2. Smooth Muscle
Anatomy
Smooth muscle is considered to be
much more primitive than either cardiac
or skeletal muscle. Muscle striations
are not visible in smooth muscle, so the
sarcomere relationship of myosin to
actin does not exists in smooth muscle.
However, per cross sectional area
smooth muscle is as strong as skeletal
muscle and smooth muscle is highly
resistant to fatigue.
3. Smooth Muscle
Anatomy
I. Smooth Muscle
– 1. Fibers are smaller than skeletal muscle
– 2. Involuntary (Usually)
– 3. No apparent myofibrils under the light
microscope - No cross striations
– 4. Fibers are thickest in the middle and
have tapered ends
– 5. Centrally located single oval nucleus
5. Smooth Muscle
Anatomy
– 6. Sarcoplasm contains thick and thin filaments
a. Not in an orderly pattern
b. 10 – 20 times more thin filaments than thick filaments
– 7. Dense bodies
a. Have thin filaments attached to them
b. Function is similar to Z disks
c. Dispersed throughout the sarcoplasm or attached to
the sarcolemma
d. Thin filaments stretch from one dense body to another
7. B. Generation of Contraction
– 1. Sliding filament mechanism involving thick and
thin filaments generates tension that is transmitted
to the thin filaments
– 2. Does not contain Troponin complex
– 3. Actin and myosin pull on the dense bodies
attached to the sarcolemma
– 4. Shortening of the smooth muscle fiber is
lengthwise, the middle of the fiber thickens
Smooth Muscle
Anatomy
9. Smooth Muscle
Anatomy
– 5. Shortening causes bubble like expansion
of the sarcolemma
– 6. Shortening is corkscrew like - the fiber
twists in a helix as it shortens and rotates in
the opposite direction as it lengthens
10. Smooth Muscle Anatomy
Structure of Smooth Muscle
II. Types of Smooth Muscle - Single
unit and multi unit
Size - Smooth muscle is much smaller
than either skeletal or cardiac muscle
Organization into bundles or sheets
(Single unit)
Response to different types of stimuli
– Nerves, hormones, mechanical stretch,
endothelial mediators (EDNO)
11. Smooth Muscle Anatomy
Structure of Smooth Muscle
Characteristics of innervation - Innervated by
ANS - Swellings along the Axon are synapses -
Continuous firing the firing rate determines the
concentration of neurotransmitter, hence the
degree of physiological response
Function - Control the size of organs - cause
the release of substances - Control the rate of
flow (blood, contents of GI tract, urine) - etc.
12. Smooth Muscle Anatomy
Structure of Smooth Muscle
Types of smooth
muscles -
Single Unit - Large
aggregates of
smooth muscle cells
which act as a single
unit.
Multi Unit - Single
smooth muscle cells
usually with a single
nerve connection
13. Smooth Muscle Anatomy
Structure of Smooth Muscle
Multi Unit - Single
smooth muscle cells
usually with a single
nerve connection
Examples - Arrector
pili muscles, Vas
deferens and iris of
the eye.
14. Smooth Muscle Anatomy
Structure of Smooth Muscle
A. Multi-unit smooth muscle
– 1. Individual fibers - independent of all other fibers
– 2. Fibers contain their own motor neuron terminals
with few gap junctions and control is mostly
nervous
– 3. Covered by a thin layer of basement membrane
of fine collagen and glycoprotein fibrillae that help
to insulate the fiber
15. Smooth Muscle Anatomy
Structure of Smooth Muscle
– 4. Action potential causes the stimulation of one
single muscle fiber
– 5. Seldom exhibit spontaneous contractions
– 6. Found in:
a. Walls of large arteries
b. Airways of lungs (Bronchioles)
c. Arrector pili muscles attached to hair follicles
d. Radial and circular muscles of the iris of the
eye
16. Smooth Muscle Anatomy
Structure of Smooth Muscle
Single Unit - Large
aggregates of
smooth muscle
cells which act as
a single unit..
These usually line
the hollow organs
such as blood
vessels or the
gastrointestinal
tract.
17. Smooth Muscle Anatomy
Structure of Smooth Muscle
Single Unit - Unitary Smooth Muscle -
(Syncytial smooth muscle or visceral
smooth muscle)
– 1. Most common - sheets or bundles of
large numbers of smooth muscle fibers
that act as if they were a single unit
(Syncytium)
– 2. Cell membranes adhere to multiple
points and gap junctions join cells
(electrical continuity)
18. Smooth Muscle Anatomy
Structure of Smooth Muscle
–3. Location;
–a. Form part of the walls of both large
and small arteries and veins
b. Hollow viscera - Stomach,
intestines, uterus, urinary bladder
–4. Muscle forms large networks and an
action potential causes contraction of
the entire organ
19. Smooth Muscle
Contractile Mechanism
C. Contractive Process in Smooth Muscle
– 1. Chemical basis for smooth muscle contraction
a. Contains actin and myosin filaments
similar in structure and interaction to
skeletal muscle
b. No troponin complex - mechanism for
contraction is different
c. Calcium influx activates the contractile
process
20. Smooth Muscle
Contractile Mechanism
E. Major differences between smooth muscle
and skeletal muscle
Physical organization of smooth muscle
Excitation-contraction-coupling
Control of the contraction process by calcium
Duration of contraction
Amount of energy required for contraction
– 2. Physical basis for smooth muscle
contraction
21. Smooth Muscle
Contractile Mechanism
A. Smooth muscle is not striated
b. Dense bodies - attachment for actin
fibers - some attach to cell membrane
others located throughout the cytoplasm -
dense bodies in one cell may join with a
dense body in the adjacent cell
c. Few myosin fibers are located in the
actin fibers (15:1 ratio of actin to myosin)
22.
23. Smooth Muscle
Contractile Mechanism
3. Comparison of smooth muscle
contraction with skeletal muscle
contraction
– a. Starts slower and lasts longer than
striated muscle fiber, smooth muscle has a
prolonged contraction - up to hours to days
– b. Can shorten and stretch to a greater
extent than striated muscle
– c. Contraction is initiated by calcium influx
into the sarcoplasm (Outside Calcium)
24. Smooth Muscle
Contractile Mechanism
– D. Sarcoplasmic reticulum in smooth muscle is
sparse - 3 to 5 % of cell volume
– e. Calcium flows into sarcoplasm from
extracellular fluid
– f. No T-tubules in smooth muscle - Therefore,
calcium movement is slow
– g. Smooth muscle tone - occurs due to the slow
movement of calcium from the cell
– h. Smooth muscle has less ATPase activity and
therefore, less degradation of ATP
25. Smooth Muscle
Contractile Mechanism
– I. Smooth muscle only needs 1/10 to 1/300 of the
energy that skeletal muscle requires
– j. Only one ATP is required per contraction cycle no
matter how long it lasts
– k. Smooth muscle reaches full contraction about
1/2 second after stimulation
– l. Contractile force reaches maximum within 1 - 2
seconds after stimulation
26. Smooth Muscle
Contractile Mechanism
– M. Rate of contraction is 30 times slower
than skeletal muscle
– n. Contractions can last from 0.2 to 30
seconds
– o. Smooth muscle force of contraction can
be approximately 2X that of skeletal
muscle
– p. Smooth muscle can shorten to a
greater degree than skeletal muscle
reduces lumen of organs to almost zero
27. Smooth Muscle
Contractile Mechanism
4. “Latch Mechanism - prolonged holding in
smooth muscle
– a. After contraction is initiated, less stimulus and
energy are needed to maintain the contraction
(Energy conservation)
– b. Can maintain prolonged tonic contractions for
hours with little energy and little excitatory signal
from nerves or hormones
28. Smooth Muscle
Contractile Mechanism
– C. Mechanism: lower activation of
enzymes, myosin head remains attached to
actin for long periods of time but large
numbers are attached and the force is
great
5. Stress - Relaxation of Smooth
Muscle
– Important characteristic of visceral smooth
muscle
29. Smooth Muscle
Contractile Mechanism
Stress - Relaxation response - ability to return nearly
to the original force of contraction seconds to minutes
after it has been elongated or shortened
– a. When smooth muscle is initially stretched - it will
contract and increase tension (Myogenic response)
– b. Smooth muscle fibers can stretch and still
maintain their contractile function
30. Smooth Muscle
Contractile Mechanism
– C. Smooth muscle can undergo great
changes in length and still retain the ability
to contract effectively
– d. This response allows vessels and hollow
organs to change size but maintain the
pressure within the structure at a constant
level (Probably related to the “latch
mechanism”)
31. Smooth Muscle
Contractile Mechanism
– D. Regulation of Contraction by Calcium Ions
a. ICF calcium is the initiating event for
smooth muscle contraction
b. An increase in calcium influx can be
caused by: nerve stimulation, hormones,
chemical changes in the environment
(Ligands), and mechanical stretch of the
fiber
32. Smooth Muscle
Contractile Mechanism
2. Role of calmodulin in excitation-
contraction-coupling in smooth muscle
– a. Smooth muscle has no Troponin but it
does have a regulatory protein called
calmodulin
– b. Calmodulin is similar in structure to
Troponin and like Troponin combines with 4
calcium ions causing activation
– c. Activated calmodulin-Ca++ complex
activates myosin light chain kinase
34. Smooth Muscle
Contractile Mechanism
– d. The activated myosin light chain kinase
phosphorylates the myosin light chains (regulatory
proteins on the myosin heads) using an ATP unit
– e. The myosin heads now engage actin and cross
bridge cycling proceeds using the same process as
in skeletal muscle
– f. Cessation of contraction
1. As [Ca++] drops below a critical level
2. Myosin phosphatase removes the phosphate
from the myosin light chains and contraction
36. Smooth Muscle
Contractile Mechanism
Regulation of the
myosin light chain and
smooth muscle
contraction -
While the MLCK adds a
P group to the MLC
dependent on [Ca++]
myosin light chain
phosphatase removes it
deactivating the myosin
head. The phosphatase
is continually active.
37. SMOOTH MUSCLE
STIMULATION
Smooth muscle responds to stimulation
from a number of different physiological
systems.
1. Nerves
2. Hormones
3. Mechanical manipulation
4. Self stimulation (Automaticity)
39. SMOOTH MUSCLE
STIMULATION
2. Excitatory and inhibitory transmitter
substances at NMJ
– a. Ach & norepinephrine are never secreted by the
same nerve fiber
– b. Ach can be excitatory or inhibitory - determined
by the type of receptor expressed by the target cell
– c. Ach and NE usually cause the opposite reaction
at a target cell (If Ach is stimulatory then NE will
most likely be inhibitory)
40. SMOOTH MUSCLE
STIMULATION Unitary
B. Membrane Potential and AP in smooth
muscle (Slow waves and Spike Potentials)
– 1. Unitary smooth muscle
a. Slow waves - In smooth muscle the resting
membrane potential is variable - usually about -
50 to -60 mV
b. Variable resting membrane potential is called
the basic electrical rhythm or BER or sometimes
slow waves
42. SMOOTH MUSCLE
STIMULATION Unitary
– B. Slow waves and spontaneous generation of
action potentials
1. Slow waves are not action potentials - they
are local unstable resting membrane potential
and they determine the rhythmicity of smooth
muscle contractions
2. Slow waves can initiate true action potentials
called spike potentials (Ca++ voltage gated)
3. Spike potentials are generated whenever BER
exceeds threshold about -35 mV
43. SMOOTH MUSCLE
STIMULATION Unitary
Spike Potentials
– a) Spike potentials cause rhythmic
contractions of smooth muscle
– b) Increase the number of spike potentials
and increase the force of smooth muscle
contraction
– c) Regulation - mechanical stretch,
hormones, and Ach cause membrane to
become less negative (Hypopolarize the
cells)
45. Depolarization of Multiunit
Smooth Muscle w/o AP
– Smooth muscle contraction in response to local
tissue factors
– 1) Arterioles, metarterioles & precapillary
sphincters have little or no nerve supply
– 2) Highly contractile smooth muscle responds
rapidly to local factors
Lack or a decrease in O2 levels - Increase in CO2
- Increase in hydrogen ions - Decrease in ECF
Ca++ - Adenosine or increased lactic acid –
Cause Vasodilation
46. Depolarization of Multiunit
Smooth Muscle w/o AP
C. Effect of hormones on smooth muscle contraction
– 1) Most hormones affect smooth muscle through
second messengers
– 2) Important hormones - Norepinephrine,
epinephrine, Ach, angiotensin II, oxytocin,
vasopressin, serotonin and histamine
– 3) Action of hormones is controlled by the type of
receptors expressed by the target cell - hormone
(Ligand) gated excitatory and inhibitory receptors
(Ca++ or K+ channels)
47. Smooth Muscle
SUMMARY
1. How does smooth muscle differ from
skeletal muscle?
2. How is excitation contraction coupling
control different in smooth muscle from skeletal
muscle?
3. What are slow waves or BER and spike
potentials?
4. How is BER controlled?