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
Where is phasic smooth muscle most abundant?
Walls of hollow organs which push contents through them
What is the structure of multi-unit smooth muscle?
- No/few gap junctions
- Activated by autonomic nerve input from very branched neuron (neurogenic)
- Units can act independently but commonly dont
- Found in hair follicles and in the eye
What is the structure of single-unit smooth muscle?
- Electrically coupled with gap junctions
- Self exitable (no nervous input)
- Tension modulated by the autonomic nervous system
- Often found in walls of hollow organs
What is phasic single unit smooth muscle?
- Clusters of specialised non-contractile cells within syncytium display spontaneous ryhthmic electrical activity
- Similar mechanism to cardiac muscle (funny current which spreads through gap junctions)
How is Ca2+ regulated in phasic muscle?
- Mostly enters through ECF, no need for T-tubules as muscle is thin
- Enters through v-dependent dihyfropyridine receptors (L-type Ca2+ channels)
How is Ca2+ regulated in tonic muscle?
- Extrinsic ligands control te release of Ca2+ from SR
- Ligand binds to G-protein receptor coupled in membrane
- Activates phospholipase C causing an increase in inositol triphosphate (IP3)
- Activates IP3 receptor Ca2+ channel in SR releasing Ca2+ into the cytosol
What 4 things dictate the gredation of smooth muscle contraction?
- Autonomic nerve activity
- Hormones
- Local metabolites
- Mechanical stretch
How do neural inputs enter smooth muscle?
- Transmitter released from varicosities along nerve branches (local synapses)
- Receptors are diffuse across muscle cell membrane
What is the effect on the instestine if a) ACh is released b) no ACh is released
a) Membrane depolarisation, slow waves cross threshold more often, muscle becomes more active
b) Membrane hyperpolarisation, less slow waves, muscle is less active
How do hormones change the condition of the uterus during pregnancy and labour?
Uterus is poorly innervated - affected most by hormones
pregnancy - high levels of progesterone decrease expression of proteins involved in gap junction formation, reduces muscle activity
labour - high oestrial levels increase gap junction formation
