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BL2305 > Smooth muscle > Flashcards

Smooth muscle Flashcards

1
Q

What are the main features of smooth muscle and what is its function?

A
  • 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
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2
Q

What are the features of smooth muscle at the cellular level?

A
  • Spindle shaped
  • No T-tubules
  • No striations
  • Same contractile elements however with a different arrangement
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3
Q

How does the actin and myosin in smooth muscle differ from other types?

A
  • Arranged in diamond-shaped lattice
  • Dense bodies anchor actin filaments which overlap from either end
  • Myosin heads sprout out of the entire strand
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4
Q

Why can smooth muscle shorten more than skeletal?

A
  • Myosin can pull in opposite directions at the same place
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5
Q

How is contraction regulated?

A
  • 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+)
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6
Q

By which process does an increase in Ca2+ levels lead to the phosphorylation of MLCs?

A
  • Binds to calmodulin (CaM)
  • Activates myosin light chain kinase (MLCK)
  • phosphorylates light chains and increases myosin ATPase activity
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7
Q

How is the relaxation of smooth muscle regulated?

A
  • 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
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8
Q

Where is ATP used the smooth muscle cycling? Is this energetically expensive?

A
  • Used for cross-bridge cycling and to phosphorylate MLC

- Not expensive as the process is very slow

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9
Q

Describe the latch phenomenon

A
  • 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
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10
Q

What allows smooth muscle to stretch to near maximal tension?

A
  • Higher actin:myosin ratio means that if one actin filament does not overlap with a myosin then another will
  • No sarcomeres
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11
Q

Why does smooth muscle need to be so elastic?

A
  • So that it can exist over a range of lengths with little change in tension
  • So hollow organs can adjust their size
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12
Q

Name 4 locations in which smooth muscle is found?

A
  1. Hollow organs
  2. Tubes
  3. Eye (iris) - radial/circular
  4. Skin - bundles attached to hair follicles
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13
Q

What two different types of contraction can smooth muscle exhibit?

A
  • Phasic

- Tonic (continuous)

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14
Q

What are the two forms of sources of excitation?

A
  • Multi-unit (different independent units in a muscle)

- Single-unit

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15
Q

What are the characteristics of tonic smooth muscle?

A
  • Partially contracted at all times
  • Low resting potential (-55/-40 mV)
  • Activity can be modulated above or below tonic level
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16
Q

Where is phasic smooth muscle most abundant?

A

Walls of hollow organs which push contents through them

17
Q

What is the structure of multi-unit smooth muscle?

A
  • 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
18
Q

What is the structure of single-unit smooth muscle?

A
  • Electrically coupled with gap junctions
  • Self exitable (no nervous input)
  • Tension modulated by the autonomic nervous system
  • Often found in walls of hollow organs
19
Q

What is phasic single unit smooth muscle?

A
  • 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)
20
Q

How is Ca2+ regulated in phasic muscle?

A
  • Mostly enters through ECF, no need for T-tubules as muscle is thin
  • Enters through v-dependent dihyfropyridine receptors (L-type Ca2+ channels)
21
Q

How is Ca2+ regulated in tonic muscle?

A
  • 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
22
Q

What 4 things dictate the gredation of smooth muscle contraction?

A
  • Autonomic nerve activity
  • Hormones
  • Local metabolites
  • Mechanical stretch
23
Q

How do neural inputs enter smooth muscle?

A
  • Transmitter released from varicosities along nerve branches (local synapses)
  • Receptors are diffuse across muscle cell membrane
24
Q

What is the effect on the instestine if a) ACh is released b) no ACh is released

A

a) Membrane depolarisation, slow waves cross threshold more often, muscle becomes more active
b) Membrane hyperpolarisation, less slow waves, muscle is less active

25
Q

How do hormones change the condition of the uterus during pregnancy and labour?

A

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

BL2305 (17 decks)

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