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SMOOTH MUSCLE Dr. Ayisha Qureshi MBBS, MPhil Assistant Professor CLASSIFICATION OF SMOOTH MUSCLES 1. 2. 3. 4. UNITARY/ SINGLE UNIT/SYNCYTIAL/VISCERAL Muscles of visceral organs .e.g. GIT, uterus, ureters & some of the smaller blood vessels. Form a sheet or bundles of tissue. Cell membranes show gap junctions that allows AP to pass rapidly from cell to cell. AP spreads rapidly throughout the sheet of cells – cells contract as a single unit. MULTI-UNIT 1. 2. 3. 4. Iris & Ciliary body of the eye, large arteries, Piloerector muscles Showing discrete, individual smooth muscle fibers. Smooth muscle cells not electrically linked. Each muscle fiber innervated by a single nerve ending. NT itself can spread and lead to an AP. Selective activation of each muscle fiber that can then contract independently of each other. Latch mechanism It is a state in which the dephosphrylated myosin remains attached to actin for prolonged period of time. This produces sustained contraction without consuming ATP & thus enables the smooth muscle to sustain long-term maintenance of tone without fatigue. E.g. urinary bladder full of urine. COMPARISON OF SKELETAL & SMOOTH MUSCLE CONTRACTION: COMPARISON OF SKELETAL & SMOOTH MUSCLE CONTRACTION: S.NO CHARACTERISTICS SKELETAL MUSCLE SMOOTH MUSCLE 1. Location Attached to the skeleton Blood vessels, hollow organs, eye 2. Mechanism of contraction Sliding filament Sliding filament 3. Nerve supply Motor neurons Autonomic nervous system 4. Level of control Under Voluntary control Under Involuntary control 5. Presence of Thick & Thin filaments Yes Yes 6. Presence of Tropomyosin Yes Yes 7. Presence of Troponin Yes No 8. Presence of T-tubules Yes No 9. Level of development of Sarcoplasmic Reticulum Well Developed Poorly Developed S.no CHARACTERISTICS SKELETAL MUSCLE SMOOTH MUSCLE 10. Source of Calcium Sarcoplasmic Reticulum (SR) ECF & SR 11. Overlap of Actin & Myosin Overlap of actin & myosin seen only in part of the Aband Continuous overlap of actin & myosin, b/c of side-polar cross bridges 12. ATP used directly by contractile apparatus Yes Yes 13. Myosin ATPase activity; speed of contraction Fast or slow depending on type of fiber Very slow 14. Site of Calcium regulation Troponin in thin filaments Myosin in thick filaments 15. Mechanism of Calcium action Physically repositions troponin-tropomyosin complex to uncover actin cross-bridge binding site Chemically brings about phosphorylation of myosin cross bridges so they can bind with actin S.no CHARACTERISTICS SKELETAL MUSCLE SMOOTH MSUCLE 16. Duration of contraction 100- 200 msec Upto 5 sec 17. Presence of Gap junctions No Yes 18. Latch mechanism No Yes 19. NMJ Yes No 20. Neurotransmitter Ach only Ach & Norepinephrine