Essentials of Anatomy and Physiology

(cerebrovascular accident) occurs in the frontal
lobes of the cerebrum. Without nerve impulses, skele-
tal muscles become paralyzed, unable to contract.
Paralyzed muscles eventually atrophy, that is, become
smaller from lack of use. Other disorders that affect
muscle functioning are discussed in Box 7–3: Muscular
Dystrophy and Box 7–4: Myasthenia Gravis.

RESPONSES TO EXERCISE—

MAINTAINING HOMEOSTASIS

Although entire textbooks are devoted to exercise
physiology, we will discuss it only briefly here as an
example of the body’s ability to maintain homeostasis.

Engaging in moderate or strenuous exercise is a phys-

iological stress situation, a change that the body must

cope with and still maintain a normal internal envi-

ronment, that is, homeostasis.

Some of the body’s responses to exercise are dia-

grammed in Fig. 7–6; notice how they are related to

cell respiration. As you can see, the respiratory and

cardiovascular systems make essential contributions to

exercise. The integumentary system also has a role,

since it eliminates excess body heat. Although not

shown, the nervous system is also directly involved, as

we have seen. The brain generates the impulses for

muscle contraction, coordinates those contractions,

and regulates heart rate, breathing rate, and the diam-

eter of blood vessels. The next time you run up a flight

The Muscular System 147

ARelaxed muscle

B Contracted muscle

Sarcolemma

T tubule

Sarcoplasmic

reticulum

Calcium

ions

Calcium ions released from

sarcoplasmic reticulum

Calcium ions

bonded to

troponin

Myosin cross bridges

attach to actin

Actin

Myosin-binding site

Tropomyosin

Troponin

Figure 7–5. Sliding filament mechanism. (A) Sarcomere in relaxed muscle fiber.

(B) Sarcomere in contracted muscle fiber. See text for description.

QUESTION:During contraction, which filaments do the pulling?