HUMAN BIOLOGY

108 Chapter 6

how muscles Contract

n bones move when the skeletal muscles attached to them

contract and pull them.

a muscle contracts when its cells shorten

A skeletal muscle contracts when the individual muscle

fibers in it shorten. In turn, each muscle fiber shortens

when units of contraction inside its myofibrils shorten.

Each of these basic units of contraction is a sarcomere.

Bundles of fibers in a skeletal

muscle run parallel along the mus-

cle’s length (Figure 6.7A). Looking

a bit deeper, each of the myofibrils

in a muscle fiber is divided into

bands. The bands appear as an

alternating light–dark pattern when

they are stained and viewed under

a microscope. Bands in neighboring

myofibrils line up closely, which is

why a skeletal muscle fiber looks

striped. The dark bands are called

Z lines. They mark the ends of

each sarcomere.

Inside a sarcomere are many

filaments, some thick, others thin.

These thick and thin filaments are

arranged in an overlapping array

shown in the image in Figure 6.7A.

Each thin filament is like two

strands of beads, twisted together, with one end attached to

a Z line. The “beads” are molecules of actin (Figure 6.7B),

a globular protein that can contract.

Each thick filament is made of molecules of the protein

myosin. A myosin molecule has a tail and a double head.

In a thick filament many of them are bundled together so

that all the heads stick out (Figure 6.7C), away from the

sarcomere’s center.

As you can see in Figure 6.7, muscle bundles, muscle

fibers, myofibrils, and their filaments all run in the same

direction. This alignment focuses the force of a contract-

ing muscle. All sarcomeres in all fibers of a muscle work

together and pull a bone in the same direction.

muscle cells shorten when actin

filaments slide over myosin

A sliding filament mechanism explains how interactions

between thick and thin filaments allow muscle fibers to

contract. In a contraction, all the myosin filaments stay in

place. They use short “power strokes” to slide the sets of actin

Figure 6.7 Animated! This diagram zooms down through

skeletal muscle from a biceps to filaments of the proteins

actin and myosin. (A: top image, From Starr/Evers/Starr, Biology Today and

Tomorrow with Physiology, 3E. © 2010 Cengage Learning; top art, © Cengage Learning 2015;

bottom right photo, © Don Fawcett/ Visuals Unlimited; bottom left art, From Starr/ Taggart/

Evers/Starr, Biology, 13E. © 2013 Cengage Learning; B–C: © Cengage Learning)

6.3

actin Beadlike contractile
protein in muscle fibers.
myosin Contractile protein
in muscle fibers that has a
tail and a double head.
rigor mortis Stiffening of
muscles after death due to
the lack of ATP energy to
release muscle contraction.
sarcomere The basic unit
of contraction in skeletal
muscles.
sliding filament mechanism
The mechanism by which
skeletal muscles contract;
sarcomeres contract
(shorten) when myosin fila-
ments slide along and pull
actin filaments toward the
center of the sarcomere.

C Myosin molecules in the thick filaments.

B Actin molecules in the thin filaments. The green lines are

proteins associated with actin.

part of

a thin

filament

one actin

molecule

part of a

myosin

molecule

part of

a thick

filament

A

tendon

mitochondrion

thin

filament

(actin)

thick

filament

(myosin)

sarcomere

one of many

nuclei

muscle in

connective

tissue sheath

bundle of

muscle fibers

skeletal muscle fiber

myofibril

sarcomere

Z line Z line

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