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Skeletal Muscular Systems Campbell, 5th ed, Chapter 49 Nancy G. Morris Volunteer State Community College Intro to Sensory Reception Action potentials that reach the brain via sensory neurons are called sensations. Interpretation of the sensation by the brain is perception. Perceptions (colors, smells, sounds, tastes) are constructed in the brain & do not exist outside it. An age old question… If a tree falls in the forest and no one is there to hear it, is there a sound? The fall produces pressure waves in the air, but if we define sound as perception, then there can be no sound unless sensory receptors detect the waves & an animal’s brain perceives them. Sensory reception Sensations, & their perceptions in the brain, begin with sensory reception, the detection of the stimulus by sensory cells. Sensory receptors: specialized neurons or epithelial cells existing singly or in groups Exteroreceptors – detect stimuli from outside the body (heat, light, pressure, etc.) Interoreceptors – detect stimuli within the body (blood pressure , body position) Skin Receptors that detect the sense of touch are called mechanoreceptors. It also contains: thermoreceptors, pain receptors, chemoreceptors (gustatory, olfactory) Figure 49.1 Sensory receptors in human skin Functions of the Integument 1) 2) 3) 4) 5) 6) Largest organ of the body Protection Waterproofing layer Temperature regulation Sensory response to stimuli Source of vitamin D (ultraviolet cholesterol) rays convert Horny layer – dead, filled with keratin, constantly sloughed off Continuous division at the basement membrane Hearing & equilibrium The mammalian hearing organ is within the inner ear The inner ear also contains organs of equilibrium A lateral line system & inner ear detect pressure waves in most fishes & aquatic amphibians Many invertebrates have gravity sensors & are sound-sensitive Ear and hearing … Movement & locomotion Movement is the hallmark of animals Locomotion is active movement from one place to another Locomotion Animals may swim, crawl, walk, run, hop, or fly In all forms, locomotion requires that an animal expend energy to overcome two forces that tend to keep it stationary: friction and gravity. Skeletons 1) Protection (skull, ribs cage, etc.) 2) Support 3) Movement (lever systems) In vertebrates: 4) Responsible for blood cell production 5) Store minerals Three main types of skeletons: Hydrostatic skeletons Exoskeletons earthworms arthropods Endoskeletons vertebrates Endoskeletons: Consist of hard supporting elements, such as bones, buried within the soft tissues of the animal Sponges – spicules Echinoderms – hard dermal plates beneath the skin and ossicles Figure 49.23 Peristaltic locomotion in the earthworm Figure 49.25 Exoskeleton of an arthropod Endoskeletons: Found only in Chordates Composed of cartilage, bone, or combination Mammalian skeleton has approximately 200 major bones Major Divisions of Human Skeleton Axial Skeleton • Cranium, hyoid, vertebral column, sternum and ribs Appendicular Skeleton Pectoral girdle & bones of upper appendages • Clavicle, scapula, humerus, ulna, radius, phalanges, metacarpals, carpals Pelvic girdle & bones of lower appendages • Pubis, ilium, ischium, femur, patella, tibia, fibula, tarsals, metatarsals, phalanges Figure 49.24 The human skeleton A cast of your skull, sir, until the original is available, would " be an ornament to any anthropological museum. It is not my intention to be fulsome, but I confess that I covet your skull. " The Hound of the Baskervilles, Chapter 1, Sir Arthur Conan Doyle Major Joints of Human Skeleton Ball-and–socket joint Rotation • Shoulder & hip joints Hinge joint Restrict movement to a single plane • Knee & elbow Pivot joint Rotation • Ulna, radius & tibia, fibula Muscles Move skeletal parts by contracting Action of the muscle is always to contract. (Muscles only pull -NEVER push.) Arranged in antagonistic pairs with each muscle working against the other Figure 49.25 Cooperation of muscles & skeletons in movement Structure & Function of Vertebrate Skeletal Muscle skeletal muscle characterized by smaller and smaller parallel units bundles of long fibers running the length of the muscle each fiber is a multinucleated single cell each fiber is a bundle of smaller myofibrils each myofibril is composed of two myofilaments: Actin (thin) & Myosin (thick) Figure 49.26 The structure of skeletal muscle Skeletal muscle striated (repetition of light & dark bands) each repeating unit is a sarcomere, the functional unit of muscle contraction borders of sarcomeres, Z lines, are lined up in adjacent myofibrils thin actin filaments attach to the Z lines & project toward the center thick myosin filaments are centered in the sarcomere and “stitched together” at the M line Actin & Myosin filaments ACTIN Thin filaments Composed of many globular actin molecules (beads) assembled in a long chain (necklace) Two protein chains are wound around one another to produce a single actin filament Contain troponin & tropomyosin proteins which in the presence of Ca2+ “uncover” binding sites on actin MYOSIN Thick filaments Longest known protein chain: 1,800 amino acids 200 or more parallel protein molecules with free globular “heads” Myosin heads: 1) binding sites for contraction and 2) contain enzymes that split ATP to power the contraction Ultra Structure of the Sarcomere M line – connection between the thick myosin filaments H zone – (from Latin ‘hell’ meaning bright or clear) the central zone in the relaxed sarcomere containing only myosin filaments I band – zone around the Z line that contains only actin filaments A band – marks the extent of the myosin filaments in the sarcomere Z line – the dark stripe in the center of the I band (bulkhead) Skeletal Muscle Sliding Filament Theory 1 Contraction involves the sliding of thin actin filaments between thick myosin filaments. Innervation by the motor neuron stimulates the muscle fiber. The neurotransmitter, acetylcholine, acts as the chemical mediator diffusing across the membrane. Acetylcholine generates electrical depolarization (by pumping Ca2+ out) in the sarcoplasmic reticulum of the entire muscle. Ca2+ binds to troponin of the thin actin filaments causing tropomyosin to “uncover” the binding sites. Figure 49.30 Roles of sarcoplasmic reticulum & T tubules in contraction Figure 49.29 The control of muscle contraction Role of calcium in contraction Sliding Filament Theory 2 Myosin’s globular heads, acting like hooks, attach to the uncovered binding sites on actin. The result is a temporary cross-bridge. These cross-bridges form, break, & reform rapidly as one filament slides (or is pulled) past another. Myosin heads contain enzymes that release the energy in ATP (ADP + Pi ) to power contraction. It is the chemical combination with the next ATP that releases the myosin head from the actin binding site breaking the temporary cross-bridge. Rigor mortis results when the cross-bridges are “locked” in place because no more ATP is available to release myosin from its binding site. Sliding Filament Theory 3 Because the cross-bridges are forming, breaking, & reforming, the actin filaments are pulled toward the center of the H zone causing contraction of the sarcomere. The filaments themselves do not change length. In response to the stimulus to contract, the filaments slide past one another and increase the amount by which they overlap, thereby shortening (contracting) the sarcomere. Figure 49.28 Interaction of actin & myosin in muscle contraction Figure 49.27 The slidingfilament model of muscle contraction What zones & bands are missing in the contracted sarcomere? Skeletal Muscle Motor Units in Vertebrates Each muscle fiber (cell) has a single neuromuscular junction, or synaptic connection, with the motor neuron that controls it. Each motor neuron branches & controls several muscle fibers. A motor neuron & all the fibers it controls constitute a motor unit. Figure 48.32 Motor units in vertebrate muscle A motor unit … Figure 49.31 Temporal summation of muscle cell contractions A muscle twitch results from a single stimulus. More rapidly delivered signals produce a graded contraction obtained by summation. Tetanus is a state of smooth & sustained contraction obtained when motor neurons deliver a volley of action potentials Smooth Muscle Tissue Review Found throughout the body particularly lining vessels & hollow organs; responsible for peristalsis Single nucleated cell with tapered ends Non-striated because actin & myosin filaments are not regularly arranged Contracts slowly, but greater range than striated Cardiac Muscle Tissue Review Found only in the heart Multinucleated Striated, branching cells electrically connected by intercalated discs (specialized gap junctions that couple cells electrically). An actin potential generated in one part of the heart will spread to all the cardiac muscle cells, & the whole heart will contract. Generates action potentials without neural input Plasma membrane has pacemaker properties