1. Dr. Abdelrahman Mustafa
بسم الله الرحمن الرحيم
SMOOTH MUSCLES
Dr. Abdelrahman Mustafa
Department of Basic Medical Sciences
Division of Physiology
Faculty of Medicine
Almaarefa Colleges

2. Learning Objectives By the end of this lecture you should be able to
Describe the Molecular Base of Smooth Muscle
Describe mechanism of smooth muscle contraction
Compare between Role of Calcium at Contraction in Smooth Muscle and Skeletal Muscle
List the types of smooth muscles
Identify smooth muscle mechanics

3. Smooth Muscle
Found in walls of
hollow organs
gland and
tubes.

4. Molecular Base of Smooth Muscle
No striations
Not arranged in sarcomere pattern found in skeletal muscle
Spindle-shaped cells with single nucleus
Cells usually arranged in sheets within muscle Cell
Has 3 types of filaments
Thick myosin filaments
Longer than those in skeletal muscle
Thin actin filaments
Contain tropomyosin
Calmodulin ( but no troponin)
Filaments of intermediate size = Intermediate Filaments
Do not directly participate in contraction
Form part of cytoskeletal framework that supports cell shape

5. Molecular Base of Smooth Muscle
Dense bodies containing protein
Dense bodies are also attached to the internal surface of the plasma membrane.
The actin filaments are anchored to the dense bodies.
More actin is present in smooth muscle cells than in skeletal
10 to 15 thin filaments for each thick filament
Smooth muscles cells has no T tubules
And poorly developed sarcoplasmic reticulum

6. Mechanism OF Contraction

7. Calcium Activation of Myosin Cross Bridge in Smooth Muscle

8. Schematic Representation of the Arrangement of Thick and Thin
Filaments in a Smooth Muscle Cell In Contracted and Relaxed States

9. Comparison of Role of Calcium In Bringing About Contraction in Smooth Muscle and Skeletal Muscle

10. Types Of Smooth Muscle
On the basis of timing of contraction & source for cytosolic Ca2+increase
- Tonic
- Phasic
On the basis of the unit that act
Multiunit smooth muscle
Single-unit smooth muscle
On the basis of generation of action potential
- Myogenic
- Neurogenic

11. Phasic Smooth muscle: Contracts in burst triggered by action potential
Located in the walls of hollow organs like GIT
Source of cytosolic Calcium
ECF
voltage gated dihydropyridine receptors in plasma membrane functions as calcium channels
Sarcoplasmic reticulum ECF calcium entering in the cell trigger release of calcium from sarcoplasmic reticulum

12. Tonic smooth muscle : Maintain state of partial contraction constantly
Voltage gated Ca2+ channels are open all the time because of low resting membrane potential( -55 to -40 )
Example : smooth muscle in walls of arterioles.
Source of cytosolic Calcium
Binding of chemical messenger (norepinephrine or various hormones ) to G-Protein couples receptors on surface membrane.
Activation of IP3/Ca2+ second messenger pathway
Binding of IP3 to receptors ( Ca2+ channels ) on membrane of sarcoplasmic reticulum stimulate more release of calcium

13. Multiunit Smooth Muscle
Are Neurogenic
Consists of discrete units that function independently of one another
Units must be separately stimulated by nerves to contract( autonomic nerves)
All multi unit smooth muscles are phasic
Found
In walls of large blood vessels
In small airways to lungs
In ciliary muscle of eye that adjusts lens for near or far vision
At base of hair follicles

14. Single-unit Smooth Muscle
Self-excitable (does not require nervous stimulation for contraction)- myogenic
Also called visceral smooth muscle
Fibers become excited and contract as single unit
Cells electrically linked by gap junctions
Can also be described as a functional syncytium
Contraction is slow and energy-efficient
Well suited for forming walls of distensible, hollow organs
Single unit smooth muscle can be phasic or tonic

15. Single-unit Smooth Muscle
Found in
Digestive tract
Reproductive tract
Uterus
Bladder
Ureter
Bile duct
Small blood vessels

16. SMOOTH MUSCLE MECHANICS
SMOOTH MUSCLE ARE SLOW AND ECONOMICAL
LENGTH TENSION RELATION SHIP
STRESS RELAXATION PROCESS

17. Smooth muscle are slow and economical
SLOW CYCLING OF MYOSIN CROSS BRIGDES: ATP splitting by myosin ATPase is much slower in smooth muscle.
because of slow cycling cross bridge remain attached for more time “LATCH MECHANISM”
LESS ENERGY REQUIRES TO SUSTAIN THE CONTRACTION
SLOWNESS OF ONSET OF CONTRACTION AND RELAXATION

18. Length tension relation ship
smooth muscle can still develop tension even when stretch up to 2.5 times because
Resting length is much shorter than the optimal length ( lo)
E.g. even though muscle fiber in urinary bladder considerably stretch when it fills with urine , they still maintain tone and even can develop further tension to empty the bladder

19. STRESS RELAXATION
Smooth muscle when stretch , initially increase tension much like the rubber band , but slowly tension comes back to resting level due to readjustment of cross bridge attachment.

20. Q1 the primary Function of Intermediate Filaments is:
A)Act directly in muscle contraction
B) Form part of cytoskeletal
C)Ca releasing
D)Initiate Action potential

21. Q2 Ca in smooth muscle contraction will bind to A)Tropinin
B)Tropomysin
C)Calmdulin
D)Actin

22. Q3 Single-unit Smooth Muscle found in :
A)In walls of large blood vessels
B)In small airways to lungs
C)In ciliary muscles
D) In the Uterus

23. Q4 Smooth muscle are slow and economical
A) Length tension relationship
B) Stress relaxation process
C) Latch mechanism
D) Mechanism contraction of smooth muscle

24. References Human physiology by Lauralee Sherwood, 7th edition
Text book physiology by Guyton &Hall,12th edition
Text book of physiology by Linda .s contanzo,third edition