Where are smooth muscle cells found?
- In the walls of hollow organs and tubes
- Like the digestive system and vasculature
Where are cardiac muscle cells found?
The heart
What do smooth muscle cells lack?
- Sarcomeres
- T-tubules
- Have very little sarcoplasmic reticulum
Thick myosin filaments - smooth muscle cells
Longer than those in skeletal muscle
Thin actin filaments - smooth muscle cells
Contain tropomyosin but not troponin
Intermediate filaments - smooth muscle cells
Do not directly support contraction but rather the cytoskeletal framework
What are dense bodies in smooth muscle cells?
- Dense bodies replace Z-lines
- Serve as anchor points for both intermediate and contractile filaments
- They’re scattered throughout the cell and on the membrane, with filaments arranged at angles forming a diamond-like pattern
Myosin cross-bridge activation in smooth muscle - Step 1
During excitation, Ca2+ enters the smooth muscle cell and binds to calmodulin
Myosin cross-bridge activation in smooth muscle - Step 2
The Ca2+-calmodulin complex binds to and activates myosin light chain kinase
Myosin cross-bridge activation in smooth muscle - Step 3
Once activated, this kinase phosphorylates the myosin light chain, which allows the myosin cross-bridge to bind actin
Calcium sources in smooth muscle - Ca2+ entry from the ECF
- Voltage-gated dihydropyridine receptors function as Ca2+ channels
- If smooth muscle cell depolarizes enough to open these channels, Ca2+ will enter the cell from the ECF
Calcium sources in smooth muscle - Ca2+ release from the SR
Once Ca2+ enters it can activate calmodulin or cause a further increase of Ca2+ by stimulating the SR to release Ca2+ by a process called CIRC
Why is calcium important for muscle function?
- Calcium is essential for both skeletal and smooth muscle contraction
- The body tightly regulates calcium levels through absorption (intestines), storage (bones), and excretion or reabsorption (kidneys)
Single unit smooth muscle
- Majority of smooth muscle
- Muscle fibres are all connected via gap junctions so they become excited and contract as a single unit (functional syncytium)
Where are single unit smooth muscle found?
- Digestive system
- Reproductive system
- Urinary tracks
- Small blood vessels
Multiunit smooth muscle
- Consists of separate groups of cells innervated by autonomic nerves
- It’s neurogenic, meaning groups of smooth muscle cells innervated by autonomic nerves to contract
Where are multiunit smooth muscle found?
- Walls of large blood vessels
- Small airways to the lungs
- The base of hair follicles of the skin
- In the eye to adjust the lens shape and iris
Myogenic
- Single unit smooth muscle is myogenic
- It is self-excitable and does not require nerve stimulation
2 types of spontaneous depolarization in single unit smooth muscle
- Pacemaker potentials
- Slow-wave potentials
Pacemaker potentials
Membrane gradually depolarizes until it reaches threshold, then fires an action potential
What initiates pacemaker depolarization?
Autorhythmic cells have If channels permeable to Na⁺ and K⁺ that open, allowing gradual depolarization until threshold is approached
What happens as the pacemaker cell becomes more positive?
If channels close and Ca²⁺ channels open, continuing depolarization until threshold is reached and an action potential fires
Slow-wave potentials
- Caused by the active transport of Ca2+ across the membrane
- As ions are transported they cause an oscillating wave of alternating hyperpolarization and depolarization
- If threshold is reached an action potential will be generated
How is single-unit smooth muscle innervated?
- Both branches of the ANS innervate smooth muscle to modify contraction rate and strength
- Postganglionic autonomic neurons release neurotransmitters from varicosities, allowing one neuron to influence many smooth muscle cells
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Module 1: Section 1 + 256
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Module 1: Section 316
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Module 1: Section 413
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Module 1: Section 531
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Module 2: Section 120
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Module 2: Section 216
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Module 2: Section 332
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Module 2: Section 451
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Module 2: Section 511
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Module 4: Section 127
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Module 4: Section 235
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Module 4: Section 333
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Module 4: Section 426
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Module 4: Section 59
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Module 4: Section 628
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Module 5: Section 115
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Module 5: Section 234
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Module 5: Section 352
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Module 5: Section 47
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Module 5: Section 533
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Module 5: Section 67
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Module 6: Section 121
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Module 6: Section 223
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Module 6: Section 344
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Module 6: Section 428
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Module 6: Section 512
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Module 6: Section 613
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Module 6: Section 743
