Sarcomere Structure Flashcards
What are myoblasts ?
Embryonic mesoderm cells
What is a myotube ?
Immature multinucleate muscle fibre
Briefly describe the development of muscle
Myoblasts undergo cell division to increase in number and enlarge.
Several myoblasts fuse together to form a myotube.
Myotube matures into skeletal muscle fibre.
Outline the steps involved in muscle contraction
Neuronal stimulus
Transfer of signal to muscle cell
Relay of signal inside the cell
Conversion of electrical signal to mechanical signal
Muscle contraction
What is the sarcomere ?
The sarcomere is the main contractile unit of muscle fiber in the skeletal muscle.
Can be known as the structure between 2 Z discs.
Thick filaments
Myosin
Thin filaments
Actin
What is each sarcomere composed of ?
Myofilaments containing actin and myosin.
Bundles of myofilaments are called myofibrils.
A bands
Dark bands that contain whole thick filaments (myosin)
What does the outer edge of the A band contain ?
Thick and Thin filaments which overlap.
Thick - myosin
Thin - actin
I bands
Light bands that contain only the thin filaments (actin) and are located between the two thick filaments.
Z disc
The area that traverses the I bands and marks the point of the connection between the two neighboring actin filaments.
M line
Marks the middle of the sarcomere and contains the protein called myomesin.
H zone
The H zone contains only myosin.
Name some supporting proteins
Alpha-Actinin
Titin
Nebulin
Dystrophin
At muscle rest describe the bands of the sarcomere
The A and I bands are similar widths
During muscle contraction describe the bands of the sarcomere
Myosin binds to actin, pulling inwards shortening the sarcomere.
The Z discs move closer together.
The I band reduces in size.
The A band remains the same width, but has greater actin and myosin overlap.
What does each myosin-II molecule consist of ?
2 Heavy intertwined chains
2 Essential light chains (which stabilises the myosin head)
2 Regulatory light chains (which regulates ATPase activity of myosin)
What does each actin consist of ?
2 intertwined alpha-helical chains of actin
Tropomyosin (prevents myosin binding to actin in the relaxed state)
Troponin (binds actin, tropomyosin and calcium)
Function of tropomyosin
Prevents myosin binding to actin in the relaxed state
Function of TnC
Binds Ca2+
Function of TnT
Binds Tropomyosin
Function of TnI
Binds actin : Inhibits contraction
Explain what happens after neuronal stimulation and depolarisation of the muscle cell.
Ca2+ is released from the sarcoplasmic reticulum and binds TnC.
This binding causes a conformational change in TnI and TnT.
This rotates tropomyosin, to reveal myosin binding sites on actin.
In presence of ATP, myosin binds to actin.
Sarcomeres shorten and muscle contracts.
Describe what happens when myosin binds to actin
The sarcomeres shorten and muscle contracts.
Calcium couples the electrical stimulation into mechanical contraction.
Function of calcium in muscle contraction
Calcium couples the electrical stimulation into mechanical contraction.
Function of TnT in relation to tropomyosin
TnT rotates tropomyosin to reveal myosin bonding sites on actin.
What is cross bridge formation ?
Energised myosin head attaches to an actin myofilament, forming a cross-bridge.
What is the power (working) stroke ?
ADP + Pi are released, the myosin head pivots and bends, changing to its bent low-energy state.
As a result of this it pulls the actin filament towards the M line.
What happens in cross bridge detachment ?
After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches.
i.e. the cross bridge “breaks”
What happens in the cocking of the myosin head ?
As ATP is hydrolysed to ADP + Pi, the myosin head returns to its pre stroke high-energy, or cocked position.
Explain what happens at the neuromuscular junction.
Action potential travels down motor neurone to the terminal motor end plate.
Voltage gated calcium channels open and influx of Ca2+ enters.
This initiates vesicles containing acetylcholine to fuse with membrane.
Ach is released into the synaptic cleft.
Ach attaches to Nicotonic Ach receptors.
Acetylcholinesterase rapidly breaks down Ach in the synaptic cleft;
Function of acetylcholinesterase
Rapidly breaks down Ach in the synaptic cleft
What are T tubules ?
T Tubules are extensions of the sarcolemma - invaginate into the cell.
What is the function of T tubules ?
They transmit the electrical impulse deep within the cell structure.
They are closely associated with the sarcoplasmic reticulum to stimulate release of Ca2+, which enables the whole cell to contract almost simultaneously.
What is the sarcoplasmic reticulum ?
An elaborate smooth endoplasmic reticulum.
A vast network of interconnecting tubules surrounding myofibril.
What is the function of the sarcoplasmic reticulum ?
Stores calcium —> releases on stimulation —> contraction.
Explain what happens at the end of muscle contraction.
The action potential stops
Ca2+ is pumped back into the sarcoplasmic reticulum by active transport. SERCA
Calcium may also be transported extracellularly via symporters.
Name some major Ca2+ binding proteins in skeletal muscle, within the sarcoplasmic reticulum.
Within the sarcoplasmic reticulum, calsequestrin and calreticulin are major Ca- binding proteins in skeletal muscle.
What are the 2 phases in muscle contraction ?
- Motor neuron stimulates muscle fibre.
- Excitation-contraction coupling occurs.
Summarise what happens in Phase 1 of muscle contraction.
Action potential arrives at axon terminal at neuromuscular junction.
ACh released; binds to receptors on sarcolemma.
Ion permeability of sarcolemma changes.
Local change in membrane voltage (depolarisation) occurs.
Local depolarisation ignites action potential in sarcolemma.
Summarise what happens in Phase 2 of muscle contraction.
Action potential travels across the entire sarcolemma.
Action potential travels along T tubules
Sarcoplasmic reticulum releases Ca2+; Ca2+ binds to troponin; myosin binding sites on actin are exposed.
Myosin heads bind to actin; contraction begins.
