Action Potential

The first step in muscle contraction is when the __ reaches the neuromuscular junction.

Action potential

The action potential triggers the release of __ into the synaptic cleft.

Acetylcholine (ACh)

ACh binds to receptors on the __, leading to depolarization.

Sarcolemma

The muscle action potential travels along the sarcolemma and down the __.

T-tubules

The signal from the action potential causes the sarcoplasmic reticulum to release __ into the cytosol.

Ca²⁺ ions

Ca²⁺ binds to __, causing a change that exposes myosin-binding sites on actin filaments.

Troponin

The binding of myosin heads to actin forms __.

Cross-bridges

During the power stroke, myosin heads pivot and pull actin filaments inward; this is called the __.

Power stroke

After the power stroke, a new __ binds to myosin, causing it to detach from actin.

ATP molecule

Hydrolysis of ATP results in __, which helps re-cock the myosin head for the next cycle.

ADP + Pi

Calcium reuptake occurs when Ca²⁺ is __ back into the sarcoplasmic reticulum.

Pumped

As calcium levels drop, __ changes back to block myosin-binding sites on actin.

Troponin

Without Ca²⁺, cross-bridges __ and the muscle relaxes.

Detach

The entire process of muscle contraction begins with an __.

Action potential

The action potential triggers the release of __ from the motor neuron.

Acetylcholine (ACh)

The entry of Na⁺ through the sarcolemma leads to __.

Depolarization

The action potential travels down the T-tubules to signal the __.

Sarcoplasmic reticulum

Calcium ions bind to troponin to expose __ sites on actin.

Myosin-binding

The __ of the myosin head allows for the power stroke to occur.

Pivoting

The process requires ATP to allow __ to detach from actin.

Myosin heads

To continue the cycle of contraction, calcium must be __ in the cytosol.

Present

Active transport is needed to pump Ca²⁺ back into the __.

Sarcoplasmic reticulum

As Ca²⁺ levels decrease, tropomyosin __ the binding sites on actin.

Covers

Cross-bridges can only break in the presence of ATP and the lack of __.

Calcium

Muscle relaxation occurs when calcium is __ from the cytosol back into the SR.

Reabsorbed

The muscle contraction cycle is primarily dependent on __ ions.

Calcium

ACh acts on the receptors of the __ to initiate the contraction process.

Sarcolemma

Upon stimulation, the release of Ca²⁺ leads to muscle __.

Contraction

The contraction cycle can repeat as long as __ is present.

Calcium

When a muscle is relaxed, the cross-bridges are __ from actin.

Detaching

The energy for myosin head re-cocking comes from the hydrolysis of __.

ATP

The process of muscle contraction involves a cycle of binding, __, and relaxing.

Detaching

Muscle fibers are depolarized when __ enters the cell.

Sodium (Na⁺)