Muscle Structure, Contraction, Neuromuscular Junctions & Disorders

What are muscle fibers?

Individual muscle cells arranged in bundles within a muscle.

What are myofibrils?

Many small structures within each muscle fiber, divided into repeating units called sarcomeres.

What is a sarcomere?

The segment of a myofibril that extends from one Z line to the next.

What do thick filaments consist of?

Myosin.

What do thin filaments consist of?

Actin.

What happens to sarcomeres during muscle contraction?

They become shorter and the Z lines move closer together.

Do actin and myosin filaments get shorter during contraction?

No, they do not get shorter.

What role does calcium (Ca2+) play in muscle contraction?

It binds to the troponin complex, causing tropomyosin to shift and expose myosin-binding sites on actin.

Why must myosin-binding sites be exposed for muscle contraction?

Myosin heads need to bind to actin to form cross-bridges and pull the thin filaments.

What happens to ATP during muscle contraction?

ATP is broken down into ADP and Pi, which energizes the myosin head.

What is the neuromuscular junction?

The site where a motor neuron communicates with a muscle fiber.

What neurotransmitter is released at the neuromuscular junction?

Acetylcholine (ACh).

What is the function of acetylcholinesterase at the neuromuscular junction?

It breaks down acetylcholine to terminate the signal.

What is a motor unit?

A motor neuron and all the muscle fibers it innervates.

What is Myasthenia Gravis?

An autoimmune disorder that leads to weakness of the skeletal muscles.

What is Gowers sign?

Using the hands to push up on the legs when standing, indicative of muscle weakness.

What is Duchenne Muscular Dystrophy?

A sex-linked recessive disorder that primarily affects males, characterized by muscle weakness.

What are early symptoms of Myasthenia Gravis?

Weakness of the eye muscles, difficulty swallowing, and slurred speech.

What changes might occur in standing or walking as muscle weakness progresses?

Standing may require pushing up on the legs (Gowers sign) and walking becomes more difficult.

What is the role of the sarcoplasmic reticulum in muscle contraction?

It releases calcium ions (Ca2+) into the cytosol to initiate contraction.

What is the function of the Ca2+ ATPase pump?

It returns calcium to the sarcoplasmic reticulum using ATP.

What happens to the overlap between actin and myosin during contraction?

The overlap increases as the sarcomere shortens.