Smooth muscle

• Where is smooth muscle found?

  • Smooth muscle is found in the walls of hollow organs such as blood vessels, the gastrointestinal (GI) tract, respiratory tract, bladder, and uterus.
• What is the primary function of smooth muscle in the body?

  • Smooth muscle regulates the diameter of hollow organs (e.g., blood vessels), propels substances (e.g., food in the GI tract), and maintains sustained contractions (e.g., bladder control).
• What is the shape of smooth muscle cells?

  • Smooth muscle cells are spindle-shaped (fusiform) and relatively short (30–200 µm in length, 5–10 µm in diameter).
• How many nuclei do smooth muscle cells have?

  • Smooth muscle cells have a single, centrally located nucleus.
• Do smooth muscles have striations like skeletal muscles?

  • No, smooth muscle does not have striations, which is why it is called "smooth."
• What are the main sources of calcium for smooth muscle contraction?

  • Calcium comes from the extracellular fluid (ECF) and the sarcoplasmic reticulum (SR).
• What protein does calcium bind to in smooth muscle to initiate contraction?

  • Calcium binds to calmodulin, which activates myosin light chain kinase (MLCK).
• Does smooth muscle rely solely on electrical stimulation for activation?

  • No, smooth muscle can also be activated chemically by hormones or local factors (e.g., adrenaline, acetylcholine).
• What are the steps of smooth muscle excitation-contraction coupling?

  1. Calcium enters the cytosol from the ECF or SR.
  2. Calcium binds to calmodulin.
  3. The calcium-calmodulin complex activates MLCK.
  4. MLCK phosphorylates myosin light chains, allowing the myosin heads to bind to actin.
  5. The cross-bridge cycle leads to contraction.
• What structural differences exist between smooth and skeletal muscle sarcomeres?

  • Smooth muscle lacks Z-lines. Instead, it has dense bodies that anchor actin filaments.
• What are gap junctions, and what role do they play in smooth muscle?

  • Gap junctions allow ions and action potentials to pass between adjacent smooth muscle cells, enabling coordinated contractions in single-unit smooth muscle.
• What is ephaptic conduction in smooth muscle?

  • Ephaptic conduction refers to the transmission of excitation through chemical signals without direct electrical contact.
• Does smooth muscle use troponin for calcium binding?

  • No, smooth muscle does not use troponin. Calcium binds to calmodulin instead.
• What are the differences between single-unit and multi-unit smooth muscle?

  • Single-unit: Contracts as a single unit due to numerous gap junctions (e.g., GI tract, uterus).
  • Multi-unit: Contracts independently with few gap junctions, allowing finer control (e.g., large blood vessels, airways).
• What are the primary calcium sources for contraction in each muscle type?

  • Skeletal muscle: Sarcoplasmic reticulum.
  • Smooth muscle: Extracellular fluid and sarcoplasmic reticulum.
  • Cardiac muscle: Extracellular fluid and sarcoplasmic reticulum
• What proteins regulate calcium binding in each muscle type?

  • Skeletal muscle: Troponin.
  • Smooth muscle: Calmodulin.
  • Cardiac muscle: Troponin.
• Which muscle types have gap junctions?

  • Skeletal muscle: None.
  • Smooth muscle: Present in single-unit smooth muscle.
  • Cardiac muscle: Present.
• Which muscle type has the ability to generate pacemaker potentials?

  • Smooth muscle (single-unit) and cardiac muscle.
• What are the contractile proteins in smooth muscle?

  • Actin and myosin, similar to skeletal and cardiac muscle.
• How does smooth muscle contraction differ under load compared to skeletal muscle?

  • Smooth muscle can maintain tension over a wide range of lengths, unlike skeletal muscle, which has a fixed length-tension relationship.
• What are dense bodies in smooth muscle?

  • Dense bodies are anchoring points for actin filaments in smooth muscle, analogous to Z-lines in skeletal muscle.
• What enzyme is critical for smooth muscle contraction?

  • Myosin light chain kinase (MLCK).
• What neurotransmitters influence smooth muscle contraction?

  • Excitatory: Acetylcholine, adrenaline, noradrenaline.
  • Inhibitory: Nitric oxide.
• How does the sarcoplasmic reticulum in smooth muscle compare to that in skeletal muscle?

  • The sarcoplasmic reticulum in smooth muscle is less developed than in skeletal muscle.