Human Physiology, 14th edition (2016)

Muscle 361

an endomysium ( fig. 12.1 ). The endomysium is the basement
membrane, or basal lamina, of the muscle fiber. Because the
connective tissue of the tendons, epimysium, perimysium, and
endomysium is continuous, muscle fibers do not normally pull
out of the tendons when they contract.
Despite their unusual elongated shape, muscle fibers have the
same organelles that are present in other cells: mitochondria, endo-
plasmic reticulum, glycogen granules, and others. Unlike most
other cells in the body, skeletal muscle fibers are multinucleate—
they contain multiple nuclei. This is because each muscle fiber is
a syncytial structure (chapter 1, section 1.3). That is, each muscle
fiber is formed from the union of several embryonic myoblast cells.
The most distinctive feature of skeletal muscle fibers, however, is
their striated appearance when viewed microscopically ( fig. 12.2 ).
The striations (stripes) are produced by alternating dark and light
bands that appear to span the width of the fiber.
The dark bands are called A bands, and the light bands
are called I bands. At high magnification in an electron
microscope, thin dark lines can be seen in the middle of the

I bands. These are called Z lines (see fig. 12.6 ). The labels A,

I, and Z—derived in the course of early muscle research—

are useful for describing the functional architecture of muscle

fibers. The letters A and I stand for anisotropic and isotropic,

respectively, which indicate the behavior of polarized light as

it passes through these regions; the letter Z comes from the

German word Zwischenscheibe, which translates to “between

disc.” These derivations are of historical interest only.

Motor End Plates and Motor Units

In vivo, each muscle fiber receives a single axon terminal from

a somatic motor neuron. The motor neuron stimulates the

muscle fiber to contract by liberating acetylcholine at the neu-

romuscular junction (chapter 7, section 7.4). The specialized

region of the sarcolemma of the muscle fiber at the neuromus-

cular junction is known as a motor end plate ( fig. 12.3 ).

Contraction of a skeletal muscle rapidly follows its stimu-

lation by somatic motor axons. Depolarization of a motor axon

Periosteum covering

the bone

Tendon

Skeletal

muscle

Fascia

Epimysium

Perimysium

Fasciculus

Endomysium

Muscle

fiber (cell)

Endomysium

Striations

Sarcolemma

Sarcoplasm

Filaments

Nuclei

Myofibrils

Figure 12.1 The structure of a

skeletal muscle. The relationship between

muscle fibers and the connective tissues of

the tendon, epimysium, perimysium, and

endomysium is depicted in the upper figure.

Below is a close-up of a single muscle fiber.