Unit 3 Chapter 9 Muscle Overview The three types of muscle tissue are skeletal cardiac and smooth These types differ in structure location function and means of activation Muscle Similarities Skeletal and smooth muscle cells are elongated and are called muscle fibers Muscle contraction depends on two kinds of myofilaments actin and myosin Muscle terminology is similar Sarcolemma muscle plasma membrane Sarcoplasm cytoplasm of a muscle cell Prefixes myo mys and sarco all refer to muscle Skeletal Muscle Tissues Packaged in skeletal muscles that attach to and cover the bony skeleton Has obvious stripes called striations Is controlled voluntarily i e by conscious control Contracts rapidly but tires easily Is responsible for overall body motility Is extremely adaptable and can exert forces over a range from a fraction of an ounce to over 70 pounds Cardiac Muscle Tissue Occurs only in the heart Is striated like skeletal muscle but is not voluntary Contracts at a fairly steady rate set by the heart s pacemaker Neural controls allow the heart to respond to changes in bodily needs Smooth Muscle Tissue Found in the walls of hollow visceral organs such as the stomach urinary bladder and respiratory passages Forces food and other substances through internal body channels It is not striated and is involuntary Functional Characteristics of Muscles Excitability or irritability the ability to receive and respond to stimuli Contractility the ability to shorten forcibly Extensibility the ability to be stretched or extended Elasticity the ability to recoil and resume the original resting length Muscle Function Skeletal muscles are responsible for all locomotion Cardiac muscle is responsible for coursing the blood through the body Smooth muscle helps maintain blood pressure and squeezes or propels substances i e food feces through organs Muscles also maintain posture stabilize joints and generate heat Skeletal Muscle Nerve and Blood Supply Each muscle is served by one nerve an artery and one or more veins Each skeletal muscle fiber is supplied with a nerve ending that controls Contracting fibers require continuous delivery of oxygen and nutrients via contraction arteries Wastes must be removed via veins Skeletal Muscle Each muscle is a discrete organ composed of muscle tissue blood vessels nerve fibers and connective tissue The three connective tissue wrappings are Epimysium an overcoat of dense regular CT that surrounds the entire Perimysium fibrous CT that surrounds groups of muscle fibers Endomysium fine sheath of CT composed of reticular fibers surrounding muscle called fascicles each muscle fiber Skeletal Muscle Attachments Muscles span joints and are attached to bone in at least two places When muscles contract the movable bone the muscle s insertion moves toward the immovable bone the muscle s origin Muscles attach aponeurosis sarcolemma Directly epimysium of the muscle is fused to the periosteum of a bone Indirectly CT wrappings extend beyond the muscle as a tendon or Microscopic Anatomy of a Skeletal Muscle Fiber Each fiber is a long cylindrical cell with multiple nuclei just beneath the Fibers are 10 to 100 m in diameter and up to hundreds of centimeters long Each cell is a syncytium produced by fusion of embryonic cells Sarcoplasm has numerous glycosomes and a unique oxygen binding protein called myoglobin tubules Fibers contain the usual organelles myofibrils sarcoplasmic reticulum and T Myofibrils Myofibrils are densely packed rodlike contractile elements They make up most of the muscle volume The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident Sarcomeres The smallest contractile unit of a muscle The region of a myofibril between two successive Z discs Composed of myofilaments made up of contractile proteins Myofilaments are of two types thick and thin Myofilaments Banding Pattern Thick filaments extend the entire length of an A band Thin filaments extend across the I band and partway into the A band Z disc coin shaped sheet of proteins connectins that anchors the thin filaments and connects myofibrils to one another Thin filaments do not overlap thick filaments in the lighter H zone M lines appear darker due to the presence of the protein desmin Ultrastructure of Myofilaments Thick Filaments Thick filaments are composed of the protein myosin Each myosin molecule has a rodlike tail and two globular heads Tails two interwoven heavy polypeptide chains Heads two smaller light polypeptide chains called cross bridges Ultrastructure of Myofilaments Thin Filaments Thin filaments are chiefly composed of the protein actin Each actin molecule is a helical polymer of globular subunits called G actin The subunits contain the active sites to which myosin heads attach during contraction Tropomyosin and troponin are regulatory subunits bound to actin Arrangement of the Filaments in a Sarcomere Longitudinal section within one sarcomere Sarcoplasmic Reticulum SR SR is an elaborate smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril Paired terminal cisternae form perpendicular cross channels Functions in the regulation of intracellular calcium levels Elongated tubes called T tubules penetrate into the cell s interior at each A band I band junction T tubules associate with the paired terminal cisternae to form triads T Tubules T tubules are continuous with the sarcolemma They conduct impulses to the deepest regions of the muscle These impulses signal for the release of Ca2 from adjacent terminal cisternae Contraction of Skeletal Muscle Fibers Contraction refers to the activation of myosin s cross bridges force Shortening occurs when the tension generated by the cross bridge exceeds generating sites forces opposing shortening Contraction ends when cross bridges become inactive the tension generated declines and relaxation is induced Sliding Filament Model of Contraction Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree In the relaxed state thin and thick filaments overlap only slightly Upon stimulation myosin heads bind to actin and sliding begins Each myosin head binds and detaches several times during contraction acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere As this event occurs throughout the sarcomeres the muscle shortens Regulation of Contraction In order to