1. Contraction and excitation of smooth muscle
Smaller fibers
Principle of contraction
Same as skeletal muscle in many aspects
Attraction between myosin and actin
Different arrangement of muscle fibers

2. Types of smooth muscle Different among different organs Two types
Physical dimensions
Organization (bundle/sheet)
Responsiveness to different stimuli
Innervation
Function
Two types
Multi-unit
Unitary (single-unit)

3. Multi-unit smooth muscle
Composed of discrete muscle fibers
Innervated by a single nerve ending
Covered with basement membrane
Separation of individual fiber
Independent contraction
Controlled by nerve signal
No spontaneous contraction

4. Unitary/single-unit smooth muscle
Mass of muscle cells contracting as one unit (syncytical smooth muscle)
Aggregated into sheet or bundle
Adhered together
Membrane (force)
Gap junctions (action potential)
Critical for contraction
Found in the wall of digestive, urinary, and reproductive tracts, and blood vessel

5. http://classes. aces. uiuc

6. Contractile process Chemical basis Actin and myosin filaments
No troponin complex
Interaction of filaments
Similar to that of skeletal muscle
Activated by Ca ions
Require ATP

7. Contractile process Physical basis No stratiation Actin filaments
Attached to dense bodies
Myosin filaments
Interspersed among actin filaments
Ratio of actin filaments to myosin filaments
More actin in smooth muscle
Greater ratio of fiber length

9. Individual contractile unit
Large number of actin filaments (attached to two dense bodies)
Single myosin fiber (half-way between two dense bodies)
Dense body (Z-disc)
No regularity
Side-polar arrangement of myosin cross-bridges

11. Individual contractile unit
Side-polar arrangement of myosin cross-bridges
One side of myosin heads pull actin fiber in one direction
Greater contraction (80 % of the length)

12. Smooth muscle contraction
Comparison with skeletal muscle
Prolonged tonic contraction
Different chemical characteristics

13. Smooth muscle contraction
Cycling of myosin cross-bridge
Slower repeated attachment and release of actin by cross-bridge (1/10 to 1/300 of skeletal muscles)
Increased time of interaction
Lower concentrations of ATPase in the cross-bridge heads

14. Smooth muscle contraction
Energy requirements
Much less (1/10 to 1/300) compared to skeletal muscles
Slow attachment and detachment of myosin heads
Critical for energy conservation
Indefinite contraction of visceral smooth muscles

15. Smooth muscle contraction
Slower onset of contraction and relaxation
Typical smooth muscle
Contraction initiated 50 to 100 mSec after excitation
Full contraction achieved 0.5 sec later
Decline in contractile force 1 to 2 sec
Total time for process 1 to 3 sec

16. Smooth muscle contraction
Slower onset of contraction and relaxation
30 times slower than skeletal muscles
Various type of smooth muscles
Slower detachment and attachment of cross-bridges
Slower response to elevation in Ca concentrations

17. Prolonged holding of contraction
Force of contraction
Greater maximum force of contraction
4 to 6 kg/cm2 compared to 3 to 4 kg/cm2 for skeletal muscles
Prolonged attachment of myosin to actin
Prolonged holding of contraction
Latch mechanism
Low energy consumption
Continuous stimulation

18. Stress-relaxation
Ability to return to near the original force of contraction shortly after elongation or contraction
Increased stretching of hollow organ
Immediate rise in pressure
Pressure returns to normal even when the organ is continuously stretched

19. Reverse stress-relaxation
Sudden decrease in pressure
Immediate decrease in pressure
Pressure returns to normal immediately after the decrease in size

20. Increase in size
Increase in pressure
Normal size
Normal pressure
Decreased size
Normal pressure
Increased size
Normal pressure
Decrease in size
Decrease in pressure

21. Role of Ca ion Initiating factor Increase in intracellular Ca
Rise in intracellular Ca
Increase in intracellular Ca
Nerve impulses
Hormones
Physical
External environment

22. Role of Ca ion
No tropinins
Different mechanisms

23. Role of Ca ion Calmodulin as a regulatory protein
Binds Ca
Activation of myosin kinase by Ca-calmodulin complex
Phosphorylation of myosin light chain
Regulatory chain within the mysoin head
Interaction of myosin head with actin filament

24. Cessation of contraction
Loss of Ca
Myosin head must be dephosphorylated
Myosin phosphatase
Timing of cessation and relaxation
Myosin phosphatase concentration dependent

25. Source of Ca ions Less-developed SR compared to skeletal muscle
Influx of Ca ions fromextracellular fluid
Concentration gradient (<10-7 M inside vs. > 10-3 M outside)
Needs 200 to 300 mSec for diffusion (latent period)
Contraction of smooth muscle is highly dependent on extracellular Ca

26. Role of SR Sacroplasmic tubules in some large smooth muscle cells
Caveole (invagination of the membrane)
Rudimentary analog of T-tubules
Stimulates release of Ca by sacroplasmic tubules when the action potential reaches
More extensive the sacroplasmic reticulum, rapid contraction of muscle

28. Removal of Ca Relaxation of smooth muscle Ca pump
Move Ca from the inside to the outside or into sacroplasmic tubules/reticulum
Slow-acting (longer contraction)

29. Regulation of latch phenomenon
Mechanism
Myosin kinase and myosin phosphatase
Activation of both enzymes
Increased velocity of contraction (increased cycle frequency)
Inactivation of enzymes
Decrease velocity of contraction
Increased interaction between myosin head and actin filament (per cycle)
Increased number of myosin heads interacting with actin filament (increased static force of contraction)

30. Nervous and hormonal control
Skeletal muscle
Exclusives stimulated by nerve fiber
Smooth muscle
Contains receptors for various stimuli
Stimulatory
Inhibitory

31. Neural stimulation Neuromusclular junction
Innervated by autonomic nerve fibers
Only on the top layer
Excitation potentials travel toward the inner layer by action potential conduction or additional diffusion of neurotransmitter
Nerve fibers branch diffusely on top of sheet
No direct contact
Diffusion junction (few nm to few mm away from muscle membrane)

33. Formation of varicosities
No motor end plate
Vesicles containing neurotransmitters
Acetylcholine
Norepinephrine
Formation of contact junction
Close proximity of varicosities to the muscle membrane (20-30 nm)
Faster contraction compared to diffusion junction

34. Release of excitatory and inhibitory substances
Acetylcholine
Norepinephrine
Secreted by different fibers (either/or, not both from the same fiber)

35. Acetylcholine Norepinephrine Excitatory in some muscles
Inhibitory in other muscles
Norepinephrine
Excitatory in muscles where acetylcholine is an inhibitor
Inhibitory in muscles where acetylcholine is a positive stimulator

36. Differential responses to acetylcholine and norepinephrine
Types of receptors

37. Membrane and action potential
Membrane potential
-50 to -16 mV at resting state
Action potential
Similar to skeletal muscle in unitary smooth muscle
Absent in multi-unit smooth muscles

38. Action potential in unitary smooth muscle
Spike potential
Action potential with plateau
Initiated by factors other than neural signals

39. Spike potential Same as the action potential in skeletal muscle
10 to 50 mSec in duration

40. Action potential with plateaus
Delayed depolarization
As much as 1 second
Prolonged contraction of the muscle

41. Role of Ca channels Numerous voltage-gated Ca channels
Few voltage-gated Na channels
Action potential generated by the influx of Ca ions
Ca channels open slower than Na channels and remain open longer
Prolonged plateaus
Ca ions more critical for smooth muscle contraction

42. Slow wave potential Self-excitation of some smooth muscles
Absence of stimuli
Slow wave potential
Slow wave (pacemaker wave)
Not action potential (no spreading)
Localized phenomenon

43. Cause of slow wave Importance Unknown Initiation of action potential
One or more action potential per each peak of slow wave
Rhythmical contraction of smooth muscle
Initiation of action potential by stretching

44. Depolarization of multi-unit smooth muscle
Contraction of multi-unit smooth muscle
Input from autonomic nerve fiber
Binding of neurotransmitters to their receptors
Depolarization
No action potential
Nerve fibers are too small to initiate action potential
Junctional potential (local depolarization)
Electronic transmission to other muscle fibers

45. Role of tissue factors and hormones
Non-nervous factors
Do not generate action potential
Local tissue factors (often associated with smooth muscle surrounding blood vessels)
Oxygen
Carbon dioxide
Hydrogen ions

46. Hormones
Requires presence of excitatory/inhibitory receptors
Mechanisms
Changes in membrane potentials
Release of Ca ions from SR
Removal of Ca ions
Alteration of enzyme activity