Smooth muscle is an involuntary non-striated muscle, found within
- the tunica media layer of large and small arteries and veins,
- the bladder,
- uterus and male and female reproductive tracts,
- gastrointestinal tract,
- respiratory tract,
- the ciliary muscle and iris of the eye.
- the glomeruli of the kidneys contain a smooth muscle-like cell called the mesangial cell.
Smooth muscle is fundamentally different from skeletal muscle and cardiac muscle in terms of structure, function, excitation-contraction coupling, and mechanism of contraction.
Compared to skeletal muscle, smooth-muscle cells are small. They are spindle-shaped, about 50 to 200 microns long and only 2 to 10 microns in diameter. They have no striations or sarcomeres. Instead, they have bundles of thin and thick filaments (More details).
They are connected by gap-junctions and form a syncitium. They plasma membrane is rich of vescicles involved in endocytosis (Caveolae).
Unlike skeletal-muscle cells, they have no troponin, tropomyosin or organized sarcoplasmic reticulum, while calmodulin (which takes on the regulatory role in smooth muscle), caldesmon and calponin are significantly expressed.
As in skeletal-muscle cells, contraction in a smooth-muscle cell involves the forming of crossbridges and thin filaments sliding past thick filaments. However, because smooth muscle is not as organized as skeletal muscle, shortening occurs in all directions. During contraction, the smooth-muscle cell's intermediate filaments help to draw the cell up, like closing a drawstring purse.
Calcium mediated contraction
Calcium ions regulate contraction in smooth muscle, but they do it in a slightly different way than in skeletal muscle:
1. Calcium ions come from outside of the cell.
2. Calcium ions bind to an enzyme complex on myosin, called calmodulin-myosin light chain kinase.
3. The enzyme complex breaks up ATP into ADP and transfers the Pi directly to myosin.
4. This Pi transfer activates myosin.
5. Myosin forms crossbridges with actin (as occurs in skeletal muscle).
6. When calcium is pumped out of the cell, the Pi gets removed from myosin by another enzyme.
7. The myosin becomes inactive, and the muscle relaxes.
This process is called myosin-regulated contraction.
The sarcolemma possess microdomains specialized to cell-signaling events and ion channels called caveolae. These invaginations in the sarcoplasma contain a host of:
- muscarinic receptors
- adrenergic receptors
- second messenger generators
- adenylate cyclase
- Phospholipase C
- G proteins
- Rho kinase-ROCK
- Protein kinase C
- Protein Kinase A
- ion channels in close proximity
- L type Calcium channels
- ATP sensitive Potassium channels
- Calcium sensitive Potassium channels
The caveolae are often in close proximity to sarcoplasmic reticulum or mitochondria, and have been proposed to organize signaling molecules in the membrane.
Response to deformation
Differential requirement for MEK/ERK and SMAD signaling in PAI-1 and CTGF expression in response to microtubule disruption. 2009
Requirements underlying colchicine-stimulated expression of PAI-1 and CTGF, two prominent cell deformation-sensitive fibrosis-initiating proteins, were evaluated in vascular smooth muscle cells. Microtubule disruption rapidly induced EGFR transactivation (at the src kinase-sensitive EGFR site) in a ROS-dependent manner
Histone deacetylases modulate vascular smooth muscle cell migration induced by cyclic mechanical strain. 2009
EC and SM
Epinephrine effect: dose dependance