Neuromuscular Junction and Muscle Contraction Initiation

Which event initiates the interaction between a motor neuron and a muscle fiber at the neuromuscular junction?

• Arrival of an action potential at the axon terminal

• Release of calcium from the sarcoplasmic reticulum

• Binding of myosin heads to actin

• Repolarization of the sarcolemma

Place the early neuromuscular junction events in the correct order from first to last.

• ACh binds receptors → action potential arrives → ion permeability changes

• Action potential arrives → ACh binds receptors → ion permeability changes

• Ion permeability changes → action potential arrives → ACh binds receptors

• ACh binds receptors → ion permeability changes → myosin binds actin

At the neuromuscular junction, acetylcholine (ACh) primarily functions to do which of the following?

• Trigger calcium release from the sarcoplasmic reticulum

• Change ion permeability by binding receptors on the sarcolemma

• Detach myosin heads from actin to end contraction

• Carry oxygen to the muscle fiber

During depolarization of the sarcolemma at the end plate, which ion movement predominates and what is the effect on membrane voltage?

• Potassium rushes in; membrane becomes more negative

• Sodium rushes in; membrane becomes more positive

• Calcium rushes out; membrane becomes neutral

• Chloride rushes in; membrane hyperpolarizes

Which statement best defines an action potential in this context?

• A permanent reversal of charge that stores energy in the membrane

• A chemical messenger that crosses the synaptic cleft

• An electrical signal that temporarily reverses charge and travels along a neuron or muscle cell

• A mechanical wave that stretches the sarcolemma

Which sequence correctly traces excitation–contraction coupling after the end plate potential ignites an action potential in the sarcolemma?

• AP travels along T tubules → AP spreads across sarcolemma → SR releases Ca2+

• SR releases Ca2+ → AP travels across sarcolemma → AP travels along T tubules

• AP spreads across sarcolemma → AP travels along T tubules → SR releases Ca2+

• Myosin binds actin → SR releases Ca2+ → AP travels along T tubules

Once calcium is released from the sarcoplasmic reticulum in skeletal muscle, what is its immediate binding target and effect?

• Troponin; exposes myosin-binding sites on actin

• Tropomyosin; blocks active sites on actin

• Myosin heads; detaches cross-bridges

• Acetylcholine receptors; opens sodium channels

Which event directly marks the beginning of contraction in this sequence?

• ACh release into the synaptic cleft

• Depolarization of the sarcolemma

• Myosin heads binding to actin to form cross-bridges

• Opening of potassium channels to repolarize the membrane

Match each phase or step term with its description.

• AP spreads; T tubules conduct signal; SR releases Ca2+

• T tubules

• Local depolarization that ignites an AP

• Phase 1: Motor neuron stimulates muscle fiber

• Conduits carrying the AP deep into the fiber

• Phase 2: Excitation–contraction coupling

• AP arrives; ACh released; ion permeability changes

• End plate potential

Which pair correctly matches the ion and its direction of movement during the early depolarization at the neuromuscular junction?

• Na+ moves in; K+ moves out

• Na+ moves out; K+ moves in

• K+ and Na+ both move out

• K+ and Na+ both move in