Smooth Muscles

Nerve - Muscle Physiology

1. Smooth Muscle

2. Structure of Smooth Muscle Lacks visible cross-striations Actin and myosin-II are present but not arranged in regular arrays Actin 5-10 times more than Myosin Dense bodies instead of Z lines In the cytoplasm and attached to cell membrane Connected to actin filaments by α - actinin Interspersed among the actin filaments are myosin filaments Some of the dense bodies connected to adjacent cells by intercellular protein bridges – transmits force of contraction from one cell to the next

3. Spindle shaped cells, found in the walls of tubular structures, hollow viscera Smaller fibres , Diameter = 1 to 5 micrometers length = 15 micron (blood vessels) to 200 micron (uterus) Structure Tropomyosn present NO Troponin

4. Structure of Smooth Muscle Myosin filaments have “ sidepolar ” cross-bridges Arranged so that bridges on one side hinge in one direction and those on other side hinge in opposite direction Allows myosin to pull an actin filament in one direction on one side while simultaneously pulling another actin filament in the opposite direction on the other side Allows smooth muscle cells to contract as much as 80% of their length instead of 30% (skeletal muscle)

5. Smooth muscle contraction

6. Structure of Smooth Muscle Contains tropomyosin , but troponin absent Thus, mechanism for control of contraction is different Regulatory protein is calmodulin instead of troponin Sarcoplasmic reticulum less extensive Few mitochondria depends, to a large extent, on glycolysis for their metabolic needs Divided into 2 main sub-types

7. Structure

8. Single unit Smooth Muscle a.k.a Unitary or visceral smooth muscle Mass of hundreds to thousands of fibers that contract together as a single unit Large sheets with low-resistance gap junctions between individual muscle cells functions in a syncytial fashion Resembles cardiac muscle undergo rhythmic, spontaneous contractions in the absence of nerve or hormonal input Present in walls of hollow viscera ( Intestinal smooth muscle, Ureters , Uterus, small arteries)

9. Multi Unit Smooth Muscle Individual units with few (or no) gap junctional bridges Leads to discrete, fine localized contractions Resembles skeletal muscle but involuntary Each fiber operates independently of others Often innervated by a single nerve ending fibers Found in ( Iris & ciliary muscle of eye, larger arteries, Some areas of intestine, Reproductive system, Pilomotor muscle (skin) ) Blood vessels have both single unit and multi unit types

10. Single Unit vs Multi Unit

11. Types of smooth muscles Single unit Large sheets, syncitial fashion Low resistance gap junctions Own myogenic tone, pacemaker regions Respond to stretch ANS, hormones,local tissue factors, pH, temp Multi unit Each fiber is independent Outer glycoprotein rich cell membrane, no gap junctions Can respond without AP, no pacemaker Do not respond to stretch Neurogenic stimulation

12. SINGLE UNIT SMOOTH MUSCLE MULTI – UNIT SMOOTH MUSCLE Eg . – Muscle of GIT, bronchi, urinary bladder and uterus Eg . – Ciliary muscles, muscles of iris and pilomotor muscles in hair follicles Pacemaker tissue present – responsible for rhythmic contraction & relaxation of muscle No pacemaker tissue Autonomic nervous system can modify the response Only show contraction as per the discharge in autonomic nerves supplying them Stretch of the muscle causes reflex contraction No effect of stretch on the muscle Low resistance bridges are present in between the cells so acting as functional synctium No such bridges Contracts as a single unit and there is a widespread contraction Contraction is more discrete, fine and localized

13. N-M Junction in Smooth Muscle Neurons are part of the autonomic nervous system rather than somatic nervous system Neuron makes multiple contacts with a smooth muscle cell (no direct contact) At each contact point, the axon diameter expands to form a varicosity that contains the vesicles (can contain ACh or NE) Varicosity is in close proximity to postsynaptic membrane (relatively little specialization) Receptors are spread more widely across the postsynaptic membrane

14. N-M Junction in Smooth Muscle

15. N-M Junction in Smooth Muscle No typical end feet as seen in skeletal muscle, axons have multiple varicosities distributed along their axes V aricosities are about 5 μ m apart, with up to 20,000 varicosities per neuron Transmitter is liberated at each varicosity, ie, at many locations along each axon This arrangement permits one neuron to innervate many effector cells The type of contact in which a neuron forms a synapse on the surface of another neuron or a smooth muscle cell and then passes on to make similar contacts with other cells is called a synapse en passant

16. NMJ: Smooth muscles

17. Unitary Smooth Muscle RMP : -50mV (-20 to -65mV (oscillates) – No true value -Unstable Superimposed on the membrane potential are (Divergent electrical activity) Slow sine wave like fluctuations – few microvolts in magnitude Spikes – Duration 50msec - AP has prolonged plateau during repolarization In addition, Pacemaker potentials Shows Continuous irregular contractions that are independent of nerve supply Maintained state of partial contraction is called Tone or tonus Generated in multiple foci that shift from place to place

18. Excitation & action potential in smooth muscle fibers Transmission of impulse from terminal nerve fibers to smooth muscle fiber is same as in S.K. muscle When an action potential reaches the terminal or excitatory nerve fibril there is a latent period of 50sec Resting potential -50 mv Threshold potential for smooth muscle is -30 to-35 mv

19. Role of Ca ++ ions causing smooth muscle action potential Depolarization process during action potential of muscle & nerve fiber is caused by rapid influx of Na + , but in action potential of smooth muscle fibers, rapid influx of ions include Na + as well as Ca ++ Caused mainly by influx of Ca ++ than Na + More voltage gated Ca ++ channels Ca ++ channels open more slowly and also remain open much longer- resp. for plateau Also calcium acts directly on contractile mechanism to cause contraction

20. Types of Smooth Muscle AP

21. Spike potential Typical smooth muscle action potential (spike potential) elicited by an external stimulus Observed in GIT Due to L-type calcium channels

22. Spike potentials with slow waves Some smooth muscles are self excitatory No extrinsic stimulus is required Usually associated with basic slow wave rhythms (BER) of membrane potentials not an action potential & do not cause contraction local property of smooth muscle fiber Cause of slow wave rhythm is unknown When potentials of slow wave rises above -35mv an AP develops & contraction occurs On Each peak of slow wave one or more action potentials occurs

23. Spike potentials with slow waves Observed in GIT – smooth muscle of intestinal wall Duration of Spike Potential is 10-50 millisec

24. Membrane potential of smooth muscles Membrane potential Tension Slow waves- Leaky cation channels

25. Spike potentials with slow waves Causes series of rhythmical contractions of the smooth muscle mass These slow waves are also called pace maker waves found in gut, ureter (tubular hollow viscera). Spread of action potential through visceral smooth muscle is via gap junctions Visceral smooth muscles generate spontaneous action potential by stretch also

26. Spike potentials with slow waves e.g if action potential begins at upper end of intestine it spreads downwards along the intestine wall creating a constriction ring that moves forwards. This constriction ring propels the intestinal contents forward - process is called peristalsis when a gut is overstretched by intestinal contents a local automatic contraction sets up a peristaltic wave that moves the contents away form the over stretched area

27. Action Potential with Plateau Instead of rapid repolarization of muscle fiber membrane the repolarization is delayed for several hundred to several thousands millisecs Importance of plateau - can account for prolonged period of contraction which occurs in ureter , uterine muscles Due to L-type Ca channels and K channels

28. Stimulus for AP/Contraction Final stimulus is Increased levels of Calcium Can be brought about by Nerve stimulation Hormonal stimulation Stretch of fiber Change in chemical environment Or can be spontaneously generated in muscle fiber itself

29. 3 Sources of Calcium Influx Entry from ECF Major pathway time required for diffusion - averages 200- 300 millisecs - latent period ( 50 times greater in sk. musc .) From Sarcoplasmic Reticulum (poorly dev.) Via ligand gated and voltage gated channels Via IP3 mediated calcium release via G-protein coupled receptors Store-operated Ca 2+ channels in plasma membrane Eventual depletion of calcium stores in SR stimulates influx from SOCC Release via 2 & 3 – Pharmacomechanical Coupling because independent of AP generation

30. Stimuli (stretch, cooling) Action potential (mechanoreceptors , thermoreceptors ) Entry of calcium (CaV) Electro-mechanical coupling

31. Stimuli ( chemical- Ach,Oxytocin ) Binding to receptor Activation of G-proteins Release of calcium from stores/ECF Activation of PLC Pharmaco -mechanical coupling

32. Excitation Contraction Coupling Slow onset of Contraction & Relaxation Begins to contract 50 to 100 millisecs after it is excited (can be as much as 500msec) Reaches full contraction about 0.5 sec later Declines in contractile force in another 1 -2 secs Total contraction time of 1 to 3 secs (30 times longer than skeletal muscle) Due to slowness of attachment and detachment of the cross-bridges with actin filaments Initiation of contraction in response to calcium ions is much slower than in skeletal muscle

33. Excitation Contraction Coupling

34. Release of calcium Binding of calcium to Calmodulin Activation of MLCK by Ca- CaM Phosphorylation of light chain of myosin Binding of myosin to actin Contractile mechanism MLCK – myosin light chain kinase

35. Contractile mechanism

36. Relaxation mechanism Another enzyme, Myosin light chain phosphatase removes the phosphate from the myosin But dephosphorylation of myosin light chain kinase does not necessarily lead to relaxation of the smooth muscle Latch bridge by which myosin cross bridge remains attached to actin for sometime even after cytoplasmic Ca ++ conc falls Produces sustained contraction with little expenditure of energy Relaxation finally occurs when Ca 2+ - Calmodulin complex dissociates – slower process

37. Latch Bridge Phenomenon – Possible mechanism When myosin kinase and myosin phosphatase enzymes are both strongly activated cycling frequency of myosin heads & velocity of contraction are great As activation of enzymes decreases Cycling frequency decreases but at the same time, deactivation of enzymes allows the myosin heads to remain attached to actin filament for a longer and longer proportion of the cycling period Number of heads attached to the actin filament at any given time remains large Because the number of heads attached to the actin determines the static force of contraction, tension is maintained, or “latched”; yet little energy is used by the muscle

38. Slow cross bridge cycling : leading to ‘Latch’ state 1/10 to 1/300 the rate of skeletal muscle Latch bridge mechanism

39. Force of Muscle Contraction Maximum force of contraction of smooth muscle is often greater than that of skeletal muscle As great as 4 to 6 kg/cm 2 cross-sectional area for smooth muscle, in comparison with 3 to 4 kg for skeletal muscle Due to prolonged period of attachment of the myosin cross bridges to actin filaments

40. Energy Requirement Energy required for sustained smooth muscle contraction is very little due to Fewer myosin filament in smooth muscle as compared to S.K. Muscle Lower myosin ATPase activity Lower rate of cross bridge cycling (only 1 ATP used per cycle irrespective of duration of cross bridge) Only 1/10 to 1/300 as much energy is required to sustain the same tension of contraction as in skeletal muscle Imp – need to work indefinitely

41. L-type Ca channel IP3 receptor

42. Contraction-Relaxation (cont.) Figure 8-5 Figure 8-3

43. Depolarization of multi unit smooth muscle without action potential Transmitter substances (Ach or NE) cause depolarization of the smooth muscle membrane ( junctional potential) S preads “ electrotonically ” over the entire fiber and is all that is needed to cause muscle contraction alters Vm and affects the entry of Ca 2+ through voltage-gated slow (L-type) Ca 2+ channels Action potentials usually do not develop fibers are too small to generate an AP

44. Multi-Unit Smooth Muscle Normally contract mainly in response to nerve stimuli (Transmitter substances -Ach or NE) Action potentials usually do not develop Fibers are too small Local depolarization ( Junctional potentials) develop, spread electrotonically over the entire fiber, and are enough to cause muscle contraction

45. Effect of Ach and NE If epinephrine or norepinephrine is added to a preparation of intestinal smooth muscle arranged for recording of intracellular potentials in vitro membrane potential usually becomes larger, spikes decrease in frequency, and muscle relaxes Ach has opposite effect membrane potential decreases, spikes become more frequent. The muscle becomes more active, with an increase in tonic tension and the number of rhythmic contractions Other factors which depolarize the membrane are Stretch and specific gastrointestinal hormones

46. Excitation/inhibition of smooth muscle

47. Excitation/inhibition of smooth muscle

48. Factors which increase Relaxation In addition to cellular mechanisms that increase contraction, certain mechanisms lead to its relaxation Endothelial cells that line the inside of blood cells release a substance called (endothelial derived relaxation factor, EDRF) or nitric oxide (NO) NO directly activates a soluble guanylate cyclase to produce another second messenger molecule, cyclic guanosine monophosphate ( cGMP ) Activate cGMP -specific protein kinases that can affect ion channels, Ca 2+ homeostasis, or phosphatases , or all of those mentioned, that lead to smooth muscle relaxation

49. Effect of Local Tissue Factors In the normal resting state, many of small blood vessels remain contracted When extra blood flow is needed, multiple factors can relax the vessel wall Local feedback control system Factors causing Vasodilation Lack of oxygen in the local tissues Excess carbon dioxide Increased hydrogen ion concentration Adenosine, lactic acid, increased potassium ions, diminished calcium ion concentration, and increased body temperature

50. Applied Aspect During an asthma attack – bronchoconstriction can be relieved by rapid response inhaler drugs ( eg , ventolin , albuterol , sambuterol ) which target β -adrenergic receptors in the airway smooth muscle to elicit a relaxation In Erectile dysfunction - specific inhibitors of PDE ( sildenafil , tadalafil , and vardenafil ) are used NO is a natural signaling molecule that relaxes smooth muscle by raising cGMP . This signaling pathway is naturally down-regulated by the action of phosphodiesterase (PDE), which transforms cGMP into a nonsignaling form, GMP. Oral administration of these drugs can block the action of PDE V, increasing blood flow

51. Relation of length to tension If a piece of visceral smooth muscle is stretched, it 1st exerts increased tension However if muscle is held at the greater length after stretching, the tension gradually decreases, called Plasticity Sometimes the tension falls to or below the level exerted before muscle was stretched Thus, no resting length can be assigned Impossible to correlate length and developed tension accurately

52. Advantage of Plasticity Ability to return to nearly its original force of contraction seconds or minutes after it has been elongated or shortened Importance is that, except for short periods of time, they allow a hollow organ to maintain about the same amount of pressure inside its lumen despite long-term, large changes in volume

53. Stress Relaxation & Reverse Stress Relaxation

54. Role in urinary bladder Stress Relaxation

55. Demonstration of Plasticity in Urinary Bladder

56. Advantage of Plasticity Accommodating more blood/fluid by vessels/viscera without increasing pressure (stress relaxation & reverse stress relaxation) Accommodating more urine by the bladder Accommodating more food by stomach without increase in pressure. (receptive relaxation)

57. Characteristics of smooth muscle Difference in Structure Unstable RMP Slow excitation contraction coupling Marked shortening during contraction Latch phenomenon for prolonged contraction Plasticity : variability of tension exerted at any given length Less energy required to sustain contraction Can respond to stretch in absence of extrinsic innervation

58. Thank you References: Guyton- Textbook of Medical Physiology Ganong’s - Review of Medical Physiology Boron-Medical Physiology Kandel -Principles of Neural Science Silbernagl -Color atlas of Physiology Ira Fox- Medical Physiology