Skeletal Muscle Pt1 Flashcards
After card 36 these are flash cards from end of lecture starting at plasticity of muscle
Skeletal muscle fnxs
- move its body and its of body
- weight bearing and maintenance of posture
- for its thoracic and abdominal cavities and contribute to visceral fxs (breathing, change abdominal pressure)
- crucial role whole body metabolism
- contributes to basal energy metabolism
- storage depot carbs and amino acids
- produces heat to maintain core body temp
major consumer oxygen and fuel during exercise
skeletal muscle
skeletal muscle consists of
fibers (or myocytes), ct, blood vessels, lymphatics, nerves
musculotendinous junction
myocyte connected to tendon fibers at longitudinal ends ; allows muscle fiber to transmit mechanical force to skeleton; this
- this is vulnerable to injury
epimysium
dense ct sheet covers muscle as whole
perimysium
- binds groups of muscle fibers and associated tendon fibers into fascicles; perimysium is continuous with epimysium
- provides passage for nerves and small blood vessels
- allows for movement of fascicles against each other
endomysium
- surrounds each muscle fiber
- consists of fine reticular fibers supporting capillaries and nerve fibers
- allows individual fibers within fascicle to move against each other
outside to inside coverings muscle
epimysium, perimysium, endomysium
feed artieries
- beginning of vascular supply to skeletal muscle
- last branches arterial system
- begins externaly to tissue
arteriolar skeletal muscle
- origin= site at which feed artery enters muscle
- within muscle arterioles undergo branching until reach terminal arterioles
microvascular unit
- group of capillaries supplied by terminal arterioles
- each unit = 15-20 capillaries running in parallel with muscle fibers
- muscle fibers= longer than capillaries -> many microvascular units arranged in series to span distance each fiber
collecting venule
capillaries of microvascular unit drain into this
pennation
- in most muscles fibers join tendon at acute angle= pinnation
- orientation muscle fiber relative to tendon affects fx muscle
pennation fx
- allows greater number fibers packed into given volume -> increased muscle power
- greater pinnation -> greater lifting capacity and greater capacity for prolonged contraction
increased pinnation cost
- decreased fiber length
- decreased lifting height
myocyte general description
- multinucleate cell resulting from fusion myoblasts during development
nuclei of myocyte
- each nucleus control protein synth in that region of cell
protein expression myocyte
generally protein expression coordinated so same protein isoforms expressed for full length fiber
myofibrils
- bulk cytoplasm of myocyte= myofibril
- cylindrical structures composed of long repeated chains sarcomeres
sarcomere consists of
- array myofilamentz anchored at each end by z-discks
microscopic appearance sarcomere
- dark band= A band = thick myosin filaments
- light band= I band= thin actin filaments
Z disk fx
- transmit mechanical force generated by individual actin-myosin cross bridges to myotendinous junction
z discs consist of
- multiple proteins some of which serve as attachment points for actin myofilamentz, some connect to sarcolemma and elements in ECM
accessory proteins myofilaments
- accessory proteins organize myofilamentz
- responsible for uniformity of organization, length, spacing of filament in sarcomere
titin
- large elastic protein
- attaches Z disc fo myosin filaments
- serves as template for sarcomere assembly
- acts as molecular spring
titin template sarcomere assembly
- positions thick filaments midway between z discs
titin as molecular spring
- prevents over stretching muscle fibers retiring myosin filament to original position following muscle contraction
cytoplasm of muscle cell
sarcoplasm
additional cellular elements in sarcoplasm of muscle fiber
- transfer tubular system (T-tubules)
- sarcoplasmic reticulum
- mitochondria
- glycogen garnules
- lipid droplets
T-tubules
- invaginations sarcolemma (plasma membrane)
- penetrate deep into muscle fibers
sarcoplasmic reticulum fx
- storage, release, reuptake Ca2+
terminal cisternae
- specialized regions of repeating patterns that form mesh like network enveloping myofibril (this = sarcoplasmic reticulum)
- these in close association with specific regions each sarcomere
triad
- each T tubule flanked by 2 cisternae
- 2 triads per sarcomere
Muscle mass depends on
balance between protein synthesis and protein degradation which are influenced by physical activity, nutritional status, hormone balance, and injury or dx
increase in muscle mass depends on
hypertrophy (increase cell size) rather than hyperplasia (cell division) b/c muscle cells are post-mitotic
stem cells and muscle cells
muscle cells contain stem cell population of stellite cells that can proliferate and differentiate into new muscle fibers under certain circumstances
exercise effect on muscle
- induces significant structural and metabolic changes in muscle tissue
- nature of changes depends on particular type of training (endurance or strength)
what does endurance training require of muscles
- activates large muscle groups to sustain activity for prolonged period without excess fatigue
- requires efficient delivery O2 to muscle fiber and enhanced ability fiber to utilize O2
response of muscle to endurance training
- increase capillary supply
- increased number and size mitochondria in myocytes
- fat and glycogen storage enhanced
- concentration oxidative enzymes for aerobic production ATP increased
- does not cause changes muscle size or force-generating capacity
strength training effect on muscle
- causes increased capacity to generate force
- size individual muscle fibers increases bc increased protein synth
- increased protein synth -> addition of myofilaments, sarcomeres, and myofibrils
- satellite cells activated -> new fibers produced
mechanisms by which increased load leads to changes in protein expression and muscle growth
- not yet fully eludicated
- mTOR pathway involved
- GPCR pathway involved
- myostatin implicated
atrophy
- reduction in muscle mass bc decrease in size and/ or umber of muscle fibers
atrophy main causes
- disuse
- denervation
- cachexia
- sarcopenia
denervation and disuse (lack of activity)
mechanical stimulation of muscle normally -> activation signaling pathway that promotes protein synthesis lack of stimulation from denervation and disuse -> fewer stimuli for protein production so muscles turn over and are not replaced
Cachexia
- loss lean body mass associated with many chronic dxs
- multiple factors contribute to weight loss:
- increased energy demapnds
- decreased energy intake
- decreased energy absorption
- shift to amino acid use for source of energy
Sarcopenia at whole body level
- loss lean body mass associated with gaining in absence of dx
- often loss lean body mass accompanied by increased fat mass
Sarcopenia at cellular level
- decrease myosin synthesis
- post translational modifications myosin -> decreased ability to form cross-bridges
- sarcoplasmic reticulum undergoes structural change so calcium stores no longer in close proximity to sarcomeres
- muscle fibers contain fewer mitochondria
- satellite cells do not respond as effectively to myogenic stimuli
