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Decks by Naomi Justus (1)

Muscle Tissue

Muscle TissueCharacteristics of ALL muscles Contractility: Contract and relax to make movement Extensibility: Extend, or stretch, allow muscles to return to rest Excitability: Respond to regulatory signals from nerves, hormones & stimuli Functions of skeletal muscles Posture, heat production, movement, protection Types of Muscle Tissue Skeletal muscle Structure (anatomy): multinucleated, regular arrangement of actin and myosin fibers into bands of light and dark (striated) Function (physiology): move the skeleton, especially the limbs Location: usually connected to bones or fascia Cardiac muscle Structure (anatomy): 1-2 nuclei, branching with intercalated disks organized as a syncytium to allow for coordinated contraction Function (physiology): Pump blood through the circulatory system Location: Heart Smooth muscle Structure (anatomy): 1 nucleus, no regular arrangement of actin and myosin proteins in cytoplasm (non-striated/smooth) Function (physiology): Goosebumps, moves food through digestive tract, blood through circulatory system Location: parts of viscera (organs, ducts) throughout the body Characteristics of Cardiac Muscle Highly coordinated contractions of cardiac muscle Similarity with skeletal muscle: striated, organized into sarcomeres Differences with skeletal muscle: only 1-2 nuclei, multiple mitochondria and myoglobin, extensively branched fibers cells, intercalated discs. Intercalated discs have sarcolemma with gap junctions & desmosomes (allow heart to work as a pump by coordinating cardiac contraction) Gap junctions: channels between adjacent cells that allow ions to flow from one cell to another quickly. Depolarization spreads quickly between cells to allow for coordinated contraction of entire heart, creates a syncytium (unit of contraction). Desmosomes anchor the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting Smooth Muscle Similarity to skeletal muscle: actin & myosin contractile proteins, thick & thin filaments. Differences with skeletal muscle: 1 nucleus, spindle-shaped, no striations, sarcomere, troponin, tropomyosin Thin filaments are anchored by dense bodies (similar to Z-discs) attached to sarcolemma. Ca2+ enters sarcoplasm from SR and ECF and binds to regulatory protein calmodulin Structure of a skeletal muscle 3 layers of connective tissue enclose a muscle to provides structure Epimysium Dense, irregular connective tissue around muscle organ Allows a muscle to contract/move; structural integrity separates muscle from other regional tissues; allows movement Perimysium Middle layer of connective tissue Allows nervous system to trigger a movement of a muscle by activating fascicle Endomysium Thin layer of collagen and reticular fibers around muscle fiber Organizes muscle fibers into fascicle (individual bundles) Has ECF and nutrients supplied by blood Skeletal Muscle Fibers (Cells) Skeletal muscle cells (muscle fibers) are long and cylindrical. During early development, embryonic myoblasts, each has own nucleus, with 100s of other myoblasts to form the multinucleated skeletal muscle fibers (myofibrils) with multiple copies of genes to allow bulk production of proteins and enzymes for muscle contraction. Sarcolemma: plasma membrane of muscle fibers Sarcoplasm: cytoplasm of muscle fibers Sarcoplasmic reticulum (SR): specialized smooth ER: stores, releases Ca2+ ions Sarcomere: Unit of skeletal muscle fiber: highly organized arrangement of contractile proteins (actin, myosin myofilaments) and regulatory proteins (troponin, tropomyosin) Sarcomere (Unit of Skeletal Muscles) 3D cylinder striations (bands of light & dark due to actin and myosin myofilaments) Each myofibril can contain 100-1000s sarcomeres connected end to end All sarcomeres within a myofibril contracts (and relaxes) simultaneously, contracting (and relaxing) the entire myofibril & muscle cell Thin filament Starts from Z-discs and projects partway to the center Has thinner actin strands and its troponin-tropomyosin complex Thick filament Starts from the center and projects partway to the Z-discs Has thicker strands and their multiple heads Z-discs (Z-lines) Forms the boundary of sarcomeres at both ends Anchored to actin myofilaments Myofilaments Each myofibril has 1000s myofilaments 4 different kinds of protein molecules make up myofilaments Protein molecules Actin (thin filaments): Has active sites (myosin binding sites) bind to myosin heads Myosin (thick filament): Has myosin heads that are attracted to actin and forms cross bridges with actin Tropomyosin (regulatory protein): at rest, it blocks the myosin binding sites on actin Troponin (regulatory protein): at rest, it holds tropomyosin in place, bind to Ca2+ ions The Neuromuscular Junction Location: Nerve ending meets the muscle fiber All cells have membrane potential (gradients across membrane): -60 to -90 mV When the membrane potential is LESS NEGATIVE: Depolarization occurs and AP starts Membrane potentials change when ions either enter or leave the cell via ion channels (open and close depending on the stimuli) This change generates AP. An action potential (AP) in a nerve at the NMJ releases a NT which leads to the start of an AP in the muscle. This AP in the muscle causes muscle contraction (Excitation-contraction coupling) Every skeletal muscle fiber is innervated by a motor neuron at the NMJ A signal from the motor neuron can cause the contraction of skeletal muscle fibers Each motor neuron can innervate from 10s to 1000s skeletal muscle fibers Contraction of Skeletal Muscle AP reaches the end of the motor neuron NT (acetylcholine or ACh) is released into NMJ ACh binds to receptors on ligand ion channels for Na+ on the skeletal muscle fiber Na+ channels open: Na+ enters sarcoplasm of muscle fiber Membrane potential of muscle fiber changes AP starts along the sarcolemma of muscle fiber: AP travels into the interior of the cell via T-tubules (extensions of the sarcolemma) AP starts with the sarcolemma of muscle fiber: AP travels into the interior of the skeletal muscle cell via T-tubules (extensions of the sarcolemma) Action potential depolarizes the cell membrane Voltage-gated Ca2+ channels in SR Ca2+ diffuses out of SR into sarcoplasm Ca2+ binds to troponin on thin filament: Troponin-tropomyosin moves to myosin sites Myosin binds actin at its myosin-binding site to form cross-bridge: ADP & inorganic phosphate (Pi) generated in the previous contraction cycle are released Myosin head pivots toward M-line at center of the sarcomere- power stroke New ATP attaches to the myosin head Cross-bridge is detached ATPase in myosin head hydrolyzes ATP to ADP Pi, releases energy Angle of myosin head moves into a cocked position (re-cock), ready to form another crossbridge with next myosin binding site Sliding Filament Model of Contraction Skeletal muscle fiber contracts when thin filaments are pulled and then slide past the thick filaments within the fiber’s sarcomeres Requires Ca2+ and ATP Ca2+ starts contraction by exposing actin-binding site to form myosin crossbridges ATP sustains contraction: Each cycle in cross-bridge needs energy by hydrolysis of ATP Without ATP, the myosin head will be still attached to actin: rigor mortis Myosin is in a high-energy configuration when myosin head is cocked: this energy is used during the power stroke Relaxation of a Muscle Fiber Muscle contraction usually stops when Nerve signal stops Muscle runs out of ATP and becomes fatigued Process Nerve signal stops Release of ACh stops Ligand gated Na+ channels close Sarcolemma and T-tubules repolarizes Voltage-gated Ca2+ channels in the SR close Ca2+ ions are pumped back into SR using ATP Tropomyosin moves to cover myosin-binding sites Thick and thin filament interaction relaxes Sources of ATP Skeletal muscle stores only small amount of ATP, requiring rapid replacement to sustain contraction. Sources of ATP: Creatine Phosphate: Transfers energy from excess ATP to make ADP and creatine. It can quickly regenerate ATP but only sustains energy for about 15 seconds. Glycolysis: An anaerobic process that breaks down glucose to produce ATP at a slower rate than creatine phosphate, providing a burst of energy for up to 1 minute. Aerobic Respiration: The breakdown of glucose or other nutrients in the presence of oxygen, resulting in carbon dioxide, water, and ATP. This is a more efficient process that produces approximately 95% of the ATP needed by muscles. Motor Units Each skeletal muscle fiber is controlled by one motor neuron, but one motor neuron can control multiple muscle fibers, depending on the muscle type. Motor Unit: Group of muscle fibers innervated by a single motor neuron Small Motor Units: Control fewer than 10 muscle fibers, allowing for precise movements, like eye movements. They use smaller, more excitable motor neurons. Larger Motor Units: Control thousands of muscle fibers and are involved in larger movements, like those in the thigh. They use bigger, less excitable motor neurons. Recruitment Process: Activating smaller motor units before large ones to INCREASE muscle contraction. As more motor units are recruited, the strength of the contraction increases, allowing variations, such as lifting a feather versus a heavy weight.