Figure 1140

Innervation of a single-unit smooth muscle is often restricted to only a few fibers in the muscle. Electrical activity is conducted from fiber to fiber throughout the muscle by way of the gap junctions between the fibers.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Muscle CHAPTER ELEVEN

Muscle CHAPTER ELEVEN

TABLE 11-6 Characteristics of Muscle Fibers

Smooth Muscle

Characteristic

Skeletal Muscle

Single Unit

Multiunit

Cardiac Muscle

Thick and thin filaments

Yes

Yes

Yes

Yes

Sarcomeres—banding pattern

Yes

No

No

Yes

Transverse tubules

Yes

No

No

Yes

Sarcoplasmic reticulum (SR)*

+ + + +

+

+

+ +

Gap junctions between fibers

No

Yes

Few

Yes

Source of activating calcium

SR

SR and extracellular

SR and extracellular

SR and extracellular

Site of calcium regulation

Troponin

Myosin

Myosin

Troponin

Speed of contraction

Fast-slow

Very slow

Very slow

Slow

Spontaneous production of action potentials by pacemakers

No

Yes

No

Yes in certain fibers, but most not spontaneously active

Tone (low levels of maintained tension in the absence of external stimuli)

No

Yes

No

No

Effect of nerve stimulation

Excitation

Excitation or inhibition

Excitation or inhibition

Excitation or inhibition

Physiological effects of hormones on excitability and contraction

No

Yes

Yes

Yes

Stretch of fiber produces contraction

No

Yes

No

No

*Number of plus signs (+) indicates the relative amount of sarcoplasmic reticulum present in a given muscle type.

*Number of plus signs (+) indicates the relative amount of sarcoplasmic reticulum present in a given muscle type.

It must be emphasized that most smooth muscles do not show all the characteristics of either single-unit or multiunit smooth muscles. These two prototypes represent the two extremes in smooth-muscle characteristics, with many smooth muscles having characteristics that overlap the two groups.

Table 11-6 compares the properties of the different types of muscle. Cardiac muscle has been included for completeness although its properties are discussed in Chapter 14.

SECTION B SUMMARY

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