Classification of smooth muscles
• multi unit smooth muscle (A)
– Cells work independently of
one another
– Each cell receives it own
innervation
• unitary smooth muscle (B)
– gap-junctions between cells
– innervation in forms of
varicosities
– In the wall of hollow organs
Classification of smooth muscles
• multi unit smooth muscle (A)
– Cells work independently of
one another
– Each cell receives it own
innervation
• unitary smooth muscle (B)
– gap-junctions between cells
– innervation in forms of
varicosities
– In the wall of hollow organs
The smooth muscle AP I.
• Major characteristics
– resting membrane potential (if
exists) -50 and -60 mV
– amplitude 60 and 70 mV
– The time between AP and the
start of contraction (latency) >
100 ms
The smooth muscle AP II.
The contractile proteins
• no sarcomere
– Instead of the Z line
“dense bodies”
• thin filament
– actin
– tropomyosin
• thick filament
– myosin
The acto-myosin cycle
•
•
•
•
[Ca2+]i increases
Ca2+ - calmodulin (CaM)
CaM kinase (CaMK) gets activated
myosin light chain (MLC) gets
phosphorylated
• actin-myosin interaction →
contraction
• MLC gets dephosphorylated
(phosphatases) → relaxation
Regulation of contraction I.
the role of the thin filament and calponin
• In the dephosphorylated state calponin binds to the thin filament
– the number of possible cross-bridges decreases → tension ↓
• In the phosphorylated state calponin detaches from the thin filament
– the number of possible cross-bridges increases → tension ↑
Regulation of contraction II.
the role of the phosphatases
change in the
phosphorylation state of
the regulatory proteins
tension – [Ca2+]i
function shifts
Regulation of contraction III.
the „latch-bridge”
• “latch-bridge” = sustained contraction with decreased energy consumption
– Dephosphorylation of MLC takes places in the AM state → myosin ATPase
activity ↓ → AM is maintained → maintained tension
• Plasticity → smooth muscles have no resting length
Regulation of smooth muscle function
humoral and hormonal effects
Tissue factors
• hypoxia
• hipercapnia
• tissue metabolites (e.g. lactic
acid, adenosine)
• NO
Other humoral effects
• acetylcholine (mAchR)
• histamine (H1 and H2 receptors)
• serotonin
• kinins and other vasoactive
polypeptides
•
•
•
•
noradrenalin (α-adrenerg)
adrenalin (β-adrenerg)
ADH (vasopressin)
oxytocin (uterus, ducts of the
mammary gland)
• estrogens
• progesterone
• permissive effect of
glukocorticoids, thyroid
hormones
Comparison of different muscle types
Striated m.
Smooth m.
Heart m.
thin and thick
filaments
yes
yes
yes
sarcomeres
yes
no
yes
speed of
contractions
fast & slow
very slow
slow
SR
ER & extracellular
SR & extracellular
place of activating
Ca2+
troponin
myosin & ?
troponin
stretch increases
muscle activity
no
yes
yes
excitation
excitation or
inhibition
excitation or
inhibition
some
large
large
activating Ca2+
effect of nerve
stimulation
effects of hormones