1-Striated Muscle Skeletal Flashcards
basal lamina
purpose
important structural element serves as anchor point for proteins than span cell membrane
link endomysium to sarcolemma
T-tubules
definition
specialized invaginations in muscle cell membrane conduct AP (depolarization) from cell surface to calcium reservoir (@sarcoplasmic reticulum)
SR surrounds myofilaments
membrane = sarcolemma
muscle development sequence
skeletal muscle
- pluripotent stem cells diff to myogenic progenitor cells
- myoblasts
- committed myocytes OR dormant satellite cells to be acitvated later
- myocytes aggregate into myotubes
- mature fiber w/ nuclei @ periphery
mature muscle organization
- myofilaments
- myofibrils
- muscle fiber
- muscle fascicle
- muscle
layers of CT
- endomysium: around indiv muscle cells/fibers
- perimysium: divides into bundles aka surrounds fascicles
- epimysium: attachs directly to bone or tendon aka surrounds entire muscle group
basal lamina/BM composition
collagen + proteoglycans + glycoproteins/complex polysaccharides
terminal cisternae of SR
swelling at end of reticular network of sarcoplasmic reticulum where Ca stored
2 terminal cisternae + T tubule = triad
SR surrounds fibrils
contractile elements
sarcomere
- thin filaments (actin)
- thick filaments (myosin)
- titin
structural elements
- Z disc (est boundaries of sarcomeres)
- M line (center line)
- Titin
bands/zones
sarcomere
- I band (light color) = thin filaments only
- A band (dark color, wider) = thin + thick
- H zone (middle of A) = thick only
thick filaments
bipolar (head and tails, tails medial) structure of a lot of myosin molecules
filaments wrap together in quasi helical fashion
myosin molecule
2 heavy chains in alpha-helix + 2 light chains (regulatory and essential) per chain so 4 total
neck of chain allows for hinge to articulate with thin filaments
myosin head has ATPase activity
thin filaments
double strand of G actin sub units twisted a helix
regulatory proteins with thin filaments
- tropomyosin
- troponins
T = bind tropomyosin
C = bind calcium
I = inhibitory, changes conformation when C binds Ca and moves T so tropomyosin out of active site on actin
titin
gigantic muscle protein that anchors thick filaments to Z discs, spans 1/2 sarcomere
gives structural integrity/elastic stabilization of thick/thin filaments, passive tension
bi-directional spring to help sarcomere return to og
Z disc/Z line
lattice like structural element at both ends of sarcomere
anchor point for thin filaments and titin filaments
connect to Z discs of adjacent myofibrils by intermediate filaments (desmin)
M line/M disc
lattice like structural element at center of sarcomere
anchor for thick filaments and titin
-M protein: stabilized proteins in M line
-obscurin: regulate sarcomere/SR interaction
-myomesin: anchors myosin with titin
costameric protein complex
aka dystrophin-glycoprotein complex + integrins = costamere
stabilizes muscle cell membrane during contraction
scaffolding for myofibrils by linking cytoskeleton w/ ECM and anchors Z disk (w/ contracticle proteins) to basal lamina
outcomes of disease
clinical relevance
- respiratory insufficiency/failure
- inc suscept to respiratory infection
- spinal curvature
- cardiomyopathy
- metabolic abnormalities
- reduced mobility/independence
muscular dystrophy
mutations in genes that encode dystrophin or other components of costamere
results: disorganized costameres
enhanced membrane leak
edema/inapp cytosolic Ca generation
ECM deposition
duchenne muscular dystrophy
no dystophin protein present
fatal by age 30 via cardio respiratory failure
becker muscular dystrophy
dystrophin present but reduced in amount
less serious, usually not fatal
limb girgle dystrophy
mutations for sarcoglycans or other dystrophin-glycoprotein complex
motor unit
one motor neuron and all the muscle fibers that it innervates
neurotransmission at triad
- depolarization down t tubule
- conformation change in L type Ca channel (LTCC)
- 4 LTCC attach to 4 RYR = tetrad, opens
- termination when Ca sequestered in myoplasm
Ca reuptake mechanism
SERCA major way, ATP dependent
Calreticulin and Calsequestrin are SR proteins that bind Ca+
sliding filament theory
thin filaments slide over thick filaments bc thick pull toward center of sarcomere
thin pull Z discs toward each other and sarcomere shortens
thin filament regulation
Ca triggers contraction by interacting with troponin C = conformational change
tropomyosin shifted deeper into actin groove to expose active site for myosin from thick to bind
cross bridge cycle
- ATP binds to myosin
- dissociation of AM complex
- ATP hydrolyzed = conform change aka cocked state
- cross bridge formed when active site exposed
- Pi released = conform change and power stroke
- ADP released and cross bridge detach from myosin if ATP present
magnesium
EC coupling
co factor for myosin ATPase mediated ATP hydrolysis
deficiency can cause muscle weakness and cramping, common in GI disease, diabetes, alcoholism, aging
modulating force
- recruit more and bigger motor units if inc stimulus intensity
- frequency summation aka high frequency stimulation = repeated contractions of progressively greater force until reaches sustained maximal force production (fused tetanus)
progressive inc in myoplasmic Ca w/ each stimuli until max Ca levels reached
type I slow twitch fibers
-aerobic pathway for ATP
-resistant to fatigue
-low/slow force output
similar to cardiac muscle with diff myosin isoforms
type II fast oxidative fibers
aerobic or anaerobic ATP
resistant to fatigue
moderate/rapid force output
type II fast glycolytic
anaerobic pathway for ATP
fatigue quickly
high/rapid force output
