Muscle 1 Flashcards
What are myofibrils?
Fibrils containing filaments e.g. actin and myosin
Describe myofiber
A cell, endomysium, multinucleate
What is epimysium ?
This surrounds muscle
What is perimysium?
Surrounds fascicles, form septa
What is the function of connective tissue in muscle?
Conveys muscle force to tendons
What makes up the thick filaments of muscle?
Myosin-molecular motor
Myosin movement is actin-dependent in striated muscles
What are the parts of muscle thick filaments?
- head portion
- tail portion
- neck domain (s2)
Describe the head portion of thick muscle filament
Head portion of myosin
- 2 globular heads(cross-bridges)
- Binds actin and hydrolyzes ATP
- ATPase activity Ca2+ dependent
Describe the tail portion of thick muscle filament
Tail portion of myosin
2 heavy chains form double helix structure or “tail”
Describe the Neck domain of thick muscle filaments
Neck domain (S2) of myosin
- Connects head and tail
- Lever arm rotates during head movement
- 4 light chains- modulate myosin activity
What are the components of thin filament?
- Actin
- Tropomyosin
- Troponin
Describe actin thin filament in muscle
Globular actin polymerized into double-stranded helical filamentous actin (F-Actin). Contains myosin binding sites
Describe tropomyosin of thin filament
Filamentous protein blocks myosin-binding sites on actin
Describe troponin of thin filament of muscle
Consists of 3 globular proteins
I) troponin T(T-tropomyosin): Attaches troponin complex to tropomyosin
II) troponin (I=inhibition): positions tropomyosin over myosin binding site on actin
III) Troponin C (C=Ca2+): Ca2+ binds calcium. Results movement of tropomyosin. Important for initiation of contraction
Summarize events of excitation contraction coupling
Neurotransmitters released diffuses across the synaptic cleft and attaches to ACh receptors on the sacrolemma
- Net entry of Na+ initiates an action potential which is propagated along the sacrolemma and down the T tubules
- Action potential in T tubule activates voltage-sensitive receptors, which in turn trigger Ca2+ release from terminal cisternae of SR into cytosol
- Calcium ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed
- Contraction; myosin heads alternately attach to actin and detach, pulling the actin filaments toward the center of the sacramer; release of energy by ATP hydrolysis powers the cycling process
- Removal of Ca2+ by active transport into the SR after the action potential ends
- Tropomyosin blockage restored, blocking myosin binding sites on actin; contraction ends and muscle fiber relaxes
How is endplate potential converted to action potential?
- Net entry of Na+ through voltage gated Na+ channels initiates AP
- voltage gated action potential generated
- AP propagates across muscle surface (sacrolemma)-positive feedback, no change in amplitude
- Migrates down the transverse tubules
- (T-tubules= invagination of sacrolemma )
What are transverse tubules?
- Na+ channel propagated depolarization travels along T-tubule deep inside large, long cell
- T-tubule in close proximity to Sarcoplasmic reticulum
Continuous with cell surface, spread throughout entire cell
Why is myosin (thick filament) a molecular motor?
Allows movement
What are the two myosin binding sites?
Actin and ATP binding sites
What is the cause of the end plate potential?
Caused by influx of sodium in post synaptic membrane
Differentiate ligand and voltage gated channel
Ligand needs a neurotransmitter
Voltage gated- needs action potential
End plate potential is a ….
Graded potential, because the amplitude diminishes as it goes on
Explain the functioning of the t-tubule
- Muscle action potential travels down the T-tubule
- Action potential is SENSED by Dihydropurine(DHP) receptor/L-typevvoltage gated calcium channel
- This activates Ryanodine receptor and it causes the opening of this receptor
- The opening of this receptor opens and allows for the flow of calcium from the sarcoplasm into the cytosolic space
- Increase cytosolic Ca2+ results in muscle contraction
Why is DHP is a sensor and not a channel in skeletal muscles?
In cardiac muscles there are extracellular sources for calcium while in skeletal muscles, the only sources for calcium are the SR
Therefore in cardiac muscle, it is a channel and a sensor in skeletal muscle