Organization of Skeletal Muscle Cell
•Beneath the connective tissue of the endomysium is the plasma membrane (sarcolemma) of an individual muscle cell
•The cytoplasm (sarcoplasm) of a skeletal muscle fiber is chocked full of contractile proteins arranged in contractile bands called myofibrils
–These are the sites that physically shorten in order to produce muscle tension |
Other Important Sarcomere Proteins
•Elastic filaments
–Composed of protein titin
–Holds thick filaments in place; helps filaments recoil after stretch
•Also resists excessive stretching
•Dystrophin
–Links thin filaments to proteins of sarcolemma
•Nebulin, myomesin, C proteins bind filaments or sarcomeres together
–Important in maintenance of alignment |
Other Important Sarcomere Proteins
•Elastic filaments
–Composed of protein titin
–Holds thick filaments in place; helps filaments recoil after stretch
•Also resists excessive stretching
•Dystrophin
–Links thin filaments to proteins of sarcolemma
•Nebulin, myomesin, C proteins bind filaments or sarcomeres together
–Important in maintenance of alignment |
Other Important Sarcomere Proteins
•Elastic filaments
–Composed of protein titin
–Holds thick filaments in place; helps filaments recoil after stretch
•Also resists excessive stretching
•Dystrophin
–Links thin filaments to proteins of sarcolemma
•Nebulin, myomesin, C proteins bind filaments or sarcomeres together
–Important in maintenance of alignment |
Other Important Sarcomere Proteins
•Elastic filaments
–Composed of protein titin
–Holds thick filaments in place; helps filaments recoil after stretch
•Also resists excessive stretching
•Dystrophin
–Links thin filaments to proteins of sarcolemma
•Nebulin, myomesin, C proteins bind filaments or sarcomeres together
–Important in maintenance of alignment |
Myofibril Banding Pattern
•Orderly arrangement of actin and myosin myofilaments within sarcomere
–Actin myofilaments = thin filaments
•Extend across I band and partway in A band
•Anchored to Z discs
–Myosin myofilaments = thick filaments
•Extend length of A band
•Connected at M line |
Muscle Fiber Structures
•Myofibril
–Densely packed, rod-like elements
– ~80% of cell volume
–Contain sarcomeres
- contractile units
•Sarcomeres contain myofilaments (contractile proteins of muscle)
–Exhibit striations
- perfectly aligned repeating series of dark A bands and light I bands
•Transverse (T)-Tubules
–Tunnels of sarcolemma that run from the surface of the muscle cell to the inner regions
–Open to the outside of the fiber and are filled with interstitial fluid
–Muscle AP’s travel along sarcolemma and down into the T-tubules
•Allows for quick spreading of AP throughout the muscle fiber and almost equal instantaneous excitation |
Functions of Muscular Tissue
•Muscles makes up a large percentage of the body’s weight
–Nearly half
•Their main functions are to:
–Create motion
•muscles work with nerves, bones, and joints to produce body movements
–Stabilize body positions and maintain posture
•Sustained contractions of your neck muscles keep your head upright while you are paying attention in lecture!!!
–Store substances within organs using sphincters
•Sphincters in your bladder keep you from micturating all over yourself
–Move substances throughout the body by peristaltic contractions
•Moving food down your esophagus or through the intestines.
–Generate heat through thermogenesis
•Shivering is involuntary contractions of skeletal muscle to increase the rate of heat production |
Organization of Skeletal Muscle Tissue
•In groups of muscles, the epimysium continues to become thicker forming a fascia which covers many muscles
–This graphic shows the fascia lata enveloping the entire group of quadriceps and hamstring muscles |
Organization of Skeletal Muscle Tissue
•Each muscle served by one artery, one nerve, and one or more veins
–Enter/exit near central part and branch through connective tissue sheaths
–Every skeletal muscle fiber supplied by neuron ending that controls its activity
–High metabolic rate when contracting
•Uses large amounts of ATP
•Huge nutrient and oxygen need
•Generates large amount of waste |
Types of Muscle
•Myo, mys, and sarco
- prefixes for muscle
•Three main types of muscle in the human body
–Skeletal
–Cardiac
–Smooth |
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Organization of Skeletal Muscle Tissue
•Many large muscle groups are encased in both a superficial and deep fascia |
Organization of Skeletal Muscle Tissue
•The epimysium, perimysium, and endomysium all are continuous with the connective tissues that form tendons, ligaments, and muscle fascia (connect muscles to other muscles to form groups of muscles)
–Connective tissue sheaths of skeletal muscle
•Support cells; reinforce whole muscle
•External to internal
–Epimysium: dense irregular connective tissue surrounding entire muscle; may blend with fascia
–Perimysium: fibrous connective tissue surrounding fascicles (groups of 10-100 muscle fibers)
»Fascicles form the “grain” in meat
–Endomysium: fine areolar connective tissue surrounding each individual muscle fiber |
Organization of Skeletal Muscle Cell
•You will need to learn the names of the internal structures of the muscle fiber
–Sarcolemma
–Sarcoplasm
–Myofibril
–T-tubules
–Triad
–Terminal cisterns
–Sarcoplasmic reticulum
–Sarcomere |
Muscle Fiber Structures
•Sarcoplasmic Reticulum
–Similar to the smooth edoplasmic reticulum of the typical cell
–Stores and releases calcium ions, amongst many other functions
•Terminal Cisternae
–Dilated end sacks of the sarcoplasmic reticulum that butt against the T-tubules
–Allow for quick release of Ca2+ from SR into sarcoplasm when stimulated
•Triad
–Formed from a T-tubule and two terminal cisterns |
Thin Filaments
•Twisted double strand of fibrous protein F actin
•F actin consists of G (globular) actin subunits
•G actin bears active sites for myosin head attachment during contraction
•Tropomyosin and troponin
- regulatory proteins bound to actin |
Types of Muscle
•Skeletal muscles
–Organs attached to bones and skin
–Elongated cells called muscle fibers
•Skeletal muscle fiber and skeletal muscle cell are the same thing
•Some are quite long
–The Sartorious muscle contains single fibers that are at least 30 cm long
–Striated (striped)
•Microscopic arrangement of contractile units give striated appearance
–Multinucleate
–Voluntary (i.e., conscious control)
–Require nervous system stimulation for contraction |
Properties of Muscular Tissue
•Like nervous tissue, muscles are excitable, or "irritable”
–they have the ability to respond to a stimulus
•Unlike nerves, however, muscles are also:
–Contractible
–Extensible
–Elastic |
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Organization of Skeletal Muscle Tissue
•Skeletal muscles attach in at least two places
–Insertion
– movable bone
–Origin
– immovable (less movable) bone
•Attachments can be direct or indirect
–Direct
—epimysium fused to periosteum of bone or perichondrium of cartilage
–Indirect
—connective tissue wrappings extend beyond muscle as rope like tendon or sheetlike aponeurosis |
Organization of Skeletal Muscle Tissue
•An aponeurosis is essentially a thick, flat fascia that connects two muscle bellies.
–The epicranial aponeurosis connects the muscle bellies of the occipitalis and the frontalis to form “one” muscle: The occipitofrontalis |
Skeletal Muscle Fiber Structures
•Sarcolemma
–The plasma membrane of the muscle cell
•Sarcoplasm
–The cytoplasm of the muscle cell
–Glycosomes for glycogen storage
•Contain a lot of glycogen
–Glucose polymer that can be hydrolyzed to provide glucose for ATP production when sarcoplasmic glucose levels fall during contraction
–Myoglobin
•Globular protein found only in muscle cells
•Binds oxygen that diffuses into the muscle cell from the interstitial fluid (fluid directly outside the cell)
–Similar to hemoglobin of red blood cells
•Releases oxygen when mitochondria need it to make ATP
•Function as quick oxygen reserve when sarcoplasmic O2 levels decline from high contractile rate leading to decrease in blood flow |
Muscle Fiber Structure
•Increasing the level of magnification, the myofibrils are seen to be composed of sarcomeres
–The smallest contractile unit (functional unit) of skeletal muscle fibers
•Align along myofibril like boxcars of a train
•Composed of thick and thin myofilaments made of contractile proteins
•Contains A band with ½ I band at each end•Z-discs form sarcomere boundary |
Types of Muscle
•Cardiac muscle
–Only in heart; bulk of heart walls
–Branched short cells
–Striated–Uni or binucleate
–Can contract without nervous system stimulation
–Involuntary (not under conscious control)
–More detail in Chapter 18
•Smooth muscle
–In walls of hollow organs, e.g., stomach, urinary bladder, and airways
–Non-striated
–Uninucleate
–Can contract with or without nervous system stimulation
–Involuntary |
Thick Filaments
•Composed of protein myosin
•Each composed of 2 heavy and 4 light polypeptide chains
–Myosin tails contain 2 interwoven, heavy polypeptide chains
–Myosinheads contain 2 smaller, light polypeptide chains per head that act as cross bridges during contraction
•Binding sites for G-actin of thin filaments
•Binding sites for ATP
•ATPase enzyme activity |
Properties of Muscular Tissue
•Electrical excitability
–Respond to certain stimuli by producing electrical signals called action potentials (APs)
•Contractility
–Muscle tissue contracts forcefully when stimulated by action potential
–Muscle contraction generates tension (force of contraction) while pulling on it’s attachment points
–ATP used to power contraction
•Extensibility
–Muscle tissue is able to stretch, to a certain point, without being damaged.
•Elasticity
–Muscle is able to return to it’s normal length after being stretched or shortened (contracted). |
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