HUMAN BIOLOGY

the MUSCULar SYSteM 109

hoW does a skeletal muscle fiber contract?

• A skeletal muscle fiber contracts when its sarcomeres shorten.
  Thus sarcomeres are the basic units of muscle contraction.


• Powered by ATP, interactions between myosin and actin filaments
  shorten the sarcomeres of a muscle fiber (the muscle cell).


• All the sarcomeres in all fibers of a muscle contract in unison,
  pulling a bone in the same direction.

takE-homE mEssagE

Although rigor mortis can be a useful tool in assessing

the time of death, various factors can affect how fast

it develops. Using reliable Web sources, do your own

investigation. What physiological characteristics of a

deceased person can skew the onset of rigor mortis?

What other factors do forensic scientists use to fill out

the picture of when—and even where—a person died?

ThiNk OuTsiDE ThE bOOk

filaments over them, toward the sarcomere’s center. Pulling
both sets of filaments shrinks the length of the sarcomere:

Each power stroke is driven by energy from ATP.
Each myosin head repeatedly “grabs” binding sites on a
nearby actin filament (Figure 6.8A). The head is an ATPase,
a type of enzyme. It binds ATP and catalyzes a phosphate-
group transfer that powers the reaction.
A rise in the concen tration of calcium ions causes the
myosin head to attach to the actin (Figure 6.8B). This link
tilts the myosin head and pulls the actin filament toward
the sarcomere’s center (Figure 6.8C and 6.8D). Next, with
the help of energy from ATP, the myosin head’s grip on
actin is broken and the head returns to its starting position
(Figure 6.8E). Each time a sarcomere contracts, hundreds
of myosin heads make a series of short strokes down the
length of actin filaments.
When someone dies, her or his body cells stop mak-
ing ATP. In muscles this means that the myosin cross-
bridges with actin can’t break apart after a power stroke.
As a result skeletal muscles “lock up,” a stiffening called
rigor mortis (“stiffness of death”). Rigor mortis lasts
for 24 to 60 hours, or until the natural decomposition of
dead tissues gets under way. Understanding rigor mortis
helps crime investigators determine when a suspicious
death occurred.

Figure 6.8 Animated! Sarcomeres shorten when actin and

myosin filaments interact. This interaction is the sliding filament

mechanism of muscle contraction. (From Starr, Biology, 8E. © 2011 Cengage

Learning)

ATP myosin head ATP

A A myosin filament in a resting muscle. All the myosin

heads were energized earlier by the binding of ATP.

ADP, Pi ADP, Pi

b Calcium released from a cellular storage system

allows myosin to bind to actin filaments.

ADP ADP

c Binding makes each myosin head tilt toward the center

of the sarcomere and slide the actin filaments along with it.

D Using energy from ATP, the myosin heads drag the

actin filiments inward, pulling the Z lines closer together.

ATP ATP

e New ATP binds to the myosin heads and they detach

from actin. The myosin heads return to their original

orientation, ready to act again.

contracted sarcomere

relaxed sarcomere

muscle

contraction

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