1. U N I T II Textbook of Medical Physiology, 11th Edition GUYTON & HALL Copyright © 2006 by Elsevier, Inc. Contraction and Excitation of Smooth Muscle

2. Copyright © 2006 by Elsevier, Inc. CLASSIFICATION OF THE MUSCULAR TISSUE  Non-striated muscle (smooth muscle)  Striated muscle (cardiac, skeletal muscle)

3. Copyright © 2006 by Elsevier, Inc. SMOOTH MUSCLE  No cross striations  Location: in the walls of blood vessels, hollow viscera  Function: controls the size of the lumen.  Shape: each fiber is fusiform in shape.  Diameter: 1-5 um  Length: 20-500 um  No well developed sarcotubular system. Caveolae are present.  Nucleus: single, rod- shaped nucleus located in the centre of each cell.

4. Copyright © 2006 by Elsevier, Inc. Types of Smooth Muscle  MULTI-UNIT Smooth Muscle  UNITARY Smooth Muscle ( syncytial/visceral )

5. Copyright © 2006 by Elsevier, Inc. MULTI-UNIT AND UNITARY SMOOTH MUSCLES

6. Copyright © 2006 by Elsevier, Inc. MULTI-UNIT AND UNITARY SMOOTH MUSCLES  Location  Type of NM Junctions  Arrangement  Control  Membrane potential.

7. MULTI-UNIT AND UNITARY SMOOTH MUSCLES 1: LOCATION :  UNITARY: Wall of GIT, ureter, bile duct, uterus and large blood vessels.  MULTIUNIT: Ciliary muscle of eye, Iris of eye and pilo-erector muscle, vas deferens

8. MULTI-UNIT AND UNITARY SMOOTH MUSCLES 2: ARRANGEMENT:  UNITARY: Sheaths or bundles with gap junctions in between, so act as single unit. If one part is excited  whole is excited.(functional syncytium)  MULTIUNIT: Individual muscle fibers. Each fiber has insulating outer membrane with glycoprotein and collagen consistency. Independently each muscle fiber is excited.

9. Copyright © 2006 by Elsevier, Inc. MULTI-UNIT AND UNITARY SMOOTH MUSCLES 3: TYPE OF NM JUNCTION:  UNITARY: Diffuse junction. No contact with nerve fiber. Neurotransmitter is released near the fiber.  MULTIUNIT: Contact junction. One of the autonomic nerves form junction with muscle fiber and releases neurotransmitter like nor-epinephrine

10. Copyright © 2006 by Elsevier, Inc. MULTI-UNIT AND UNITARY SMOOTH MUSCLES 4: CONTROL:  UNITARY:  Mainly non-nervous stimuli (hormonal stimuli like oxytocin and serotonin).  Nervous stimuli are less important  Show spontaneous contractions.  MULTIUNIT:  Mainly through nervous stimuli.  Does not show spontaneous contraction

11. MULTI-UNIT AND UNITARY SMOOTH MUSCLES Types of potential in unitary smooth muscles 5: MEMBRANE POTENTIAL:  UNITARY: 3 types:  Spike potential,  Action potential with Plateau and  spikes superimposed on Slow wave.  MULTIUNIT:  action potential is not produced.  Only localized depolarization in response to excitation.

12. Contractile mechanism in smooth muscle CHEMICAL BASIS FOR Smooth Muscle Contraction: Contain both actin and myosin Doesn’t contain the normal troponin complex

13. Copyright © 2006 by Elsevier, Inc. Contractile mechanism of smooth muscle fiber  E/M shows that actin and myosin are present in the cytoplasm.  Actin: diamter is 5-7 nm  Myosin: diameter 12-16 nm  actin/myosin ratio: 15/1  Desmin intermediate filaments are also present in the cytoplasm. (dia= 10 nm)  Dense bodies: intermediate filaments cross over one another to form darkly staining patches.

14. Copyright © 2006 by Elsevier, Inc. EXCITATION- CONTRACTION COUPLING OF SMOOTH MUSCLE FIBERS

15. Copyright © 2006 by Elsevier, Inc. EXCITATION-CONTRACTION COUPLING OF SMOOTH MUSCLE FIBERS

16. Copyright © 2006 by Elsevier, Inc. Contraction-Relaxation – myosin based regulation Ca 2+ calmodulin MLCK MLC active (phosphorylated) MLC inactive (dephosphorylated) Relaxation Contraction MLCP MLCK, myosin light chain kinase MLCP, myosin light chain phosphatase

17. Copyright © 2006 by Elsevier, Inc. Mechanisms That Increase Intracellular Ca 2+ Concentration in Smooth Muscle

18. Copyright © 2006 by Elsevier, Inc. Comparison of Smooth muscle contraction and Skeletal muscle contraction  Slow cycling of the myosin cross-bridges  Low energy requirement to sustain smooth muscle contraction  Slowness of onset of contraction and relaxation of smooth muscle contraction  Maximum force of contraction is often greater in smooth muscle than in skeletal muscle

19. Copyright © 2006 by Elsevier, Inc.  “LATCH” Mechanism facilitates prolonged holding of contractions of smooth muscle  Stress-Relaxation of smooth muscle

20. Copyright © 2006 by Elsevier, Inc.