3.5 - Smooth Muscle

Question 1

What’s a sphincter?

Answer 1

A ring that can open or close

Question 2

TRUE OR FALSE:

Smooth muscles are found attached to bones of skeleton

Answer 2

FALSE

Question 3

Where can smooth muscles be found in the body?

Answer 3

• walls of hollow organs & tubes

Question 4

Give some examples of smooth muscles

Answer 4

• bladder sphincter
• intestine
• walls of blood vessels

Question 5

TRUE OR FALSE

Smooth muscles are part of the peripheral nervous system

Answer 5

FALSE

• part of the autonomic nervous system

Question 6

How are smooth muscle cells arranged?

Answer 6

• single unit

- multi unit

Question 7

Define single unit smooth muscle cells. Where are they found?

Answer 7

• coupled by gap junctions
• not necessary to stimulate individual fibre as cells are coupled
• found on walls of internal organs, e.g. blood vessels

Question 8

Define multi unit smooth muscle cells. Where are they found?

Answer 8

• not coupled by gap junctions
• individual muscle fibre separately innervated
• e.g. found in iris, parts of reproductive organs

Question 9

State the difference between smooth and skeletal muscle:

i) on whole muscle level

Answer 9

smooth muscle:

• contraction changes both muscle shape and muscle length
• develops tension/force slowly
• can maintain contraction longer w/o fatiguing e.g. bladder sphincter

Question 10

State the difference between smooth and skeletal muscle:

i) on cellular level

Answer 10

smooth muscle:

• fibres are much smaller: about same diameter as single myofibril in skeletal muscle fibre
• actin and myosin NOT arranged in sarcomeres: no banding pattern (no striations)
• actin and myosin arranged in long bundles diagonally around periphery of the cell
• actin anchored at cell membrane structures called dense bodies: not attached to Z lines in skeletal
• caveolae (invaginations of sarcolemme for cell signalling) instead of T tubules and SR
• force of contraction related to amount of Ca2+ released

Question 11

What is the effect of not having T-tubules in smooth muscle cells?

Answer 11

Ca2+ comes from outside the cell to cause Ca2+ release from the SR

Question 12

State the difference between smooth and skeletal muscle:

i) on molecular level

Answer 12

in smooth muscle cells:

• myosin and actin filaments are longer and overlap more
• less myosin per actin unit
• myosin ATPase activity much slower
• myosin heads are located along all parts of myosin molecule
• no troponin

Question 13

What is the major difference between contraction of smooth muscle and cardiac muscle?

Answer 13

• the role of phosphorylation in regulating smooth muscle contraction

Question 14

List the steps of smooth muscle contraction:

Answer 14

i) intracellular Ca2+ concentrations increase as Ca2+ enters the cell and is released from the SR and caveolae
ii) Ca2+ binds to calmodulin (CaM)
iii) Ca2+/CaM activates myosin light chain kinase (MLCK)
iv) MLCK phosphorylates light chains in myosin heads and increases myosin ATPase activity by using energy and Pi from ATP
v) activated myosin crossbridges slide along actin and create muscle tension

Question 15

What are 3 ways that calcium enters the cell?

Answer 15

i) voltage gated channels: open when cells depolarize
ii) stretch activated channels: open when membrane stretched
iii) chemically gated channels: open in response to hormones

Question 16

What is the difference between smooth muscle and skeletal muscle contraction regulation?

Answer 16

• smooth muscle: myosin is regulated via phosphorylation

- skeletal muscle: actin is regulated via troponin/tropomysoin interactions

Question 17

List the steps of smooth muscle relaxation:

Answer 17

i) Free Ca2+ in cytosol decreases when Ca2+ is pumped out of the cell OR back into the SR through Ca2+ ATP-ase or Ca2+ Na- antiport
ii) decrease causes Ca2+ to unbind from calmodulin, thus inactivating MLCK
iii) myosin light chains are dephosphorylated by MLCP, thus decreasing myosin ATPase activity
iv) smooth muscle enters latch state (tension is maintained) myosin still bound to actin, but with minimal ATP consumption

Question 18

TRUE OR FALSE:

Dephosphorylation of myosin automatically relaxes the smooth muscle

Answer 18

FALSE