296
UNIT TWO
contraction throughout the body, which helps to maintain
body temperature. Homeostatic mechanisms promote heat
loss when the temperature of the internal environment
begins to rise (see chapters 1 and 6, pp. 9–10 and 181–183,
respectively).
PRACTICE
10
What are the sources of energy used to regenerate ATP?
11
What are the sources of oxygen required for the aerobic reactions
of cellular respiration?
12
How do lactic acid and oxygen debt relate to muscle fatigue?
13
What is the relationship between cellular respiration and heat
production?
9.4
MUSCULAR RESPONSES
One way to observe muscle contraction is to remove a sin-
gle muscle F ber from a skeletal muscle and connect it to a
device that senses and records changes in the F ber’s length.
An electrical stimulator is usually used to promote muscle
contraction.
Threshold Stimulus
When an isolated muscle F
ber is exposed to a series of stim-
uli of increasing strength, the F ber remains unresponsive
until a certain strength of stimulation called the
threshold
stimulus
(thresh
old stim
u-lus) is applied. Once threshold is
reached, an action potential is generated, resulting in a mus-
cle impulse that spreads throughout the muscle F
ber, releas-
ing enough calcium ions from the sarcoplasmic reticulum to
activate cross-bridge binding and contract that F
ber. A single
nerve impulse in a motor neuron normally releases enough
ACh to bring the muscle F bers in its motor unit to threshold,
generating a muscle impulse in each muscle F
ber.
Recording of a Muscle Contraction
The contractile response of a single muscle F ber to a muscle
impulse is called a
twitch.
A twitch consists of a period of
contraction, during which the F
ber pulls at its attachments,
followed by a period of relaxation, during which the pull-
ing force declines. These events can be recorded in a pat-
tern called a myogram
(f g. 9.15)
. A twitch has a brief delay
between the time of stimulation and the beginning of con-
traction. This is the
latent period,
which in human muscle
may be less than 2 milliseconds.
The length to which a muscle F
ber is stretched before
stimulation affects the force it will develop. If a muscle
F
ber is stretched well beyond its normal resting length, the
force will decrease. This is because sarcomeres of that F
ber
become so extended that myosin cross-bridges cannot reach
binding sites on the thin F
laments and cannot contribute to
contraction. Conversely, at very short F
ber lengths, the sar-
comeres become compressed, and further shortening is not
possible
(f g. 9.16)
. During normal activities, muscle F bers
The runners are on the starting line, their muscles primed for a sprint.
Glycogen will be broken down to release glucose, and creatine phos-
phate will supply high-energy phosphate groups to replenish ATP
stores by phosphorylating ADP. The starting gun F
res. Energy comes
first from residual ATP, but almost instantaneously, creatine phos-
phate begins donating high-energy phosphates to ADP, regenerat-
ing ATP. Meanwhile, oxidation of glucose ultimately produces more
ATP. But because the runner cannot take in enough oxygen to meet
the high demand, most ATP is generated in glycolysis. ±ormation
of lactic acid causes fatigue and possibly leg muscle cramps as the
runner crosses the F
nish line. Already, her liver is actively converting
lactic acid back to pyruvic acid and storing glycogen. In her muscles,
creatine phosphate levels begin to return to normal.
Muscle Fatigue
A muscle exercised persistently for a prolonged period may
lose its ability to contract, a condition called
fatigue.
This
condition has a number of causes, including decreased blood
fl ow, ion imbalances across the sarcolemma from repeated
stimulation, and psychological loss of the desire to continue
the exercise. However, muscle fatigue is most likely to arise
from accumulation of lactic acid in the muscle from anaer-
obic ATP production. The lowered pH from the lactic acid
prevents muscle F
bers from responding to stimulation.
Occasionally a muscle fatigues and cramps at the same
time. A cramp is a sustained, painful, involuntary muscle
contraction. Cramps may result when changes, particularly
a decreased electrolyte concentration, occurring in the extra-
cellular fl uid surrounding the muscle F bers and their motor
neurons trigger uncontrolled stimulation of the muscle.
As muscle metabolism shifts from aerobic to anaerobic
ATP production, lactic acid begins to accumulate in muscles
and to appear in the bloodstream (lactic acid threshold).
This leads to muscle fatigue. How quickly this happens var-
ies among individuals, although people who regularly exer-
cise aerobically produce less lactic acid than those who do
not. The strenuous exercise of aerobic training stimulates
new capillaries to extend into muscles, supplying more oxy-
gen and nutrients to the muscle F bers. Such physical train-
ing also adds mitochondria, increasing the ability of muscle
F bers to produce ATP aerobically. Some muscle F
bers may
be more likely to accumulate lactic acid than others, as
described in a later section entitled “±ast- and Slow-Twitch
Muscle ±ibers.”
Heat Production
All active cells generate heat, which is a by-product of cellu-
lar respiration. Muscle tissue constitutes such a large propor-
tion of total body mass, that it is a major source of heat.
Less than half of the energy released in cellular res-
piration is available for use in metabolic processes; the
rest becomes heat. Blood transports the heat from muscle
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