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

Smooth Muscle Contraction Flashcards

(49 cards)

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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

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A

They lead to IP3 receptor activation and CIRC

21
Q

Describe excitation contraction coupling

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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

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A

They differ in :

  • Amino acid sequence
  • Arrangement of myosin heads
23
Q

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

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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

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A

Alpha-SMA : vascular
Gamma-SMA : GI tract

No troponin

25
What is calmodulin ?
A key regulatory protein enabling myosin to interact with actin.
26
Briefly describe activation of myosin
Phosphorylation of myosin needed for cross-bride formation with actin. Via calmodulin / myosin light chain kinase (MLCK)
27
Briefly describe relaxation of myosin
Myosin must be de-phosphorylated for relaxation to occur. Whilst phosphorylated, it will continue to form cross-bridges with actin.
28
Explain the process of contraction in smooth muscle
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
Explain the process of relaxation in smooth muscle
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
What does the time of relaxation depend on ?
Determined by the amount of active myosin phosphate in cells.
31
How is calcium transported out of the sarcoplasm ?
Active transport via : - Membrane Ca2+ ATPase - Ca2+ ATPase (SERCA) Passively via : - Na+ / Ca2+ exchangers
32
What monitors calcium levels in the SR ?
Stim1 senses calcium levels in the SR and activates STORE OPERATED CALCIUM CHANNELS for influx of calcium back into the cell.
33
Function of store-operated calcium channels (SOCs)
Enables the SR to refill Influx of Ca2+ back into the cell
34
What does smooth muscle lack ?
Highly specialised neuromuscular junctions
35
Discuss the innervation of smooth muscle
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
What is a varicosity ?
Varicosities are swellings which release neurotransmitter in the general area of smooth muscle cells.
37
Where do varicosities originate from ?
Postganglionic fibres of both sympathetic and parasympathetic neurons.
38
Name some neurotransmitters in smooth muscle
Acetylcholine Noradrenaline
39
What do smooth muscle cell membranes contain ?
Receptors which can initiate or inhibit contraction.
40
Function of neurotransmitters
They can either stimulate or inhibit contractile activity.
41
Describe a neurotransmitter that may produce opposite effects in different smooth muscle tissues
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
Describe how smooth muscle contraction is regulated
ANS stimulation - Spontaneous electrical activity - Stretch - Hormones - Local chemicals within extracellular fluid
43
State some local chemicals within extracellular fluid
Oxygen Carbon dioxide Acidity Ion concentration Nitric oxide
44
What is smooth muscle resting membrane potential ?
-50 to -60 mV
45
How can unitary smooth muscle spike potentials be generated by ?
Electrical stimulation Hormones Stretch Spontaneous depolarisation from pace-maker cells of the intestinal wall
46
Why is smooth muscle contraction prolonged ?
Due to slower ATPase activity / cross-bridge formation AND Slower response to influx of Ca2+ ions
47
Why is the force of contraction in smooth muscle greater than that of skeletal muscle ?
Due to the longer cross-bridge attachments between actin and myosin.
48
Function of the latch mechanism
Maintains prolonged contraction, with minimal ATP use - only 1 ATP required for each cycle
49
When does the latch mechanism occur ?
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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