Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 5Excitable Tissue: Muscle 107

MECHANICAL PROPERTIES

CONTRACTILE RESPONSE

The contractile response of cardiac muscle begins just after the
start of depolarization and lasts about 1.5 times as long as the
action potential (Figure 5–16). The role of Ca2+ in excitation–
contraction coupling is similar to its role in skeletal muscle

(see above). However, it is the influx of extracellular Ca2+

through the voltage-sensitive DHPR in the T system that trig-

gers calcium-induced calcium release through the RyR at the

sarcoplasmic reticulum. Because there is a net influx of Ca2+

during activation, there is also a more prominent role for plas-

ma membrane Ca2+ ATPases and the Na+/Ca2+ exchanger in

recovery of intracellular Ca2+ concentrations. Specific effects

FIGURE 5–15 Cardiac muscle. A) Electron photomicrograph of cardiac muscle. Note the similarity of the A-I regions seen in the skeletal
muscle EM of Figure 3-2. The fuzzy thick lines are intercalated disks and function similarly to the Z-lines but occur at cell membranes (× 12,000).
(Reproduced with permission from Bloom W, Fawcett DW: A Textbook of Histology, 10th ed. Saunders, 1975.) B) Artist interpretation of cardiac muscle as seen under
the light microscope (top) and the electron microscope (bottom). Again, note the similarity to skeletal muscle structure. N, nucleus. (Reproduced
with permission from Braunwald E, Ross J, Sonnenblick EH: Mechanisms of contraction of the normal and failing heart. N Engl J Med 1967;277:794. Courtesy of Little, Brown.)

SARCOMERE

FIBRIL

Capillary

FIBER

Fibrils

Sarcolemma

10 μm

2 μm

Sarcoplasmic reticulum

T system

Terminal

cistern

Mitochondria Intercalated disk

Nucleus

Intercalated disk

NN

NN

B

A