Smooth Muscle

Question 1

What are the main differences between skeletal muscle and smooth muscle

Answer 1

skeletal

• voluntary
• longer
• multinuclei
• limited ability to divide once differentiated
• banding
• similar max tension

smooth

• involuntary
• smaller
• one nucleus
• can divide throughout life of individual
• no banding or regular alignment
• diagonal filaments

Question 2

What filaments do smooth muscle fibres contain

Answer 2

actin (thin) -contains tropomyosin but no tropnin, also associates with caldesmon
-more than in skeletal

myosin (thick) -less than in skeletal

Question 3

What regulates cross-bridge cycling?

Answer 3

A Ca2+ regulated enzyme that phosphorylates myosin

Question 4

What occurs after an increase in cytosolic ca2+?

Answer 4

1. Calcium ion binds to calmodulin
2. The Ca2+ - calmodulin complex binds to myosin light-chain kinase. this activates the enzyme
3. Active myosin light-chain kinase then uses ATP to phosphorylate myosin light chains in the globular head of myosin
4. Phosphorylation of myosin drives the cross-bridge way from the thick filament backbone, allowing it to bind to actin
5. Cross-bridges go through repeated cycles of force generation as long as myosin light chains are phosphorylated
6. To relax a contracted smooth muscle myosin must be dephosphorylated because then it will be unable to bind to myosin
7. dephosphorylation of myosin is regulated by the enzyme myosin light-chain phophatase, this is continously active during rest and contraction

smooth muscle myosin has a low rate of ATPase activity, this rate determines the rate of cross-bridge cycling and shortening velocity, smooth muscle shortening is much slower than skeletal muscle Due to this slow use of energy usage smooth muscle does not undergo fatigue during prolonged activity like skeletal muscle

Question 5

What is the structure of cardiac muscle?

Answer 5

Cardiac muscle combines the properties of skeletal and smooth muscle

• it has a striated apperance
• thick myosin
• thin actin (troponin and tropomyosin)
• T-tubule system
• relatively small
• usually just one nucleus
• cells are joined at the end by intercalated disks
• desmosomes hold the cells together, myofibrils are attached to desmosomes
• gap junctions