Muscle Structure and Contraction

What is the structure of actin?

Made up of F-actin, troponin and tropomyosin. Join to form an actin filament that is twisted into a helix. On each actin molecule there is a myosin-binding site, where a myosin head can attach. Tropomyosin covers myosin-binding sites on actin molecules, preventing myosin from binding to actin. Troponin's role is binding to Ca⁺ ions, subsequently beginning muscle contraction.

What neurotransmitter is release at the NMJ?

Acetylcholine

What are the 4 steps of the contraction cycle?

1) ATP hydrolysis 2) Attachment of myosin to actin forming cross bridges 3) Power stroke 4) Detachment of myosin from actin

What are the connective tissue structures in muscle?

1) Skeletal muscle cells are surrounded by a connective tissue sheath called the endomysium. 2) Cells are grouped in fascicles surrounded by perimysium. 3) Fascicles are grouped together to form the muscle which is surround by epimysium.

What % of body weight does skeletal muscle make up?

40%

What are the T-tubules in striated muscle?

A T-tubule (or transverse tubule) is a deep invagination of the sarcolemma, which is the plasma membrane of skeletal muscle and cardiac muscle cells. These invaginations allow depolarization of the membrane to quickly penetrate to the interior of the cell.

What is the sarcomere?

A structural unit of a myofibril in striated muscle, consisting of a dark band and the nearer half of each adjacent pale band. Contained between the 2 Z-lines.

Describe the detachment of myosin from actin.

At the end of the power stroke, the cross-bridge remains firmly attached to actin until it binds another molecule of ATP. As ATP binds to the ATP-binding site on the myosin head, the myosin head detaches from actin.

Where do muscle action potentials arise?

At the neuromuscular junction - the synapse between a somatic motor neuron and a skeletal muscle fibre.

What is the typical structure of skeletal muscle?

Cells are very large (10-100 microns in diameter and several centimetres in length). Muscle fibres have a multinucleate syncytial structure.

What is the structure of myosin?

Composed of a myosin tail and myosin heads. The tail points towards the M line in the centre of the sarcomere. The heads project outwards from the shaft in a spiralling fashion.

What are the 3 types of muscle proteins?

Contractile proteins (actin and myosin), regulatory proteins (troponin and tropomyosin) and structural proteins (titian, α-actin, dystrophin).

What is excitation-contraction coupling?

How an action potential causes muscle contraction. As a muscle action potential propagates along the sarcolemma and into the T-tubules, the AP alters conformation of DHP receptor (Dihydropyridine). This acts as a voltage sensor which causes the Ryanodine receptor in the SR to open its Ca²⁺ channels. The released Ca²⁺ ions combine with troponin and the contraction cycle begins.

What is the ratio of thin to thick filaments?

There are 2 thin filaments for every thick filament.

What are the thick filaments made up?

These are 16nm in diameter and 1-2µm long and composed mostly of the protein myosin.

What are motor proteins and give an example

They pull various cellular structures to achieve movement by converting the chemical energy in ATP to the mechanical energy of motion, e.g. Myosin.

What are the thin filaments made up of?

Thin filaments are 8nm in diameter and 1-2 µm long and composed mostly of the protein actin.

Describe the power stroke.

This occurs after the cross-bridges are formed. The site on the cross bridge where ADP is still bound now opens. As a result, the cross-bridge rotates and releases the ADP. The cross-bridge generates force as it rotates towards the centre of the sarcomere, sliding the thin filament past the thick filament towards the M-line.

Where are triads located in skeletal muscle?

Triads are located at the junction of the A and I bands. Therefore, there are 2 triads per sarcomere.

When does relaxation occur in muscle?

When the sarcoplasmic Ca²⁺ concentration returns to normal by active transport of Ca²⁺ back into the SR where it is bound to calsequestrin. Calsequestrin helps hold calcium in the cisterna of the sarcoplasmic reticulum after a muscle contraction.

What does the extent of overlap between the thick and thin filaments depend on?

Whether the muscle is related, contracted or stretched.

What are the 3 types of muscle?

Skeletal, cardiac and smooth

What are the thin and thick filaments both involved in?

The contractile process.

Describe the formation of cross bridges.

The energised myosin head attach to the myosin binding site on actin and releases the previously hydrolysed phosphate group. When the myosin heads attach to actin during contraction, they are referred to as cross-bridges.

Describe ATP hydrolysis at the start of the sliding filament mechanism.

The myosin head includes an ATP-binding site and an ATPase, an enzyme that hydrolyses ATP into ADP + P. This hydrolysis reaction reorients and energises the myosin head. ADP + P are still attached to the myosin head.

Describe the structure of the sarcomere when the muscle is contracted.

Muscle contraction occurs when the myosin heads attach to and 'walk' along the thin filaments at both ends of the sarcomere, progressively pulling the thin filaments towards the M line. The thin filaments slide inwards and meet at the centre of the sarcomere, they may even overlap. As the thin filaments move inward, the I band and H zone narrow and eventually disappear altogether when the muscles is maximally contracted. The width of the A band and the individual lengths of the thick and thin filaments remain unchanged. The Z discs come closer together and the sarcomere shortens.

What is the structure of the thin filament in relaxed muscle?

Myosin is blocked from binding to actin because strands if tropomyosin cover the myosin-binding sites on actin. The tropomyosin strands in turn are held in place by troponin molecules.

Describe the structure of the sarcomere.

The H zone bisects the A band. Thin filaments = actin, thick filaments = myosin. The darker, middle part of the sarcomere is the A band which extends the entire length of the thick filaments. Towards each end of the A band there is a zone of overlap where the thick and thin filaments lie side by side. The H zone is in the centre of each A band and contains thick filaments only. The M line is in the middle of the sarcomere and bisects the H zone. The I band is the lighter, less dense area that contain the rest of the thin filaments. The I band is bisected by the Z-line.

What is the sarcoplasmic reticulum?

The sarcoplasmic reticulum is a specialized type of smooth ER that regulates the calcium ion concentration in the cytoplasm of striated muscle cells.

What occurs at the onset of contraction?

The sarcoplasmic reticulum releases Ca+ ions into the sarcoplasm. There, they bind to troponin which then undergoes a conformational change in shape. This change moves tropomyosin away from the myosin-binding sites on actin and muscle contraction subsequently begins.

What is the function of alpha-actin, dystrophin and titin?

α-actin maintains the actin lattice. Dystrophin anchors actin filaments to the sarcolemma. Titin connects the Z disc to the M line of the sarcomere, helping to stabilise the thick filament position.