Muscle contraction requires lots of energy. The energy
comes from ATP, but stores of ATP can be exhausted with-
in seconds. The body must be able to make ATP quickly to
keep up with the energy demand
of the working muscle. ATP can
come
from
three
sources:
cre-
atine phosphate
, anaerobic res-
piration or
glycolysis
, and aerobic
respiration (
Figure 6.7
).
Anaero-
bic respiration
is respiration that
does not require oxygen; oxygen is
required for
aerobic respiration
.
When muscles can no longer
contract, they become fatigued.
Although muscle fatigue can come from a decrease in
the release of calcium (calcium depletion), it most often
results from insufficient supplies of ATP (due to depleted
amounts of creatine phosphate and
glycogen
, inadequate
delivery of oxygen to the muscle, or build-up of
lactic
acid
). Sometimes, as ATP levels decrease and lactic acid
levels increase, muscle cramping may also occur because
muscles cells cannot relax. This is because the active
transport process that moves calcium out of the cell also
requires ATP.
After strenuous exercise, the oxygen consumption by
working muscles continues to be elevated. This elevated
oxygen consumption is referred to as oxygen debt. Follow-
P hases o f a m uscle tw itch • Figure 6.8
ing vigorous physical activity, the heart rate and breath-
ing rate generally remain elevated for a period of time to
move more oxygen into the tissue, which helps “repay”
this oxygen debt. The excess oxygen delivered to the tis-
sues converts lactic acid back to glucose and glycogen,
remakes creatine phosphate, and replaces the oxygen lost
from myoglobin during aerobic respiration. (
Myoglobin
is a muscle protein that stores oxygen within the muscle
cell.) Oxygen is also used for the following processes:
Increased body heat speeds up other reactions in the body
that use ATP, so oxygen is used to replenish these stores.
Oxygen
feeds
hard-working
heart
and
breathing
muscles, which use more ATP.
Oxygen replenishes ATP used in tissue repair processes.
Muscle Contraction Comes in Many
Forms: Twitch, tetanus, isotonic,
and isometric
There are two categories of muscle contraction: twitch
and tetanus (contracture). A
twitch
is a single muscle con-
traction in response to a single nerve impulse. It consists
of three periods: the
latent period
,
contraction period
,
and
relaxation period
. Each period is associated with the
events of excitation-contraction coupling (
Figure 6.8
).
glycolysis
(glT-KOL-i-
sis) A series of anaero-
bic chemical reactions
in the cytosol of a cell
in which a molecule
of glucose is split
into two molecules of
pyruvic acid, with the
net production of two
ATPs.
A myogram records the properties of a twitch in response to a single
stimulus (arrow), starting with the latent period, moving to the contrac-
tion period, and then to the relaxation period. Notice that the twitch
lasts only about 50 ms, or 0.05 s.
1. Latent period. There is a time delay during which
the muscle action potential travels down the muscle
and calcium ions get released from the sarcoplasmic
reticulum.
2. Contraction period. Contraction cycles generate
force.
3. Relaxation period. Ionized calcium levels in the cell
return to normal and contraction cycles decrease.
Muscles Are Built to Move 163
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