Smooth muscles are found in visceral organs and blood vessels, characterized by the absence of sarcomeres and unique properties like prolonged contractions with low energy expenditure. They exist in two types: single-unit smooth muscle, where fibers are connected by gap junctions allowing synchronized contractions, and multi-unit smooth muscle, where contractions do not spread between cells. The contraction mechanism involves calcium influx that activates myosin through calmodulin, leading to muscle shortening and relaxation regulated by changes in calcium concentration.

1. SMOOTH MUSCLES
•
Smooth muscles are mainly present in the visceral organs and
blood vessels
.
•
The distinctive features of smooth muscles
:
•
1
.
They are characterized by absence of sarcomere and the
regular pattern of cross-striations as found in skeletal and
cardiac muscles, hence the name smooth muscles
.
•
2
.
The nerve supply to smooth muscles is through the branches
of autonomic nervous system
.
•
3
.
The length tension relationship is not linear as in skeletal
and cardiac muscle
.
•
4
.
Smooth muscles can remain contracted for relatively longer
periods without expenditure of much energy, a property that
endows them with a better metabolic economy
.

2. •
A. Morphological properties of smooth muscle
•
1
-
Each smooth muscle fiber is a spindle-shaped cell, 100–300
µm long and 5–10 µm in diameter at the middle
•
2
.
There are no sarcomeres and striations are not visible
•
3
.
Absence of troponin
•
4
.
Absence of T ­
tubules
•
5
.
Absence of well­developed neuromuscular junctions

3. Smooth muscle cell
•
Morphology of smooth muscle cell
:
•
-
cell membrane
•
-
nucleus
•
-
myofilament(actin, myosin ,intermediate
•
-
dense body(alpha actin protein)
•
-
In addition to the filaments, the cytoplasm contains a
calcium-binding protein called calmodulin, (Note ,that
troponin protein complex is absent)

5. Types of smooth muscle
•
A-Single-unit Smooth Muscle
•
Properties of single-unit smooth muscle are
:
•
1
-
The muscle fibers in single-unit smooth muscles are
connected to each other by gap junctions
.
•
2
.
The action potential generated at one place spreads rapidly to
all the fibers of that unit. Therefore, contraction occurs
simultaneously ,synchronous electrical and mechanical activity
in all the fibers
.
•
3
-
They can be contracted by myogenic activity
•
4
-
They form the walls of hollow viscera such as the
(gastrointestinal tract, from the esophagus to the rectum,
including the gallbladder and ducts of digestive glands; the
ureters and urinary bladder; the uterus and small diameter
blood vessels.)
•

6. •
B-Multi-unit Smooth Muscle
•
Properties of multi-unit smooth muscle are
:
•
1
-
Multiunit smooth muscle cells do not possess gap junctions,
and contraction does not spread from one cell to the next, but
is instead confined to the cell that was originally stimulated
. .
•
2
.
They are abundantly innervated by the fibers of the
autonomic nervous system
.
•
3
.
The intrinsic muscles of the eye (ciliary body and iris),
muscle in the large airways to the lungs,, and pili erector
muscles are examples of multiunit smooth muscles

10. •
-
Myofilaments of smooth muscle
•
The cytoplasm contains three types of myofilaments
:
•
1
-
Thick filament ,is composed of protein myosin
,
•
2
-
Thin filament is composed of protein actin , and
•
3
-
intermediate (which is present between the thick and thin
filaments) is composed of 2 protein ,desmin and vemetin
.
•
.

11. Mechanism of contraction
•
The contraction of the thin and thick filament causes the dense
bodies to move closer, witch causes the shortening of the
intermediate filament found throughout the cell
.
•
Since some of the filament are connected to membrane, this
cause the cell to get smaller and contract as whole

14. Functional Organization
•
Smooth muscle fibers types
:
•
1
.
Circular: Circular pattern of arrangement is seen in the
blood vessels and in the airways of the lungs where
contraction of the muscles narrows the diameter of the passage
and reduces the flow. In sphincters, the contraction can totally
block the passage and stop the flow
.
•
2
.
Circular and longitudinal: is found in GI tract, where an
inner thick circular muscle layer and an outer thin longitudinal
muscle layer regulate the many types of movements of
intestine and promote the mixing and forward movement of
the digested food
.
•
3
.
Circular, longitudinal and oblique: is typically found in the
uterus and urinary bladder. When the increased contents of
these viscera stretch these muscles, they contract and decrease
the volume by expelling the contents
.

16. •
-
Role of Calcium in smooth muscle contraction
•
1
.
The strength of the stimulus (the degree of stretch or the
amount of neurotransmitter/hormone reaching the cell
membrane) acting on the smooth muscle fiber can be graded.
Accordingly, the change in cytosolic calcium concentration
may be minute, moderate or very high, producing different
degrees of contraction
.
•
2
.
The cytosolic calcium concentration is increased mainly by
entry of calcium from the interstitial fluid (calcium influx) or to
some extent by its release from cytoplasmic calcium stores
(calcium release)
.
•
3
.
However, the concentration of calcium in cytosol decreases
due to binding of calcium to calmodulin, pumping back of
calcium to the SR and calcium efflux
.

17. •
Calcium Influx
•
Calcium influx occurs through voltage-gated Ca++ channels,
ligand-gated Ca++ channels and a few leaky channels
.
•
The ECF calcium concentration is about 12,000 times higher
than that of the cytoplasm (1,200,000 nmol/L versus 100
nmol/L)
.
•
This high inwardly directed gradient favors calcium entry into
the smooth muscle as soon as calcium channels open
.

18. The mechanism of excitation–contraction coupling

22. The mechanism of excitation–contraction coupling
Is different from that in skeletal muscle.
There is no troponin;instead,Ca2+ regulates myosin on the thick filament
1-Depolarization of the cell membrane opens voltage-gated Ca2+
channels and Ca2+ flows into the cell down its electrochemical gradient,
increasing the intracellular [Ca2+].
Hormones and neurotransmitters may open ligand-gated Ca2+ channels
in the cell membrane. Ca2+ entering the cell causes release of more
Ca2+ from the SR in a process called Ca2+induced Ca2+ release.
Hormones and neurotransmitters also directly release Ca2+ from the SR
Ca2+ channels.

23. • 2-Intracellular[Ca2+] increases.
• 3- Ca2 binds to calmodulin.The Ca2+–calmodulin complex
binds to and activates myosin light chain kinase.
• When activated, myosin light chain kinase phosphorylates
myosin and allows it to bind to actin, thus initiating cross-
bridge cycling.
• The amount of tension produced is proportional to the
intracellular Ca2+ concentration.
• 4-A decrease in intracellular [Ca2+] produces relaxation.

24. Mechanism of Relaxation
•
1
.
During contraction, the rate of MLCK-induced
phosphorylation exceeds the rate of MLCP-induced
dephosphorylation and therefore, amount of phosphorylated
myosin increases in the cell, which allows the contraction to
proceed
.
•
2
.
When the cytoplasmic calcium concentration decreases,
calcium is detached from calmodulin resulting in decline in
MLCK activity. Consequently, the activity of MLCP
predominates causing dephosphorylation of myosin; the
myosin ATPase enzyme remains inactive and relaxation
ensues
.
•
3
.
Thus, phosphorylation and dephosphorylation of the myosin
light-chain is regulated by respective rise and fall of cytosolic
calcium concentration
.