminology: o Sarcolemma: Plasma membrane of muscle fibers. o Sarcoplasm: Cytoplasm of muscle fibers. o Sarcoplasmic Reticulum (SR): Specialized ER that stores and regulates calcium ions. The Sarcomere 1. Functional Unit Structure: o Composed of actin and myosin filaments arranged in a specific pattern. o Each sarcomere is about 2 μm long, bordered by Z-discs. 2. Z-Discs (Z-Lines) Function: Serve as attachment points for the thin filaments (actin). Structure: Composed of proteins such as alpha-actinin, which anchor the actin filaments, and connect adjacent sarcomeres. Role in Contraction: As sarcomeres shorten during contraction, the Z-discs are pulled closer together. 3. Thin Filaments (Actin) Composition: Primarily made of globular actin (G-actin) molecules that polymerize to form filamentous actin (F-actin). Regulatory Proteins: o Troponin: A complex of three proteins (TnC, TnI, TnT) that binds Ca++. When Ca++ binds to TnC, it causes a conformational change that moves tropomyosin away from myosin-binding sites on actin. o Tropomyosin: A long protein that wraps around actin filaments, blocking myosin- binding sites when the muscle is relaxed. Length: Thin filaments extend from the Z-discs toward the center of the sarcomere but do not reach the M-line. 4. Thick Filaments (Myosin) Composition: Made up of myosin proteins, each with a long tail and a globular head. Myosin Heads: The heads contain ATPase activity, allowing them to hydrolyze ATP for energy during muscle contraction. Each head can bind to actin, forming cross-bridges during contraction. Structure: Thick filaments are anchored at the M-line and extend toward the Z-discs, partially overlapping with thin filaments. Attached to Titan protein 6. M-Line Function: The M-line is the central line of the sarcomere where thick filaments are anchored. Structure: Composed of proteins such as myomesin, which help stabilize the position of myosin filaments. 7. A-Band Definition: The region of the sarcomere that contains thick filaments (myosin) and overlapping thin filaments (actin). Appearance: Dark band in a striated muscle under a light microscope due to the density of myosin and overlapping actin. Role in Contraction: The length of the A-band remains constant during contraction as the thin filaments slide over the thick filaments. 8. I-Band Definition: The light band in the sarcomere that contains only thin filaments (actin). Structure: Each I-band is bisected by a Z-disc. Role in Contraction: The I-band shortens during muscle contraction as the actin filaments slide toward the M-line. 9. H-Zone Definition: The central region of the A-band that contains only thick filaments (myosin) without overlapping thin filaments. Appearance: Appears lighter than the rest of the A-band under a microscope. Role in Contraction: The H-zone becomes narrower during contraction as the thin filaments slide past the thick filaments. Neuromuscular Junction and Excitation-Contraction Coupling 1. Neuromuscular Junction (NMJ) Structure: o Connection point between a motor neuron and a muscle fiber. o Synaptic Cleft: Gap where neurotransmitters diffuse. 2. Action Potential Mechanism: o ACh released into the synaptic cleft binds to receptors on the sarcolemma. o Initiates depolarization of the muscle fiber, leading to the generation of an action potential. 3. T-Tubules and Calcium Release T-Tubules: Invaginations of the sarcolemma that ensure the action potential reaches deep into the muscle fiber.  Triad Structure: o Formed by a T-tubule and two adjacent terminal cisternae of the SR, allowing rapid Ca++ release. Sliding Filament Model of Contraction 1. Process Overview Initiation: Ca++ binds to troponin, displacing tropomyosin, exposing myosin-binding sites. Cross-Bridge Formation: Myosin heads attach to exposed binding sites on actin. 2. Cross-Bridge Cycle Steps: o Binding: Myosin heads attach to actin. o Power Stroke: Myosin pulls actin towards the M-line, shortening the sarcomere. o Release: ATP binds to myosin, causing it to detach from actin. o Re-cocking: ATP is hydrolyzed, re-energizing the myosin head for another cycle. 3. Continuous Contraction Occurs as long as Ca++ remains in the sarcoplasm and ATP is available. Muscle Relaxation 1. Mechanism of Relaxation ACh Breakdown: Acetylcholinesterase degrades ACh in the synaptic cleft, stopping action potential generation. Ca++ Re-uptake: Ca++ is pumped back into the SR, leading to tropomyosin re-covering actin-binding sites. 2. Muscle Fatigue Results from depletion of ATP or accumulation of metabolic byproducts (e.g., lactic acid). Can also be influenced by neural factors, such as reduced motor neuron firing.