335
CHAPTER NINE
Muscular System
7. Energy sources for contraction
a. ATP supplies the energy for muscle F
ber
contraction.
b. Creatine phosphate stores energy that can be used
to synthesize ATP as it is decomposed.
c. Active muscles require cellular respiration for
energy.
8. Oxygen supply and cellular respiration
a. Anaerobic reactions of cellular respiration yield
few ATP molecules, whereas aerobic reactions of
cellular respiration provide many ATP molecules.
b. Hemoglobin in red blood cells carries oxygen from
the lungs to body cells.
c. Myoglobin in muscle cells temporarily stores some
oxygen.
9. Oxygen debt
a. During rest or moderate exercise, oxygen is
sufF
cient to support the aerobic reactions of
cellular respiration.
b. During strenuous exercise, oxygen deF
ciency may
develop, and lactic acid may accumulate as a result
of the anaerobic reactions of cellular respiration.
c. The amount of oxygen required to react
accumulated lactic acid to form glucose and to
restore supplies of ATP and creatine phosphate is
called oxygen debt.
10. Muscle fatigue
a. A fatigued muscle loses its ability to contract.
b. Muscle fatigue is usually due to the effects of
accumulation of lactic acid.
c. Athletes usually produce less lactic acid than
nonathletes because of their increased ability to
supply oxygen and nutrients to muscles.
11. Heat production
a. Muscular contraction generates body heat.
b. Most of the energy released by cellular respiration
is lost as heat.
9.4
MUSCULAR RESPONSES (PAGE 296)
1. Threshold stimulus is the minimal stimulus needed to
elicit a muscular contraction.
2. Recording of a muscle contraction
a. A twitch is a single, short contraction of a muscle
F
ber.
b. A myogram is a recording of the contraction of an
electrically stimulated isolated muscle or muscle
F
ber.
c. The latent period is the time between stimulus and
responding contraction.
d. During the refractory period immediately following
contraction, a muscle F
ber cannot respond.
e. The length to which a muscle is stretched before
stimulation affects the force it will develop.
(1) Normal activities occur at optimal length.
(2) Too long or too short decreases force.
f. Sustained contractions are more important than
twitch contractions in everyday activities.
3. Summation
a. A rapid series of stimuli may produce summation
of twitches and sustained contraction.
b. ±orceful, sustained contraction without relaxation
is a tetanic contraction.
f. When a F
ber is at rest, troponin and tropomyosin
molecules interfere with linkage formation.
Calcium ions remove the inhibition.
g. Transverse tubules extend from the cell membrane
into the cytoplasm and are associated with the
cisternae of the sarcoplasmic reticulum.
9.3
SKELETAL MUSCLE CONTRACTION
(PAGE 289)
Muscle F
ber contraction results from a sliding movement
of actin and myosin F laments overlapping that shortens
the muscle F
ber.
1. Neuromuscular junction
a. Motor neurons stimulate muscle F
bers to contract.
b. The motor end plate of a muscle F
ber lies on one
side of a neuromuscular junction.
c. One motor neuron and the muscle F
bers
associated with it constitute a motor unit.
d. In response to a nerve impulse, the end of a motor
nerve F
ber secretes a neurotransmitter, which
diffuses across the junction and stimulates the
muscle F
ber.
2. Stimulus for contraction
a. Acetylcholine released from the end of a motor
nerve F
ber stimulates a muscle F
ber.
b. Acetylcholinesterase decomposes acetylcholine,
preventing continuous stimulation.
c. Stimulation causes a muscle F
ber to conduct
an impulse that travels over the surface of the
sarcolemma and reaches the deep parts of the F
ber
by means of the transverse tubules.
3. Excitation contraction coupling
a. A muscle impulse signals the sarcoplasmic
reticulum to release calcium ions.
b. Calcium ions combine with troponin, causing the
tropomyosin to shift and expose active sites on the
actin for myosin binding.
c. Linkages form between myosin and actin, and
the actin F
laments move inward, shortening the
sarcomere.
4. The sliding F
lament model of muscle contraction
a. The sarcomere, deF ned by striations, is the
functional unit of skeletal muscle.
b. When the overlapping thick and thin myoF laments
slide past one another, the sarcomeres shorten.
The muscle contracts.
5. Cross-bridge cycling
a. A myosin cross-bridge can attach to an actin
binding site and pull on the actin F
lament. The
myosin head can then release the actin and
combine with another active binding site farther
down the actin F
lament and pull again.
b. The breakdown of ATP releases energy that
provides the repetition of the cross-bridge cycle.
6. Relaxation
a. Acetylcholinesterase rapidly decomposes
acetylcholine remaining in the synapse preventing
continuous stimulation of a muscle F
ber.
b. The muscle F
ber relaxes when calcium ions are
transported back into the sarcoplasmic reticulum.
c. Cross-bridge linkages break and do not re-form—
the muscle F
ber relaxes.
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