Chapter 10 – Muscle Tissue Muscle cells are highly specialized for contraction. There are three types of muscle tissue: skeletal, cardiac, smooth Skeletal Muscle Organs composed primarily of skeletal muscle tissue, but also contain connective tissue, nerves, and blood vessels. Each cell in skeletal muscle tissue is a single muscle fiber Skeletal muscles perform six functions: Functional Anatomy of Skeletal Muscle Three layers of connective tissue are part of each muscle: epimysium – perimysium – fascicle endomysium – muscle fiber – At each end of the muscle, the collagen fibers of the epimysium, perimysium, and endomysium come together to form either a bundle (tendon) or a broad sheet (aponeurosis). Both of these typically attach skeletal muscle to bones. The blood vessels and nerves that supply the muscle fibers are located in the connective tissues of the epimysium and perimysium. An extensive vascular system exists in skeletal muscle due to the large energy requirements necessary for movement. Skeletal muscles contract only under stimulation from the ________________________. Axons (nerve fibers) penetrate the epimysium, perimysium, and endomysium to innervate individual muscle fibers. Skeletal fibers are called __________________ muscles because we have voluntary control over their contractions. Many skeletal muscles, such as the diaphragm, may also be controlled on the subconscious level. Skeletal Muscle Fibers Myoblasts Myosatellite cells Internal Organization of Muscle Fibers The Sarcolemma Sarcoplasm - Transverse Tubules (T tubules) Myofibrils Myofilaments Types of myofilaments: a. thin filaments – b. thick filaments - Sarcoplasmic Reticulum Terminal cisternae Triad Calcium ions Sarcomeres Muscle Striations A Bands Contains the thick filaments and portions of the thin filaments Contain the following three subdivisions: 1. M line 2. H zone 3. Zone of Overlap I Bands Z line Z line to Z line is a ________________ Titin Thin filaments 5-6 nm in diameter Contains four proteins: F actin, nebulin, tropomyosin, and troponin 1. F actin 2. Nebulin 3. Tropomyosin 4. Troponin Initiating contraction A contraction cannot occur unless the position of the troponin-tropomyosin complex changes, exposing the active sites on F actin. The necessary change in position occurs when calcium ions bind to receptors on the troponin molecules. Thick filaments Cross-bridge = when a myosin head interacts with thin filaments during contraction. This connection functions as a hinge that lets the head pivot at its base. When pivoting occurs the head swings toward or away from the M line All myosin heads are arranged toward the M line The H zone has a central region that contains no myosin heads Sliding Filament Theory states that the thin filaments are sliding toward the center of the sarcomere, alongside the thick filaments. During a contraction, sliding occurs in every sarcomere along the myofibril. When a skeletal muscle fiber contracts the following happens: a. the H zones and I bands get smaller b. the zones of overlap get larger c. the Z lines move closer together d. the width of the A bands remains constant The Contraction of Skeletal Muscle Skeletal muscle fibers contract only under the control of the nervous system. Communication between the nervous system and a skeletal muscle fiber occurs at a specialized intercellular connection known as a neuromuscular junction. Each skeletal muscle fiber is controlled by a neuron at a single neuromuscular junction midway long the fiber’s length. A single axon branches many times, and each one ends at a synaptic terminal. The cytoplasm of the synaptic terminal contains mitochondria and vesicles filled with acetylcholine (ACh). ACh is a neurotransmitter that alters the permeability of the sarcolemma and triggers the contraction of the muscle fiber. A narrow space (synaptic cleft) separates the synaptic terminal of the neuron from the opposing sarcolemmal surface. This surface, which contains membrane receptors that bind ACh, is called the motor end plate. The enzyme acetylchoninesterase (AChE) is also found here. It breaks down ACh. A neuron stimulates a muscle fiber through a series of five steps: Step One: The arrival of an action potential at the synaptic terminal. Step Two: Acetylcholine is released is released into the synaptic cleft. Step Three: ACh diffuses across the synaptic cleft and binds to the ACh receptors on the surface of the sarcolemma at the motor end plate. The binding changes the permeability of the motor end plate to sodium ions making sodium ions rush into the sarcoplasm. This influx continues until AChE removes ACh. Step Four: The influx of sodium results in the generation of an action potential in the sarcolemma. The electrical impulse originates at the edges of the motor end plate, sweeps across the entire membrane surface, and travels inward along each T tubule. Step Five: AChE breaks down ACh to return everything back to the initial state. The link between the generation of an action potential in the sarcolemma and the start of a muscle contraction is called excitation-contraction coupling. The contraction cycle Step One: Once the action potential reaches the triad Calcium is released from the cisternae of the sarcoplasmic reticulum and binds to the troponin. The troponin molecule changes position, pulling the tropomyosin molecule away from the active sites on actin and allowing interaction with the energized myosin heads. Step Two: When the active sites are exposed, the energized myosin heads bind to them, forming cross-bridges. Step Three: At rest, the myosin head points away from the M line. When the cross-bridge forms the myosin head pivots toward the M line. This is called the power stroke. Once the head pivots, the ADP + P are released. Step Four: When another ATP binds to the myosin head, the link between the active site on the actin molecule and the myosin head is broken. The active site is now exposed and able to form another cross-bridge. Cocking the myosin head (pointing away from the M line) requires energy, which is obtained by breaking down ATP into ADP + P. Step Five: Myosin reactivation occurs when the free myosin head splits the ATP into ADP and a phosphate group. The energy released in this process is used to recock the myosin head. The duration of a contraction depends on: Steps that end a contraction Step One: Action potential generation ceases as ACh is broken down by AChE. Step Two: The sarcoplasmic reticulum reabsorbs calcium ions, and the concentration of calcium ions in the sarcoplasm declines Step Three: When calcium concentrations approach normal resting levels, the calcium ions detach from the troponin, and the troponin-tropomyosin complex returns to its normal position. The tropomyosin re-covers the active sites and prevents further cross-bridge interaction. Step Four: Without cross-bridge interactions, further sliding cannot take place, and the contraction ends. Step Five: Muscle relaxation occurs, and the muscle returns passively to its resting length. Rigor Mortis Energy Use and Muscular Activity ATP and CP reserves The primary function of ATP is the transfer of energy from one location to another rather than the long-term storage of energy. At rest, a skeletal muscle fiber produces more ATP than it needs. Under these conditions, ATP transfers energy to creatine, a small molecule that muscle cells assemble from fragments of amino acids. The transfer creates another high-energy compound called creatine phosphate (CP). When the myosin head breaks down ATP into ADP + P, the creatine phosphate is used to convert the ADP back into ATP. The enzyme that makes this happen is creatine phosphokinase (CPK) Cells in the body generate ATP through aerobic metabolism in mitochondria and glycolysis in the cytoplasm. Aerobic metabolism Glycolysis Peak exertion Muscle fatigue Recovery period Cori Cycle Oxygen Debt Muscle Performance Two major factors determine the performance capabilities of any skeletal system: the types of muscle fibers in the muscle and physical conditioning. A. Types of Skeletal Muscle Fibers The human body has three major types of skeletal muscle fibers: fast, slow, and intermediate. 1. Fast Fibers 2. Slow Fibers 3. Intermediate Fibers White muscle Red muscle Human muscle Muscle hypertrophy Muscle atrophy Cardiac Muscle Tissue (heart) Contain many organized myofibrils and the presence of many aligned sarcomeres which give the cells a striated appearance. Structural characteristics of Cardiac Muscle Cell Intercalated Discs Function- Functional Characteristics of Cardiac Muscle Tissue 1. Contract without neural stimulation (automaticity). The timing of contractions is normally determined by specialized cardiac muscle cells called pacemaker cells. 2. Innervation by the nervous system can alter the pace established by the pacemaker cells and adjust the amount of tension produced during a contraction. 3. Contractions last roughly 10 times longer than skeletal muscle fibers, have longer refractory periods, and do not readily fatigue. 4. Individual twitches cannot undergo wave summation and cannot produce titanic contractions. If it did, it would not be able to pump blood. Smooth Muscle Tissue Forms sheets, bundles, or sheaths around tissues in almost every organ Examples include: around blood vessels, arrector pili muscle, respiratory passageways to change resistance to airflow, etc Cells are relatively long and slender Centrally located nucleus Has NO _________________________________________ Possess myosin and actin The lack of sarcomeres gives makes them twist like corkscrews when they contract and gives them plasticity (ability to function over a wide range of lengths) Excitation-Contraction Coupling of Smooth Muscle On stimulation, a blast of calcium ions enters the cell from the extracellular fluid, and additional calcium ions are released by the sarcoplasmic reticulum. Once in the sarcoplasm, the calcium ions interact with ___________________ (a calcium binding protein). The calmodulin activates the enzyme _________________________________________ which enables the attachment of myosin heads to actin.