P: Smooth muscle

Question 1

Describe the structure of smooth muscle

Answer 1

No sarcomeres
No troponin
Lots of myosin, caldesmon, calponin (tonically inhibit actin-myosin interaction)
No t-tubules, instead caveolae
Thick filament regulated

Question 2

Function of dense bodies

Answer 2

Link smooth muscle together

Question 3

Types of smooth muscle

Answer 3

Single unit (visceral)
Multi-unit

Question 4

Features of smooth muscle

Answer 4

Single unit has many gap junctions - pacemaker activity
NMJ of smooth muscle not as structured as skeletal muscle
Multiunit has few gap junctions – nerve stimulation required
Both types regulated by ANS not somatic N

Question 5

What kind of calcium channels does smooth muscle have

Answer 5

Voltage gated
Ligand gated
Stretch activated

Question 6

Smooth muscle contraction/ relaxation

Answer 6

Excitation of the muscle via AP, hormone, mechanical stimulus etc
Increase in cytosolic Ca2+ (ECF and SR)
Calmodulin activates MLCK (myosin light chain kinase) –> cross bridge formation
Increase in activity of Myosin ATPase with cross-bridge formed
Contraction of muscle
MLCK dephosphorylated by MP (myosin phosphatase)
Relaxation of Muscle
Ca2+ homeostasis restored

Question 7

How is force regulated with tonic smooth muscle

Answer 7

• Slows rate of cross-bridge cycling
• Increased sensitivity to calcium by decreasing activity of MP (myosin phosphatase) and increasing activity of MLCK
• Sustained tonic activity (latch state) allows for less ATP consumed.

Question 8

Single unit smooth muscle

Answer 8

• Visceral organs
• Pacemaker activity + cells electronically coupled via gap junctions

Question 9

Multiunit smooth muscle

Answer 9

• Not electrically coupled –> controlled by nerve signals
• Individual muscle fibres may be stimulated by individual nerve fibres
• Important for fine control of these muscles

Question 10

Phasic contraction vs tonic contraction:

Answer 10

Phasic - intermittent contraction
Tonic - continuously contracting

Question 11

Regulation of SR Calcium in smooth muscle

Answer 11

• Neurotransmitter/ chemical binds to G protein
• Phospholipase C (PLC) converts PIP2 into Inositol tri-phosphate (IP3)
• Stimulates calcium release from SR

Question 12

Why is the lack of sarcomeres important for the mechanism of contraction in smooth muscle

Answer 12

Muscle must often contract when already contracted

Question 13

What links smooth muscle cells

Answer 13

Dense bodies

Question 14

What is poorly developed in smooth muscle

Answer 14

SR, therefore extracellular Ca++ is very important together with SR Ca+

Question 15

What is calmodulin

Answer 15

Ca++ binding protein that activates MLCK

Question 16

What is MLCK

Answer 16

Myosin light chain kinase causes phosphorylation of the regulatory chains near the myosin head groups which activates them so that they can bind to the actin molecules.