293
CHAPTER NINE
Muscular System
Relaxation
When nerve impulses cease, two events relax the muscle f ber.
First, an enzyme called
acetylcholinesterase
rapidly decom-
poses acetylcholine remaining in the synapse. This enzyme,
present in the synapse and on the membranes o± the motor
end plate, prevents a single nerve impulse ±rom continuously
stimulating a muscle f ber. Second, when ACh breaks down,
the stimulus to the sarcolemma and the membranes o± the
muscle f ber ceases. The calcium pump (which requires ATP)
quickly moves calcium ions back into the sarcoplasmic retic-
ulum, decreasing the calcium ion concentration o± the cyto-
sol. The cross-bridge linkages break (see f g. 9.10
6
—this also
requires ATP, although it is not broken down in this step),
and tropomyosin rolls back into its groove, preventing cross-
bridge attachment (see f g. 9.10
1
). Consequently, the muscle
f ber relaxes.
Table 9.1
summarizes the major events leading
to muscle contraction and relaxation.
If acetylcholine receptors at the motor end plate are too few, or
blocked, muscles cannot receive the signal to contract. This may occur
as the result of a disease, such as myasthenia gravis, or exposure to a
poison, such as nerve gas. A drug called pyridostigmine bromide is
used to treat myasthenia gravis. The drug inhibits the enzyme (ace-
tylcholinesterase) that normally breaks down acetylcholine, keep-
ing the neurotransmitter around longer. It was given to veterans of
the F
rst Gulf War who reported muscle aches in the months following
their military service. Health o±
cials reasoned that the drug’s e²
ect on
myasthenia gravis might also help restore muscle function if the vet-
erans’ symptoms arose from exposure to nerve gas during the war.
Acetylcholinesterase inhibitors are also used as insecticides. The
buildup of acetylcholine causes an insect to twitch violently, then die.
It is important to remember that ATP is necessary ±or
both muscle contraction and ±or muscle relaxation. The trig-
ger ±or contraction is the increase in cytosolic calcium in
response to stimulation by ACh ±rom a motor neuron.
A few hours after death, the skeletal muscles partially contract, F
xing
the joints. This condition, called
rigor mortis,
may continue for seventy-
two hours or more. It results from an increase in membrane perme-
ability to calcium ions, which promotes cross-bridge attachment, and
a decrease in availability of ATP in the muscle F
bers, which prevents
cross-bridge release from actin. Thus, the actin and myosin F
laments of
the muscle F
bers remain linked until the muscles begin to decompose.
PRACTICE
5
Describe a neuromuscular junction.
6
DeF
ne motor unit.
7
List four proteins associated with myoF
brils, and explain their
structural and functional relationships.
8
Explain how the F
laments of a myoF
bril interact during muscle
contraction.
9
Explain how a motor nerve impulse can trigger a muscle contraction.
FIGURE 9.11
When a skeletal muscle contracts (
a
), individual
sarcomeres shorten as thick and thin F
laments slide past one
another. (
b
) Transmission electron micrograph showing a sarcomere
shortening during muscle contraction (23,000×).
Z line
(a)
Z line
Sarcomere
A band
Contracting
Thin
filaments
Thick
filaments
Fully contracted
Relaxed
2
3
1
Z line
(b)
Z line
Sarcomere
A band
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