Smooth muscle Flashcards
What are the main features of smooth muscle and what is its function?
- Composed of single cells which do not extend the full length and are arranged in circular/longitudinal sheets held together by cell bands
- They surround other tissues
What are the features of smooth muscle at the cellular level?
- Spindle shaped
- No T-tubules
- No striations
- Same contractile elements however with a different arrangement
How does the actin and myosin in smooth muscle differ from other types?
- Arranged in diamond-shaped lattice
- Dense bodies anchor actin filaments which overlap from either end
- Myosin heads sprout out of the entire strand
Why can smooth muscle shorten more than skeletal?
- Myosin can pull in opposite directions at the same place
How is contraction regulated?
- Triggered by increase in Ca2+ from ECF and SR
- Lightweight protein chains attached to myosin (MLCs)
- Myosin can only interact with actin when these chains are phosphorylated (regulated by Ca2+)
By which process does an increase in Ca2+ levels lead to the phosphorylation of MLCs?
- Binds to calmodulin (CaM)
- Activates myosin light chain kinase (MLCK)
- phosphorylates light chains and increases myosin ATPase activity
How is the relaxation of smooth muscle regulated?
- Ca transported out of cell by Na/Ca pump
- Ca-CaM dissociates
- Myosin phosphatase removes phosphate from myosin, decreasing myosin ATPase activity
- Decreased muscle tension
Where is ATP used the smooth muscle cycling? Is this energetically expensive?
- Used for cross-bridge cycling and to phosphorylate MLC
- Not expensive as the process is very slow
Describe the latch phenomenon
- ATP binding to myosin head is necessary for deactivating actin
- Myosin head takes longer to detatch due to reduced affinity for ATP after de-phosphorylating MLC
- Helps muscle produce sustained contractions
What allows smooth muscle to stretch to near maximal tension?
- Higher actin:myosin ratio means that if one actin filament does not overlap with a myosin then another will
- No sarcomeres
Why does smooth muscle need to be so elastic?
- So that it can exist over a range of lengths with little change in tension
- So hollow organs can adjust their size
Name 4 locations in which smooth muscle is found?
- Hollow organs
- Tubes
- Eye (iris) - radial/circular
- Skin - bundles attached to hair follicles
What two different types of contraction can smooth muscle exhibit?
- Phasic
- Tonic (continuous)
What are the two forms of sources of excitation?
- Multi-unit (different independent units in a muscle)
- Single-unit
What are the characteristics of tonic smooth muscle?
- Partially contracted at all times
- Low resting potential (-55/-40 mV)
- Activity can be modulated above or below tonic level