Lecture 14 - Muscle Tissue Flashcards
What are the different classifications of muscle tissues?
- skeletal (voluntary/striated)
- cardiac
- smooth (involuntary)
What are the histological characteristics of skeletal muscle?
- multinulceated syncytium formed by fusion of embryonic myotubes
- peripheral nuclei
- sarcomeric arrangement
- innervated by a single motor axon
- contains troponin C
- “all-or-none” contraction
- 50-60μm
What are the characteristics of Type I myofibers?
- intense staining for oxidative enzymes
- rich in NADH transferase and myoglobin
- many mitochondria
- primarily uses oxidative phosphorylation
- slow, continuous contractions
- “red” or “dark” fibers
What are the characteristics of Type IIA myofibers?
- intermediate staining for oxidative enzymes
- uses aerobic and anaerobic respiration
- contract more rapidly than Type I but slower than Type IIB
- resistant to fatigue
What are the characteristics of a Type IIB myofiber?
- light staining for oxidative enzymes
- utilize anaerobic respiration primarily
- rich in ATPase
- contract most rapidly
- fatigue quickly
- “white” or “light” fibers
What are the histological characteristics of smooth muscle?
- single, mononucleated cells
- no sarcomeric arrangement
- innervated by ANS
- no “all-or-none” response
- connected via gap junctions
What are the histological characteristics of cardiac muscle?
- single, mononucleated cells
- centrally located nuclei
- often branched
- sarcomeric arrangement
- connected via gap junctions (intercalated discs)
- not directly innervated
Describe the hierarchal structure of skeletal muscle (include connective tissue).
Myofilament (actin/myosin filaments that make up a sacromere) -> myofibril (chain of sarcomeres) -> myofiber (muscle cell containing many myofibrils) -> sarcolemma (cell membrane of myofiber) -> endomysium (connective tissue surrounding each myofiber) -> fascicle (bundle of myofibers) -> perimysium (connective tissue surrounding each fascicle) -> muscle (multiple fascicles) -> epimysium (connective tissue around each muscle
What are Z disks?
- separates adjacent sarcomeres
- anchor actin filaments
What are A bands?
- consist of both actin and myosin
- represents the length of myosin filaments
- flanked by I bands and contains H band
- does not change length
What is the I band?
- portion of actin filaments not overlapping with myosin
- in between A band and Z disc
- changes length
What is the H band?
- portion of myosin not overlapped by actin
- in the middle of the A band
- changes length
Describe thick myofilaments.
Myosin:
-dimeric protein with long tails and two heads
-two pairs of light chains
—essential light chains (stability of head?)
—regulatory (stability of myosin II and sites for PO4+ binding)
Heads contain:
- actin binding region
- ATP-binding region
- light-chain binding site
Describe thin myofilaments.
F actin:
- polymer of G actin
- two polymers wound in an alpha helix
- plus end inserts into Z-disk
Actin-associated molecules: -troponin —troponin I (inhibits binding of actin and myosin) —troponin C (binds calcium) —troponin T (binds tropomyosin) -tropomyosin —sits in groove of actin strands —spans 7 actin monomers
Where are T tubules located in the sarcomere in skeletal muscle?
-A-I junctions (two per sarcomere)
What is αβ-crystalline?
-heat shock protein that protects desmin from mechanical stress
What is dystrophin?
- links α-actinin/desmin complex to sarcolemma
- anchors actin to sarcolemma
- reinforces sarcolemma
What is the dystroglycan complex?
-links dystrophin (intracellular) to laminin-2 (extracellular)
What is α-actinin?
-attaches thin filaments to Z line
What is nebulin?
- extends from Z disc to end of actin filaments
- regulates length of actin filament
What is titin?
- large, fibrous protein
- extends from Z-disk to middle of H-band and connects ends of thick filaments to Z-line
- provides myosin elasticity
- centers thick filaments in the sarcomere
What is desmin?
- intermediate filaments
- framework of desmin filaments which surrounds Z disc
- links myofibrils laterally and to the sarcolemma
What is plectin?
-binds desmin filaments
What is muscular dystrophy?
- X-linked genetic disease resulting in progressive degeneration of skeletal muscles
- result of the absence of or abnormal dystrophin which anchors actin to the sarcolemma and stabilizes muscles during contraction
What are satellite cells?
-reserve “stem” cells found in muscle tissue
What are the contents of a neuromuscular spindle?
- extrafusal fibers (muscle fiber)
- intrafusal fibers (nuclear bag, sensory)
- alpha motor neurons (supply extrafusal fibers)
- gamma motor fibers (supply intrafusal fibers)
- primary and secondary afferent fibers
What are the histological characteristics of cardiac muscle tissue? (6)
- single, mononucleated cells
- central nuclei
- sarcomeric (striated)
- communicate via gap junctions (intercalated discs)
- not innervated
- one T-tubule per cisterna (diad)
What is the composition of an intercalated disc?
Macula adherentes (desomsomes)
- link intermediate filaments (desmin)
- transverse component
Fascia adeherentes
- anchor actin filaments
- transverse component
Gap junctions
-longitudinal component
Where does ANP come from and what does it do?
Atrial natriuretic peptide
-secreted as prohormone by cardiac muscle cells in atria when the atria stretches
- regulates fluid electrolytes
- relaxes vascular smooth muscle
What are the histological characteristics of smooth muscle tissue?
- singe, mononucleated cells
- central nuclei
- no sarcomeric arrangement
- innervated by ANS or enteric system
- not “all-or-none”
- actin/myosin bundles in meshwork arrangement
- dense bodies
- no T-tubules
- no extensive SR
- pinocytic vesicles transporting Ca++
- caveolae and lipid rafts
What is the function of dense bodies in smooth muscle tissues?
- anchors actin bundles
- communicate force of contraction to cytoskeleton and cell membrane
What is the function of pinocytic vesicles in smooth muscle cells?
- source of Ca++ in absence of T-tumble system and extensive SR
- Ca++ is extracellular
What are the caveolae and lipid rafts?
- depressions in cell membrane involved in electrolyte transfer
- forms vesicles
