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Smooth Muscle Contraction and Excitation: Classification and Mechanisms

This chapter discusses the classification of muscular tissue, focusing on smooth muscle, its location, function, and characteristics. It explores the types of smooth muscle, their arrangements, control mechanisms, and membrane potential. The chapter also examines the contractile mechanism of smooth muscle, including the chemical basis for contraction and the excitation-contraction coupling. Additionally, it compares smooth muscle contraction with skeletal muscle contraction and discusses the "latch" mechanism and stress-relaxation of smooth muscle.

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Smooth Muscle Contraction and Excitation: Classification and Mechanisms

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  1. Contraction and Excitation of Smooth Muscle Chapter 8:

  2. CLASSIFICATION OF THE MUSCULAR TISSUE • Non-striated muscle (smooth muscle) • Striated muscle (cardiac, skeletal muscle)

  3. 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. Types of Smooth Muscle • MULTI-UNIT Smooth Muscle • UNITARY Smooth Muscle ( syncytial/visceral )

  5. MULTI-UNIT AND UNITARY SMOOTH MUSCLES

  6. 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. 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. 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 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. Types of potential in unitary smooth muscles

  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. 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. EXCITATION-CONTRACTION COUPLING OF SMOOTH MUSCLE FIBERS

  15. EXCITATION-CONTRACTION COUPLING OF SMOOTH MUSCLE FIBERS

  16. MLC inactive (dephosphorylated) MLC active (phosphorylated) Contraction-Relaxation – myosin based regulation Ca2+ Ca2+ calmodulin Relaxation MLCP MLCK Contraction MLCK, myosin light chain kinase MLCP, myosin light chain phosphatase

  17. Mechanisms That Increase Intracellular Ca2+ Concentration in Smooth Muscle

  18. 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. “LATCH” Mechanism facilitates prolonged holding of contractions of smooth muscle • Stress-Relaxation of smooth muscle

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