163
CHAPTER FIVE
Tissues
5.5
MUSCLE TISSUES
General Characteristics
Muscle tissues
are contractile; they can shorten and
thicken. As they contract, muscle cells pull at their attached
ends, which moves body parts. The cells that comprise mus-
cle tissues are sometimes called
muscle f
bers
because they
are elongated. The three types of muscle tissue, skeletal,
smooth, and cardiac, are introduced here and discussed fur-
ther in chapter 9.
Skeletal Muscle Tissue
Skeletal muscle tissue
(f g. 5.28)
forms muscles that usually
attach to bones and are controlled by conscious effort. For
this reason, it is often called
voluntary
muscle tissue. Skeletal
muscle cells are long—up to or more than 40 mm in length—
and narrow—less than 0.1 mm in width. These threadlike cells
have alternating light and dark cross-markings called
striations.
Each cell has many nuclei (multinucleate). A nerve cell can
stimulate protein ± laments in the muscle cell to slide past one
another, which contracts the cell. The muscle cell relaxes when
stimulation stops. Skeletal muscles move the head, trunk, and
limbs and enable us to make facial expressions, write, talk, and
sing, as well as chew, swallow, and breathe.
Smooth Muscle Tissue
Smooth muscle tissue
(f g. 5.29)
is called smooth because
its cells lack striations. Smooth muscle cells are shorter than
those of skeletal muscle and are spindle-shaped, each with
a single, centrally located nucleus. This tissue comprises the
walls of hollow internal organs, such as the stomach, intes-
tines, urinary bladder, uterus, and blood vessels. Unlike skel-
etal muscle, smooth muscle usually cannot be stimulated to
contract by conscious effort. Thus, its actions are
involuntary.
Smooth muscle tissue moves food through the digestive tract,
constricts blood vessels, and empties the urinary bladder.
5.4
TYPES OF MEMBRANES
After discussing epithelial and connective tissues, sheets of
cells called membranes are better understood.
Epithelial
membranes
are thin structures that are usually composed of
epithelium and underlying connective tissue, covering body
surfaces and lining body cavities. The three major types of
epithelial membranes are
serous, mucous,
and
cutaneous.
Serous
(se
rus)
membranes
line the body cavities that do
not open to the outside and reduce friction between the organs
and cavity walls. They form the inner linings of the thorax
and abdomen, and they cover the organs in these cavities (see
± gs. 1.11 and 1.12). A serous membrane consists of a layer of
simple squamous epithelium (mesothelium) and a thin layer
of loose connective tissue. Cells of a serous membrane secrete
watery
serous fl
uid,
which helps lubricate membrane surfaces.
Mucous
(mu
kus)
membranes
line the cavities and
tubes that open to the outside of the body. These include
the oral and nasal cavities and the tubes of the digestive,
respiratory, urinary, and reproductive systems. A mucous
membrane consists of epithelium overlying a layer of loose
connective tissue. However, the type of epithelium varies
with the location of the membrane. For example, strati± ed
squamous epithelium lines the oral cavity, pseudostrati-
±
ed columnar epithelium lines part of the nasal cavity, and
simple columnar epithelium lines the small intestine. Goblet
cells within a mucous membrane secrete
mucus.
Another epithelial membrane is the
cutaneous
(ku-ta
ne-us)
membrane,
more commonly called
skin.
It is part
of the integumentary system described in detail in chapter 6.
A type of membrane composed entirely of connective
tissues is a
synovial
(sı˘-no
ve-al)
membrane.
It lines joints
and is discussed further in chapter 8 (p. 264).
PRACTICE
21
Name the four types of membranes, and explain how they diF
er.
Striations
Portion of a
muscle fiber
Nuclei
(a)
(b)
FIGURE 5.28
Skeletal muscle tissue is composed of striated muscle ±
bers with many nuclei (700×).
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