Myofilaments, Sarcomere, Sliding Filament Theory

Myofilaments

Protein bundles that make up myofibrils

There are three types:

thick filaments, thin filaments, and elastic filaments

Three functional types of muscle proteins in myofilaments

Contractile proteins produce tension

regulatory proteins control when contraction can occur

structural proteins hold myofilaments in place and stabilize the muscle fiber

Thick filaments

Largest-diameter myofilaments

composed primarily of the contractile protein myosin

Myosin

Contractile protein of thick filaments

each molecule has two globular heads and two intertwining chains forming a tail

The myosin heads interact with actin during contraction

Thin filaments

Contain contractile, regulatory, and structural proteins including actin, tropomyosin, troponin, and nebulin

Actin

Bead-shaped contractile protein of thin filaments

Each actin subunit has an active site that can bind to a myosin head

actin “string” appears as two intertwining strands in the functional thin filament

Tropomyosin

Long, ropelike regulatory protein that spirals around two actin strands

at rest, it covers the active sites on actin and prevents myosin from binding

Troponin

small globular regulatory protein that helps hold tropomyosin in place

works with tropomyosin to switch muscle contraction on and off

Nebulin

Structural protein believed to help determine the length of the thin filament by limiting how many actin subunits can make up the filament

similar to the string that holds the beads of a necklace

Elastic filaments & titin

Elastic filaments are the thinnest myofilaments and are composed of the springlike structural protein titin

Titin stretches and recoils, resists excessive stretching, and helps the muscle fiber return to its original length

titin runs through the core of the thick filament

Sarcomere

Repeating functional unit of a myofibril

The dark and light areas noticeable through the microscope are I, A, H, M, Z

extends from one Z-disc to the next Z-disc and shortens during muscle contraction

I band

Light region of a striation of the sarcomere that contains only thin filaments, so more light passes through

A band

Dark region of a striation

contains thick filaments which block more light

portions of the A band also contain overlapping thin filaments

H zone

Middle portion of the A band that contains only thick filaments

M line

Dark line in the middle of the A bands

consists of structural proteins that hold thick filaments in place

Z-discs

composed of structural proteins

Form the boundaries of each sarcomere

anchor thin filaments

provide attachment for elastic filaments

attach neighboring myofibrils to one another

Zone of overlap

Region where thick and thin filaments overlap, allowing myosin heads to interact with actin

Sarcomere from Z-disc to center

Z-disc → I band → A band → H zone → M line

the pattern is mirrored on the other half of the sarcomere

Sliding-filament mechanism

Thin filaments slide past thick filaments toward the M line, generating tension and shortening the sarcomere

the filaments themselves do NOT shorten

The Z-discs move closer together as the sarcomere shortens

The I band narrows as the thin filaments slide toward the M line

The H zone narrows as the thin filaments move farther into the center of the sarcomere

The A band remains the same width because the thick filaments do not change length

The zone of overlap between the thick and thin filaments increases

Neither thick or thin filament shortens

The thin filaments slide past the thick filaments toward the M line

How does shortening of sarcomeres shorten the whole muscle fiber?

Sarcomeres are arranged end to end within each myofibril

When many sarcomeres shorten simultaneously, the myofibrils and ultimately the entire muscle fiber shorten