334
UNIT TWO
the muscle layers in the walls of veins thicken, making the
vessels more rigid and less elastic. Gradually, the muscles
become smaller, drier, and capable of less forceful contrac-
tion. Connective tissue and adipose cells begin to replace
some muscle tissue. By age eighty, nearly half the muscle
mass has atrophied, due to a decline in motor neuron activ-
ity. Diminishing muscular strength slows refl
exes.
Exercise can help maintain a healthy muscular system
throughout life, countering the less effective oxygen deliv-
ery that results from the decreased muscle mass that accom-
panies aging. Exercise can even lead to formation of new
muscle by stimulating skeletal muscle cells to release inter-
leukin-6 (IL-6), a type of proinfl ammatory molecule called a
cytokine. The IL-6 stimulates satellite cells, which function
as muscle stem cells. They divide and migrate, becoming
incorporated into the muscle F
ber. Exercise also maintains
the fl
exibility of blood vessels, which helps to keep blood
pressure at healthy levels. A physician should be consulted
before starting any exercise program.
According to the National Institute on Aging, exercise
should include strength training and aerobics, with stretching
before and after. Strength training consists of weight lifting or
using a machine that works speciF
c muscles against a resis-
tance, performed so that the same muscle is not exercised on
consecutive days. Strength training increases muscle mass,
and the resulting stronger muscles can alleviate pressure on
the joints, which may lessen arthritis pain. Aerobic exercise
improves oxygen use by muscles and increases endurance.
Stretching increases fl exibility and decreases muscle strain,
while improving blood fl
ow to all muscles. A side beneF t of
regular exercise, especially among older individuals, is fewer
bouts of depression.
PRACTICE
31
What changes are associated with an aging muscular system?
32
Describe two types of recommended exercise.
Invertor
The
tibialis posterior
(tib
e-a
lis pos-te
¯r
e-or) is the deepest
of the muscles on the back of the leg. It connects the F
bula
and tibia to the ankle bones by means of a tendon that curves
under the medial malleolus. This muscle assists in inversion
and plantar fl exion of the foot (see F g. 9.43). The dorsifl
exor
extensor hallucis longus,
because it pulls up on the medial
portion, also inverts the foot (see F
g. 9.41).
Evertor
The
f
bularis
(peroneus)
longus
(F b
u-la
ris long
gus) is a
long, straplike muscle located on the lateral side of the leg.
It connects the tibia and the F bula to the foot by means of
a stout tendon that passes behind the lateral malleolus. It
everts the foot, assists in plantar fl
exion, and helps support
the arch of the foot (see F
gs. 9.42 and 9.44).
As in the wrist, fascia in various regions of the ankle
thicken to form retinacula. Anteriorly, for example,
extensor
retinacula
connect the tibia and F bula as well as the calcaneus
and fascia of the sole. These retinacula form sheaths for ten-
dons crossing the front of the ankle (see F gs. 9.41 and 9.42).
Posteriorly, on the inside, a
flexor retinaculum
runs
between the medial malleolus and the calcaneus and forms
sheaths for tendons passing beneath the foot (see F g. 9.43).
Fibular retinacula
connect the lateral malleolus and the cal-
caneus, providing sheaths for tendons on the lateral side of
the ankle (see F
g. 9.42).
9.9
LIFE-SPAN CHANGES
Signs of aging in the muscular system begin to appear
in one’s forties, although a person can still be active. At a
microscopic level, supplies of the molecules that enable mus-
cles to function—myoglobin, ATP, and creatine phosphate—
decline. The diameters of some muscle F
bers may shrink, as
2. Skeletal muscle F
bers
a. Each skeletal muscle F
ber is a single muscle cell,
the unit of contraction.
b. Muscle F bers are cylindrical cells with many
nuclei.
c. The cytoplasm contains mitochondria,
sarcoplasmic reticulum, and myoF
brils of actin
and myosin.
d. The arrangement of the actin and myosin F
laments
causes striations. (I bands, Z lines, A bands, H
zone and M line)
e. Cross-bridges of myosin F
laments form linkages
with actin F
laments. The reaction between actin
and myosin F
laments provides the basis for
contraction.
CHAPTER SUMMARY
9.1
INTRODUCTION (PAGE 285)
All movements require muscles. The three types of muscle
tissue are skeletal, smooth, and cardiac.
9.2
STRUCTURE OF A SKELETAL MUSCLE
(PAGE 285)
Skeletal muscles are composed of nervous, vascular
and various other connective tissues, as well as skeletal
muscle tissue.
1. Connective tissue coverings
a. ±ascia covers each skeletal muscle.
b. Other connective tissues surround cells and groups
of cells within the muscle’s structure (epimysium,
perimysium, endomysium).
c. ±ascia is part of a complex network of connective
tissue that extends throughout the body.
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