is glycogen. When glycogen is needed to provide
energy for sustained contractions (more than a few
seconds), it is first broken down into the glucosemol-
ecules of which it is made. Glucose is then further bro-
ken down in the process of cell respiration to produce
ATP, and muscle fibers may continue to contract.
Recall from Chapter 2 our simple reaction for cell
respiration:
Glucose O 2 →CO 2 H 2 O ATP heat
Look first at the products of this reaction. ATP will
be used by the muscle fibers for contraction. The heat
produced will contribute to body temperature, and if
exercise is strenuous, will increase body temperature.
The water becomes part of intracellular water, and the
carbon dioxide is a waste product that will be exhaled.
Now look at what is needed to release energy from
glucose: oxygen. Muscles have two sources of oxygen.
The blood delivers a continuous supply of oxygen
from the lungs, which is carried by the hemoglobinin
red blood cells. Within muscle fibers themselves there
is another protein called myoglobin, which stores
some oxygen within the muscle cells. Both hemoglo-
bin and myoglobin contain the mineral iron, which
enables them to bond to oxygen. (Iron also makes
both molecules red, and it is myoglobin that gives
muscle tissue a red or dark color.)
During strenuous exercise, the oxygen stored in
myoglobin is quickly used up, and normal circulation
may not deliver oxygen fast enough to permit the
completion of cell respiration. Even though the respi-
ratory rate increases, the muscle fibers may literally
run out of oxygen. This state is called oxygen debt,
and in this case, glucose cannot be completely broken
down into carbon dioxide and water. If oxygen is not
present (or not present in sufficient amounts), glucose
is converted to an intermediate molecule called lactic
acid, which causes muscle fatigue.
In a state of fatigue, muscle fibers cannot contract
efficiently, and contraction may become painful. To be
in oxygen debt means that we owe the body some oxy-
gen. Lactic acid from muscles enters the blood and
circulates to the liver, where it is converted to pyruvic
acid, a simple carbohydrate (three carbons, about
half a glucose molecule). This conversion requires
ATP, and oxygen is needed to produce the necessary
ATP in the liver. This is why, after strenuous exercise,
the respiratory rate and heart rate remain high for a
time and only gradually return to normal. Another
name proposed for this state is recovery oxygen
uptake, which is a little longer but also makes sense.
Oxygen uptake means a faster and deeper respiratory
rate. What is this uptake for? For recovery from stren-
uous exercise.MUSCLE FIBER—
MICROSCOPIC STRUCTUREWe will now look more closely at a muscle fiber, keep-
ing in mind that there are thousands of these cylindri-
cal cells in one muscle. Each muscle fiber has its own
motor nerve ending; the neuromuscular junctionis
where the motor neuron terminates on the muscle
fiber (Fig. 7–2). The axon terminalis the enlarged tip
of the motor neuron; it contains sacs of the neuro-
transmitter acetylcholine(ACh). The membrane of
the muscle fiber is the sarcolemma, which contains
receptor sites for acetylcholine, and an inactivator
called cholinesterase. The synapse(or synaptic cleft)
is the small space between the axon terminal and the
sarcolemma.
Within the muscle fiber are thousands of individual
contracting units called sarcomeres, which are
arranged end to end in cylinders called myofibrils.
The structure of a sarcomere is shown in Fig. 7–3:
The Z lines are the end boundaries of a sarcomere.
Filaments of the protein myosinare in the center of
the sarcomere, and filaments of the protein actinare
at the ends, attached to the Z lines. Myosin filaments
are anchored to the Z lines by the protein titin.
Myosin and actin are the contractile proteins of a
muscle fiber. Their interactions produce muscle con-
traction. Also present are two inhibitory proteins, tro-
poninand tropomyosin, which are part of the actin
filaments and prevent the sliding of actin and myosin
when the muscle fiber is relaxed.
Surrounding the sarcomeres is the sarcoplasmic
reticulum, the endoplasmic reticulum of muscle cells.
The sarcoplasmic reticulum is a reservoir for calcium
ions (Ca^2 ), which are essential for the contraction
process.
All of these parts of a muscle fiber are involved in
the contraction process. Contraction begins when a
nerve impulse arrives at the axon terminal and stimu-
lates the release of acetylcholine. Acetylcholine gener-
ates electrical changes (the movement of ions) at the
sarcolemma of the muscle fiber. These electrical
changes initiate a sequence of events within the mus-
cle fiber that is called the sliding filament mecha-
nism of muscle contraction. We will begin our
discussion with the sarcolemma.142 The Muscular System