cylindrical. Like other cells, muscle fibers have mitochon-
dria to provide energy and a plasma membrane to sepa-
rate the inside of the cell from the outside environment.
However, skeletal muscle cells differ from other cells in
several ways:
A muscle cell has more than one nucleus.
A muscle cell’s plasma membrane invaginates regu-
larly into the deep parts of the muscle fiber to form
a transverse tubule, or T-tubule.
The endoplasmic reticulum of a muscle
cell is called the
sarcoplasmic reticulum
.
The primary function of the sarcoplasmic
reticulum is to serve as a reservoir to
store calcium ions between muscle con-
tractions. The regularly structured sarco-
plasmic reticulum ends as small sacs near
the T-tubules.
The arrangement of myofilaments (threadlike proteins)
and other proteins within the muscle fibers allows them to
shorten. The proteins and the surrounding connective tis-
sue are stretchable and allow muscle cells to lengthen. Let’s
take a closer look at this arrangement of the myofilaments,
which allows muscle cells to both contract and relax.
The Sliding Filament Theory Explains
Muscle Contraction
Early on,
scientists
thought
that
muscle
cells
fold
when they contract, thereby causing mus-
cles to shorten. However, when scientists in
the 1950s examined relaxed and contracted
muscle cells with electron microscopes, they
found that the lengths of the thick and thin
filaments do not change; only the length of
the sarcomere changes. Physiologists A. L.
Hodgkin and H. E. Huxley explained these
findings with a hypothesis called the
s lid in g f i l a m e n t th e-
ory,
in which thin filaments slide past the thick fila-
ments (Figure 6.3).
sarcoplasmic
reticulum
(sar'-ko-
PLAZ-mik re-TIK-Q-lum)
A network of saccules
and tubes surround-
ing myofibrils of a
muscle fiber.
Sliding filam ent th e o ry o f m uscle co n tractio n
• Figure 6.3
When muscles contract, thin filaments slide along the thick filaments toward the
M-line, thereby shortening the sarcomere. When the thick filaments butt up
against the Z discs, the sarcomere cannot shorten anymore.
2 Sarcomeres
H zone
I band
A band
I
Thick filament
Z disc Thin filament
Z disc
M line
Z disc
a.
Relaxed muscle
Z disc
M line
Z disc
H zone
I----------------- ---------------------------- 1---------------------------- ----------------- 1—
I band
A band
I band
b. Partially contracted muscle
c.
Maximally contracted muscle
158 CHAPTER 6
The Muscular System
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