Sarcomere structure & Muscle Contraction Flashcards
Describe the development of muscle
- Embryonic mesoderm cells called myoblasts undergo cell division and enlarge
- Several myoblasts fuse together to form a myotube
- Myotube matures into skeletal muscle fibre
What is a sarcomere
The function units of myofibres.
Z discs form boundaries between sarcomeres
What are the contractile proteins of a muscle
Actin - thin filaments
Myosin - Thick filaments
2:1 ratio of actin to myosin
What are the supporting proteins for contraction?
- A-Actinin - bind in actin to keep the Z discs in place
- Titin - stabilises myosin
- Nebulin
- Dysotrophin
What is are bands of the sarcomere?
I band is the lighter region only actin is present
A band is the darker region with overlap of actin and myosin
Describe myosin
- Bipolar assemblies of multiple myosin- II molecules ( double helix)
- Each myosin - II molecule consists of:
- Two intertwined heavy chains (mhc)
- Two essential light chains (MLC-1) stabilises myosin head
- Two regulatory light chains (MLC - 2) regulates ATPase activity of myosin to genetrate ADP
Describe Actin
Two intertwined a-helical chains of actin (F actin)
Explain the interaction of myosin and actin in sarcomere shortening
- At rest: the A and I bands are a similar width
- During contraction: the myosin binds to actin, pulling inwards shortening the sarcomere and the Z discs move closer
- The I band reduces in size
- The A band remains the same width however more actin and myosin overlap
- ADP and P
Explain the role of Tropomyosin
Prevents myosin binding to actin in a relationship relaxed state
Explain the role of troponin
Troponin - binds actin, tropomyosin and calcium
- TnC - binds to Ca 2+
- TnT - binds to tropomyosin
- TnI - binds to actin which inhibits contraction
Explain the role of calcium in muscle contraction
- Following neuronal stimulation and depolarisation of the muscle cell, Ca2+ is released from the SR and binds to TnC
- This causes conformational change in TnI and TnT rotates tropomyosin to reveal myosin binding sites on actin
- In the presence of ATP, myosin can bind to actin - sarcomeres shorten and muscle contracts
- Calcium couples the electrical stimulation into mechanical contraction - Excitation- contraction coupling
Describe the process of cross bridge formation and power stroke
- Energized myosin head attaches to actin myofilament, turning into cross bridge
- ADP and P are released an the myosin head pivots and bends, changing to its bent low-energy state. As a result, it pulls the actin filament towards to the M line.
- In the absence of ATP, myosin heads will not detach, causing rigor mortis
- After ATP attaches to myosin, the link between myosin and actin weakens and the myosin head detaches; breaking the cross bridge
- As ATP is hydrolysed to ADP and P, the myosin head returns to its original cocked position
Describe events occuring at the neuromuscular junction
- Action potential travels down motor neurone to the terminal motor end of the plate
- V gated Ca2+ channels open and influx of Ca2+ initiates movement of vesicles of Ach to undergo endocytosis
- Ach is released into the synaptic cleft
- Ach attaches to Nicotinic Ach receptors ( nAchR)
- Acetylcholinesterase rapidly breaks down Ach in synaptic cleft
Describe the T tubules
- Extensions of the sarcolemma that invagintate into the cell
- Transmits the electrical impulse deep within the cell structure
- Closely associated with the sarcoplasmic reticulum to stimulate Ca2+ release. This enables the whole cell to contract simultaneously
- SR stores Ca2+
Describe the Sarcoplasmic Reticulum
- Elaborate smooth endoplasmic reticulum
- Vast network of interconnecting tubules surrounding each myofibril
- Stores calcium > releases on stimulation > contraction
Describe the end of contraction
- Action potential stops, Ca2+ is pumped back into the SR by active transport through the use of ‘sarco-endoplasmic reticulum calcium ATPase (SERCA)’
- Within the SR, calsequestrian and calreticulin are major Ca - binding proteins in skeletal muscle
- Located predominantly at triad junction
- Calcium may also be transported
