Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 5Excitable Tissue: Muscle 103

store is built up. During exercise, the phosphorylcreatine is
hydrolyzed at the junction between the myosin heads and ac-
tin, forming ATP from ADP and thus permitting contraction
to continue.

CARBOHYDRATE & LIPID BREAKDOWN

At rest and during light exercise, muscles utilize lipids in the
form of free fatty acids as their energy source. As the intensity
of exercise increases, lipids alone cannot supply energy fast
enough and so use of carbohydrate becomes the predominant
component in the muscle fuel mixture. Thus, during exercise,
much of the energy for phosphorylcreatine and ATP resynthe-
sis comes from the breakdown of glucose to CO 2 and H 2 O.
Glucose in the bloodstream enters cells, where it is degraded
through a series of chemical reactions to pyruvate. Another

source of intracellular glucose, and consequently of pyruvate,

is glycogen, the carbohydrate polymer that is especially abun-

dant in liver and skeletal muscle. When adequate O 2 is

present, pyruvate enters the citric acid cycle and is metabo-

lized—through this cycle and the so-called respiratory enzyme

pathway—to CO 2 and H 2 O. This process is called aerobic

glycolysis. The metabolism of glucose or glycogen to CO 2 and

H 2 O forms large quantities of ATP from ADP. If O 2 supplies

are insufficient, the pyruvate formed from glucose does not

enter the tricarboxylic acid cycle but is reduced to lactate. This

process of anaerobic glycolysis is associated with the net pro-

duction of much smaller quantities of energy-rich phosphate

bonds, but it does not require the presence of O 2. A brief over-

view of the various reactions involved in supplying energy to

skeletal muscle is shown in Figure 5–13.

TABLE 5–2 Classification of fiber types in skeletal muscles.

Type 1 Type IIA Type IIB

Other names Slow, Oxidative (SO) Fast, Oxidative, Glycolytic (FOG) Fast, Glycolytic (FG)

Color Red Red White

Myosin ATPase Activity Slow Fast Fast

Ca2+-pumping capacity of sarcoplasmic reticulum Moderate High High

Diameter Small Large Large

Glycolytic capacity Moderate High High

Oxidative capacity High Moderate Low

Associated Motor Unit Type Slow (S) Fast Resistant to Fatigue (FR) Fast Fatigable (FF)

Membrane potential = –90 mV

Oxidative capacity High Moderate Low

FIGURE 5–12 Creatine, phosphorylcreatine, and creatinine
cycling in muscle. During periods of high activity, cycling of phos-
phorylcreatine allows for quick release of ATP to sustain muscle activity.

Rest

Exercise

HN C——O

HN——C

CH 2

PO 3

CH 3 N

HN

H 2 N+——C + ADP

CH 3 NCH 2 COO−

Phosphorylcreatine

Creatinine

H 2 N

H 2 N+——C + ATP

CH 3 NCH 2 COO−

Creatine

FIGURE 5–13 ATP turnover in muscle cells. Energy released

by hydrolysis of 1 mol of ATP and reactions responsible for resynthesis

of ATP. The amount of ATP formed per mole of free fatty acid (FFA)

oxidized is large but varies with the size of the FFA. For example, com-

plete oxidation of 1 mol of palmitic acid generates 140 mol of ATP.

ATP + H 2 O ADP + H 3 PO 4 + 7.3 kcal

Phosphorylcreatine + ADP Creatine + ATP

Glucose + 2 ATP (or glycogen + 1 ATP)

Glucose + 2 ATP (or glycogen + 1 ATP)

Anaerobic

2 Lactic acid + 4 ATP

Oxygen

6 CO 2 + 6 H 2 O + 40 ATP

Oxygen

FFA CO 2 + H 2 O + ATP