Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 5

Excitable Tissue: Muscle 95

STRIATIONS

Differences in the refractive indexes of the various parts of the
muscle fiber are responsible for the characteristic cross-stria-
tions seen in skeletal muscle when viewed under the micro-
scope. The parts of the cross-striations are frequently
identified by letters (Figure 5–2). The light I band is divided by
the dark Z line, and the dark A band has the lighter H band in
its center. A transverse M line is seen in the middle of the H
band, and this line plus the narrow light areas on either side of
it are sometimes called the pseudo-H zone. The area between
two adjacent Z lines is called a
sarcomere.
The orderly arrange-
ment of actin, myosin, and related proteins that produces this
pattern is shown in Figure 5–3. The thick filaments, which are

about twice the diameter of the thin filaments, are made up of

myosin; the thin filaments are made up of actin, tropomyosin,

and troponin. The thick filaments are lined up to form the A

bands, whereas the array of thin filaments extends out of the A

band and into the less dense staining I bands. The lighter H

bands in the center of the A bands are the regions where, when

the muscle is relaxed, the thin filaments do not overlap the

thick filaments. The Z lines allow for anchoring of the thin fil-

aments. If a transverse section through the A band is exam-

ined under the electron microscope, each thick filament is

seen to be surrounded by six thin filaments in a regular hexag-

onal pattern.

The form of myosin found in muscle is myosin-II, with two

globular heads and a long tail. The heads of the myosin

FIGURE 5–2
Electron micrograph of human gastrocnemius muscle.
The various bands and lines are identified at the top (
×
13,500).
(Courtesy of Walker SM, Schrodt GR.)

A band I band H band Z line M line

FIGURE 5–3
A)
Arrangement of thin (actin) and thick (myosin) filaments in skeletal muscle (compare to Figure 5–2).
B)
Sliding of actin on
myosin during contraction so that Z lines move closer together.
C)
Detail of relation of myosin to actin in an individual sarcomere, the functional
unit of the muscle.
D)
Diagrammatic representation of the arrangement of actin, tropomyosin, and troponin of the thin filaments in relation to a
myosin thick filament. The globular heads of myosin interact with the thin filaments to create the contraction. Note that myosin thick filaments
reverse polarity at the M line in the middle of the sarcomere, allowing for contraction.
(
C
and
D
are modified with permision from Kandel ER, Schwartz JH,
Jessell TM [editors]:
Principles of Neural Science
, 4th ed. McGraw-Hill, 2000.)

A band

Sarcomere

Z line Thick

filament

Thin

filament

Myosin

Relaxed Contracted

Actin

Z line

Actin

Z line

Myosin

M line

Actin

Tropomyosin Troponin

IC

T ICT

Actin

AB

CD