Histology Of Muscular System pdf - Dr. Elham Majed

TUCOM

By-Dr.Elham Majed

HISTOLOGY OF MUSCULAR
SYSTEM

SKELETAL MUSCLE

MUSCLE

Motion, as a reaction of multicellular organisms to changes in the

internal and external environment, is mediated by muscle cells

.
The basis for motion mediated by muscle cells is the conversion of

chemical energy (ATP) into mechanical energy by the contractile

apparatus of muscle cells. The proteins actin and myosin are part of the
contractile apparatus. The interaction of these two proteins mediates
the contraction of muscle cells. Actin and myosin
form myoﬁlaments arranged parallel to the direction of cellular

contraction

.
A further specialization of muscle cells is an excitable cell
membrane which propagates the stimuli which initiate cellular

contraction

.
Three structurally and functionally distinct types of muscle are found in
body

Skeletal muscle

Smooth muscle and

cardiac muscle

Skeletal Muscle

Skeletal muscle consists of very long tubular cells (also called muscle ﬁbers)

.
The average length of skeletal muscle cells in humans is about 3 cm (sartorius
muscle up to 30 cm, stapedius muscle only about 1 mm). Their diameters vary

from 10 to 100 µm

Skeletal muscle ﬁbers contain many peripherally placed nuclei

.
Up to several hundred rather small nuclei with 1 or 2 nucleoli are located just

beneath the plasma membrane

Skeletal muscle ﬁbers show in many preparations characteristic cross-striations. It

is therefore also called striated muscle

Skeletal muscle is innervated by the somatic nervous system

Skeletal muscle makes up the voluntary muscle

Structure of skeletal muscle

Muscle ﬁbers in skeletal muscle occur in bundles, fascicles, which make up the

muscle. The muscle is surrounded by a layer of connective tissue, the epimysium,

which is continuous with the muscle fascia. Connective tissue from the epimysium
extends into the muscle to surround individual fascicles (perimysium) from which a

delicate network of reticular ﬁbers surrounds each individual muscle ﬁber

(endomysium). The connective tissue transduces the force generated by the

muscle ﬁbers to the tendons

.
The insertion into the tendon of the connective tissue ﬁbers surrounding the

muscle ﬁbers, i.e. the muscle-tendon junction

Location of skeletal muscle

Tongue, Skeletal Muscle, human
Skeletal muscle in the tongue is arranged in bundles
which typically run at right angles to each other. Both
longitudinally and transversely . In both section plans
the nuclei are located in the periphery of the muscle
ﬁber. Striations are visible in longitudinally cut ﬁbers.
They stand out more clearly if you close the iris
diaphragm of the microscope. Numerous capillaries
between the muscle ﬁbers supply the muscle with
oxygen and nutrient. The muscle surrounding the
upper one-third of the esophagus is skeletal muscle.

Smooth muscle surrounds its lower one-third. In
section of the middle of the esophagus it is usually
possible to identify both muscle types and their

The Contractile Apparatus of Skeletal Muscle

The spatial relation between the ﬁlaments that make up the myoﬁbrils

within skeletal muscle ﬁbers is highly regular. This regular organization of the

myoﬁbrils gives rise to the cross-striation, which characterizes skeletal and

cardiac muscle. Sets of individual "stria" within a myoﬁbril correspond to

the smallest contractile units of skeletal muscle, the sarcomeres

.
Depending on the distribution and interconnection of myoﬁlaments a number

of "bands" and "lines" can be distinguished in the sarcomeres

I-band - actin ﬁlaments

A-band - myosin ﬁlaments which may overlap with actin ﬁlaments

H-band - zone of myosin ﬁlaments only (no overlap with actin ﬁlaments) within

the A-band

,
Z-line - zone of apposition of actin ﬁlaments belonging to two neighboring

sarcomeres

M-line - band of connections between myosin ﬁlaments .The average length of

a sarcomere is about 2.5 µm

Motor end plate

The area of contact between the end of a motor nerve and

a skeletal muscle cell is called the motor end plate.
Numerous ﬁne branches of the motor nerve make plate-like
contacts (buttons) with the muscle cell. The excitatory

transmitter at the motor end plate is acetylcholine. The

space between the buttons and the muscle ﬁber is called
primary synaptic cleft. Numerous infoldings of the

sarcolemma in the area of the motor end plate form

secondary synaptic clefts

The spread of excitation over the sarcolemma is mediated

by voltage-gated ion channels

Invaginations of the sarcolemma form the T-tubule
system which "leads" the excitation into the muscle ﬁber,
close to the border between the A- and I-bands of the
myoﬁbrils. Here, the T-tubules are in close apposition with
cisternae formed by the sarcoplasmic reticulum. This

association is called a triad

Types of Skeletal Muscle

Skeletal muscle cells respond to stimulation with a brief maximal

contraction - they are of the twitch type. Individual muscles ﬁbers cannot

maintain their contraction over longer periods. The sustained contraction
of a muscle depends on the "averaged" activity of often many muscles

ﬁbers, which individually only contract for a brief period of time

.
The functional properties of the muscle can be "ﬁne-tuned" further to the

tasks the muscle performs by blending functionally diﬀerent types of

muscle ﬁbers

Red or type I ﬁbresRed muscles contain predominantly
(but not exclusively) red muscle cells. Red muscle cells are
comparatively thin and contain large amounts of myoglobin
and mitochondria. Red ﬁbers contain an isoform of myosin with
low ATPase activity, i.e. the speed with which myosin is able to
use up ATP. Contraction is therefore slow. Red muscles are
used when sustained production of force is necessary, e.g. in

the control of posture

White or type II ﬁbresWhite muscle cells, which are
predominantly found in white muscles, are thicker and contain
less myoglobin. ATPase activity of the myosin isoform in white
ﬁbres is high, and contraction is fast. Type IIa ﬁbres contain
many mitochondria and are available for both sustained
activity and short-lasting, intense contractions. Type IIb ﬁbres
contain only few mitochondria. They are recruited in the case of
rapid accelerations and short lasting maximal contraction.
Type IIb ﬁbres rely on anaerobic glycolysis to generate the ATP

needed for contraction

.
Skeletal muscle ﬁbres do not contract spontaneously. Skeletal
muscle ﬁbres are not interconnected via GAP junctions but
depend on nervous stimulation for contraction. All muscle

ﬁbres of a motor unit are of the same type

Muscle Spindles
Muscle spindles are sensory specialization of the muscular
tissue. A number of small specialized intrafusal muscle
ﬁbers (nuclear bag ﬁbers and nuclear chain ﬁbers) are
surrounded by a capsule of connective tissue. The intrafusal
ﬁbers are innervated by eﬀerent motor nerve ﬁbers.
Aﬀerent sensory nerve ﬁbers surround the intrafusal muscle

ﬁbers

.
If the muscle is stretched, the muscle ﬁbers in the muscle
spindle are stretched, sensory nerves are stimulated, and a
change in contraction of the muscle is perceived. Diﬀerent
types of intrafusal ﬁbers and nerve endings allow the
perception of position, velocity and acceleration of the

contraction of the muscle

.
The contraction of the intrafusal ﬁbers, after stimulation by the
eﬀerent nerve ﬁbers, may counteract or magnify the changes
imposed on the muscle spindle by the surrounding muscle. The
intrafusal ﬁbers and the eﬀerent nerves can in this way set the

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