smooth muscle: properties each fibre is much smaller than in skeletal muscle found in walls of...
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Smooth Muscle: Properties• Each fibre is much smaller than in skeletal muscle• Found in walls of hollow organs and tubes.• Usually found in two different layers:• Circular – to squeeze or dilatee.g-blood vessels.• Longitudinal – to stretch or shortene.g-GI tract.Metabolic economy-Uses less energy, Low oxygen
consumption thus allows to Maintain force for long periods, e.x-urinary and esophageal sphincters.
• Smooth muscle is different:-– It has more variety– Anatomy is different.– It is controlled by hormones, paracrines, and
neurotransmitters.• Unitary and Multiunit Smooth Muscle• There are two distinct categories of smooth
muscle determined by the processes they use to coordinate contraction with their neighboring cells
Single unit/Visceral smooth muscle
• This represents the majority• Made up of groups of cells joined together by
gap junctions– FUNCTIONAL SYNCYTIUM• may be innervated but does not always require
nervous stimulation for contraction• Allows for coordinated contractions– Uterus, GIT
Multi unit smooth muscle
• Found in large blood vessels, large airways and ciliary muscle
• Made up of discrete units– Similar to skeletal muscle
• Must be separately stimulated by nerves• Autonomic stimulation
Smooth Muscle
• Has actin and myosin filaments but Lacks the Regular Sarcomere Structure of Skeletal Muscle.
• Myosin light chain has regulatory role• Have intermediate filaments- dense bodies,
analogous to Z-line• Has less sarcoplasmic reticulum– IP3-receptor channel is the primary calcium channel.• Calcium storage function of sarcoplasmic reticulum is
supplemented by Caveolae analogous to ….. In skeletal muscle?
Molecular Mechanism of Smooth Muscle Contraction. In order for smooth muscle to contract there must be some connection between the myofilaments and the cell (to have the same role as the Z line in skeletal muscle).•This connection is provided by the dense bodies found within the smooth muscle cells.
The thick filament is made of myosin as in skeletal muscle (though of a different form) and so has two heavy chains, including the crossbridge region with the 4 light chains found on the heads. These light chains have an important role to play in smooth muscle contraction because it is phosphorylation of the regulatory light chains found on the myosin heads that initiate contraction.
•When the myosin is phosphorylated it binds to the thin filaments and pulls them towards the center of the thick filament moving the two dense bodies connected to the thin filaments closer together, shortening the smooth muscle cell.
Figure 12-28
Smooth Muscle ContractionECF
Ca2+Ca2+
Ca2+
Sarcoplasmicreticulum
CaM Pi
Pi
ActiveMLCK
CaM
ADP +
Active myosinATPase
Actin
PP
Intracellular Ca2+
concentrations increase when Ca2+ enters cell and is released from sarcoplasmic reticulum.
Ca2+ binds to calmodulin (CaM).
Ca2+–calmodulin activates myosin lightchain kinase (MLCK).
MLCK phosphorylates light chains in myosinheads and increases myosin ATPase activity.
Active myosin crossbridges slide along actin and create muscle tension.
ATP
Increasedmuscletension
Ca2+
Inactive myosin
InactiveMLCK
1
2
3
4
5
1
2
3
4
5
Figure 12-28, step 1
Smooth Muscle ContractionECF
Ca2+Ca2+
Sarcoplasmicreticulum Intracellular Ca2+
concentrations increase when Ca2+ enters cell and is released from sarcoplasmic reticulum.
Ca2+
11
Figure 12-28, steps 1–2
Smooth Muscle ContractionECF
Ca2+Ca2+
Ca2+
Sarcoplasmicreticulum
CaM Pi
Pi
CaM
Intracellular Ca2+
concentrations increase when Ca2+ enters cell and is released from sarcoplasmic reticulum.
Ca2+ binds to calmodulin (CaM).
Ca2+
1
2
1
2
Figure 12-28, steps 1–3
Smooth Muscle ContractionECF
Ca2+Ca2+
Ca2+
Sarcoplasmicreticulum
CaM Pi
Pi
ActiveMLCK
CaM
Intracellular Ca2+
concentrations increase when Ca2+ enters cell and is released from sarcoplasmic reticulum.
Ca2+ binds to calmodulin (CaM).
Ca2+–calmodulin activates myosin lightchain kinase (MLCK).
Ca2+
InactiveMLCK
1
2
3
1
2
3
Figure 12-28, steps 1–4
Smooth Muscle ContractionECF
Ca2+Ca2+
Ca2+
Sarcoplasmicreticulum
CaM Pi
Pi
ActiveMLCK
CaM
ADP +
Active myosinATPase
PP
Intracellular Ca2+
concentrations increase when Ca2+ enters cell and is released from sarcoplasmic reticulum.
Ca2+ binds to calmodulin (CaM).
Ca2+–calmodulin activates myosin lightchain kinase (MLCK).
MLCK phosphorylates light chains in myosinheads and increases myosin ATPase activity.
ATP
Ca2+
Inactive myosin
InactiveMLCK
1
2
3
4
1
2
3
4
Figure 12-28, steps 1–5
Smooth Muscle ContractionECF
Ca2+Ca2+
Ca2+
Sarcoplasmicreticulum
CaM Pi
Pi
ActiveMLCK
CaM
ADP +
Active myosinATPase
Actin
PP
Intracellular Ca2+
concentrations increase when Ca2+ enters cell and is released from sarcoplasmic reticulum.
Ca2+ binds to calmodulin (CaM).
Ca2+–calmodulin activates myosin lightchain kinase (MLCK).
MLCK phosphorylates light chains in myosinheads and increases myosin ATPase activity.
Active myosin crossbridges slide along actin and create muscle tension.
ATP
Increasedmuscletension
Ca2+
Inactive myosin
InactiveMLCK
1
2
3
4
5
1
2
3
4
5
Figure 12-29
Relaxation in Smooth MuscleCa2+
ECFCa2+
Ca2+Ca2+
Ca2+
Na+
Na+
CaM
CaM
Inactive myosin Myosin ATPaseactivity decreases.
ADP +
Myosinphosphatase
PP
ATP
Decreasedmuscletension
Sarcoplasmicreticulum
Free Ca2+ in cytosol decreases whenCa2+ is pumped out of the cell or backinto the sarcoplasmic reticulum.
Ca2+ unbinds from calmodulin (CaM).
Myosin phosphatase removes phosphate from myosin, which decreases myosin ATPase activity.
Less myosin ATPase results in decreased muscle tension.
1
2
3
4
1
2
3
4
ATP
ATP
Figure 12-29, step 1
Relaxation in Smooth MuscleCa2+
ECFCa2+
Ca2+Ca2+
Na+
Na+
Sarcoplasmicreticulum
Free Ca2+ in cytosol decreases whenCa2+ is pumped out of the cell or backinto the sarcoplasmic reticulum.
1
1
ATP
ATP
Figure 12-29, steps 1–2
Relaxation in Smooth MuscleCa2+
ECFCa2+
Ca2+Ca2+
Ca2+
Na+
Na+
CaM
CaM
Sarcoplasmicreticulum
Free Ca2+ in cytosol decreases whenCa2+ is pumped out of the cell or backinto the sarcoplasmic reticulum.
Ca2+ unbinds from calmodulin (CaM).
1
2
1
2
ATP
ATP
Figure 12-29, steps 1–3
Relaxation in Smooth MuscleCa2+
ECFCa2+
Ca2+Ca2+
Ca2+
Na+
Na+
CaM
CaM
Inactive myosin Myosin ATPaseactivity decreases.
ADP +
Myosinphosphatase
PP
ATP
Sarcoplasmicreticulum
Free Ca2+ in cytosol decreases whenCa2+ is pumped out of the cell or backinto the sarcoplasmic reticulum.
Ca2+ unbinds from calmodulin (CaM).
Myosin phosphatase removes phosphate from myosin, which decreases myosin ATPase activity.
1
2
3
1
2
3
ATP
ATP
Figure 12-29, steps 1–4
Relaxation in Smooth MuscleCa2+
ECFCa2+
Ca2+Ca2+
Ca2+
Na+
Na+
CaM
CaM
Inactive myosin Myosin ATPaseactivity decreases.
ADP +
Myosinphosphatase
PP
ATP
Decreasedmuscletension
Sarcoplasmicreticulum
Free Ca2+ in cytosol decreases whenCa2+ is pumped out of the cell or backinto the sarcoplasmic reticulum.
Ca2+ unbinds from calmodulin (CaM).
Myosin phosphatase removes phosphate from myosin, which decreases myosin ATPase activity.
Less myosin ATPase results in decreased muscle tension.
1
2
3
4
1
2
3
4
ATP
ATP
Smooth Muscle Relaxation
Ca2+Deactivating
MLCK
myosinmyosin-Pi
RELAXATION
Myosin lightchain phosphatase
predominates
Smooth Muscle Regulation
• Many smooth muscles have dual innervation– Controlled by both sympathetic and
parasympathetic neurons• Hormones and paracrines also control smooth
muscle contraction– Histamine constricts smooth muscle of airways– Nitric oxide affects regulation of diameter of blood
vessels
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