Muscle Physiology and Sarcomere Structure

These flashcards cover the molecular structure of muscle fibers, the proteins involved in contraction and regulation, the structure of the sarcomere, and the concept of motor units.

What is the difference between a myofilament and a myofibril?

A myofilament is an individual strand of protein, while myofibrils are bundles of myofilaments.

Which protein is known as the thick filament in skeletal muscle?

Myosin

Which protein is known as the thin filament in skeletal muscle?

Actin

Which two proteins are classified as contractile proteins in muscle tissue?

Actin and Myosin

Which two proteins are classified as regulating proteins in muscle tissue?

Troponin and Tropomyosin

What is the function of tropomyosin in a relaxed muscle cell?

It acts as a ribbon-like protein that blocks the active binding sites on actin, preventing myosin from binding.

What is the function of troponin in muscle regulation?

It acts like staples or tacks to hold the tropomyosin in place over the actin binding sites.

What is a sarcomere?

The functional subunit of skeletal and cardiac muscle that extends from one $Z$ disc to another $Z$ disc.

To which structure in the sarcomere does myosin attach?

The $M$ line

To which structure in the sarcomere does actin attach?

The $Z$ discs

What creates the dark and light stripping (striation) appearance in muscle cells?

The overlap of thick and thin filaments creates dark zones, while areas with fewer layers (no overlap) appear as light zones.

According to the sliding filament theory, how do the lengths of actin and myosin change during contraction?

The proteins do not change their length; the overall arrangement of the sarcomere shortens as the proteins slide past each other.

Why are troponin levels measured in the bloodstream during a suspected heart attack?

Troponin is a very small protein that is the first to leak out of heart muscle cells as they break down or die from lack of oxygen.

What is the physiological cause of delayed onset muscle soreness after a workout?

It is caused by inflammation and increased fluid pressure (swelling) around the cells that squashes simple bare nerve endings, rather than damage to the proteins themselves.

What is a motor unit?

A motor unit consists of one motor neuron and all of the individual muscle cells it controls or innervates.

How do motor units differ between areas requiring fine motor control versus gross movement?

Areas for fine motor control (like the face) have small motor units with one neuron controlling only a few cells (e.g., $4$), while areas for gross movement (like the calf) have large motor units where one neuron controls thousands of cells (e.g., $1,000$).

What is the neuromuscular junction ($NMJ$)?

The point where a motor neuron and a muscle cell meet to communicate via neurotransmitters, separated by a small gap.

What is denervation atrophy?

The shrinking and breakdown of muscle tissue that occurs when the functional nerve signal to the muscle is lost, such as in paralysis.