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physiology for PHYSIOTHERAPY > Smooth muscle > Flashcards

Smooth muscle Flashcards

1
Q

Describe the structure of a smooth muscle cell

A
  • spindle/ rod shaped
  • unnucleated
  • able to proliferate throughout lifetime, unlike skeletal muscle
  • no striations
  • arranged in sheets
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2
Q

What’s the difference between thin and thick filaments on skeletal vs smooth?

A
  • smooth muscle contains thick myosin filaments and thin actin filaments
  • instead of the troponin protein on actin in skeletal muscle, it contains calmodulin that binds to Ca2+
  • Myosin has hinged heads along its length
  • Actin and myosin loosely arranged in cell held in place by dense bodies
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3
Q

What is a dense body?

A

Thin filaments that are anchored to cytoplasmic structures

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4
Q

What dictates function?

A
  • All structurally similar but altered expression of genes can alter excitability and activation
  • eg voltage gated channels/ stretch - activated ion channels / hormone receptors
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5
Q

What is important for smooth muscle cell communication?

A
  • Gap junctions for electrical and chemical communications
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6
Q

What controls smooth muscle?

A
  • Automatic nervous system
  • hormonal secretions
  • spontaneous electrical activity
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7
Q

How is the smooth muscle cell membrane activated?

A
  • NO NMJ
  • membrane may express voltage gated ion channels (GI tract / Uterus) , mechanically gated (eg stomach)
  • membrane may express hormone receptors eg adrenaline induced mobilisation of Ca2+ in SR
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8
Q

What is spontaneous electrical activity/ Pace maker electrical potentials ?

A
  • spontaneous AP in absence of neural or hormonal input eg peristalisis
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9
Q

How are smooth muscle cells innervated?

A
  • Autonomic neuron
  • Arteries ( blood vessels ) generally sympathetic
  • other tissues are both sympathetic and para symp
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10
Q

Describe the 2 types of smooth muscle

A
  • Single unit - synchronous activity (electrical and mechanical) , gap junctions, may include pace maker cells to start AP EG ) Uterus , GI tract
  • Multi unit - Do not usually spread AP’s, richly innervated, no or few gap junctions eg arteries and airways
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11
Q

What is the difference between skeletal muscle SR and smooth muscle SR?

A
  • Skeletal muscle cells have organised SR and T tubules
  • Smooth muscle cells dont have organised SR and T tubules
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12
Q

How are calcium levels controlled in smooth muscle cells?

A
  1. Action potential
  2. If smooth muscle cell dosen’t require APs, then there are 2nd messengers that are released from membrane or made in cytosol that trigger release of Ca2+
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13
Q

What is cross bridge cycling in smooth muscle cells controlled by?

A
  • Enzyme called myosin- light chain kinase
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14
Q

How is cross bridge regulation different in skeletal and smooth muscle?

A
  • skeletal muscle contains tropomyosin which blocks inactivated troponin from binding to actin when Ca2+ levels are low
  • Smooth muscle does not contain troponin, so therefore tropomyosin never blocks anything… contains regulatory enzyme instead
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15
Q

Describe the sequence of events of a cross bridge cycle in smooth muscle.

A
  • Ca2+ binds to calmodulin
  • Ca2+ calmodulin then binds to enzyme - myosin light chain kinase
  • enzyme is activated
  • enzyme uses ATP to phosphorylate myosin light
  • chains
  • Myosin ATPase enzyme hydrolyses ATP - ADP
  • Phosphorylated myosins bind to actin - cross bridge
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16
Q

What controls smooth muscle relaxation?

A
  • myosin light chain phosphatase
  • when cytosolic Ca2+ levels are low again , calmodulin unbinds to ca2+
  • Myosin… phosphatase removes phosphate from myosin which then cannot bind to actin
  • this decreases myosin ATPase activity = less tension
17
Q

What is the difference between smooth muscle and skeletal muscle contraction?

A
18
Q

What is the latch state in smooth muscle?

A
  • The smooth muscle holds tension at low rates of ATP hydrolysis
  • Eg sphincter muscles, Uterus

When contraction is persistant and sustained, Ca2+ conc remains elevated and rate of ATP hydrolysis is maintained

19
Q

What does dephosphorylating the RLCS of myosin cause?

A
  • inhibits myosin cross bridge formation with actin, it induces the latch state
20
Q

What are the 2 sources of cytosolic calcium in smooth muscle?

A
  1. SR (only small amounts from SR)
  2. extracellular Ca2+ entering the cell through plasma membrane - voltage or ligand gated ion channels (most)
21
Q

Describe Calcium voltage gated channels

A
  1. change conformation / activated by AP’s or graded potential
  2. in vascular systems, sufficient influx of Ca2+ activates MLCK
  3. important in cardiac muscle

physiology for PHYSIOTHERAPY (25 decks)

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