How Muscles Contract

Muscle cells (or muscle fibers) are filled with long, thin structures called myofibrils.

Muscle cells (or muscle fibers) are filled with long, thin structures called myofibrils.

The myofibrils are little microscopic bundles of myofilaments.

(You do NOT need to draw the next illustration!)

There are two types of myofilaments:

There are two types of myofilaments:

Thick myofilaments are made of a protein called myosin.

There are two types of myofilaments:

Thick myofilaments are made of a protein called myosin. They appear as the dark bands or striations (stripes) in skeletal muscle.

Thin myofilaments are made of a protein called actin.

Thin myofilaments are made of a protein called actin.

They appear as the light bands or striations in skeletal muscle.

Magnified skeletal muscle

A unit of a dark band and light band is called a sarcomere.

A unit of a dark band and a light band is called a sarcomere.

This is the functional unit of a muscle that contracts.

A unit of a dark band + a light band (really, two halves of a light band) is called a sarcomere.

This is the contracting unit of a muscle.

A muscle fiber contains many sarcomeres in a repeating pattern, which is why you see “stripes”.

A sarcomere seen with very strong magnification.

A diagram of a sarcomere. The light bands are pink, the dark bands are

green.

A sarcomere contracts through chemical reactions triggered by the release of calcium ions stored in the muscle fiber’s sarcoplasmic reticulum, which wraps around the sarcomere.

When many, many sarcomeres contract, the muscle contracts.

The filaments of the light and dark bands link and pull together, sliding past one another.

The muscle gets shorter and thicker.

When the filaments unlink, the muscle relaxes and gets longer.

That’s how you get movement!