💪Sarcomere Anatomy + Sliding Filament Theory

Sliding filament theory

The process of muscle contraction in which the thin filaments slide toward the center of the sarcomere while the thick and thin filaments themselves do not shorten.

Sarcomere

The basic functional unit of a muscle fiber that extends from one Z line to the next and shortens during muscle contraction.

Z line

A structure at each end of a sarcomere that anchors the thin filaments; the two of these structures move closer together when the sarcomere contracts.

H zone

The central region of a sarcomere that contains only thick filaments; it becomes smaller as the thin filaments slide toward the center during contraction.

M line

A structure in the center of the sarcomere that helps hold the thick filaments in place.

A band

The darker region of a sarcomere that corresponds to the entire length of the thick filaments; its length stays the same during contraction.

I band

The lighter region of a sarcomere containing thin filaments but no thick filaments; it becomes smaller as the sarcomere contracts.

Actin

The protein that forms the thin filament and is attached to the Z lines; it slides toward the center of the sarcomere during contraction.

Myosin

The protein that forms the thick filament and contains heads that attach to actin and pull it toward the center of the sarcomere.

Thin filament

A filament made primarily of actin that is attached to the Z lines and slides toward the center of the sarcomere during contraction.

Thick filament

A filament made primarily of myosin that stays near the center of the sarcomere while its heads pull the thin filaments inward.

Myosin head

The knob-like portion of a myosin molecule that attaches to actin and pulls the thin filament during contraction.

Myosin tail

The long portion of a myosin molecule that helps form the thick filament.

Actin subunits

The individual protein molecules that join together to form the actin filament.

Myosin-binding site

The specific location on an actin filament where a myosin head attaches to form a cross-bridge.

Actin-binding site

The region on a myosin head where it interacts with actin during the formation of a cross-bridge.

Flexible hinge region

The bendable part of a myosin molecule that allows its head to change position during the contraction cycle.

Cross-bridge

The attachment between a myosin head and a myosin-binding site on actin.

Power stroke

The movement in which an attached myosin head pivots and pulls the actin filament toward the center of the sarcomere.

Relaxed sarcomere

A sarcomere in which the Z lines are farther apart and tropomyosin covers the myosin-binding sites on actin.

Contracted sarcomere

A sarcomere in which the Z lines are closer together because the thin filaments have been pulled toward the center.