Muscle Contraction Physiology

What is the main energy source for muscle contraction?

ATP

What neurotransmitter stimulates skeletal muscle contraction?

Acetylcholine (ACh)

What happens when acetylcholine reaches the motor end plate?

Sodium channels open and Na⁺ enters the muscle cell.

What does sodium entering the muscle cell cause?

Depolarization

What does the sarcoplasmic reticulum store?

Calcium (Ca²⁺)

What happens when the muscle cell depolarizes?

The sarcoplasmic reticulum releases calcium.

What is excitation-contraction coupling?

The process linking muscle depolarization to calcium release and muscle contraction.

What does calcium bind to?

Troponin

What happens when calcium binds to troponin?

Troponin causes tropomyosin to move away from the myosin-binding sites on actin.

What is tropomyosin's job at rest?

It blocks myosin-binding sites on actin.

What are the two main contractile proteins?

Actin and myosin

Which protein is the thin filament?

Actin

Which protein is the thick filament?

Myosin

What happens when myosin binds to actin?

A cross-bridge forms.

What is a cross-bridge?

A myosin head attached to actin.

What is the power stroke?

Myosin pulls actin toward the center of the sarcomere.

What is the recovery stroke?

Myosin returns to its original position.

What does ATP allow myosin to do?

Detach from actin and reset for another cycle.

What is a sarcomere?

The basic contractile unit of skeletal muscle between two Z lines.

What happens to the sarcomere during contraction?

It shortens.

What is cross-bridge cycling?

Myosin binds to actin, pulls actin, releases, and resets.

What are the four steps of cross-bridge cycling?

Binding → power stroke → release → recovery

What is a T-tubule?

A tube-like structure that carries the electrical signal deep into the muscle cell.

What is a triad?

One T-tubule and two terminal cisternae.

What is the sarcolemma?

The membrane surrounding a muscle cell.

What are terminal cisternae?

Widened portions of the sarcoplasmic reticulum that store calcium.

What is the overall muscle contraction sequence?

ACh → Na⁺ enters → depolarization → Ca²⁺ released → Ca²⁺ binds troponin → tropomyosin moves → myosin binds actin → power stroke → sarcomere shortens

What does calcium do during contraction?

Allows myosin to bind to actin by moving tropomyosin.

What does ATP do during contraction?

Provides energy and allows myosin to detach from actin.

What happens when calcium is removed from the sarcoplasm?

Tropomyosin blocks the myosin-binding sites and contraction stops.