289
CHAPTER NINE
Muscular System
Actin, myosin, troponin, and tropomyosin are abundant in muscle
cells. Scarcer proteins are also vital to muscle function. This is the case
for a rod-shaped muscle protein called
dystrophin
. It accounts for only
0.002% of total muscle protein in skeletal muscle, but its absence
causes the devastating inherited disorder Duchenne muscular dystro-
phy, a disease that only aF
ects males. Dystrophin binds to the inside
face of muscle cell membranes, supporting them against the powerful
force of contraction. Without even these minute amounts of dystro-
phin, muscle cells burst and die. Other forms of muscular dystrophy
result from abnormalities of proteins to which dystrophin attaches.
Neuromuscular Junction
Recall from chapter 5 (p. 164) that neurons establish commu-
nication networks throughout the body. Each neuron has a
process called an axon, which extends from the cell body and
is capable of conducting a nerve impulse. Neurons that control
effectors, including skeletal muscle, are called
motor neurons.
Each skeletal muscle F
ber is functionally (but not physi-
cally) connected to an axon of a motor neuron that passes
outward from the brain or the spinal cord, in much the same
way that you can talk into a cell phone although your mouth
is not in direct physical contact with it. The site of this func-
tional connection is called a
synapse.
It is a space through
which information can pass. Neurons communicate with the
cells that they control by releasing chemicals, called
neu-
rotransmitters
(nu
ro-trans-mit-erz), at a synapse. Normally
a skeletal muscle F ber contracts only upon stimulation by a
motor neuron.
Although muscle ±
bers and the connective tissues associated with
them are ²
exible, they can tear if overstretched. This type of injury,
common in athletes, is called a
muscle strain
. The seriousness of the
injury depends on the degree of damage the tissues sustain. In a mild
strain, only a few muscle ±
bers are injured, the fascia remains intact,
and little function is lost. In a severe strain, many muscle ±
bers as well
as fascia tear, and muscle function may be lost completely. A severe
strain is very painful and is accompanied by discoloration and swell-
ing of tissues due to ruptured blood vessels. Surgery may be required
to reconnect the separated tissues.
PRACTICE
1
Describe how connective tissue is associated with a skeletal muscle.
2
Describe the general structure of a skeletal muscle ±
ber.
3
Explain why skeletal muscle ±
bers appear striated.
4
Explain the physical relationship between the sarcoplasmic
reticulum and the transverse tubules.
9.3
SKELETAL MUSCLE
CONTRACTION
A muscle F ber contraction is a complex interaction of several
cellular and chemical constituents. The result is a movement
within the myofibrils in which the filaments of actin and
myosin slide past one another, shortening the sarco meres.
When this happens, the muscle F
ber shortens and pulls on
its attachments.
Nucleus
Sarcoplasmic
reticulum
Openings into
transverse tubules
Mitochondria
Thick and thin
filaments
Sarcolemma
Sarcoplasm
Nucleus
Cisternae of
sarcoplasmic reticulum
Transverse tubule
Triad
Myofibrils
FIGURE 9.7
Within the sarcoplasm of a skeletal muscle ±
ber is
a network of sarcoplasmic reticulum and a system of transverse
tubules.
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