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+ BIOL 370 > Lecture 7: Smooth Muscle > Flashcards

Lecture 7: Smooth Muscle Flashcards

1
Q

Why aren’t smooth muscles studied as much as skeletal muscles? (5)

A
  • highly variable in structure and function
  • anatomy makes functional studies difficult
  • controlled by hormones, paracrines, and neurotransmitters
  • has variable electrical properties
  • multiple pathways influence contraction and relaxation
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2
Q

How do smooth muscles compare with striated muscles? (6)

A
  • much smaller fibres – around 250 long and 5 μM wide
  • not arranged in sarcomeres
  • has longer actin and myosin filaments
  • much slower myosin ATPase activity
  • myosin light chain plays regulatory role
  • has very little sarcoplasmic reticulum – Ca2+ also enters cell from extracellular fluid
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3
Q

Describe the structure of smooth muscle.

A
  • mononucleate, spindle-shaped cells
  • no visible striations – thick and thin filaments are not organized into sarcomeres
  • SR is less extensive than in striated muscle
  • no t-tubules
  • adjoining cells can be connected via gap junctions, but depends on muscle type
  • orientation of cells in relation to each other differs in relation to the needed direction of contraction (ie. smooth muscles of the stomach vs. eye)
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4
Q

How are thick and thin filaments of smooth muscle organized? How does this differ from striated muscle?

A

bundles of thick and thin filaments are arranged into networks throughout the cytoplasm

  • approximately 15 thin filaments for every thick filament
  • adhesion plaques attach thin filaments to cell membrane
  • thin filaments are anchored to dense bodies, which attach to cytoskeleton

thick and thin filaments are arranged into sarcomeres in striated muscle

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5
Q

Compare the contraction duration of skeletal, cardiac, and smooth muscle.

A

from fastest to slowest contraction duration:
skeletal > cardiac > smooth

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6
Q

How is smooth muscle contraction regulated?

A
  • regulated by a variety of factors including sympathetic neurons, hormones, and stretch
  • many of these factors regulate contraction via changes in cytoplasmic [Ca2+], although some regulation of contraction is achieved independent of Ca2+
  • regulation of contraction can also be achieved by changing the sensitivity to Ca2+ rather than by changing [Ca2+]
  • Ca2+ affects both thick and thin filaments
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7
Q

Contraction Regulation – Thick Filament

A
  1. Ca2+ enters cell and is released from SR, increasing intracellular [Ca2+]
  2. Ca2+ binds to calmodulin (CM)
  3. Ca2+-CM activates myosin light chain kinase (MLCK)
  4. activated MLCK phosphorylates the light chains in myosin heads, increasing myosin ATPase activity
  5. active myosin cross-bridges slide along actin, resulting in muscle tension
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8
Q

Contraction Regulation – Thick Filament

What role do thick filaments play in the regulation of smooth muscle contraction in vertebrates?

A

myosin must be activated before it can bind

  • myosin light chain kinase (MLCK) phosphorylates the regulatory myosin light chain (MLC) – increases myosin-actin binding
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9
Q

Contraction Regulation – Thick Filament

What does myosin light chain kinase (MLCK) do?

A

phosphorylates the regulatory myosin light chain (MLC) – increases myosin-actin binding

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10
Q

Contraction Regulation – Thick Filament

What does myosin light chain phosphatase (MLCP) do?

A

dephosphorylates the regulatory myosin light chain (MLC) – deactivates/prevents myosin binding

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11
Q

Contraction Regulation – Thick Filament

What factors regulate MLCK and MLCP?

A

some pathways involve changes in [Ca2+], while others do not

  • ie. nitric oxide (NO) induces smooth muscle relaxation via pathway that activates MLCP without changing [Ca2+]
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12
Q

Contraction Regulation – Thin Filament

Smooth muscle lacks troponin. How does tropomyosin function?

A

position of tropomyosin on the actin filament is regulated by caldesmon

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13
Q

Contraction Regulation – Thin Filament

How is the myosin binding site blocked in relaxed smooth muscle?

A

binding of both caldesmon and tropomyosin to actin

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14
Q

Contraction Regulation – Thin Filament

How does contraction occur?

A
  • cytoplasmic [Ca2+] increases
  • calmodulin binds Ca2+, then binds to caldesmon
  • calmodulin-caldesmon dissociates from actin and tropomyosin, shifting position and exposing myosin binding sites which allows for contraction
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15
Q

Contraction Regulation – Thin Filament

How does relaxation occur?

A
  • cytoplasmic [Ca2+] decreases
  • Ca2+ dissociates from calmodulin
  • caldesmon binds to actin
  • tropomyosin shifts position to block myosin bindings sites which allows relaxation
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16
Q

Contraction Regulation – Thin Filament

What regulates caldesmon activity? How?

A

hormones

  • many of the hormones that affect smooth muscle function act via signaling cascades that lead to phosphorylation or dephosphorylation of caldesmon
  • ie. signaling cascade that leads to phosphorylation of caldesmon by MAP kinase – phosphorylated caldesmon will not bind to actin, even with low cytoplasmic [Ca2+]
17
Q

How does Ca2+ regulate contraction via both thick and thin filaments?

A
  • with increased cytoplasmic [Ca2+], calmodulin binds Ca2+
  • Ca2+-calmodulin binds to caldesmon, causing it to detach from actin, exposing myosin binding sites on the thin filaments
  • Ca2+-calmodulin binds to MLCK such that it phosphorylates the myosin light chain, activating the myosin of the thick filaments
18
Q

What does the contraction and relaxation of vascular smooth muscle change? What does this affect?

A

changes vessel diameter

  • allows regulation of the distribution of blood flow throughout the body
  • contributes to regulation of blood pressure
19
Q

Describe a hormone that affects vascular smooth muscle.

A

angiotensin

  • hormone produced by the liver that increases blood pressure
  • in vascular smooth muscle, angiotensin receptors are G protein-coupled receptors – when they bind angiotensin, they generate a signaling cascade that releases Ca2+ from SR, increasing cytoplasmic [Ca2+]
20
Q

What are phasic smooth muscles?

A

muscles that contract in bursts triggered by APs that cause increased cytosolic Ca2+

  • ie. gastrointestinal and urogenital systems
21
Q

What are tonic smooth muscles?

A

muscles that are partially contracted at all times

  • varies its contraction according to cytosolic Ca2+ level
  • ie. large arteries and veins
22
Q

What are two ways in which smooth muscle is organized?

A
  • multi-unit smooth muscle
  • single-unit smooth muscle
23
Q

Describe multi-unit smooth muscles.

A

cells must each be separately stimulated by nerves to contract (myocytes are not electrically coupled with each other)

  • found surrounding arteries, respiratory airways, and in the eye
  • contractile activity is neurogenic and phasic
  • can be initiated to contract by the autonomic nervous system and hormone signaling
  • rarely contain gap junctions between cells
24
Q

Describe single-unit smooth muscles.

A

muscle fibres are self-excitable and contract as a single unit

  • more common arrangement found in walls of most visceral organs
  • gap junctions electrically link neighbouring cells (functional syncytium)
  • contractile activity is myogenic and may be phasic (pacemaker potentials) or tonic (slow-wave potentials)
  • modified by autonomic nervous system
25
Q

What occurs to smooth muscle during an asthma attack?

A

smooth muscle surrounding the bronchioles contracts, decreasing bronchiole diameter and restricting air flow

26
Q

What is the treatment for asthma?

A
  • smooth muscle cells surrounding the bronchioles express beta-2 adrenergic receptors
  • epinephrine is used as treatment for asthma
  • but salbutamol is an agonist for beta-2 adrenergic receptors – avoids many of the side effects of epinephrine
27
Q

How do skeletal, cardiac, and smooth muscle differ in their location?

A
  • skeletal: attached to skeleton
  • cardiac: heart
  • smooth: blood vessels and eyes (multi-unit), walls of visceral organs (single-unit)
28
Q

How do skeletal, cardiac, and smooth muscle differ in their mechanism of contraction?

A
  • skeletal: sliding filament
  • cardiac: sliding filament
  • smooth: sliding filament
29
Q

How do skeletal, cardiac, and smooth muscle differ in their initiation of contraction?

A
  • skeletal: neurogenic
  • cardiac: myogenic
  • smooth: neurogenic/myogenic
30
Q

Do skeletal, cardiac, and smooth muscle have sarcomeres?

A
  • skeletal: yes
  • cardiac: yes
  • smooth: no
31
Q

Do skeletal, cardiac, and smooth muscle have troponin and tropomyosin?

A
  • skeletal: yes
  • cardiac: yes
  • smooth: tropomyosin only
32
Q

Do skeletal, cardiac, and smooth muscle have t-tubules?

A
  • skeletal: yes
  • cardiac: yes
  • smooth: no
33
Q

How do skeletal, cardiac, and smooth muscle differ in their site of Ca2+ regulation?

A
  • skeletal: troponin on thin filament
  • cardiac: troponin on thin filament
  • smooth: myosin thick filament
34
Q

Do skeletal, cardiac, and smooth muscle have gap junctions?

A
  • skeletal: no
  • cardiac: yes
  • smooth: depends

+ BIOL 370 (15 decks)

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