skeletal muscle Flashcards
myocyte
indiv. muscle cell
muscle fiber
many myocytes fused together
myofibrils
contractile elements
sarcoplasm
muscle cytoplasm
muscle plasma membrane
sarcolemma
skeletal muscle appearance
-cells are multinucleate
-nuclei migrate to periphery of cell
-striated fibers
-assembled into sarcomeres
can skeletal muscle proliferate?
no
endomysium
CT that surrounds individual muscle fibers
fascicles
several fibers bound together
perimysium
CT that surrounds fascicles
muscle
formed from many fascicles
epimysium
CT that surrounds entire muscle, continuous with tendinous attachment
where do blood vessels tend to follow in skeletal muscle?
along CT (epi- and perimysium for support)
two major types of skeletal Mm fibers
fast twitch & slow twitch
type I skeletal muscle fibers
slow twitch
slow twitch fibers
-“slow” myosin
-small fibers w/ large amount of myoglobin
-use 1’ aerobic respiration for oxidative metabolism
-large # of mitochondria (ATP for energy)
-resistant to fatigue
-generate only moderate muscle tension
-common in peripheral limbs
type II skeletal muscle fibers
fast twitch
fast twitch fibers
-“fast” myosin
-large fibers with less myoglobin & fewer mitochondria
-use 1’ anaerobic glycolysis for energy production
-abundant glycogen
-extensive sarcoplasmic reticulum for rapid Ca release
-fatigue rapidly, but generate high muscle tension for short bursts of activity
what types of skeletal muscle fibers present in muscle?
both (type I and type II)
where are you most likely to see more type I skeletal muscle fibers?
in muscles used constantly
ex: leg muscles for standing
type IIA skeletal muscle fibers
oxidative & fatigue resistant
type IIB skeletal muscle fibers
glycolytic & fatigue sensitive
what happens during peak periods of exertion for skeletal muscle?
both type I and type II fibers metabolize glycogen via anaerobic glycolysis to produce ATP –> intermediate metabolites (e.g., lactic acid) –> precipitate as crystals in Mm –> tearing of Mm fibers & pain after heavy exertion
what happens during severe oxygen debt?
ischemia, muscle cramps, and even cell death
extreme exertion can lead to…
rhabdomyolysis (breakdown of actin & myosin) & subsequent kidney failure due to release of myoglobin & clogging of glomeruli
during normal exercise Mm fibers can develop…
micro-tears
hypertrophy
increase in cell size
what causes hypertrophy of muscle cells?
exercise causes an increase in number of mitochondria & increase in volume of contractile proteins; splitting & branching of individual Mm fibers
hyperplasia
production of new Mm fibers
relatively rare
atrophy
decrease in cell size
what causes atrophy of muscle cells?
-disuse
-immobilization can cause denervation
sarcopenia
loss of skeletal muscle fibers and Mm mass
comes with increasing age
satellite cells
-small cells adjacent to sarcolemma
-proliferate following injury and then differentiate into myoblasts
what are the two types of mechanoreceptors in skeletal muscle?
neuromuscular spindles & neurotendinous spindles
what do mechanoreceptors do in skeletal muscle?
-prevent overstretching & tearing of Mm
-used in postural reflexes
-coordination
neuromuscular spindles
-located within belly of Mm
-sensitive to changes in length
neurotendinous spindles
-located within the tendon
-sensitive to changes in tension
skeletal muscle ultrastructure
-individual Mm fibers composed of myofibrils
-myofibrils composed of numerous myofilaments (or contractile proteins) in parallel bundles
-myofibrils arranged in sarcomeres
what are the two types of myofilaments?
actin & mysoin
actin
thin filament
myosin
thick filament
sarcomere structure
-A band: actin & myosin overlap
-I band: only actin
-Z-discs: located in I band
-M line: middle line of sarcomere; located in A band
Z-discs
act as anchoring points for actin myofilaments
during Mm contraction ___ shorten, but ____ remain the same length
sarcomeres; myofilaments
sliding filament theory of Mm contraction
-thick & thin filaments slide over one another via energy from ATP
-sarcomere is shortened due to repeated binding & unbinding of actin & myosin filament
-ratchet-like, or “walk-along” mechanism
neuromuscular junction
site where skeletal muscle innervated by nerves
motor end plate (NMJ)
terminal portion of axon surrounded by myelin that rests on sarcolemma
terminal bouton
similar to motor end plate by unmyelinated
synaptic cleft
space b/w motor end plate & sarcolemma
what bridges the synaptic cleft?
chemical neurotransmitter
generally acetylcholine
transverse tubule system (T-tubules)
-extensive network of tubules continuous with sarcolemma
-indirectly links extracellular space with ER & intracellular environment
triad
ends of T-tubules bounded by terminal cisternae of ER on either side
what travels down the T-tubules?
electrical stimulation of neuron –> influx of Na+ into neuron –> wave of depolarization down axon –> release of neurotransmitter, binding to receptor proteins –> wave of depolarization of Mm fiber
Ca reservoirs in skeletal muscle
sarcoplasmic reticulum & terminal cisternae
sliding filament mechanism
influx of Na+ ions into cytoplasm from T-tubules triggers depolarization of sarcolemma –> release of Ca from ER & terminal cisternae into cytoplasm during contraction –> Ca causes conformational change in troponin, which interacts with tropomyosin (bound to actin), exposing myosin-binding sites on actin filament –> myosin binding causes conformational change in myosin head & sliding of myosin past actin
myosin heads repeatedly bind & unbind to actin in presence of Ca and Pi, causing contraction
what ions activate sliding filament mechanism?
Ca
each muscle fiber exhibits what type of response?
all or none; either it contracts or it doesn’t
graded response
total number of muscle fibers contracting @ any given time determine strength of overall contraction
motor unit (skeletal muscle)
-group of muscle fibers supplied by single motor neuron
-stimulation of a motor neuron results in contraction of all muscle fibers within that motor unit
recruitment (skeletal muscle)
increase in number of motor units firing within a muscle
muscular dystrophy
-degenerative wasting disease
-Mm weakness due to genetic defect in Mm protein
-can cause cell death
-one form affects the protein dystrophin
myasthenia gravis
-autoimmune disease caused by production of antibodies to Ach receptors
-causes Mm weakness
-treatment with Achase inhibitors
