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Medicine Year 1 - Semester 2 - Part 1 > Smooth Muscle Contraction > Flashcards

Smooth Muscle Contraction Flashcards

1
Q

State the approximate length of smooth muscle cells

A

30-200 micrometres in length

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2
Q

Where is smooth muscle found ?

A

Blood vessels
Digestive tract
Reproductive tract
Respiratory tract
Skin
Eyes

etc

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3
Q

Briefly describe smooth muscle cells

A

Small
Spindle shaped cells
Arranged in sheets
Less regularly organised

NO striations
Single nucleus

Involuntary - ANS, hormones and stretch

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4
Q

State the types of smooth muscle

A

Multi-unit
Unitary

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5
Q

Describe multi-unit smooth muscle

A

Discrete/separate fibres each with its own nerve ending

Independent contraction

Mainly innervated by nerve signals

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6
Q

Where can you find multi-unit smooth muscle ?

A

Ciliary muscle of the eye
Iris
Piloerector muscles
Vas deferens

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7
Q

Describe unitary smooth muscle

A

Sheets of electrically coupled cells - syncytium/visceral smooth muscle

Contract in unison

Connected by gap junctions

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8
Q

Where can you find unitary smooth muscle ?

A

GI tract
Bile ducts
Ureters
Uterus
Blood vessels

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9
Q

State a difference between multi-unit and unitary smooth muscle

A

Multi-unit : Independent contraction

Unitary : Contract in unison

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10
Q

Describe the structure of smooth muscle

A

No striations

Contain :

  • Dense bodies
  • GAP junctions
  • Focal adhesions

Do NOT contain :

  • Troponin
  • T Tubules
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11
Q

What does smooth muscle contain for structure ?

A

GAP junctions which electrically couple cells in UNITARY smooth muscle.

Focal adhesions which connect cells together mechanically.

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12
Q

What is a ‘Dense Body’ ?

A

Dense bodies correspond to Z discs, which are lattice like structures anchoring actin within the fibre and tethers contractile proteins to the sarcolemma.

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13
Q

Function of dense bodies

A

Transmit force of contraction within and between cells.

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14
Q

Difference between GAP junctions and focal adhesions

A

GAP junctions couple adjacent cells electrically
Focal adhesions couple adjacent cells mechanically

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15
Q

State the difference in sarcoplasmic reticulum of smooth muscle and skeletal/cardiac muscle

A

The SR in smooth muscle is much less developed.
The actin and myosin filaments are less regularly organised.

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16
Q

What are caveolae ?

A

Pouchlike infoldinfgs of the sarcolemma.

They contain large numbers of calcium channels.

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17
Q

What is the main trigger for smooth muscle contraction ?

A

Extracellular calcium is the main trigger for contraction.

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18
Q

State the 3 mechanisms which lead to an increase in intracellular calcium concentration

A

Voltage gated L type calcium receptors

Receptor operated calcium channels

Store operated calcium channels

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19
Q

Function of voltage gated L type calcium channels in excitation contraction coupling

A

They lead to calcium induced calcium release via the ryanodine receptor activation

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20
Q

Function of receptor operated calcium channels in excitation contraction coupling

A

They lead to IP3 receptor activation and CIRC

21
Q

Describe excitation contraction coupling

A

Following stimulation, an influx of calcium couples the excitation into mechanical contraction.

22
Q

State the differences between smooth muscle myosin and skeletal/cardiac muscle myosin

A

They differ in :

  • Amino acid sequence
  • Arrangement of myosin heads
23
Q

Describe how myosin in smooth muscle differs from skeletal/cardiac muscle

A

In smooth muscle,

  • Myosin heads are arranged along the entire length of the molecule
  • Head hinges opposing direction on the same filament - pulls in opposite directions, increasing shortening
24
Q

State the types of smooth muscle actin

A

Alpha-SMA : vascular
Gamma-SMA : GI tract

No troponin

25
Q

What is calmodulin ?

A

A key regulatory protein enabling myosin to interact with actin.

26
Q

Briefly describe activation of myosin

A

Phosphorylation of myosin needed for cross-bride formation with actin.

Via calmodulin / myosin light chain kinase (MLCK)

27
Q

Briefly describe relaxation of myosin

A

Myosin must be de-phosphorylated for relaxation to occur.

Whilst phosphorylated, it will continue to form cross-bridges with actin.

28
Q

Explain the process of contraction in smooth muscle

A

Contraction is initiated by calcium influx from extracellular fluid in the SR.

Calcium binds to calmodulin (instead of troponin as in skeletal muscle)

Ca-Calmodulin-MLCK complex lead to phosphorylation of MLC (requires ATP)

Phosphorylated myosin head binds to actin and power stroke occurs automatically.

29
Q

Explain the process of relaxation in smooth muscle

A

When stimulus ends, calcium is pumped out of the cell or into the SR.

When calcium drops below a critical level, calcium dissociates from calmodulin (inactivates MLCK)

Myosin phosphate removes phosphate from the MLC, causing detachment of the myosin head from the actin filament, causing relaxation.

30
Q

What does the time of relaxation depend on ?

A

Determined by the amount of active myosin phosphate in cells.

31
Q

How is calcium transported out of the sarcoplasm ?

A

Active transport via :

  • Membrane Ca2+ ATPase
  • Ca2+ ATPase (SERCA)

Passively via :

  • Na+ / Ca2+ exchangers
32
Q

What monitors calcium levels in the SR ?

A

Stim1 senses calcium levels in the SR and activates STORE OPERATED CALCIUM CHANNELS for influx of calcium back into the cell.

33
Q

Function of store-operated calcium channels (SOCs)

A

Enables the SR to refill
Influx of Ca2+ back into the cell

34
Q

What does smooth muscle lack ?

A

Highly specialised neuromuscular junctions

35
Q

Discuss the innervation of smooth muscle

A

Autonomic nerve fibres branch diffusely creating synaptic clefts.

Unitary muscles have wide diffuse junctions

Multi-unit muscle cell junctions are closer in proximity - contact junctions

36
Q

What is a varicosity ?

A

Varicosities are swellings which release neurotransmitter in the general area of smooth muscle cells.

37
Q

Where do varicosities originate from ?

A

Postganglionic fibres of both sympathetic and parasympathetic neurons.

38
Q

Name some neurotransmitters in smooth muscle

A

Acetylcholine
Noradrenaline

39
Q

What do smooth muscle cell membranes contain ?

A

Receptors which can initiate or inhibit contraction.

40
Q

Function of neurotransmitters

A

They can either stimulate or inhibit contractile activity.

41
Q

Describe a neurotransmitter that may produce opposite effects in different smooth muscle tissues

A

Noradrenaline stimulates contraction of most vascular smooth muscle by acting on alpha-adrenergic receptors

BUT

produces relaxation of airway smooth muscle by acting on beta2-adrenergic receptors.

42
Q

Describe how smooth muscle contraction is regulated

A

ANS stimulation

  • Spontaneous electrical activity
  • Stretch
  • Hormones
  • Local chemicals within extracellular fluid
43
Q

State some local chemicals within extracellular fluid

A

Oxygen
Carbon dioxide
Acidity
Ion concentration
Nitric oxide

44
Q

What is smooth muscle resting membrane potential ?

A

-50 to -60 mV

45
Q

How can unitary smooth muscle spike potentials be generated by ?

A

Electrical stimulation
Hormones
Stretch

Spontaneous depolarisation from pace-maker cells of the intestinal wall

46
Q

Why is smooth muscle contraction prolonged ?

A

Due to slower ATPase activity / cross-bridge formation

AND

Slower response to influx of Ca2+ ions

47
Q

Why is the force of contraction in smooth muscle greater than that of skeletal muscle ?

A

Due to the longer cross-bridge attachments between actin and myosin.

48
Q

Function of the latch mechanism

A

Maintains prolonged contraction, with minimal ATP use - only 1 ATP required for each cycle

49
Q

When does the latch mechanism occur ?

A

When myosin is de-phosphorylated while still attached to actin.

(only if [Ca2+]i remains elevated above background levels)

Medicine Year 1 - Semester 2 - Part 1 (51 decks)

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