smooth muscle: properties each fibre is much smaller than in skeletal muscle found in walls of...

WINDSOR UNIVERSITYSCHOOL OF MEDICINE

Smooth MuscleDr.Vishal Surender.MD.

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

Figure 12-25a

Types of Smooth Muscle

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

Types of Smooth Muscle

Figure 12-25b

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?

Figure 12-26

Caveolae in Smooth Muscle

Figure 12-27a–b

Anatomy of Smooth 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


Ca2+Ca2+

Sarcoplasmicreticulum Intracellular Ca2+


Ca2+

11

Figure 12-28, steps 1–2


Ca2+Ca2+

Ca2+


CaM Pi

Pi

CaM

Intracellular Ca2+



Ca2+

1

2

1

2



Ca2+Ca2+

Ca2+


CaM Pi

Pi

ActiveMLCK

CaM

Intracellular Ca2+




Ca2+

InactiveMLCK

1

2

3

1

2

3



Ca2+Ca2+

Ca2+


CaM Pi

Pi

ActiveMLCK

CaM

ADP +

Active myosinATPase

PP

Intracellular Ca2+





ATP

Ca2+

Inactive myosin

InactiveMLCK

1

2

3

4

1

2

3

4



Ca2+Ca2+

Ca2+


CaM Pi

Pi

ActiveMLCK

CaM

ADP +

Active myosinATPase

Actin

PP

Intracellular Ca2+





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

Calcium Plays a Critical Role in Smooth Muscle Activation

Smooth Muscle Contraction

Ca2+Activating

MLCK

myosinmyosin-Pi

+ATP/actin

CONTRACTION

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


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


ECFCa2+

Ca2+Ca2+

Na+

Na+



1

1

ATP

ATP



ECFCa2+

Ca2+Ca2+

Ca2+

Na+

Na+

CaM

CaM




1

2

1

2

ATP

ATP



ECFCa2+

Ca2+Ca2+

Ca2+

Na+

Na+

CaM

CaM


ADP +

Myosinphosphatase

PP

ATP





1

2

3

1

2

3

ATP

ATP



ECFCa2+

Ca2+Ca2+

Ca2+

Na+

Na+

CaM

CaM


ADP +

Myosinphosphatase

PP

ATP

Decreasedmuscletension





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

Muscles: Summary

smooth muscle: properties each fibre is much smaller than in skeletal muscle found in walls of...

Documents

smooth muscle cells

multiunit smooth muscle

types of smooth muscle

ciliary muscle

types of smooth muscle

multi unit smooth muscle

myosin filaments

b slide