💥| Chapter 12: Muscle Physiology

What is the light band of a sarcomere?

The I band

What is the dark band of a sarcomere?

The A band

Which filaments are found in the I band?

Thin filaments (actin only)

Which filaments are found in the A band?

Thick filaments (myosin) and overlapping thin filaments

What structure defines one sarcomere?

Z disc to Z disc

What structure anchors thick filaments to each other?

The M line

What is actin?

The thin filament protein

What is myosin?

The thick filament protein with ATP- and actin-binding sites

What is the role of tropomyosin?

Blocks myosin-binding sites on actin at rest

What is troponin?

A regulatory protein complex that controls tropomyosin movement

Which troponin subunit binds calcium?

Troponin C

What is titin?

Elastic protein connecting Z disc to myosin; aids recoil

What is the Z disc?

Sarcomere boundary that anchors thin filaments

What is the specialized region of the sarcolemma at the neuromuscular junction?

Motor end plate

What neurotransmitter is released at the neuromuscular junction?

Acetylcholine (ACh)

What produces an end plate potential?

ACh binding to receptors opening ligand-gated ion channels

How does an end plate potential lead to a muscle action potential?

Depolarization opens voltage-gated Na+ channels

What is a motor unit?

A motor neuron and all the muscle fibers it innervates

How does motor unit recruitment increase force?

By activating more motor units

What happens to sarcomere length during contraction?

Decreases

What happens to I band length during contraction?

Decreases

What happens to A band length during contraction?

Remains unchanged

What happens to the distance between adjacent A bands during contraction?

Decreases

Where does myosin bind actin?

At myosin-binding sites on actin

Where does myosin bind ATP?

On the myosin head

What enzyme activity does the myosin head have?

ATPase

What triggers the power stroke?

Release of inorganic phosphate (Pi)

What causes cross-bridges to detach?

ATP binding to myosin

Which steps of the cross-bridge cycle require ATP?

Myosin head cocking and detachment from actin

What molecule blocks myosin-binding sites at rest?

Tropomyosin

What causes tropomyosin to move?

Ca2+ binding to troponin C

Why is Ca2+ required for contraction?

It exposes myosin-binding sites on actin

What structures carry action potentials into the muscle fiber?

Transverse (T) tubules

What organelle stores Ca2+?

Sarcoplasmic reticulum

What channels release Ca2+ from the sarcoplasmic reticulum?

Ryanodine receptors

What is the process linking action potentials to contraction?

Excitation-contraction coupling

Why is ATP required for muscle relaxation?

To detach myosin from actin and pump Ca2+ back into the SR

Where is Ca2+ pumped during relaxation?

Into the sarcoplasmic reticulum

What pump transports Ca2+ back into the SR?

SERCA (Ca2+-ATPase)

What happens to cross-bridges if ATP is unavailable?

They remain attached (rigor)

Does cardiac muscle require nerve stimulation to contract?

No, it contracts automatically

What role do voltage-gated Ca2+ channels play in cardiac muscle?

Ca2+ entry triggers Ca2+-induced Ca2+ release

Why does blocking Ca2+ channels prevent cardiac contraction?

SR Ca2+ release cannot occur

What happens to cross-bridges when ATP is lost?

They remain attached

Can a muscle relax without ATP?

No

What happens if Ca2+-ATPase pumps stop working?

Ca2+ remains elevated, prolonging contraction

How can muscle force increase without changing stimulus strength?

Motor unit recruitment