Smooth Muscle Flashcards
Describe smooth muscle cells.
Spindle shaped.
Mononucleate.
Divides through life.
Innervated by the ANS.
How does smooth muscle contract?
Has cross-bridge cycles that use Ca2+.
Filaments are arranged diagonally across cells.
Anchored to membranes and cell structures by dense bodies.
Filaments slide over one another to contract.
Describe the smooth muscle cross-bridge cycle.
- [Ca2+] increases.
- Ca2+ binds to calmodulin. This complex binds to myosin light chain kinase.
- Kinase phosphorylates myosin cross-bridges with ATP.
- Phosphorylated cross-bridges bind to actin.
- Contraction and tension occurs.
Describe how smooth muscle relaxes.
Myosin light chain phosphatase depolarises cross-bridges.
How can some smooth muscle be constantly stimulated?
Cross-bridges can dephosphorylate while still bound to actin.
A decreased rate of ATP splitting slows the cross-bridge cycle.
Tension is maintained for long periods of time with low ATP.
Where are sources of cytosolic Ca2+?
SR (less than skeletal).
Extracellular Ca2+ (VGCs).
How is Ca2+ removed from the cytosol?
Pumped into the SR and out of the cell by Ca2+-ATPases (slower than skeletal).
Describe how smooth muscle contractions are graded.
Only some sites are activated.
Depends on the number of APs.
A basal level of Ca2+ in cells causes a constant level of tension.
What factors affect contractile activity?
Pacemaker activity.
Hormones.
Local factors.
What are examples of single unit smooth muscle?
GIT.
Small blood vessels.
Describe single unit smooth muscle.
Many cells linked by gap junctions.
Signals travel between cells.
Contracts synchronously.
May contain pacemaker cells.
Stretch evokes contractions.
Give examples of multiunit smooth muscle.
Airways.
Large arteries.
Describe multiunit smooth muscle.
Few or no gap junctions.
Richly innervated by the ANS.
Doesn’t respond to stretch.
Describe the types of smooth muscle in organs.
A mixture of single unit and multiunit cells.
Allows organs to have a mixture of properties in different areas.
Where is constantly stimulated smooth muscle useful?
Blood vessel walls.
