22. Characteristics of smooth muscle Flashcards
1
Q
General
A
- Comprises about 3% of the body mass
- There are two major types of smooth muscle:
- single-unit smooth muscle
- Multi-unit smooth muscle
- Single unit smooth muscle is found in the walls of the body’s hollow and tubular organs
- Electrically connected by gap junctions that facilitate synchronized contractions.
- Sustained contraction of the smooth muscle in walls transfers pressure to the content or the organ, while alternating contractions and relaxations lead to mixing of the content.
- Can also create waves of contraction = peristaltic movements
- Multi-unit smooth muscle is primarily located in the eyes and skin
- Lacks gap junctions – can contract independently
- Responsible for:
- Adjusting the diameter of the pupils according to the intensity of light
- Altering the curvature of the lens in the eye
- Changing the angle of hairs relative to skin surface
2
Q
Structure
A
- Diameter at the middle of the cells is less than 0,01 mm, and their length is between 0,1 and 0,2 mm.
- Develop from one cell and have only one nucleus. Can be formed throughout life.
- Contain both actin and myosin filaments, but they are not organized in sarcomers. They are longer than in skeletal muscle, and are arranged in bundles of different orientation.
- In the cytoplasm, the actin filaments are attached to dense protein lattices (dense bodies), which are analogous to the Z-discs in skeletal muscle fibers.
- Myosin filaments are located between the actin filaments. The ends on the myofilaments bundles are attached to dense protein plates in the cell membrane.
- There are also bundles of intermediate filaments between the membrane plates, and between the dense bodies in the cytoplasm.
- These filaments do not participate in the contraction, but are a part of the cytoskeleton that determined the shape of the cell.
- The intermediate filaments allow the forces developed by filament sliding to be transferred to the surface in such way that the cell shortens.
- Adjacent cells in smooth muscle are connected by proteins fibers between the protein plates in the membrane and the cells are also anchored to collagen gibers in the extracellular matrix.
- Lack T-tubules.
- Sarcoplasmic reticulum is poorly developed compared to the skeletal muscle fibers.
3
Q
Contraction of smooth muscle
A
- Stimulation of the cells result in increased calcium concentration in the cytosol, due both to calcium influx through ion channels in the cell membrane, and to calcium release from the sarcoplasmic reticulum.
- Calcium binds to calmodulin (calcium binding protein).
- Calcium-calmodulin complex activates myosin kinase.
- The activated myosin kinase transfers a phosphate group from ATP to the myosin heads.
- Phosphorylated myosin heads hydrolyze ATP and bind to actin.
- Cross-bridges are formed and broken as long as the myosin heads are phosphorylated, causing the cell to contract.
- When stimulation of the cell ceases, calcium pumps in the cell membrane and in the membrane of the sarcoplasmic reticulum will remove more calcium from the cytosol than flows in.
- The calcium concentration in the cytosol falls and calcium dissociates from calmodulin, and the activation of myosin kinase ceases.
- Myosin phosphatase dephosphorylates the myosin heads, which then lose their ATPase activity and the ability to bind to actin.
- The muscle cell relaxes.
–> Myosin heads are locked to actin for a long time before the bond is broken. The ATP consumption during isometric contractions is therefore correspondingly low, because one ATP molecule is hydrolyzed for each new cross-bridge formed.
Skeletal muscle requires several hundred times more ATP than smooth muscle in order to maintain isometric contraction.
4
Q
Signals for smooth muscle contraction/relaxation
A
(blue figures – inhibition, red figures – activation)
Relaxation of smooth muscle cells –> vasodilatation
Extension (stretching) of certain smooth muscle cells results in contraction = Bayliss effect
