L: Sarcomere structure and muscle contraction

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What do myoblasts form from?

Embryonic mesoderm cells

What do myoblasts fuse together to form?

Myotubes

What is a myotube?

Immature multinucleate muscle fibre

Give an overview of muscle contraction.

Neuronal stimulus, transfer of signal to muscle cell, relay of signal inside of the cell, conversion of electrical signal to mechanical signal, muscle contraction

What are t-tubules extensions of?

The sarcolemma

What direction do T tubules invaginate?

Into the cell

What muscle cell structure stores Ca²⁺?

The sarcoplasmic reticulum.

What is closely associated with the sarcoplasmic reticulum and what does it stimulate?

T tubules and they stimulate the release of Ca²⁺.

What do t tubules allow for contraction?

Allow the whole cell to contract almost simultaneously.

what is the triad made up of?

Sarcoplasmic reticulum cisterna, transverse tubule, sarcoplasmic reticulum cisterna

What does the sarcoplasmic reticulum surround?

All myofibrils (actin and myosin filaments)

Is sarcoplasmic reticulum smooth or rough ER?

Smooth

What does the sarcoplasmic reticulum release to initiate contraction?

Ca²⁺

What are sarcomeres the functional units of?

Myofibres (muscle cells)

What forms the barrier between sarcomeres?

2 discs

What are the two contractile proteins within sarcomeres?

Actin and myosin

Which is the thin filament (contractile protein)?

Actin

Which is the thick filament (contractile protein)?

Myosin

What are 4 supporting proteins that are critical for maintaining sarcomere structure?

alpha-actinin, tinin, nebulin, dystrophin

What is the M line of the sarcomere?

The area in the middle of the A band where myosin is tethered

What is the Z part of the sarcomere?

Where the two discs are

What would you find near the outside of the A band?

Myosin and actin

What would you find in the I band?

Actin

Which is lighter, the A band or the I band?

The I band

Which band becomes smaller during contraction

The I band

What is the H zone of a sarcomere?

The area within the A band where only myosin exists.

What is myosin bipolar assemblies of?

Multiple myosin-II molecules.

What does a myosin-II molecule consist of?

• 2 intertwined heavy chains (MCH)

• 2 essential light chains (MLC-1)

• 2 regulatory light chains (MLC-2)

What is the function of MLC-1 in myosin-II molecules?

Stabilises the myosin head.

What is the function of MLC-2 in myosin-II molecules?

Regulate ATPase activity of myosin.

What is actin made up of?

2 intertwined alpha-helical chains of actin

What regulatory proteins exist on an actin filament?

Tropomyosin and troponin.

What is the function of tropomyosin on the actin filament?

Prevents myosin binding to actin in a relaxed state.

What is the function of troponin on an actin filament?

It binds actin, tropomyosin and calcium

Which part of troponin does calcium bind to?

TnC

Which part of troponin binds tropomyosin?

TnT

Which part of troponin binds actin?

TnI

How does TnI inhibit contraction in the relaxed state?

It stops myosin binding to actin in the absence of calcium

What follows Ca²⁺ binding to TnC?

A conformational change in TnT and TnT rotates tropomyosin to reveal myosin binding sites on actin.

What molecules must be present for myosin to bind to actin?

Calcium and ATP

Which molecule couples the electrical stimulation into mechanical action and what is this called?

Calcium - excitation, contraction coupling

How do the sizes of the A and I bands of the sarcomere compare at rest?

They are similar widths

When is myosin and actin overlap the greatest - during contraction or relaxation?

Contraction

What is a cross bridge?

Myosin head attaches to actin

What is the power stroke?

ADP and Pi are released from the myosin head and the myosin head bends to a low energy state, resulting in the actin filament being pulled towards the M line

What happens in the absence of ATP once the myosin head has attached to actin?

the myosin head will not detach and this causes rigor mortis

What causes myosin to detach from actin?

ATP attaching to myosin

What happens as ATP on the myosin head is hydrolysed?

The myosin head returns to its pre-stroke position (high energy)

What happens after an action potential of a motor neurone reaches the terminal bouton?

Voltage gated calcium channels open and an influx of calcium causes vesicles containing acetylcholine to fuse with the membrane.

What does the acetylcholine attach to on the muscle cell?

Nicotinic Ach receptors (nAchR)

What breaks down acetylcholine in the synaptic cleft?

Acetylecholinesterase

What happens to calcium when the action potential stops?

It is pumped back into the SR by active transport by SERCA.

What does SERCA stand for?

Sarco-endoplasmic reticulum calcium ATPase

What calcium binding proteins exist within the SR?

Calsequestrin and calreticulin

Where are calsequestrin and calreticulin predominantly located within the SR?

The triad junction

What happens differently to calcium in cardiac and smooth muscle?

It may also be transported extracellularly after the end of contraction via symporters