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What is the hierarchy of muscle cell organization?
myofilament > myofibril > muscle fiber/cell > muscle fascicle
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What are myofibrils?
series of sarcomeres linked together like "beads on a string", align w/ bands above and below
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What is another name for muscle fiber?
- muscle cell
- multinuc w/ eccentric nuc
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What is skeletal muscle derived from?
paraxial mesoderm (forms somites) and somitomeres in developing head of embryoblast
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neuromuscular junction
ends of nerve endings interact w/ muscle fibers at motor end plates
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What are characteristics of skeletal muscle?
- multinuc, eccentric nuc
- long fibers (but not as long as muscle itself)
- voluntary control (innervated by cholinergic nerve endings)
- contraction results from shortening sarcomeres
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What are the ends of muscle fibers attached to?
CT w/in muscle or CT tendons at each end of muscle
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What is the isotropic band?
- (I band) "monorefringent"- plane of polarized light is not bent
- light in color
- bisected by Z disc
- made of only thin actin filaments
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What is the anisotropic band?
- (A band) birefringent: plane of polarized light is bent
- made of thick and thin filaments (depends on your XS as to what the composition is)
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What is the Hensen band?
- (H band); thick filaments only
- btw thin filaments of A band; at center of A band
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What is the M line?
made of only thick myosin filaments w/ cross connections of myomesin
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What is the Z disk?
bisects I band
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What is a sarcomere?
area btw 2 Z disks; basic contractile unit of striated muscle
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What composes actin filaments?
- G actin monomers that comprise filamentous F actin
- two F actins intertwine to form a helix
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What is the purpose of tropomyosin?
occupies shallow grooves btw actin helices; masks myosin-binding sites on G actin
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What is the purpose of troponin?
- located on tropomyosin molecules; 3 subunits:
- troponin T: binds troponin to tropomyosin
- troponin C: binds Ca2+ which causes conformational change in tropomyosin and exposure of myosin binding site on G actin
- troponin I: binds to actin to prevent actin-myosin interaction
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What is the structure of myosin?
- two globular heads on two helices that intertwine
- heads have ATPase activity
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What is myomesin?
holds myosins together
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What is titin?
connects ends of myosin molecules to Z line
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What is the sarcoplasmic reticulum?
sER in muscle cells; stores intracellular Ca2+ and release
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What is sarcolemma?
cell membrane of striated muscle cells
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What are transverse tubules?
invaginations of sarcolemma into interior of muscle fiber; occur at A-I junction in skeletal muscle and Z-intercalated disk in cardiac muscle
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sarcoglycans
aka TM prots
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congenital muscular dystrophies
gene mut or deletion in lamins of dystroglycans
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oculopharyngeal muscular dystrophy
due to mut in Type 1 red muscle fibers
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myasthenia gravis
due to mut in type 2 white fibers
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autosomal limb girdle muscular dystrophies
gene mut or deletion in sarcoglyans
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What are terminal cisternae?
- cisternae of sarcoplasmic reticulum which parallel T tubule
- two, one for each side of T tubule; forms a triad in skeletal muscle and diad in cardiac muscle
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How do terminal cisternae function?
have Ca2+ stored in it, released after deplolarization from AP; Ca2+ causes troponin to relieve tropomyosin inhibition
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Where are terminal cisternae located?
at A-I junction; v closely assoc w/ myofibrils for best contraction
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How are sER and calcium related?
release calcium once receives signal and re-uptakes calcium right after releasing it so that contractions do not continue to happen
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When does rigor occur?
if levels of Ca2+ become too high or ATP levels become depleted (actin and myosin become cross-linked)
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How are actin filaments anchored?
via dystrophin protein to TM proteins that are linked to basal lamina; maintains muscle integrity during contraction
dystrophin is lg prot so lots of chances for mutations (duschennes and beckers)
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Red fibers (type 1)
- lots of myoglobin and oxidative enz
- -"slow twitch", sustained force, weight-bearing, enduring
- -sm diameter
- -NADH dk staining (bc of myoglobin)
- -abundant lipids
- -minimal glycogen
- -one motor neuron innervates many muscle fibers (less precise control)
- -located in limb muscles and long back muscles
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White fibers (type 2)
- -"fast twitch", sudden movements; purposeful motion; easily fatigued
- -less myoglobin
- -NADH light staining
- -minimal lipids
- -abundant glycogen
- -few mit
- -one motor neuron innervates few muscle cells (precise control)
- -located in extraocular muscles, digits
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What does endomysium surround?
ind myocytes
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What does perimysium surround?
bundles of muscle cells (fascicles)
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What does epimysium surround?
entire muscle; many fascicles
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What is the nt for skeletal muscle?
Ach; uses cholinergic (nicotinic) receptors
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How does the motor unit work for skeletal muscle?
multipolar neuron from ventral horn of gray column synapses on ind fibers (motor end plate)
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What are some characteristics of muscle fibers and nerves?
- bundle of skeletal muscle fiber innervated by single motor neuron
- myelinated
- every muscle fiber receives a nerve terminal
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How do skeletal muscle cells regenerate?
stem cells--->myoblasts--->new muscle fibers
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How can you tell an original muscle cell from new muscle cell?
new cell has a more central nuc (bc from myoblast) whereas an original cell has an eccentric nuc
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Where are cardiac muscle nuclei located?
centrally; mononucleated
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How do cardiac muscle cells contract?
same as skeletal muscle cells
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What is an intercalated disk?
junctional complex that connects sequential cardiocytes; creates lots of attachment and communication sites for gap junctions
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What are components of intercalated disks?
- maculae adherens (desmosomes)-attachment of intF
- fascia adherens-attachment of actin thin filaments
- gap junctions-ion transport to synchronize heart contraction
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What is an intracellular bridge?
bridge between layers of cardiocytes; allows heart to contract at once
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What is the function of mit in cardiocytes?
elongated around the myofibrils; lots bc need lots ATP for heart
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What does the sarcoplasmic reticulum look like in cardiocytes?
less well-developed; diads; located at Z disks (but not intercalated disks)
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What else is in cardiocytes?
- lots of cytoplasmic glycogen
- secretory granules in atria contain ANP
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What is the function of ANP in cardiocytes?
- when the atria is stretched (bc of too much H20), cardiocytes in atria release ANP:
- osmotic balance
- distention atrial wall
- stimulates diuresis (counters ADH)
- relaxes cardio channels
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What are purkinje fibers?
- modified cardiocytes; lgr than regular cardiocytes w/ central nuc and less myofibrils peripherally located
- binucleate; lots of glycogen granules
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Where are purkinje fibers located?
subendocardium
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How are purkinje fibers organized?
nodes and bundles
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What is the function of purkinje fibers?
to facilitate conduction of contractile stimulus (not actually contract as skeletal...more of a transmitter to rest of heart)
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How are cardiac muscle cells regenerated?
replaced w/ fibrous CT
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What do smooth muscle cells look like?
- organized as bundles or sheets of cells
- spindle shaped
- involuntary control
- non-striated
- stains "frosty pink" color
- organelles (golgi and sec granules) around each end of nuc
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What can smooth muscle cells secrete?
- elastin (tunica media cells of elastic arteries)
- type 4 and 3 collagen (tunica intima cells of blood vessels)
- lamin and proteoglycans (CTs)
- renin (juxtaglomerular cells in kidneys)
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How do smooth muscles utilize gap junctions?
to act synchronously
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How are smooth muscles innervated?
- nonmyelinated, ANS
- no specialized synapse
- only a few cells are innervated
- has gap junctions to allow concerted contractions
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What are interstitial cells of Cajal?
(ICC): highly specialized pacemaker cells that modulate autonomic stimulation of smooth muscle cells and are important in frequency of rhythmic contraction (i.e. peristalsis)
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How do smooth muscles contract?
- sliding filament mechanism w/ actin-myosin
- Ca2+ dep
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What does Ca2+ do for smooth muscle contraction?
Ca2+ induces calmodulin dep act of myosin light chain kinase...initiates actin-myosin interaction
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What are some characteristics of smooth muscle contraction?
- slow
- contractions are sustained (little ATP req)
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How do smooth muscles regenerate?
undergo mitosis
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What is the conducting pathway of the heart?
SA-AV-His-Purkinje
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What is the parasympathetic innervation of the heart?
vagus nerve-slows down heart contraction
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What is the sympathetic innervation of the heart?
- speeds up contraction at SA
- stim cardiac muscle cells to contract harder
- goes to each node and cell
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What is variscosity?
bare nerve terminal of smooth muscle cells where NE is dumped onto
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