Exam #2: Muscle Flashcards
Sarcomere
Contractile unit of muscle
What are the three components of thin filaments?
1) Actin
2) Troponin
3) Tropomyosin
Actin
- Globular protein
- Main component of thin filaments
What are the three components of troponin?
TnT= attaches to tropomyosin TnC= Binds Ca++ TnI= binds actin & inhibits myosin binding
Tropomyosin
- Two polypeptide chains forming an alpha-helix
- Forms a filament that wraps around actin
- Block active sites on actin monomers to prevent myosin binding
Filamentous
Two twisted polymers of actin
What are thick filament composed of?
Myosin II (roughly 200 subunits form a thick filament)
Heavy Meromysin
- 2x globular heads
- 4x light chains
- Short twisted tail
Light Meromysin
Long twisted tail
Heavy chain
Golf club appearing chains
Light chain
Golf ball appearing chains
S1
Globular head & 2x light chains
S2
Short twisted tail
What are the thick bold lines in a sarcomere?
Myosin
What are the thin gray lines in a sarcomere?
Actin
What are the boundaries of a single sarcomere?
Z-disc–> Z-disc
A-band
- Dark band
- Contains thick & thin filaments
- Bisected by the H & M bands
- Widest band of sarcomere
I-Band
- Light band
- Thin filaments ONLY
- Formed by 2x adjacent sarcomeres
- Bisected by the z-disc
Z-disc
- Attachment for thin filaments
- Contains alpha-actinin
- Bisects the I-band
H-band
- Thick filaments only
- Bisects the A-band
Alpha-actinin
- Associated with the z-disc
- Anchors the thin filaments to the z-disc
Nebulin
- Associated with the z-disc
- Non-elastic
- Holds the thin filaments in place
Titin
- Associated with the H-band
- Anchors the thick filaments to the sarcomere (z-dics)
- Elastic
Myomesin
Holds the thick filaments to the midline
C-protein
Holds the thick filaments to the midline
Endomysium
- Connective tissue that separates muscle fibers
- Contains Reticular Fibers (Type III Collagen)
Perimysium
- Connective tissue that contains bundles of muscle fibers (fasicles)
- Collagenous
Epimysium
- Connective tissue that covers the entire muscle
- Deep fascia
- Contains blood vessels & nerve fibers
Skeletal Muscle Characteristics
- Multi-nucleated
- Nuclei pushed to the periphery
- Fibers oriented longitudinally
- Euchromatic nucleus (protein synthesis)
- External lamina is just outside/ small spaces between cells
Sarcoplasmic Reticulum
Very elaborate sER
T-tubules
- Invaginations of the sarcolemma that contain ECF
- Function to carry the nerve impulse to the sarcomere
- Run perpendicular to the longitudinal axis of the cell
Triad
- 2x T-tubules
- Terminal Cisterna
Terminal Cisterna
Contain Ca++ needed for muscle contraction
Motor end-plate
- Nerve terminal on the skeletal muscle fiber
- ACh
Motor Unit
A single nerve cell & all of the muscle fibers that it innervates
Myasthenia Gravis
Autoantiboides to AChR on the post-synaptic membrane of a muscle fiber prevent NT binding & cause muscle weakness
DMD
- Duchenne Muscular Dystrophy
- Most common & severe form of muscular dystrophy affected striated (muscle & cardiac) muscle
- Characterized by weakness of large muscle groups
- XP21 locus on the x-chromosome is mutated, which affects synthesis of “dystrophin” protein
Dystrophin
- Binds actin filaments to laminin
- Mutated (DMD), muscle contraction is inhibited
Characteristics of Cardiac Muscle
- Branching
- Intercalated Discs
- Large spaces between cells
- Nuclei at the center of the cell
- Cells contain 1-2 nuclei
What do atrial cardiac muscle cells contain?
- Diuretics enclosed in vesicles
- Atrial natriuretic peptide & brain natriuretic factor
- Influence kidney to pump out Na+ & H20 follows
- Net effect: lower blood pressure
Intercalated Discs
Function to bind cardiac muscle cells together
Transverse Portion vs. Lateral Portion of Intercalated Disc
Transverse Portion= Fascia adherens & desmosomes
Lateral Portion= Gap junctions
Purkinje Fibers
- Specialized cardiac muscle fibers that are larger than normal
- Stain paler
- Fewer myofibrils
- Contain glycogen
- Function as “batteries” that relay electrical impulses
Characteristics of Smooth Muscle
- NO Sarcomeres
- Fusiform
- Euchromatic Nucleus
- Very little extracellular space
What is the function of longitudinally oriented smooth muscle fibers in the GI tract?
- Outer fibers
- Propel luminal contents forward
What is the function of circularly oriented smooth muscle fibers in the GI tract?
- Inner fibers
- Decreases the diameter of the lumen
Caveole
Tiny pinocytotic vesicles associated with smooth muscle that contain & release Ca++ for contraction
Dense Bodies
Point of contact between myofilaments in smooth muscles
How does the morphology of a smooth muscle fiber change with contraction?
Contraction causes puckering & a “squishing” of the nucleus
What is the effect of ACh binding to the AChR on the post-synpatic sarcolemma?
Increases the permeability to Na+
How does the nerve impulse spread to the muscle cell?
T-tubules
What specifically releases Ca++ in the muscle cell in response to a nerve impulse reaching the T-tubule?
- T-tubule interaction with the sarcoplasmic reticulum
- Sarcoplasmic reticulum contains calciquestrin that contains Ca++
- Nerve impulse causes calciquestrin to release Ca++
What happens when Ca++ is released by Calciquestrin into the cytoplasm?
Binds TnC, which induces a conformational change in Troponin
What happens once there is a conformational change in Troponin?
TnT pushes Tropomyosin deeper into the actin spiral groove, exposing myosin binding sites
Describe the actin-myosin cross-bridge cycle.
1) Myosin binds actin filaments
2) ATP hydrolysis causes a conformational change in Myosin that pull the actin filament over the thick filament
Which band does not change length during muscle contraction?
A-band
M-line
- Protein structures lying between thick filaments
- Creatine Kinase is the major protein
- Bisects the H-band
