MUSCLES AND MUSCLE TISSUE SKELETAL MUSCLE TISSUE CARDIAC MUSCLE TISSUE SMOOTH MUSCLE TISSUE attach to and cover bones voluntary striations only in the heart striations intercalated discs involuntary walls of hollow visceral organs no striations involuntary Excitability responsive to stimulus Extensibility stretch Contractility shorten when stimulated Elasticity recoil and rest Muscle characteristics Muscle Functions movement locomotion and in body stabilize joints protect organs constricts eye pupils Posture and position counteract gravity generate heat heat when they contract form valves for passages form arrector pili muscles attached to hair follicles SKELETAL MUSCLE In general one nerve one artery and one or more veins go to each muscle Rich blood supply here contracting fibers need energy oxygen and nutrients from blood GROSS ANATOMY Epimysium overcoat of dense irregular connective tissue that surrounds muscle Fascicles sticks of muscle fibers grouped together bundles Perimysium within skeletal muscle perimysium surrounds each fascicle is a layer of fibrous connective tissue around the fascicles Endomysium wispy sheath of connective tissue that surrounds each individual muscle fiber made of fine areolar connective tissue Insertion the movable bone where muscle is attached ex Thigh muscles knees Origin the immovable bone where muscle is attached usually proximal to insertion ex Thigh muscles hips Direct fleshy attachement epimysium of muscle is fused to periosteum of bone or perichondrium of cartilage Indirect attachment muscle s connective tissues extend beyond the muscle as a tendon or as an aponeurosis anchored this way more common attachment also conserves space MICROSCOPIC ANATOMY Muscle fiber a muscle cell long and cylindrical between 10 and 100 micrometers in diameter and up to 30 cm long Sarcolemma plasma membrane of each muscle fiber Sarcoplasm the cytoplasm of a muscle cell contains many glycosomes which are stored glycogen that provide glucose during muscle activity also contain lots of myoglobin which is a red pigment that stores oxygen Myofibrils single muscle fiber contains thousands of these myofibrils that run parallel are densely packed in fiber and make up 80 of cell s volume Sarcomeres contractile elements of skeletal muscle extends from one z disc to the next Striations dark and light bands along each myofibril A band dark band I band light band H zone midsection of the A band M line line down the middle of the H zone formed by protein molecules Z discs indicate the edges of the sarcomere zigzagged overlap part contained in I band anchors the thin filaments Myofilaments the bands mentioned above that compose each myofibril M lines and Z discs hold myofilaments in alignment Thick filaments contain myosin and extend entire length of the A band in a sarcomere o Myosin two heavy and four light polypeptide chains a rodlike tail attached to flexible hinge of two globular heads bridges o Globular heads link thin and thick filaments together during contraction and form cross o Each filament has about 300 myosin molecules o Elastic filament is composed of giant protein called titin which runs from Z disc to thick filament holds thick filaments in place Thin filaments contain actin and extend across the I band and partway into the A band in sarcomere o Composed of protein actin which has polypeptide units called globular actin with actives sites where myosin heads from thick ends attach during contraction o G actin subunits become long actin filaments called filamentous actin o Contain regulatory proteins Tropomyasin rod shaped protein that spirals around the actin core and can block myosin binding sites on actin Troponin can bind to actin to inhibit it or bind to tropomyasin and help position it on actin third proponin component binds to calcium ions Dystrophin links thin filaments to sarcolemma Sarcoplasmic reticulum an organelle elaborate smooth reticulum that has interconnecting tubes that surround each myfobril run longitudinally along myofibril except for some Terminal cisterns the SR tubules that are larger and form perpendicular cross channels in pairs regulate calcium levels for contraction T Tubules an elongated tube at each A I band junction increase muscle s surface area help conduct impulses into sarcomere also Triad the combination grouping of a terminal cistern a T tubule and another terminal cistern signals for contraction are passed through here T tubule is voltage sensor and SR forms gated channels through which terminal cisterns release Ca2 SLIDING FILAMENT MODEL OF CONTRACTION the thin filaments slide past the thick ones so that actin and myosin overlap to a greater degree shortening means myosin s crossbridges generate enough tension on the thin filaments to overcome forces 2 1 nervous system stimulates muscle fibers must be activated by an action potential myosin heads on thick filaments latch onto myosin binding sites on actin of thin filaments sliding begins cross bridge attachments form and break several times propels thin filaments towards center of the sarcomere 3 occurs in sarcomeres all over the cell so muscle cell shortens 4 Z discs are pulled toward the M line so I bands shorten and distance between Z discs shortens 1 NERVE STIMULUS AND NEUROMUSCULAR JUNCTION EVENTS fiber is activated by nerve ending change in membrane potential electrical current action potential generated in sarcolemma action potential goes along sarcolemma intercellular calcium ion levels rise and provide final trigger for contraction Somatic motor neurons ones that activate the skeletal muscle fibers reside in brain and spinal cord axons extend to muscle cells Axons divide as they enter muscles and each one has an ending with several short branches that form a neuromuscular junction aka an end plate with a single muscle fiber Each muscle fiber has only one neuromuscular junction around midway along its length Synaptic cleft the space between the axon terminal and the muscle fiber filled with synaptic fluid that contains glycoproteins and collagen Synaptic vesicles small membranous sacs containing neurotransmitter acetylcholine Ach Junctional folds folds in the trough of sarcolemma that helps make neuromuscular junction hold Ach receptors Acetylcholinesterase after Ach is bound to receptors this enzyme terminates the neurotransmitter by breaking it down into acetic acid and choline so muscle fiber contraction can be stopped nerve impulse reaches end of axon axon