Fundamentals of Anatomy and Physiology

204

the potassium ions that were kept inside begin to move to
the outside to restore the resting potential, but they cannot
change back to the resting potential situation be-cause so
many sodium ions are rushing in.
This action potential not only travels over the sur-face
of the muscle cell membrane but passes down into the cell
by way of the T tubules and also deep into

Head

Chapter 9

all the cells that make up the muscle. This action po-tential

causes the sarcoplasmic reticulum to release stored calcium

ions into the fluids surrounding the myofibrils of the

muscle cell. Surrounding the actin myofilaments are two

inhibitor substances: troponin (TRO-poh-nin) and

tropomyosin (troh-poh-MY-oh-sin). Refer to -Figure 9-

3. These substances keep the

Two polypeptide coils wound in a supercoil

(A) Myosin molecule

(B)

Myosin myofilament

Troponin (^)
complex Tropomyosin (^)
Actin molecule

(C)

Actin myofilament

(^) ®
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Figure 9- 3 The structure of the actin and myosin myofilaments of a muscle cell. (A) Myosin molecule. (B) Myosin
myofilament.
(C) Actin myofilament.