slide smooth m

Smooth muscle
Responsible for the contractility of hollow organs
blood vessel
gastrointestinal tract
uterus
urinary bladder
smooth muscle contraction
Small fibers
Skeletal fibers 20 times bigger ,thousand times longer
Same principles of contraction
Differ in physical arrangement
Types of smooth muscle
A. MULTIUNIT SMOOTH MUSCLE
B. SINGLE UNIT SMOOTH MUSCLE
A. multiunit smooth muscle
-discrete muscle fibers
-each fiber operates independently
-Innervated by a single nerve ending
-covered by basement-like membrane substances
-control by nerve signal
-rarely exhibit spontaneous contraction
-ciliary muscle of the eye
-iris of the eye
B. single unit smooth muscle
-group of muscle fibers that contract together as a single
unit
-aggregated into sheets or bundles
-cell membranes are adherent
-impulse generated at one fiber can be transmitted to the
next fiber
-gap junction—thru which ion can flow from one cell to
the other
-syncitial smooth muscle : visceral sm m.– walls of most
viscera
contraction process
Chemical basis – actin and myosin but no troponin
Physical basis – actin attached to dense bodies
- dense bodies are attached to the
cell membranes and other structural
proteins linking them to one another
- myosin filamints are sparsely seen
- less regularity that is characteristics of
skeletal muscle
comparison of smooth and skeletal m
Skeletal muscle contraction – rapid
Smooth m. contraction – prolonged and sustained
Slow cycling of cross bridges – 1/10 to 1/300 the frequency
of skeletal m.
- myosin head lacks ATPase activity
- fraction of time of attachment of cross bridges
actin filamints is increased
Energy required to sustain smooth m. contraction:
only 1/10 to 1/300 in skeletal m.

Slowness of onset of contraction and relaxation:
- contracts 50 to 100 millisec.after it is
excited
- reaches full contraction ½ sec. later
- contraction time of 1 to 3 sec.(30x longer)
- due to slow cycling of cross bridges
Force of contraction – 4 -6 kg/cm2
Percentage of shortening of smooth m. during contraction –
- greater percentage of shortening while maintaining
full force of contraction
-important in function of hollow viscus
due to: 1. optimal overlapping of filamints
2. longer actin filamints
Prolonged holding contraction
The latch mechanism:
once the muscle has developed full contraction the
degree of activation of the muscle can be reduced to less
than the initial level
at the same time the muscle maintain its full strength
of contraction
- responsible for the prolonged tonic with minimal
energy utilization
- little excitatory signal is required
- due to prolonged attachment of filaments
Stress relaxation of smooth m
Ability to return to its original force of contraction seconds after it has been elongated or stretched
example; urinary bladder

Related to latch phenomenon
Regulation of contraction
Initiating event for contraction is increase in intracellular calcium
Increase of calcium by:
1.nervous stimulation
2.hormonal stimulation
3.mechanical factor such as stretch
4. changes in chemical environment
Mechanism of contraction
NO troponin thus different mechanism thru calmodulin
1. calmodulin binds with calcium
2. complex activates myosin kinase
3.one of the light chains of the myosin head(regulatory)
becomes phosphorylated
4. phosphorylation start binding of myosin head with
actin thus cycling process occurs
Cessation of contraction is due to
- myosin phosphatase
- splits phosphate from regulatory chain
Myosin kinase and myosin phosphatase may explain the latch phenomenon
Neural and hormonal regulation of contraction
Skeletal muscle is activated exclusively by nervous system
Smooth m. can be stimulated by
nervous
hormonal stimuli and other ways
Presence of receptor proteins in the cell membrane
Some are inhibitory receptors
Neuromuscular junction of smooth m
Terminal axon has varicosities which contain transmitter substances
Transmitter substances– acetylcholine and norepinephrine both can either be inhibitory or excitatory depending on the receptor protein
Membrane and action potential in smooth m
Value is variable from one type to the other
Usually between -50 to -60millivolts
Action potentials occur in single unit smooth m. the same way as in skeletal m.
Action potential occurs in single unit smooth m.
the same way in skeletal muscle
Action potential do not normally occur in many
or most of the multiunit type
2 types of potential in smooth m
1. spike potential
2. action potential with plateau
Spike potential
Such as those seen in skeletal m.
Can be seen in most single unit smooth m.
Can be elicited by:
1. electrical stimulation
2. hormonal stimulation
3. transmitter substance
4. spontaneous generation
Action potential with plateaus
Onset is similar to spike potential
Repolarization is delayed by several hundreds
to thousands of milliseconds
Important in prolonged contraction like:
ureter
uterus
other vascular smooth muscle
calcium channel in action potential
Cell membrane of smooth m. has more calcium channel and few sodium channel
Very little sodium participation
Influx of calcium is responsible for potential generation
Calcium channels open more slowly than sodium channel
Slow action potential
Calcium entry into the cell can act directly on the contracting mechanism
Slow wave potentials in single unit smooth muscle and spontaneous genearation of action potential
Some smooth muscle are self excitatory
Due to basic slow wave rhythm of membrane
potential
Slow wave itself is not an action potential but a local
property of smooth muscle fibers
slow potential
Cause of slow wave
1. waxing and waning of pumping of sodium
2. rhythmical increase and decrease of ionic
conductance
Importance of slow wave is that it can initiate action
potential thus also known as pacemaker waves
excitation of smooth m by stretch
When single unit smooth m. is stretched this results
to spontaneous generation of action potential
Due to normal slow wave and decrease in the
negativity of the membrane cause by the stretch
itself
Response of the hollow organ to resist stretch as
seen in the gut
depolarization of smooth m without action potential
Normally contract mainly in response to nerve stimuli
Nerve endings release transmitter substances
Half or most of all smooth m. contraction is initiated
not by action potentials but by stimulatory factors
acting directly on smooth muscle machinery
The 2 factors are
1. local tissue factors
2. various hormones
Local tissue factors
Contraction of arterioles and meta-arterioles and
precapillary sphincter as respond to rapidly changing condition in the interstitial fluid
specific control factors
1.lack of O2 in the local tissue –smooth m.relaxation
2.excess CO2 – vasodilatation
3.increased hydrogen ion conc.- vasodilatation
Hormonal factors
Norepinephrine, epinephrine,acetylcholine,
Angiotensin,oxytocin, serotonin and histamine
Hormone can cause contraction if the membrane contains hormone-gated excitatory receptor and inhibition if receptor is inhibitory
Smooth m contraction
Some hormone receptors in smooth m. open sodium
or calcium channels thus depolarization
Sometime action potential result
Enhance rhythmical action potential
Mostly no action potential into the cell
Calcium ion entry into the cell promotes contraction
Activation of some membrane receptors inhibits contraction
-due to closing of the sodium or calcium channel
-opening of the potassium channels resulting to
efflux of K ion increasing negativity inside—
HYPERPOLARIZATION
Sometimes contraction or inhibition is initiated by hormones without change in the membrane potential
-hormone activates a membrane receptor that does
open ion channel but causes an internal change
in muscle fiber: release of calcium ion
- activating enzymes like adenyl cyclase resulting
AMP as second messenger-inhibition of contraction
by affecting calcium pump in sarcoplasmic and
cell membrane
source of calcium ions
Both through the cell membrane and sarcoplasmic
reticulum
Source of calcium ions differ from that of skeletal m.
Sarcoplasmic reticulum is a rudimetary structure in
most of smooth m.
Most calcium ions in smooth m. comes from extra
cellular fluid
Because smooth m. fibers are small – calcium can
diffuse to all parts of the fiber causing contraction
Calcium can also enter inside the m. fiber through
hormone activated calcium channel
Calcium may not cause action potential because of
sodium pump but contraction can proceed
role of sarcoplamic reticulum
Moderately developed in some fibers
Situated close to the cell membrane invagination-
calveoli
Calveoli represents the T tubules in the skeletal m.
The more extensive the sarcoplasmic reticulum in the
smooth m. the faster it can contract
effect of extracellular calcium ion
Low concentration results to weakened muscular contraction
Calcium pump
In cell membrane
In organelle membrane
Removal of calcium ions result in relaxation of
contractile filamints
Calcium pump in smooth m. is slow acting –
longer duration of contraction
neural and hormonal control of smooth m contraction
Skeletal muscle is activated exclusively by nervous
system
Smooth m. can be stimulated by nervous and
hormonal stimuli and other ways
Presence of receptor proteins in the cell membrane
Some are inhibitory receptors