CHAPTER 6
Muscle
99
Smooth Muscle
Figure 6-10A
A Representation of the Organization and Characteristics of Smooth Muscle
Figure 6-10B
Smooth Muscle, Duodenum
Figure 6-11A,B
Smooth Muscle—Uterus and Bronchiole
Figure 6-11C
Clinical Correlation: Chronic Asthma
Figure 6-12
Transverse Section of the Smooth Muscle of the Trachea
Figure 6-13A
Smooth Muscle of a Medium Artery
Figure 6-13B
Schematic Diagram of the Contractile Mechanism of Smooth Muscle
Table 6-1
Muscle Characteristics
Synopsis 6-1
Pathological and Histological Terms for Muscle
Introduction and Key
Concepts for Muscle
The contraction of
muscle tissue
is the only way in which we
can interact with our surroundings and is essential to maintain-
ing life itself. There are three general types of muscles:
skel-
etal
,
cardiac
, and
smooth
. The voluntary contraction of
skeletal
muscle
allows us to move our limbs, F
ngers, and toes; to turn
our head and move our eyes; and to talk. Its name comes from
the fact that most skeletal muscle attaches to bones of the skel-
eton and functions to move the skeleton. However, exceptions
include the extraocular muscles, the tongue, and a few others.
The continuous, rhythmic contraction of
cardiac muscle
pumps
blood through our bodies, without ceasing, for our whole life-
time. Cardiac muscle contraction is involuntary, in contrast to
that of skeletal muscle, although its frequency of contraction is
modulated by the autonomic nervous system and by hormones
and neurotransmitters in the blood.
Smooth muscle
is the most
diverse type of muscle. It occurs in different subtypes in differ-
ent organs and is essential for many involuntary physiological
functions, which include regulating blood fl ow and blood pres-
sure, aiding in the digestion of food, moving food through the
digestive system, regulating air fl ow during respiration, control-
ling the diameter of the pupil in the eye, expelling the baby
during childbirth, and others.
Skeletal Muscle
A single
skeletal muscle
, such as the
biceps
, is composed of numer-
ous bundles of muscle F bers called
fascicles
. The muscle as a
whole is surrounded by a sheet of dense connective tissues, called
the
epimysium
. Each fascicle is surrounded by a sheet of moder-
ately dense connective tissues, called the
perimysium
, and each
individual muscle F ber (muscle cell) in a fascicle is surrounded by
a delicate collagen network, called the
endomysium
(±ig. 6-2). A
skeletal muscle F ber is a long (as long as 10 cm in some muscles),
thin (10–100
μ
m), tubular structure that contains many nuclei
arranged in the cytoplasm (called
sarcoplasm
) just under the cell
membrane (called
sarcolemma
). (Many words relating to muscle
are derived from the Greek word
sarx
, meaning, “fl esh.”) A single
muscle F ber contains many individual
myo± brils
, tiny bundles of
contractile proteins (±ig. 6-2).
CONTRACTION
of skeletal muscle is voluntary. Skeletal
muscle is characterized by a striped appearance when viewed
at higher powers in light microscopy. This pattern of stripes
(called
striations
), at right angles to the long axis of the muscle,
is more obvious when viewed using polarized light and is strik-
ing in electron micrographs (±ig. 6-4A,B). The striations refl
ect
a repeating pattern of contractile elements called
sarcomeres
.
Each sarcomere is made up of an orderly array of
actin
and
myosin myo± laments
(±ig. 6-4B). Each myoF lament consists
of a bundle of actin or myosin molecules together with some
additional accessory molecules. Sudden, all-or-none contraction
occurs in a skeletal muscle F
ber when a motor neuron action
potential (see Chapter 7, “Nervous Tissue”) releases acetylcho-
line at the neuromuscular junction (±ig. 6-6A,B). This causes
a similar action potential to travel along the sarcolemma, trig-
gering the release of
calcium ions
(Ca
++
) into the intracellular
space and initiating a complex interaction between the actin and
the myosin myoF laments (±ig. 6-4C) to produce shortening of
the F ber. The necessary Ca
++
is stored within the muscle F
ber
in a
modiF ed endoplasmic reticulum called the
sarcoplasmic
reticulum
(±ig. 6-5). Calcium channels in the
terminal cisterns
of the sarcoplasmic reticulum open when the electrical action
potential that is carried along the sarcolemma travels into the
interior of the cell via the
transverse tubule system
. This system
consists of many tubular invaginations of the sarcolemma that
lie between pairs of terminal cisterns and encircle each myoF
-
bril, forming
triads
. In general, skeletal muscle is specialized for
rapid contraction under neural control. Although each skeletal
muscle F ber contracts at its maximum whenever it contracts,
variations of overall force of muscle contraction are achieved
by recruiting a greater or lesser number of muscle F bers at any
given moment.
Cardiac Muscle
The
muscle of the heart
is similar to skeletal muscle in that
it is striated and the F
bers contain sarcomeres made up of
arrays of actin and myosin F
laments. However, cardiac muscle
cells are much shorter than those of skeletal muscle and typi-
cally split into two or more branches, which join end to end
(or,
anastomose
) with other cells at
intercalated disks
(±ig. 6-8B).
A transverse tubule system is present in cardiac muscle, but the
sarcoplasmic reticulum is not as highly developed as in skeletal
muscle. Each cardiac muscle F
ber does not receive direct inner-
vation as skeletal muscle F
bers do. Excitation spreads from
F ber to F
ber via
gap junctions
. Contraction is also controlled
by a system of
pacemaker nodes
and
Purkinje cells
.
Smooth Muscle
Muscle F
bers that do not display striations are termed
smooth
muscle
. This type of muscle also contracts by means of a
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