How electrical stimulation leads to calcium release and contraction/ Cross-bridge formation and the sliding-filament mechanism

What is excitation in a skeletal muscle fiber?

A) Action potentials in a motor nerve fiber leading to action potentials in the muscle fiber | B) Calcium binding to troponin on thin filaments | C) Myosin heads pulling actin toward the M line | D) Acetylcholinesterase breaking down ACh

Action potentials in a motor nerve fiber leading to action potentials in the muscle fiber

Which ion enters the axon terminal to trigger synaptic vesicles to release acetylcholine (ACh)?
A) Sodium (Na+) | B) Potassium (K+) | C) Chloride (Cl-) | D) Calcium (Ca2+)

Calcium (Ca2+)

How many ACh molecules must bind to each ligand-gated receptor on the sarcolemma to open the ion channel?

A) One | B) Two | C) Four | D) Ten

Two

What causes the rapid up-and-down voltage fluctuation at the motor end plate called the end-plate potential (EPP)?

A) Calcium diffusing out of the sarcoplasmic reticulum | B) Potassium rushing rapidly into the cell | C) Na+ flowing quickly into the cell while K+ slowly flows out | D) Voltage-gated channels closing in the T tubules

Na+ flowing quickly into the cell while K+ slowly flows out

How does an end-plate potential (EPP) create an action potential in the muscle fiber?

A) It opens voltage-gated Na+ and K+ channels on the sarcolemma next to the end plate | B) It directly binds calcium to tropomyosin | C) It triggers exocytosis of new synaptic vesicles | D) It closes all ion channels along the sarcolemma

It opens voltage-gated Na+ and K+ channels on the sarcolemma next to the end plate

What is excitation–contraction coupling?

A) The breakdown of ACh in the synaptic cleft | B) Events linking action potentials on the sarcolemma to activation of the myofilaments | C) The return of calcium into the sarcoplasmic reticulum | D) The attachment of a tendon to the periosteum

Events linking action potentials on the sarcolemma to activation of the myofilaments

How does the wave of action potentials travel from the sarcolemma deep into the interior of the muscle cell?

A) Through gap junctions | B) Down the T tubules | C) Along the endomysium | D) Through the M line

Down the T tubules

What happens when action potentials open voltage-gated ion channels in the T tubules?

A) Calcium rushes into the axon terminal | B) Sodium floods the synaptic cleft | C) Linked calcium channels in the SR terminal cisterns open, releasing Ca2+ into the cytosol | D) Tropomyosin covers the active sites on actin

Linked calcium channels in the SR terminal cisterns open, releasing Ca2+ into the cytosol

Once released into the cytosol from the sarcoplasmic reticulum, which protein does calcium bind to?

A) Myosin | B) Tropomyosin | C) Dystrophin | D) Troponin

Troponin

What happens immediately after calcium binds to troponin?

A) The troponin–tropomyosin complex changes shape and exposes active sites on actin | B) Myosin heads detach from actin | C) Calcium is pumped back into the SR | D) Acetylcholine is released into the cleft

The troponin–tropomyosin complex changes shape and exposes active sites on actin

According to the sliding filament theory, what happens to individual myofilaments during contraction?

A) Thick filaments shorten while thin filaments stay the same | B) Neither thick nor thin filaments shorten; thin filaments slide over thick ones | C) Both thick and thin filaments shrink in length | D) Thick filaments slide outward past the Z discs

Neither thick nor thin filaments shorten; thin filaments slide over thick ones

Which enzyme in the myosin head hydrolyzes ATP into ADP and Pi to "cock" the head into a high-energy position?
A) Acetylcholinesterase | B) Calsequestrin | C) Myosin ATPase | D) Myoglobin

Myosin ATPase

What is formed when a cocked myosin head binds to an exposed active site on a thin actin filament?

A) A triad | B) A motor end plate | C) A junctional fold | D) A cross-bridge

A cross-bridge

What happens during the power stroke of cross-bridge cycling?

A) Myosin releases ADP and Pi and flexes, tugging the thin filament along with it | B) A new ATP binds to break the myosin–actin bond | C) Calcium is pumped back into the sarcoplasmic reticulum | D) Tropomyosin covers the active sites on actin

Myosin releases ADP and Pi and flexes, tugging the thin filament along with it

What directly causes the myosin head to detach from actin (breaking the cross-bridge) so it can perform a recovery stroke?

A) Release of ADP | B) Binding of a new ATP molecule to myosin | C) Breakdown of ACh by AChE | D) Calcium binding to troponin

Binding of a new ATP molecule to myosin

Why doesn't the thin filament slide backward when a myosin head detaches during the recovery stroke?

A) Titin locks the thin filament in place | B) Troponin freezes the sarcomere | C) Myosin heads contract sequentially, half are holding onto actin | D) Z discs fuse together

Myosin heads contract sequentially, half are holding onto actin

Once nerve signals stop arriving at the NMJ, which enzyme breaks down remaining ACh to stop muscle stimulation?

A) Myosin ATPase | B) Tropomyosin | C) Calsequestrin | D) Acetylcholinesterase (AChE)

Acetylcholinesterase (AChE)

What happens to cytosolic Ca2+ levels when excitation ceases during relaxation?

A) The sarcoplasmic reticulum reabsorbs Ca2+; cytosolic levels fall | B) Ca2+ floods out of the T tubules | C) Ca2+ stays permanently bound to troponin | D) Ca2+ exits through gap junctions

The sarcoplasmic reticulum reabsorbs Ca2+; cytosolic levels fall

Which protein moves back to block the active sites on actin after calcium dissociates from troponin?

A) Myosin | B) Titin | C) Tropomyosin | D) Dystrophin

Tropomyosin

How does a relaxed muscle return to its original resting length?

A) Relaxation automatically pushes the Z discs apart | B) An external force (like an antagonistic muscle or gravity) must stretch it back | C) Myosin heads push the thin filaments outward | D) The epimysium springs it back

An external force (like an antagonistic muscle or gravity) must stretch it back