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MUSCLES Muscular System Muscle Tissue: Histology

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MUSCLES Muscular System Muscle Tissue: Histology MUSCLES Muscular System Muscle Tissue: histology • 3 muscle types: skeletal muscle, cardiac muscle, and smooth muscle • All muscles show similarities and differences • All muscles composed of elongated cells called fibers • Muscle cytoplasm is sarcoplasm, and muscle cell membrane is sarcolemma • Muscle fibers contain myofibrils made of contractile proteins actin and myosin Skeletal Muscle Fibers are multinucleated cells with peripheral nuclei: multiple nuclei due to fusion of mesenchyme myoblasts during embryonic development Each muscle fiber is composed of myofibrils and myofilaments Skeletal Muscle Actin and myosin filaments form distinct cross-striation patterns Light I bands contain thin actin, and dark A bands contain thick myosin filaments Dense Z line bisects I bands; between Z lines is the contractile unit, the sarcomere Skeletal Muscle Accessory proteins align and stabilize actin and myosin filaments Titin protein anchors myosin filaments, and a-actinin binds actin filaments to Z lines Titin centers, positions, and acts like a spring between myosin and Z lines Skeletal Muscle Muscle is surrounded by connective tissue epimysium Muscle fascicles are surrounded by connective tissue perimysium Each muscle fiber is surrounded by connective tissue endomysium Skeletal Muscle Voluntary muscles are under conscious control Neuromuscular spindles are specialized stretch receptors in almost all skeletal muscles Intrafusal fibers and nerve endings are found in spindle capsules Stretching of muscle produces a stretch reflex and movement to shorten muscle Skeletal Muscle Skeletal Muscle Skeletal Muscle Skeletal Muscle Skeletal Muscle:Transmission Electron Microscopy ◊ Light bands = I bands, formed by thin actin filaments: are crossed by dense Z lines ◊ Between Z lines = smallest contractile unit = sarcomere ◊ Dark bands = A bands, located in the middle of sarcomere, formed by overlapping actin and myosin filaments ◊ M bands = in the middle of A bands represent linkage of myosin filaments ◊ H bands on each side of M bands contain only myosin filaments ◊ Sarcoplasmic reticulum and mitochondria surround each sarcomere Skeletal Muscle Skeletal Muscle Skeletal Muscle: functional correlation Skeletal muscles = voluntary (under conscious control), contracting only when stimulated Motor endplates = sites of nerve innervations and transmission of stimuli to muscle Axon terminals of motor endplates contain vesicles with the neurotransmitter acetylcholine (released into synaptic cleft by action potential and combined with its receptors on muscle membrane); acetylcholinesterase neutralizes acetylcholine and prevents further contraction Skeletal Muscle Skeletal Muscle: functional correlation Before arrival of impulse, Ca+ is stored in sarcoplasmic reticulum (SR) T tubules = sarcolemma invaginations into each myofiber, carrying stimulus for muscle contraction to every myofiber, myofibril, and SR membrane Triads = 2 expanded terminal cisternae of SR and T tubules, located at A–I junctions After stimulation, SR releases Ca+ into sarcomeres, activating binding of actin and myosin, causing muscle contraction and shortening After the end of stimulus, Ca+ is actively transported and stored in SR Cardiac Muscle • Located in heart and large vessels with cross-striations of actin and myosin forming similar I bands, A bands, and Z lines as in skeletal muscle • Characterized by dense junctional complexes called intercalated disks, containing gap junctions [coupling all fibers for rhythmic contraction, forming functional syncytium], one or two central nuclei, fibers shorter and showing branching • T tubules located at Z lines and larger + SR less well developed + mitochondria larger and more abundant than in skeletal muscles • For contraction, Ca+ is imported from outside cell and from SR • Exhibit autorhythmicity and spontaneously generate stimuli • Autonomic nervous system innervates heart and influences heart rate and blood pressure Cardiac Muscle Cardiac Muscle Cardiac Muscle Cardiac Muscle Cardiac Muscle Smooth Muscle • Fibers fusiform, containing single central nuclei, found in hollow organs and blood vessels, with actin and myosin filaments without cross-striation, not showing regular arrangement or striations, but forming lattice network, and inserting into dense bodies in sarcoplasm and cytoplasm • Zonula adherens binds muscle cells, whereas gap junctions provide functional coupling • In intestines, muscles are arranged in concentric layers, and in blood vessels in a circular pattern Smooth Muscle • SR not well developed for Ca+ storage and sarcolemma containing invaginations called caveolae (controlling influx of Ca+ into cell after stimulation) • Following stimulation, calcium enters sarcoplasm from caveolae and SR • Calmodulin, a calcium-binding protein, stimulates actin and myosin interaction, contract muscle by a sliding mechanism similar to skeletal muscle • Exhibit spontaneous activity and maintain tonus in hollow organs: peristaltic contractions propel contents in the organs; innervated by postganglionic neurons of sympathetic and parasympathetic divisions; involuntary muscles regulated by autonomic nervous system, hormones, and stretching Smooth Muscle Smooth Muscle Smooth Muscle Smooth Muscle Muscle tissues Muscular System system to name skeletal muscles: in some cases, the muscle is named by its shape, and in other cases it is named by its location or attachments to the skeleton. understanding meaning of the name of the muscle, often it will help you remember its location and/or what it does, to describe how skeletal muscles are arranged to accomplish movement, and how other muscles may assist, or be arranged on the skeleton to resist or carry out the opposite movement Interactions of Skeletal Muscles in the Body To move the skeleton, tension created by the contraction of the fibers in most skeletal muscles is transferred to the tendons = strong bands of dense, regular connective tissue connecting muscles to bones (bone connection muscle called skeletal muscle). Interactions of Skeletal Muscles in the Body To pull on a bone (=to change angle at synovial joint = moving the skeleton), a skeletal muscle must also be attached to a fixed part of the skeleton moveable end of the muscle that attaches to the bone being pulled is called the muscle’s insertion, and the end of the muscle attached to a fixed (stabilized) bone is called the origin. Interactions of Skeletal Muscles in the Body prime mover, or agonist = principal muscle involved in an action, although a number of muscles may be involved To lift a cup, a muscle called the biceps brachii is actually the prime mover; however, because it can be assisted by the brachialis, this is called a synergist in this action synergist can also be a fixator that stabilizes the bone that is the attachment for the prime mover’s origin. Interactions of Skeletal Muscles in the Body Antagonist = muscle with the opposite action of the prime mover play 2 important roles in muscle function: (1) maintain body or limb position, such as holding the arm out or standing erect (2) control rapid movement also be reversed for the opposing action Interactions of Skeletal Muscles in the Body Also skeletal muscles not pulling against skeleton for movements: • muscles that produce facial expressions. • skeletal muscles in the tongue, and the external urinary and anal sphincters that allow for voluntary regulation of urination and defecation, respectively. • diaphragm contracts and relaxes to change the volume of the pleural cavities but it does not move the skeleton to do this. Patterns of Fascicle Organization Skeletal muscle enclosed in connective tissue scaffolding at 3 levels: • each muscle fiber (cell) is covered by endomysium • entire muscle is covered by epimysium • when a group of muscle fibers is “bundled” as a unit within the whole muscle (= fascicle) by an additional covering of a connective tissue called perimysium Fascicle arrangement by perimysia is correlated to the force generated by a muscle; it also affects the range of motion of the muscle. Patterns of Fascicle Organization Based on the patterns of fascicle arrangement, skeletal muscles can be classified in several ways most common fascicle arrangements Parallel muscles = fascicles arranged in the same direction as the long axis of the muscle (majority of skeletal muscles): 1. some parallel muscles are flat sheets that expand at the ends to make broad attachments. 2. other parallel muscles are rotund with tendons at one or both ends. 3. muscles that seem to be plump have a large mass of tissue located in the middle of the muscle, between the insertion and the origin, which is known as the central body (= belly). When a parallel muscle has a central, large belly that is spindle-shaped, meaning it tapers as it extends to its origin and insertion, it sometimes is called fusiform. Patterns of Fascicle Organization Circular muscles = also called sphincters (= when they relax increase the size of the opening, and when they contract shrink to the point of closure) Convergent muscle = widespread expansion over a sizable area, but then the fascicles come to a single, common attachment point [that could be a tendon, an aponeurosis (=flat, broad tendon), or a raphe (= very slender tendon)]. Pennate muscles (= “feathers”) blend into a tendon that runs through the central region of the muscle for its whole length. Patterns of Fascicle Organization Due to this design, the muscle fibers in a pennate muscle can only pull at an angle,
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