An In Depth Exploration of Muscular Tissue Structure Function and Its Crucial Role in Human Physiology and Movement

Muscular Tissue and Homeostasis

Although bones provide leverage and form the framework of the body,
they cannot move body parts by themselves. Motion results from the
alternating contraction and relaxation of muscles, which make up
40–50% of total adult body weight (depending on the percentage of
body fat, gender, and exercise regimen). Your muscular strength reflects
the primary function of muscle—the transformation of chemical energy
into mechanical energy to generate force, perform work, and produce
movement. In addition, muscle tissues stabilize body position, regulate
organ volume, generate heat, and propel fluids and food matter through
various body systems.

Overview of Muscular Tissue

OBJECTIVES

• Explain the structural diff erences among the three types of
muscular tissue.
• Compare the functions and special properties of the three types
of muscular tissue.

Types of Muscular Tissue

The three types of muscular tissue—skeletal, cardiac, and smooth—
were introduced in Chapter 4 (see Table 4.9). The scientific study of
muscles is known as myology (mı¯-OL-oˉ-je¯; myo- = muscle; -logy =
study of). Although the diff erent types of muscular tissue share some
properties, they diff er from one another in their microscopic anatomy
and location, and in how they are controlled by the nervous and endo-
crine systems.
Skeletal muscle tissue is so named because most skeletal mus-
cles move the bones of the skeleton. (A few skeletal muscles attach to
and move the skin or other skeletal muscles.) Skeletal muscle tissue is
striated: Alternating light and dark protein bands (striations) are seen
when the tissue is examined with a microscope (see Table 4.9). Skel-
etal muscle tissue works mainly in a voluntary manner. Its activity can
be consciously controlled by neurons (nerve cells) that are part of the
somatic (voluntary) division of the nervous system. (Figure 12.10 de-
picts the divisions of the nervous system.) Most skeletal muscles also
are controlled subconsciously to some extent. For example, your dia-
phragm continues to alternately contract and relax without conscious
control so that you don’t stop breathing. Also, you do not need to con-
sciously think about contracting the skeletal muscles that maintain
your posture or stabilize body positions.
Only the heart contains cardiac muscle tissue, which forms
most of the heart wall. Cardiac muscle is also striated, but its action is
involuntary. The alternating contraction and relaxation of the heart
is not consciously controlled. Rather, the heart beats because it
has a natural pacemaker that initiates each contraction. This built-in
rhythm is termed autorhythmicity (aw′-toˉ-rith-MISS-i-te¯). Several
hormones and neurotransmitters can adjust heart rate by speeding
or slowing the pacemaker.
Smooth muscle tissue is located in the walls of hollow inter-
nal structures, such as blood vessels, airways, and most organs in
the abdominopelvic cavity. It is also found in the skin, attached to
hair follicles. Under a microscope, this tissue lacks the striations of
skeletal and cardiac muscle tissue. For this reason, it looks nonstri-
ated, which is why it is referred to as smooth. The action of smooth
muscle is usually involuntary, and some smooth muscle tissue,
such as the muscles that propel food through your gastrointestinal
tract, has autorhythmicity. Both cardiac muscle and smooth mus-
cle are regulated by neurons that are part of the autonomic
(involuntary) division of the nervous system and by hormones re-
leased by endocrine glands.

Functions of Muscular Tissue

Through sustained contraction or alternating contraction and relaxa-
tion, muscular tissue has four key functions: producing body move-
ments, stabilizing body positions, storing and moving substances
within the body, and generating heat.
1. Producing body movements. Movements of the whole body such
as walking and running, and localized movements such as grasping
a pencil, keyboarding, or nodding the head rely on the integrated
functioning of skeletal muscles, bones, and joints.
2. Stabilizing body positions. Skeletal muscle contractions stabilize
joints and help maintain body positions, such as standing or sit-
ting. Postural muscles contract continuously when you are awake;
for example, sustained contractions of your neck muscles hold your
head upright when you are listening intently to your anatomy and
physiology lecture.
3. Storing and moving substances within the body. Storage is accom-
plished by sustained contractions of ringlike bands of smooth mus-
cle called sphincters, which prevent outflow of the contents of a
hollow organ. Temporary storage of food in the stomach or urine in
the urinary bladder is possible because smooth muscle sphincters
close off the outlets of these organs. Cardiac muscle contractions of
the heart pump blood through the blood vessels of the body. Con-
traction and relaxation of smooth muscle in the walls of blood ves-
sels help adjust blood vessel diameter and thus regulate the rate
of blood flow. Smooth muscle contractions also move food and
substances such as bile and enzymes through the gastrointestinal
tract, push gametes (sperm and oocytes) through the passageways
of the reproductive systems, and propel urine through the urinary
system. Skeletal muscle contractions promote the flow of lymph
and aid the return of blood in veins to the heart.
4. Generating heat. As muscular tissue contracts, it produces heat,
a process known as thermogenesis (ther′-moˉ-JEN-e-sis). Much
of the heat generated by muscle is used to maintain normal body
temperature. Involuntary contractions of skeletal muscles, known
as shivering, can increase the rate of heat production.

Properties of Muscular Tissue

Muscular tissue has four special properties that enable it to function
and contribute to homeostasis:
1. Electrical excitability (ek-sı¯ t′-a-BIL-i-te¯), a property of both mus-
cle and nerve cells that was introduced in Chapter 4, is the ability
to respond to certain stimuli by producing electrical signals called
action potentials (impulses). Action potentials in muscles are re-
ferred to as muscle action potentials; those in nerve cells are called
nerve action potentials. Chapter 12 provides more detail about
how action potentials arise (see Section 12.3). For muscle cells,
two main types of stimuli trigger action potentials. One is auto-
rhythmic electrical signals arising in the muscular tissue itself, as
in the heart’s pacemaker. The other is chemical stimuli, such as
neurotransmitters released by neurons, hormones distributed by
the blood, or even local changes in pH.
Contractility (kon′-trak-TIL-i-te¯) is the ability of muscular tissue to
contract forcefully when stimulated by an action potential. When a
skeletal muscle contracts, it generates tension (force of contraction)
while pulling on its attachment points. If the tension generated is
great enough to overcome the resistance of the object to be moved,
the muscle shortens and movement occurs.
3. Extensibility (ek-sten′-si-BIL-i-te¯) is the ability of muscular tissue to
stretch, within limits, without being damaged. The connective tissue
within the muscle limits the range of extensibility and keeps it within
the contractile range of the muscle cells. Normally, smooth muscle
is subject to the greatest amount of stretching. For example, each
time your stomach fills with food, the smooth muscle in the wall is
stretched. Cardiac muscle also is stretched each time the heart fills
with blood.
4. Elasticity (e-las-TIS-i-te¯) is the ability of muscular tissue to return
to its original length and shape aft er contraction or extension.
Skeletal muscle is the focus of much of this chapter. Cardiac
muscle and smooth muscle are described briefly here. Cardiac mus-
cle is discussed in more detail in Chapter 20 (the heart), and smooth
muscle is included in Chapter 15 (the autonomic nervous system),
as well as in discussions of the various organs containing smooth
muscle.

Structure of Skeletal
Muscle Tissue

OBJECTIVES

• Explain the importance of connective tissue components, blood
vessels, and nerves to skeletal muscles.
• Describe the microscopic anatomy of a skeletal muscle fiber.
• Distinguish thick filaments from thin filaments.
• Describe the functions of skeletal muscle proteins.
Each of your skeletal muscles is a separate organ composed of hun-
dreds to thousands of cells, which are called muscle fibers (myocytes)
because of their elongated shapes. Thus, muscle cell and muscle fiber
are two terms for the same structure. Skeletal muscle also contains
connective tissues surrounding muscle fibers, and blood vessels and
nerves (Figure 10.1). To understand how contraction of skeletal muscle
can generate tension, you must first understand its gross and micro-
scopic anatomy.

Connective Tissue Components

Connective tissue surrounds and protects muscular tissue. The sub-
cutaneous layer or hypodermis, which separates muscle from skin
(see Figure 11.21), is composed of areolar connective tissue and adi-
pose tissue. It provides a pathway for nerves, blood vessels, and lym-
phatic vessels to enter and exit muscles. The adipose tissue of the
subcutaneous layer stores most of the body’s triglycerides, serves as
an insulating layer that reduces heat loss, and protects muscles from
physical trauma. Fascia (FASH-e¯-a = bandage) is a dense sheet or
broad band of irregular connective tissue that lines the body wall and
limbs and supports and surrounds muscles and other organs of the
body. As you will see, fascia holds muscles with similar functions
together (see Figure 11.21). Fascia allows free movement of muscles;
carries nerves, blood vessels, and lymphatic vessels; and fills spaces
between muscles.
Three layers of connective tissue extend from the fascia to pro-
tect and strengthen skeletal muscle (Figure 10.1):
• Epimysium (ep-i-MI¯Z-e¯-um; epi- = upon) is the outer layer, encircling
the entire muscle. It consists of dense irregular connective tissue.
• Perimysium (per-i-MI¯Z-e¯-um; peri- = around) is also a layer of
dense irregular connective tissue, but it surrounds groups of 10
to 100 or more muscle fibers, separating them into bundles called
fascicles (FAS-i-kuls = little bundles). Many fascicles are large
enough to be seen with the naked eye. They give a cut of meat its
characteristic “grain”; if you tear a piece of meat, it rips apart along
the fascicles.
• Endomysium (en′-doˉ-MI¯Z-e¯-um; endo- = within) penetrates the in-
terior of each fascicle and separates individual muscle fibers from one
another. The endomysium is mostly reticular fibers.
The epimysium, perimysium, and endomysium are all continu-
ous with the connective tissue that attaches skeletal muscle to other
structures, such as bone or another muscle. For example, all three
connective tissue layers may extend beyond the muscle fibers to form
a ropelike tendon that attaches a muscle to the periosteum of a bone.
An example is the calcaneal (Achilles) tendon of the gastrocnemius
(calf) muscle, which attaches the muscle to the calcaneus (heel bone)
(shown in Figure 11.22c). When the connective tissue elements
extend as a broad, flat sheet, it is called an aponeurosis (ap-oˉ-noo-
RO¯-sis; apo- = from; -neur- = a sinew). An example is the epicranial
aponeurosis on top of the skull between the frontal and occipital
bellies of the occipitofrontalis muscle (shown in Figure 11.4a, c).

Nerve and Blood Supply

Skeletal muscles are well supplied with nerves and blood vessels.
Generally, an artery and one or two veins accompany each nerve that
penetrates a skeletal muscle. The neurons that stimulate skeletal
muscle to contract are somatic motor neurons. Each somatic motor
neuron has a threadlike axon that extends from the brain or spinal
cord to a group of skeletal muscle fibers (see Figure 10.9d). The axon
of a somatic motor neuron typically branches many times, each
branch extending to a diff erent skeletal muscle fiber.

Microscopic blood vessels called

capillaries are plentiful in
mus cu lar tissue; each muscle fiber is in close contact with one or
more capil laries (see Figure 10.9d). The blood capillaries bring in
oxygen and nu tri ents and remove heat and the waste products of
muscle meta b olism. Especially during contraction, a muscle fiber
synthesizes and uses considerable ATP (adenosine triphosphate).
These reactions, which you will learn more about later on, require
oxygen, glucose, fatty acids, and other substances that are delivered
to the muscle fiber in the blood.
Microscopic Anatomy of a Skeletal
Muscle Fiber
The most important components of a skeletal muscle are the muscle
fibers themselves. The diameter of a mature skeletal muscle fiber ranges
from 10 to 100 μm.* The typical length of a mature skeletal muscle fiber is
about 10 cm (4 in.), although some are as long as 30 cm (12 in.). Because
each skeletal muscle fiber arises during embryonic development from
the fusion of a hundred or more small mesodermal cells called myoblasts
(MI¯-oˉ -blasts) (Figure 10.2a), each mature skeletal muscle fiber has a
hundred or more nuclei. Once fusion has occurred, the muscle fiber loses
its ability to undergo cell division. Thus, the number of skeletal muscle
fibers is set before you are born, and most of these cells last a lifetime.
Sarcolemma, Transverse Tubules, and Sarcoplasm 
The multiple nuclei of a skeletal muscle fiber are located just beneath
the sarcolemma (sar′-koˉ-LEM-ma; sarc- = flesh; -lemma = sheath),
the plasma membrane of a muscle cell (Figure 10.2b, c). Thousands
of tiny invaginations of the sarcolemma, called transverse (T)
tubules, tunnel in from the surface toward the center of each
muscle fiber. Because T tubules are open to the outside of the fiber,
they are filled with interstitial fluid. Muscle action potentials travel
along the sarcolemma and through the T tubules, quickly spreading
throughout the muscle fiber. This arrangement ensures that an
action potential excites all parts of the muscle fiber at essentially
the same instant.
Within the sarcolemma is the sarcoplasm (SAR-koˉ-plazm), the cyto-
plasm of a muscle fiber. Sarcoplasm includes a substantial amount of
glycogen, which is a large molecule composed of many glucose mole-
cules (see Figure 2.16). Glycogen can be used for synthesis of ATP. In ad-
dition, the sarcoplasm contains a red-colored protein called myoglobin
(mı¯-oˉ-GLO¯B-in). This protein, found only in muscle, binds oxygen mole-
cules that diff use into muscle fibers from interstitial fluid. Myoglobin re-
leases oxygen when it is needed by the mitochondria for ATP production.
The mitochondria lie in rows throughout the muscle fiber, strategically
close to the contractile muscle proteins that use ATP during contraction
so that ATP can be produced quickly as needed (Figure 10.2c).
Myofibrils and Sarcoplasmic Reticulum  At high
magnification, the sarcoplasm appears stuffed with little threads.
These small structures are the myofibrils (mı¯-oˉ -FI¯-brils; myo- =
muscle; -fibrilla = little fiber), the contractile organelles of skeletal
muscle (Figure 10.2c). Myofibrils are about 2 μm in diameter and
extend the entire length of a muscle fiber. Their prominent striations
make the entire skeletal muscle fiber appear striped (striated).
A fluid-filled system of membranous sacs called the sarcoplas-
mic reticulum (SR) (sar′-koˉ-PLAZ-mik re-TIK-uˉ-lum) encircles each
myofibril (Figure 10.2c). This elaborate system is similar to smooth
endoplasmic reticulum in nonmuscular cells. Dilated end sacs of the

Q Which structure shown here releases calcium ions to trigger muscle contraction?

sarcoplasmic reticulum called terminal cisterns (SIS-terns = reser-
voirs) butt against the T tubule from both sides. A transverse tubule
and the two terminal cisterns on either side of it form a triad (tri- =
three). In a relaxed muscle fiber, the sarcoplasmic reticulum stores
calcium ions (Ca2+). Release of Ca2+ from the terminal cisterns of the
sarcoplasmic reticulum triggers muscle contraction.
Filaments and the Sarcomere  Within myofibrils are
smaller protein structures called filaments or myofilaments (Figure
10.2c). Thin filaments are 8 nm in diameter and 1–2 μm long and
composed of the protein actin, while thick filaments are 16 nm in
diameter and 1–2 μm long and composed of the protein myosin.
Both thin and thick filaments are directly involved in the contractile
process. Overall, there are two thin filaments for every thick filament
in the regions of filament overlap. The filaments inside a myofibril
do not extend the entire length of a muscle fiber. Instead, they
are arranged in compartments called sarcomeres (SAR-koˉ-me¯rs;
-mere = part), which are the basic functional units of a myofibril
(Figure 10.3a). Narrow, plate-shaped regions of dense protein ma-
te rial called Z discs separate one sarcomere from the next. Thus, a
sarcomere extends from one Z disc to the next Z disc.
The components of a sarcomere are organized into a variety of
bands and zones (Figure 10.3b). The darker middle part of the sar-
comere is the A band, which extends the entire length of the thick
filaments (Figure 10.3b). Toward each end of the A band is a zone
of overlap, where the thick and thin filaments lie side by side. The
I band is a lighter, less dense area that contains the rest of the thin
filaments but no thick filaments (Figure 10.3b), and a Z disc passes
through the center of each I band. The alternating dark A bands
and light I bands create the striations that can be seen in both

Q Which of the following is the smallest: muscle fiber, thick filament, or myofibril?
Which is largest?

myofibrils and in whole skeletal and cardiac muscle fibers. A narrow
H zone in the center of each A band contains thick but not thin fila-
ments. A mnemonic that will help you to remember the composition
of the I and H bands is as follows: the letter I is thin (contains thin
filaments), while the letter H is thick (contains thick filaments). Sup-
porting proteins that hold the thick filaments together at the center
of the H zone form the M line, so named because it is at the middle
of the sarcomere. Table 10.1 summarizes the components of the
sarcomere.

Muscle Proteins

Myofibrils are built from three kinds of proteins: (1) contractile pro-
teins, which generate force during contraction; (2) regulatory proteins,
which help switch the contraction process on and off ; and (3) struc-
tural proteins, which keep the thick and thin filaments in the proper
alignment, give the myofibril elasticity and extensibility, and link the
myofibrils to the sarcolemma and extracellular matrix.
The two contractile proteins in muscle are myosin and actin,
components of thick and thin filaments, respectively. Myosin (MI¯-oˉ-
sin) is the main component of thick filaments and functions as
motor protein in all three types of muscle tissue. Motor proteins pull
various cellular structures to achieve movement by converting the
chemical energy in ATP to the mechanical energy of motion, that
is, the production of force. In skeletal muscle, about 300 molecules
of myosin form a single thick filament. Each myosin molecule is
shaped like two golf clubs twisted together (Figure 10.4a). The
myosin tail (twisted golf club handles) points toward the M line in
the center of the sarcomere. Tails of neighboring myosin molecules
lie parallel to one another, forming the shaft of the thick filament.
The two projections of each myosin molecule (golf club heads) are
called myosin heads. Each myosin head has two binding sites
(Figure 10.4a): (1) an actin-binding site and (2) an ATP-binding site.
The ATP-binding site also functions as an ATPase—an enzyme that
hydrolyzes ATP to generate energy for muscle contraction. The
heads project outward from the shaft in a spiraling fashion, each
extending toward one of the six thin filaments that surround each
thick filament.
The main component of the thin filament is the protein actin
(AK-tin) (see Figure 10.3b). Individual actin molecules join to form an
actin filament that is twisted into a helix (Figure 10.4b). On each actin
molecule is a myosin-binding site, where a myosin head can attach.