10.3 The Sarcomere Anatomy and General Function

What are sarcomeres?

Repeating units of myofilaments that are found along the length of myofibrils

What is the function of a sarcomere?

Muscle contraction and relaxation

What are sarcomeres?

Repeating units of myofilaments that are found along the length of myofibrils

What is the function of a sarcomere?

Muscle contraction and relaxation

What are four things that make up a sarcomere?

- Thick filaments

- Thin filaments

- Proteins that stabilize the position of the myofilaments

- Proteins that regulate interactions between the myofilaments

What is the main protein that makes up the thick filament of the sarcomere?

Myosin

What is the main protein that makes up the thin filament of the sarcomere?

Actin

What are the two divisions of a sarcomere?

- A bands

- I bands

The A band is the ____ band?

a) Light

b) Dark

Dark

The I band is the ____ band?

a) Light

b) Dark

Light

What are the three subdivisions of the A band?

- M line

- H Band

- Zone of overlap

Where is the M line located?

The center of the A band

What are the two functions of the M line?

- Connects neighboring thick filaments together

- Stabilizes the positions of the thick filaments

Where is the H band located?

On either side of the M line

What types of filaments are found in the H band?

Only thick filaments

Where is the zone of overlap located?

On the outer edges of the A band

What types of filaments are found in the zone of overlap?

Thick and thin filaments

What types of filaments are found in the I band?

Only thin filaments

What structure bisects the I band?

The Z line

What are two proteins found at the Z line?

- Actinin

- Titin

What is the function of actinin?

Connects the neighboring thin filaments together

What is the function of titin?

- Stabilizes the positions of the thick and thin filaments

- Restores resting sarcomere length after contraction

What are the four proteins that make up a thin filament?

- F-actin

- Nebulin

- Tropomyosin

- Troponin

What does F-actin stand for?

Filamentous actin

What protein is F-actin made up of?

G-actin

What is a structure found on G-actin?

Active site

What is the active site on G-actin?

It is the binding site for myosin from the thick filament

What is the function of nebulin?

Holds the F-actin together

What is the function of tropomyosin?

It blocks the active sites on G-actin and prevents myosin from binding

How many active sites can one molecule of tropomysin cover?

Seven

What other molecule is tropomyosin bound to?

Troponin

How many subunits does troponin contain?

Three

What are the functions of the three subunits of troponin?

- One subunit binds to tropomyosin

- One subunit bonds to a G-actin

- One subunit has a calcium receptor

How many calcium ions can one troponin bind?

Two

What type of molecules is a thick filament composed of?

Myosin molecules

About how many myosin molecules make up a thick filament?

300

Describe the structure of a myosin molecule.

- Composed of two subunits twisted around one another

- Has a long tail and a free head composed of two globular protein subunits

What is the function of the myosin tail?

It binds to other myosin molecules in the thick filament

What is the function of the myosin head?

Binds with the active site on a thin filament

The ____ of a myosin molecule points toward the M line?

a) Tail

B) Head

Tail

What is the connection between a myosin head and the active site on a thin filament called?

Cross-bridge

What protein is found at the core of a thick filament?

Titin

How do sarcomeres shorten?

Myosin heads bind and form a cross-bridge with actin molecules. The myosin heads then pull on the actin molecules causing them to slide along the myosin filaments. Thus, sarcomeres shorten.

What is the sliding filament theory?

A theory that postulates that the thin filaments slide toward the center of a sarcomere during muscle contraction