Muscle system

• The muscular system consists of all the muscles of the body. These make approximately 42 percent of total body weight and are composed of long slender cells known as fibers. The fibers are different lengths and vary in color from white to deep red. 
• Muscle cell= muscle fiber= myocyte 
• Sarcolemma is the muscle fiber plasma membrane surrounding a muscle cell. 
• Modiefiedplasm is the muscle fiber cytoplasm.
• Muscles contain glycosides for glycogen storage and contains myoglobin for O2 storage
• Modiefied organelles include myofibrils, sarcoplasmic reticulum, and T tubules. 
• The main functions of muscle tissue are motion, heat production, posture and body support, breathing, speaking, protection of vital organs, regulating elimination of materials and regulating distribution of materials. 
• The  characteristics of skeletal muscle are excitability, conductivity, which happens after excitability, contractility, extensibility, and elasticity. 
• Excitability is the responsiveness of the muscle.
• Contractility is the ability to turn chemical energy (ATP) into mechanical energy.
• Extensibility is how the muscle can be stretched
• Elasticity is the ability for the muscle to recoil to its resting length. 
• Skeletal muscle is attached to and covers the bony skeleton. They are the longest muscle cells and are voluntary. They have the ability to contract rapidly but tire easily an are adaptable with forces ranging from a fraction of an ounce to well over 70 pounds. Microscopically these cels have striations and are multinucleate.
• The functions of skeletal muscle include: 

  Body movement: moves bones, makes facial expressions, speak, breathe, and swallow 

  Maintenances of posture: stabilize joints and postural muscles 

  Stabilizes joints 

  Heat production: help maintain body temperature 

  Skeletal muscle does not pump blood. 
• Cardiac muscle is only found in the heart and is an involuntary muscle. The function of this muscle is to pump blood throughout the body and it is under the control  of neural and hormonal regions. Microscopically the muscle is striated and contains gap junctions. 
• Smooth muscle is found in the walls of hollow visceral organs such as heart valves. The function of this muscle  is involuntary and forces food and other substances through internal body channels. Microscopically this muscle is not striated. 
• Each muscle is a discrete organ that contains muscle tissue, blood vessels, nerve fibers, connective tissue, and requires nerve stimulation.
• Each skeletal muscle is covered in connective tissue this supports cells and reinforces the whole muscle.  The sheaths in order from internal to external are endomysium, which os a fine areolar connective tissue surrounding each muscle fiber, the perimysium, which is a fibrous connective tissue surrounding fascicles (groups of muscle fibers), and the epimysium, which is dense irregular connective tissue surrounding entire muscle. 
• Myofibril —> muscle fiber (surrounded by endomysium) —> fascicle (surrounded by perimysium) —> muscle (covered externally by the epimysium) 
• Myofibrils are bundles of myofilaments enclosed in sarcoplasmic reticulum, there are hundreds to thousands of myofibrils per cell. Myofilaments are contractile proteins within myofibrils. 
• Thick filaments that consist of bundles of many myosin protein molecules. Myosin strands each have a head and tail. The myosin head binds actin to ATP. 
• Thin filaments that consist of actin protein molecules, along with troponin and tropomyosin. The actin has myosin binding sites where myosin heads attach and tropomyosin and troponin are regulatory proteins.  Tropomyosin physically blocks myosin binding and cross bridge formation. Troponin is a complex of 3 proteins, TnI, which binds to actin, TnT, which binds to tropomyosin, and TnC, which binds to Ca2+. 
• During muscle relaxation tropomyosin blocks the binding site. During muscle contraction the myosin head binds to actin. 
• Other proteins in the myofilaments are elastic filament which is composed of protein titin, which holds thick filaments in place, dystrophin, which links thin filaments to proteins of sarcolemma, nebula, myosin, and C proteins which bind filaments or sarcomeres together. 
• Insertion is attachment to moveable bone, examples include knee joints or elbow bones
• Origin is the attachment to immovable or less movable bone, examples include the shoulder bone.
• Each muscle in served by one nerve, an artery, and one or more veins.
• Contracting fibers require continuous delivery of oxygen and nutrients and wastes must be removed.
• Muscles attach to bone in at least two places. 
• Skeletal muscle actions 


  Extensor- increases the angle at a joint 

  Flexor- decreases the angle at a joint 

  Abductor- moves limb away from the midline of the body 

  Adductor- moves limb toward the midline of the bone 

  Depressor- moves insertion downward 

  Levator- moves insertion upward 

  Rotator- rotates bone along its axis 

  Sphincter- constricts an opening 
• Sarcomere is the smallest contractile unit of muscle fiber and consists of myofibril region between two z discs. The sarcomere is composed of myofilaments, thick (myosin) and thin (actin).
• I bands are light appearing regions that contain only thin filaments and are bisected by Z disc. These bands get smaller when muscle contracts. 
• A bands are dark appearing regions that contains thick filaments and overlapping thin filaments and contains H zone and M line. These bands make up the central region of sarcomere. 
• The H zone is the central portion of the A band and only thick filaments are present. This disappears with maximal contraction. 
• The M line is the middle of H zone and is a protein meshwork structure and attachment site for thick filaments.
• Dystrophin anchors some myofibrils to sarcolemma proteins and abnormalities of this protein cause muscular dystrophy. 
• sarcolemma 


  the plasma membrane, that has T-tubules (transverse tubules ) that extended deep into the cell. The sarcolemma and its T-tubules have voltage-gated ion channels that allow for conducticalsequestrincal signals. 
• Sarcoplasmic reticulum (storage site of calcium)

  internal membrane complex similar to smooth ER that contains terminal cisternae, which is where most of the calcium is stored, two cistenae with T- tubule in between = triad. The membrane also contains calcium pumps to import calcium that binds to calmodulin and calsequestrin, and calcium release channels (ryanodine receptors) that are triggered by action potential traveling down sarcolema and T-tubule; calcium is released into sarcoplasm. 
• Skeletal muscle contraction 


   the skeletal muscle must be stimulated by a nerve ending. It propagates AP along its sarcolemma and increases intracellular calcium levels. Excitation-Contraction coupling is the linking of an electrical signal to the contraction. 
• Motor neurons from spinal cord innervate numerous muscle fibers.