Exam 2: Structure of Skeletal Muscle cells Flashcards
Muscle Tissue
Musscle Tissue
Specialized for contraction
Three Types:
* Skeletal Muscle - Striated, volumtary
* Cardiac Muscle - Found in the heart, striated involuntary
* Smooth - lines hollow organs, nonstriated involuntary
Functions
Skeletal Muscle Functions
- Produce skeletal movement
- Maintain posture and bodt positions
- Support soft tissues
- Guard entrances and exits\
- Maintain body temp (cellular resp at a high rate ATP and Heat is produced)
- Nutrient Reserves (Glycogen and protein)
Gross Anatomy of Skeletal Muscles
Gross Anatomy of Skeletal Muscles
Attached to bone by tendon
* Origin - attached to bone that remains relativley stationary during movement
* Insertion - attached to the bone that moves
* Synergistic Muscles - muscles that work together
* Antagonistic Muscles - muscles that appose ech other (flexors and extensors)
* Exceptions (Circular sphincter muscles)
Orginization of connective tissue in skeletal muscles
Orginization of connective tissue in skeletal muscles
- Endomysium - covers individual muscle fibers (cells)
- Perimysium - sheaths bundles of muscle fibers (muscle Fasicles)
- Epimysium - Surrounds a muscle
- Endomysium and Perimysium contain blood vessels and nerves
Skeletal Muscle Cells
Skeletal Muscle Cells (Myo and Sacro) = muscles
- Muscle cell = muscle fiber
- Multinucleate , very long cell (each muscle cell is as long as the muscle)
- Formed during embryogenesis by end-to-end fusion of uni-necleate myoblasts (fusion of stem cells)
- Adult muscle repair is limited - new skeletal muscle cells come from stem cells called satellite cells
Structure of Skeletal Muscle cell
Structure of Skeletal Muscle Cell (Muscle Fiber)
Contains large quantities of protein filaments = myofilaments
* Actin - thin myofilament
* Myosin - thick myofilament
Myofibrils = bundles of myofilaments
Sarcoplasm = (muscle cell cytoplasm)
Sarcolemma (cell membrane) - sarcoplasmic reticulum (moddified ER)
Sarcolemma
Sarcolemma (muscle cell membrane)
Excitable membrane
* Conduct action potentials
Narrow Tubes of sarcolemma extended into cell at right angles to cell surface
* Transverse tubules (t-tubules)
* Conduct action potentials deep into cell
* Come in close contact with sarcoplasmic reticulunm
Sarcoplasmic Reticulum (SR)
Sarcoplasmic Reticulum (SR)
- Simmilar to smooth ER
- Forms a tubular network around each myofibril
- Terminal cisternae form triads with T Tubules
- Stores high concentrations of Ca2+ ions needed for muscle contrations
Sarcoplasmic Reticulum (SR)
Sarcoplasmic Reticulum (SR)
- Simmilar to smooth ER
- Forms a tubular network around each myofibril
- Terminal cisternae form triads with T Tubules
- Stores high concentrations of Ca2+ ions needed for muscle contrations
Arrangement of Myofilaments in a muscle fiber (muscle cell)
Arrangement of Myofilaments in a muscle fiber (muscle cell)
Myofilaments
* Thick (myosin)
* Thin (actin)
Myofibril - bundels of myofilaments
* anchored to inner surface of sarcolemma at either end of cell
Sarcomeres - repeting unints of myofiliments in myofibril
* Can activly shorten
* Myofibrils create sarcomers
Arrangement of Myofilaments in a muscle fiber (muscle cell)
Arrangement of Myofilaments in a muscle fiber (muscle cell)
Myofilaments
* Thick (myosin)
* Thin (actin)
Myofibril - bundels of myofilaments
* anchored to inner surface of sarcolemma at either end of cell
Sarcomeres - repeting unints of myofiliments in myofibril
* Can activly shorten
* Myofibrils create sarcomers
Striated Sarcomeres
Striated Sarcomeres
Differences in distrobution of thick anf thin myofilaments gives banded apperance
* I bands - LIght band - contains only thin filaments
* A bands - dArk band - contain thick filaments, and some overlap with thin filaments
* H band - only thick filaments
* Z disk (line) - border between sarcomeres
Sarcomere Structure and Function
Sarcomere Structure and Function
- Myofibril in muscle cell consists of thousands of sarcomeres end to end
- Interactions between thin filaments and thick filaments are responsible for muscle contration
- Thin filaments slide over thic filaments, shortening the sarcomere
- Shorterning occurs in every sarcomere in the myofibril, thus shortens the myofibril
Sliding Filament Model of Muscle Contraction
Sliding Filament Model of Muscle Contraction
- Thin actin filaments - attached to z disk
- As thin filaments move toward ceneter of sarcomere:
- Thin Filaments slide over thick filaments
- Z lines are pulled closer together
- I band and H band narrow
- A band stays the same
- Sarcomere is at maximum shortening when it is the width of the A band, no I band or H band are visible
What causes thin and thick myofilaments to slide across eachother?
What causes thin and thick myofilaments to slide across eachother?
- Myosin filaments have many short projections extending out of the filament
- These projections can bind to site an actin filaments, forming cross bridges
- Cross bridges, once formed, change shape pulling the actin past the myosin
- Cross bridges use energy of ATP to change shape and pull the actin - convert chemical energy to mechanical energy
Molecular Anatomy of thick (myosin) myofilaments
Molecular Anatomy of thick (myosin) myofilaments
Composed of many (-100) identical myosin molecules bundeld side by side, in stagerd bipolar arrays
* Myosin molecules have elongate tails, globular head - golf club shape
* Arrayed with half facing each end, center is just tails
* Heads form cross bridges during contraction
* Intersections between myosin head and actin provented by tropomyosin during rest
Molecular Anatomy of thin (actin) myofilaments
Molecular Anatomy of thin (actin) myofilaments
Composed of multiple actin molecule
* Twisted strand composed of two rows of individual globular actin molecules
* Each actin molecule in twisted strand has active to which a myosin head can bind
* Strands of Tropomyosin cover the actin active site during rest
* Tropomyosin strands attached to actin by Troponin
Silding Filament Theory
Silding Filament Theory
Explains the relationship between thick and thin filaments
Cyclic process beginning with calcium release from SR
* Calcium binds to Troponin
* Troponin moves, moving tropomyosin and exposing actin active site
* Myosin head forms cross bridge to actin, bends towards center of sarcomere, pulling the actin
* ATP allows release of cross bridge
Role of ATP in moleculare mechanism of Contraction
Role of ATP in moleculare mechanism of Contraction
ATP supplies the energy for the movement of the myosin head
* Converting chemical energy to mechanical energy of movement
myosin head in energized position binds to actin active site
* Releases ADP and P
* Pivots, pulling on actin and moving to un-energized state
Atp binds to un-energized myosin head
* Detaching mysoin from actin
* ATP is split and head is energized
Role of Calcium Ions
Role of Calcium Ions
Concentration of CA2+ around sarcomere controls sarcomere contraction
* ca2+ is low around sarcomere at rest
* Action potential in sarcolema and T tubules results in contraction
* Causes coltage gateded Ca2+ chanels of SR to open
* Releaeses Ca2+ into sarcoplasm around sarcomere
* Ca2+ binds to troponin - causes troponin to change shape and pull tropomyosin off actin active sites
* Myosin heads bind to avaliable actin sites over and over until Ca2+ level falls
* When Ca2+ levels fall - tropomyosin covers actin active site, ending contraction
Control of skeletal muscle activity occurs at the neuromuscular Junction
Control of skeletal muscle activity occurs at the neuromuscular Junction
- Motor Neuron - nerve cell that controls muscle contraction
- Neuromusclar Junction - synapse between motor neuron and muscle cell
- Action potential initiated in motor neuron in response to central nervous system commands
- Travels theough motor neuron and arraives at synaptic terminal
Control of skeletal muscle activity occurs at the neuromuscular Junction
Control of skeletal muscle activity occurs at the neuromuscular Junction
- AP in motor neuron causes the neurotransmiter Acetylcholine (ACH) to be released from motor neuron terminal
- ACh diffuses across synaptic gap
- ACh binds to receptors on chemicaly-gated sodium chanelsin muscle membrane
- Sodium flows into muscle cell
- Depolarizes the muscle cell membrane and starts an action potential in the muscle cell
Control of skeletal muscle activity occurs at the neuromuscular Junction
Control of skeletal muscle activity occurs at the neuromuscular Junction
chemically regulated gates stay open as long as ACh is present
* Acetylcholine Esterase (AChE) - enzymne
* Located in the synpatic gap
* Rapidly Breaks down acetylcholine
Excitation of Muscle cell
* Action potential is initiated which spreads across the entire muscle cell membrane including the T tubuels
Excitation/Contraction coupling
Excitation/Contraction coupling
Action potentials along T-tubule causes release of calcium from cisternae of SR
Initiates Contraction cycle
* Ca2+ binds to troponin, moving tropomysoin
* Attachment of myosin head to actin
* Pivot of mysoin head pulls on actin
* Detachment of myosin head with binding of ATP
Cycle repeats over and over until calcium ion concentration falls to resting level
How does calcium ion concentrtion return to resting level?
How does calcium ion concentrtion return to resting level?
AP depolirization ends, voltage gated Ca2+ channels in SR close
* Calcium ion diffusion into sarcoplasm stops
Ca2+ is activly transported out of sarcoplasm
* Across sarcolemma to outside of cell (some)
* Across sarcoplasmic reticulum membrane into SR (Most)
* Requires ATP for active transport protein to function
Duration of Contraction Depends on:
Duration of Contraction Depends on:
- Duration of stimulation at nerve-muscle synapse (nueromusclar junction)
* Multiple action potentials in motor neuron cause continued release of ACh and multiple AP in muscle fiber - Presence of calcium ions in sarcoplasm
* Contraction cycle continues until calcium concentration returns to resting level - Availability of ATP
* If no ATP is avalible, contraction cycle stops even if action potential and calcium ions are present
* Needed to power myosin head
Duration of Contraction Depends on:
Duration of Contraction Depends on:
- Duration of stimulation at nerve-muscle synapse (nueromusclar junction)
* Multiple action potentials in motor neuron cause continued release of ACh and multiple AP in muscle fiber - Presence of calcium ions in sarcoplasm
* Contraction cycle continues until calcium concentration returns to resting level - Availability of ATP
* If no ATP is avalible, contraction cycle stops even if action potential and calcium ions are present
* Needed to power myosin head
Contraction ends and relaxation occurs when:
Contraction ends and relaxation occurs when:
- Action potential stops in motor neuron
- Acettylcholine esterase breaks down the ACh in the neuromuscular synaptic gap
- ACh gated chanels close (sodium ion influx stops)
- Action potentials stop occuring in sarcolemma and T-tubules
Contraction ends and relaxation occurs when:
Contraction ends and relaxation occurs when:
calcium ion levels in sarcoplasm return to resting levels
* Tropomyosin covers actin sites and now no new myosin cross bridges can form
Relaxation requires ATP
* ATP needed to pump Ca2+ into the SR, pump Na+/K+
* ATP needed to disconnect myosin heads from actin
* Rigot morits - lack of ATP after death
Muscular System Disorders
Muscular System Disorders
Nevrous system disorders that affecr the coordination or control of muscle contraction
* Blockage of release of ACH (EX. Botulism)
* Interference with binding of ACh to receptors (EX. Myasthenia Gravis)
* Interference with ACh esterase activity (uncontrolled muscle contraction)
* Loss of motor Neuron (EX. polio)
* Loss of motor neuron axon (Peripheral nerve damage)
* Reduction of AP effecinecy (damage to myelin (MS))
* Excessive stimulation of motor neuron (EX. Tetanus)
