W3 skeletal muscle Flashcards
• Skeletal muscle
o Elongated multi-nucleated cells grouped into bundles surrounded by connective tissue sheaths and extend from their proximal site of origin to their distal attachment
• Epimysium (E)
o Dense connective tissue that surrounds the entire muscle
o Also called deep facia
• Perimysium (P)
o Connective tissue that surrounds groups of fibers to form fascicle
o Conveys large blood vessels and nerves into muscles
• Endomysium (En)
o Delicate connective tissue that surrounds individual muscle fibers
• Type I muscle fibers
o Show up light on histology o Slow/red muscle fibers o More efficient over long periods of time o Postural maintenance and endurance o ATP via oxidative phosphorylation o High myoglobin levels o Low speed of contractions o Large motor units (many muscle fibers innervated by single neuron)
• Type II muscle fibers
o Shows up dark on histology o Slow/white o Short bursts of speed o Fatigue quickly o ATP via anaerobic glycolysis o High levels of glycogen o Speed of contraction is high o Smaller motor units (small number of muscle fibers innervated by single neuron)
• Sarcolemma
o Plasma membrane of muscle cells
o Contains numerous ion transport proteins and receptors
o Surrounded by basal lamina
o Invaginates finger-like projections to form transverse tubule system
• T tubules
o Formed by sarcolemma
o Make contact with sarcoplasmic reticulum and transmit depolarization signal deep in the muscle
• Sarcoplasmic reticulum (SR)
o Internal membrane network with no connection to extracellular space
o Makes junctional complexes with T tubules to form triads
o SR is site of intracellular Ca++ storage and release
o Important in regulation of contraction
• Myofibrils
o Composed of thin filaments (actin) and thick filaments (myosin)
o Occupies 80% of cytoplasm
o Surrounded by mitochondria
• Desmin
o Intermediate filament
o Anchored to inside of sarcolemma and extends from one myofibril to another
o Help organize cytoplasm and facilitate coordinated contraction
• Sarcomeres
o Z lines o I band o A band o H band o M line
• Z lines
o Define boundaries of sarcomere
o Contains actin binding protein: alpha actinin
• I band
o Composed of actin thin filaments
o Width decreases during contraction
• A band
o Composed of overlapping thick and thin filaments
o Width remains constant during contraction
• H band
o Central region of A band which contains only thick filaments
o Width decreases during contraction
• M line
o Middle of H band
o Overlapping tails of thick filaments
• Thin filaments
o G actin monomers
o Form double stranded and twisted F actin filament
o G actin monomers have binding sites for myosin, regularly spaced
o Forms a complex with tropomyosin and troponin complex (I, T, C)
o Tropomyosin runs in groove of F actin filaments and bins to troponin T (TnT)
o TnI inhibits binding of myosin
o TnC binds Ca++
o Nebulin molecule extends from Z line along the thin filament and stabilizes the thin filament
• Thick filaments
o Composed of myosin (two identical HCs and two LCs)
o Each HC has a globular head that binds to actin and ATPase domain
o HC also has alpha helical coiled tail to myosin can self-assemble into a bipolar aggregate or filament
o Globular head and tail are separated by moveable hinge region
o ATP hydrolysis causes movement at hinge joint and affects interaction with actin
o LC plays a role in regulating activity of myosin head and titin
• Titin
o thick filament that extends from Z line to center of sarcomere and controls assemble of thick filaments and provides elasticity to sarcomere
• skeletal muscle in contraction
o thin filaments slide toward center of sarcomere
o length of thick and thin filaments does not change
o length of sarcomere decreases because thick and thin slide past one another causing Z lines to move closer together
o width of H and I band decrease
o sliding and tension facilitated by conformational changes in myofibrillar proteins through ATP hydrolysis and release of Ca++
• steps in a single cycle
o ATP binds to myosin head causing dissociation of myosin from actin thin filament
o ATP is hydrolyzed by myosin head ATPase causing conformation change in myosin and movement at the hinge so angle between head and tail increases. This lines up the head with a new myosin binding site on the thin filament
o Myosin head attaches to thin filament
o Binding of myosin causes the release of Pi and a conformational change in the myosin and movement at the hinge such that the angel between the head and tail decreases. This causes a sliding of thin filament toward the center of the sarcomere.
o ADP released from myosin head and new cycle begins
• Calcium cycle during contraction
o Depolarization spreads from sarcolemma to the T tubules which transport the signal to interior of muscle cell
o Depolarization of T tubule is sensed by L Type Ca++ channels in the membrane of the T tubule that induces opening of ryanodine sensitive Ca++ channels in the membrane of SR
o Ca++ stored in SR is release into cytoplasm of the muscle fiber increasing resting level of Ca++ from 10-7 mM to 10-5 mM
o Ca++ binds to TnC causing conformational change in the whole troponin complex
o Change in troponin alters conformation of tropomyosin and leads to uncovering of the myosin-binding sites on actin allowing thick filament to bind to actin
o Ca++ dependent ATPase mediates return of Ca++ to SR during relaxation
• Neuron and muscle interactions
o Each fiber is innervated by axon of motor neuron via motor end plate (neuromuscular junction)
o Contraction begins with Ach release from synaptic vesicles
o Ach binds to receptors on sarcolemma and causes depolarization
o Action potential spreads to the T tubules which transmit signal to interior muscle cell and cause Ca++ release
o Acetylcholinesterase is released from basal lamina of synaptic cleft and eliminates Ach
• Motor unit
o Motor axon and all the muscle fibers
o Can be small and large
o Small motor units have fewer muscle cells innervated per axon compared to large motor units
o Fine motor control muscles have small motor units
o Muscles for powerful movements have large motor units
o All motor units have single type of muscle fiber (can change if innervating motor neuron changes from one type to another)
• Motor unit in damage
o Can change from one type to another
o Denervation causes atrophy and become flattened and angulated
o Reinnervation restores fiber size and shape, but could be innervated by a different type
o In ongoing axonal or neuronal damage and dropout: residual motor axons may innervate increasingly large numbers of myofibers leading to enlargement of motor units
• Fiber type grouping
o Motor units being composed of one muscle fiber type
• Grouped atrophy
o If axon is damaged, all fibers in a motor unit will atrophy
• Sensory components of skeletal muscle
o Proprioceptors (sensory organs) and tendons provide feedback on contractile state of muscle, tendon tension and position of joint
o Muscle spindles have intrafusal muscle fibers and afferent nerve endings
o Stretching a muscle stimulates nerve endings and is sensed by spinal cord
o Appropriate motor neuron is stimulated and the stretched muscle contracts
o Simultaneously, sensory synapses on interneuron that inhibits motor neuron to antagonistic muscle allowing it to relax
• Golgi tendon organ
o Bare sensory nerve ending in tendons that inhibits muscle contraction when it senses excess tension placed on the tendon by a muscle
o At the same time causes contraction of the antagonistic muscle group
