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Introduction
As you read this introduction, skeletal muscles are mov-ing
your eyes to read the words. Muscles allowed you to first
pick up this book and open it to the correct page. You
walked to your desk, and you took this book off a shelf. All
of these actions allowed you to function in your en-
vironment. In addition, smooth muscle is containing the
blood in your arteries and veins, food is being pushed
through your digestive tract, and urine is being trans-ported
from your kidneys via the ureters to your bladder.
Meanwhile, cardiac muscle is pumping the blood, carry-ing
oxygen and nutrients to your body cells, and carrying away
waste.
Muscles make up about 40% to 50% of the body’s
weight. They allow us to perform extraordinary physical
feats of endurance (running, playing sports) and grace
(ballet, figure skating). When they contract, they bring
about movement of the body as a whole and cause our
internal organs to function properly. Muscles of the di-
aphragm, chest, and abdomen allow us to breathe. See
Concept Map 9-1: Muscular System.
The Types of Muscle
From the discussion of tissues in Chapter 5, you re-call that
there are three types of muscle tissue: skeletal or striated,
smooth or visceral, and cardiac. Recall that skeletal-
muscle is voluntary, that is, we can control its contraction.
Under the microscope, skeletal muscle cells are
multinucleated and striated; we can see alternating dark and
light bands. Smooth muscle, on the other hand, is
involuntary, uninucleated, and nonstriated. It is found in
places like the digestive tract. Cardiac muscle is invol-
untary, striated, and uninucleated and is found only in the
heart.
The Anatomy of Skeletal or
Striated Muscle
Mature skeletal or striated muscle cells are the longest and
most slender muscle fibers, ranging in size from 1 to 50
mm in length and 40 to 50 μm in diameter -(Figure 9-1).
Because of this unique structure of the cell, that is, their
length being much greater than their width, skeletal muscle
cells are often referred to as skeletal muscle fi-bers. In
addition, each muscle cell or fiber is multinucle-ated and is
surrounded by a special cell membrane. This cell
membrane is electrically polarized and is called a
sarcolemma (sahr-koh-LEM-ah). The sarcolemma is
sur-rounded by the first of three types of connective tissue
Chapter 9found in a muscle, the endomysium (in-do-MISS-ee-
um), which is delicate connective tissue.
As we study Figure 9-1, we see that the entire muscle
consists of a number of skeletal muscle bundles called
fasciculi (fah-SICK-you-lye). Each individual bundle of
muscle cells, or fascicle (FASS-ih-kl), is surrounded by
another layer of connective tissue called the perimysium
(pair-ih-MISS-ee-um). This is visible to the naked eye.
This perimysium connects with the coarse irregular con-
nective tissue that surrounds the whole muscle called the
epimysium (eh-pih-MISS-ee-um). These three layers of
connective tissue act like cement holding all of the mus-cle
cells and their bundles together. In addition, a layer of
areolar tissue covers the whole muscle trunk on top of the
epimysium and is called the fascia (FASH-ee-ah).
When skeletal muscle is viewed under a microscope,
the cells appear to have alternating dark and light bands
referred to as cross-striations. The striations are due to an
overlapping of the dark and light bands of protein on the
myofibrils. The dark bands are made of the thick fila-ments
of the protein myosin. Being thick, they therefore appear
dark and are called the A bands (hint to remem-ber: the
second letter in the word dark is A). The light bands are
made of the thin filaments of the protein actin; being thin,
they appear light and are called the I bands (hint to
remember: the second letter in the word light is I).
A number of other markings are important to note. A
narrow, dark staining band found in the central region of
the I band that looks like a series of the letters Z one on top
of another is called the Z line. A slightly darker sec-tion in
the middle of the dark A band is called the H band or H
zone. This is where the myosin filaments are thickest and
where there are no cross-bridges on the myosin fila-ments.
The area between two adjacent Z lines is called a
sarcomere (SAHR-koh-meer). It is here at the molecu-lar
level that the actual process of contraction occurs via
chemical interactions, which is discussed later.
Electron microscopy has also revealed the fact that
muscle fibrils (thousands of tiny units that make up a
muscle cell) are surrounded by structures made up of
membranes in the form of vesicles and tubules. These
structures constitute what is referred to as the sarcotubu-
lar (sahr-koh-TYOO-byoo-lar) system. The sarcotubular
system is made up of two components: the T system or
tubules and the sarcoplasmic reticulum (reh-TIK-
you-lum). The tubules of the T system are continuous with
the cell membrane or sarcolemma of the muscle fiber and
form a grid perforated by individual muscle fibrils. The
sarcoplasmic reticulum forms an irregular curtain around
each of the fibrils. Again refer to Figure 9-1 for these
complex structures. This T system functions in the