Smooth muscle cells are found within organs and blood vessels. There are two main types - multi-unit smooth muscle composed of separate fibers, and unitary (visceral) smooth muscle with fibers joined by gap junctions. Smooth muscle contraction is activated by calcium ions and sustained through a latch mechanism using little ATP. Contraction can be stimulated by nerves releasing acetylcholine or norepinephrine, hormones, stretch of the muscle, or local chemical factors and pacemaker potentials in some muscles. Prolonged contraction is enabled through action potentials with plateaus or slow wave potentials.

1. Smooth Muscle Cell

2. Smooth Muscle Cell:
• 1 to 5 micrometers in
diameter and only 20 to
500 micrometers in
length.
• The same attractive
forces between myosin
and actin filaments.
• The internal physical
arrangement of smooth
muscle fibers is different.

3. TYPES OF SMOOTH MUSCLE:
• The smooth muscle of each organ is distinctive from
that of most other organs in several ways:
(1) Physical dimensions.
(2) Organization into bundles or sheets.
(3) Response to different types of stimuli.
(4) Characteristics of innervation.
(5) Function.
• Smooth muscle can generally be divided into two
major types:
A. multi-unit smooth muscle.
B. unitary (or single-unit) smooth muscle.

5. A. Multi-Unit Smooth Muscle:
• Composed of discrete, separate smooth muscle fibers.
• Operates independently of the others.
• Is innervated by a single nerve ending.
• Covered by outer surface of fiber covered by a thin layer of
basement membrane–like substance, a mixture of fine
collagen and glycoprotein that helps insulate the separate
fibers from one another.
• Some examples of multi-unit smooth muscle are:
A. Ciliary muscle of the eye.
B. The iris muscle of the eye.
C. The piloerector muscles that cause erection of the hairs.

6. B. Unitary Smooth Muscle:
• Also called syncytial smooth muscle or visceral smooth
muscle.
• A mass of hundreds to thousands of smooth muscle fibers
that contract together as a single unit.
• The fibers usually are arranged in sheets or bundles.
• The cell membranes are joined by many gap junctions.
• It is found in the walls of most viscera of the body,
including:
A. Gastrointestinal tract,
B. Bile ducts,
C. Ureters,
D. Uterus,
E. Many blood vessels.

7. CONTRACTILE MECHANISM
IN SMOOTH MUSCLE:
a) Smooth muscle
contains both actin
and myosin filaments.
b) It does not contain the
troponin complex.
c) the contractile process
is activated by
calcium ions, and
adenosine
triphosphate (ATP) –
ADP.

8. Comparison of Smooth Muscle Contraction
and Skeletal Muscle Contraction:
• Most skeletal muscles contract and relax rapidly, most smooth muscle
contraction is prolonged tonic contraction, sometimes lasting hours or
even days.
1. Slow Cycling of the Myosin Cross-Bridges (Actin - myosin).
2. Low Energy Requirement to Sustain Smooth Muscle Contraction (ATP).
3. Slowness of Onset of Contraction and Relaxation of the Total Smooth
Muscle Tissue.
4. The Maximum Force of Contraction Is Often Greater in Smooth
Muscle Than in Skeletal Muscle.
5. The “Latch” Mechanism Facilitates Prolonged Holding of Contractions
of Smooth Muscle (maintain prolonged tonic contraction in smooth
muscle for hours with little use of energy).
6. Stress-Relaxation of Smooth Muscle (maintain about the same amount
of pressure inside its lumen despite sustained, large changes in
volume.).

9. Activation and subsequent contraction occur in
the following sequence:
1. Calcium concentration in the cytosolic fluid of the smooth muscle
increases as a result of the influx of calcium from the extracellular fluid
through calcium channels and/or release of calcium from the
sarcoplasmic reticulum.
2. The calcium ions bind reversibly with calmodulin.
3. The calmodulin-calcium complex then joins with and activates myosin
light chain kinase, a phosphorylating enzyme.
4. One of the light chains of each myosin head, called the regulatory chain,
becomes phosphorylated in response to this myosin kinase.
When this chain is not phosphorylated, the attachment-detachment cycling
of the myosin head with the actin filament does not occur.
However, when the regulatory chain is phosphorylated, the head has the
capability of binding repetitively with the actin filament and proceeding
through the entire cycling process of intermittent “pulls”

10. Smooth Muscle Contraction Is
Dependent on Extracellular
Calcium Ion Concentration.

12. A Calcium Pump Is Required to
Cause Smooth Muscle Relaxation.
Myosin Phosphatase Is Important
in Cessation of Contraction.

13. NERVOUS AND HORMONAL CONTROL
OF SMOOTH MUSCLE CONTRACTION
• Smooth muscle can be stimulated to contract by:
1. Nervous signals.
2. Hormonal stimulation.
3. Stretch of the muscle.
4. Several other ways.
• The smooth muscle membrane contains many types:
A. Receptor proteins that can initiate the contractile
process.
B. Still other receptor proteins inhibit smooth muscle
contraction.

14. NEUROMUSCULAR JUNCTIONS
OF SMOOTH MUSCLE:
the vesicles of the autonomic nerve
fiber endings contain acetylcholine
in some fibers and norepinephrine in
others, and occasionally other
substances as well.
Contact junctions
Diffuse junctions
the rapidity of contraction of these
smooth muscle fibers is considerably
faster than that of fibers stimulated
by the diffuse junctions.
Acetylcholine is an excitatory transmitter
substance for smooth muscle fibers in
some organs but an inhibitory transmitter
for smooth muscle in other organs. When
acetylcholine excites a muscle fiber,
norepinephrine ordinarily inhibits it.
Conversely, when acetylcholine inhibits a
fiber, norepinephrine usually excites it.

15. MEMBRANE POTENTIALS AND ACTION
POTENTIALS IN SMOOTH MUSCLE
• In the normal resting state, the intracellular
potential is usually about −50 to −60 millivolts.
• Action Potentials in Unitary Smooth Muscle:
• Same way that they occur in skeletal muscle.
• Not normally occur in most multi-unit types of
smooth muscle.
• The action potentials of visceral smooth muscle
occur in one of two forms:
(1) Spike potentials.
(2) Action potentials with plateaus.

16. Spike Potentials:
• The duration of this type of action potential is 10 to 50
milliseconds.
• In Unitary Smooth muscle
• Action potentials can be elicited in many ways:
1. Elicited by electrical stimulation:
2. Action of hormones.
3. Action of transmitter substances from nerve
fibers.
4. By stretch.
5. Spontaneous generation in the muscle fiber
itself

17. Action Potentials with
Plateaus:
• Similar to that of the typical spike potential.
• The repolarization is delayed for several hundred to
as much as 1000 milliseconds (1 second).
• The importance of the plateau is that it can account
for the prolonged contraction that occurs in some
types of smooth muscle:
A. Ureter.
B. The uterus under some conditions.
C. certain types of vascular smooth muscle.
D. Also seen in cardiac muscle fibers that have a
prolonged period of contraction.

18. Slow Wave Potentials in Unitary Smooth
Muscle Can Lead to Spontaneous
Generation of Action Potentials
• Some smooth muscle is self-excitatory
(Pacemaker)—that is, action potentials arise within
the smooth muscle cells without an extrinsic
stimulus.
• This activity is often associated with a basic slow
wave rhythm of the membrane potential.
• The slow wave itself is not the action potential.
• It is a local property of the smooth muscle fibers that
make up the muscle mass.
• The slow waves are caused by waxing and waning of
the pumping of positive ions (presumably sodium
ions) outward through the muscle fiber membrane.

19. Continueeeeeeeeee.
• The conductances of the ion channels increase and
decrease rhythmically.
• The slow waves is that, when they are strong enough,
they can initiate action potentials.
• The slow waves themselves cannot cause muscle
contraction.
• When the peak of the negative slow wave potential
inside the cell membrane rises in the positive direction
from −60 to about −35 millivolts an action potential
develops and spreads over the muscle mass and
contraction occurs.
• The slow waves are called pacemaker waves.

20. Excitation of Visceral Smooth
Muscle by Muscle Stretch:
• When visceral (unitary) smooth muscle is
stretched sufficiently, spontaneous action
potentials are usually generated.
• They result from a combination of
(1) The normal slow wave potentials
(2) A decrease in overall negativity of the
membrane potential caused by the stretch.
• This response to stretch allows the gut wall, when
excessively stretched, to contract automatically
and rhythmically.

21. DEPOLARIZATION OF MULTI-UNIT
SMOOTH MUSCLE WITHOUT
ACTION POTENTIALS

22. • The smooth muscle fibers of multi-unit smooth muscle
normally contract mainly in response to nerve stimuli.
• The transmitter substances cause depolarization of the
smooth muscle membrane, and this depolarization in turn
elicits contraction.
• Action potentials usually do not develop because the fibers
are too small to generate an action potential.
• Yet in small smooth muscle cells, even without an action
potential, the local depolarization (called the junctional
potential) caused by the nerve transmitter substance itself
spreads “electrotonically” over the entire fiber and is all
that is necessary to cause muscle contraction.

23. • Approximately half of all smooth muscle
contraction is likely initiated by stimulatory
factors acting directly on the smooth muscle
contractile machinery and without action
potentials.
• Two types of non-nervous and non - action
potential stimulating factors often involved are:
(1) local tissue chemical factors (humoral)
(2) various hormones.

24. Smooth Muscle Contraction in Response to
Local Tissue Chemical Factors
• Some of the specific control factors are as follows:
1. Lack of oxygen in the local tissues causes smooth muscle
relaxation and, therefore, vasodilation.
2. Excess carbon dioxide causes vasodilation.
3. Increased hydrogen ion concentration causes vasodilation.
4. Adenosine, lactic acid, increased potassium ions,
diminished calcium ion concentration, and increased body
temperature can all cause local vasodilation.
5. Decreased blood pressure, by causing decreased stretch of
the vascular smooth muscle, also causes these small blood
vessels to dilate.
6. Nitric oxide (NO), the endothelium-derived relaxing factor
(EDRF).

25. Effects of Hormones on
Smooth Muscle Contraction:
1. Norepinephrine,
2. Epinephrine,
3. Acetylcholine
4. Angiotensin II,
5. Endothelin,
6. Vasopressin,
7. Oxytocin,
8. Serotonin,
9. Histamine.