The Muscular System 207
K+^ K+^ Na+ Na+ (^)
Na+ (^)
(^) K (^)
(^2 3) K (^)
(^4)
1 P^
(^) ADP
K+^
Na+ (^) ATP
P (^)
Na+
K+ (^)
Na+ (^)
(^) ®
Learning
Cengage ©
Figure 9- 5 The sodium-potassium pump of the membrane of a muscle
cell.
is transferred to creatine to build up a reserve of phos-
phocreatine. During strenuous exercise, the phospho-
creatine takes up ADP to release ATP and creatine, thus
supplying the muscle with an additional supply of ATP.
The overall reaction, which goes in both directions, is
phosphocreatine 1 ADP creatine 1 ATP.
In addition, skeletal muscle cells can take up free fatty
acids from the blood and break them down as an-other
source of energy into CO 2 , H 2 O, and ATP. Of course,
during any contraction, heat is produced as a waste product.
In summary, muscle cells have four sources of ATP for
the energy of contraction:
1.^ Glucose 1 2 ATP →^ CO^2 1 H^2 O 1 38 ATP (aerobic)^
2.^ Glucose 1 2 ATP →^ 2 lactic acid 1 2 ATP (anaerobic)^
3.^ Phosphocreatine 1 ADP →^ creatine 1 ATP^
4. Free fatty acids → CO 2 1 H 2 O 1 ATP
In these processes, glycolysis, the Krebs citric acid
cycle, and electron transport play a vital role.
The Muscle Twitch
When the contraction of a skeletal muscle is studied in the
laboratory by applying an electrical charge to the muscle,
the analysis of the contraction is called a muscle twitch
(Figure 9-6). This reveals a brief latent pe-riod directly
following stimulation just before contrac-tion begins. This
latent period is followed by a period
Muscle twitch
contraction^Latent Shortening Relaxation (^)
S (^)
t (^)
i (^)
m
®^
Learning^
u
l
Cengage
(^)
u (^)
s^ ©^
Figure 9- 6 A laboratory analysis of a muscle twitch.
of contraction followed by a period of relaxation. This
latent period occurs because the resting potential of the
muscle cells must change into the electrical potential as
sodium ions rush in. This is caused by the acetylcholine
released by the nerve cell’s axon terminals into the neu-
romuscular junction. The electrical potential then be-comes
the action potential as the signal travels down the T tubules
to the sarcoplasmic reticulum. Then calcium ions get
released into the fluids around the myofibrils of actin and
myosin and contraction occurs. Once the sodium-
-potassium pump operates, calcium gets reab-sorbed and
relaxation occurs.
The strength of the contraction depends on a num-ber
of factors: the strength of the stimulus (a weak stimulus
will not bring about contraction), the duration of the
stimulus (even if the stimulus is quite strong, if it is
-applied for a millisecond it may not be applied long