Lecture 33

Question 1

Smooth muscle strucutre:

Answer 1

No t-tubules
-Dense bodies act like z-lines to anchor actin to sarcolemma

-In unitary smooth muscle cells gap junctions electrically connect the cells together

• Intermediate filament is cytoskeleton element
• Poorly developed SR

Question 2

Single unit smooth muscle:

Answer 2

Sheets of cells that are electriclaly coupled and act as one unit. Found in most blood vessels and hollow organs.

Question 3

Multiunit:

Answer 3

Tissue made of discrete bundles of independent cells which are densely innervated and contract only in response to its innervation (eg vas deferens, iris protectors)

Question 4

Contractile proteins:

Answer 4

No striations, contain actin and myosin filaments

-Less organized, allows for greater shortening

Question 5

How does contraction initiate?

Answer 5

• Trigger is an increase in intracellular calcium. The Ca2+ entering through channels in the cell membrane is a very important source of calcium.
• Can be neural, hormonal or spontaneous.

Question 6

Source of calcium in smooth muscle:

Answer 6

Extracellular and SR

Question 7

Calcium regulation in smooth muscle: VDCC:

Answer 7

Voltage dependent Ca2+ channels

Question 8

Calcium regulation in smooth muscle: SOC:

Answer 8

Store operated Ca2+ channel

Question 9

Calcium regulation in smooth muscle: ROC:

Answer 9

Receptor operated Ca2+ channel (often regulated by DAG second messenger pathway)

Question 10

Calcium regulation in smooth muscle: Hormones or neurotransmitter acting on G-protein receptors to incre

Answer 10

hormones such as angiotensin II or neurotransmitter (eg noradrenaline) acting on G-protein receptors to increase IP3 levels, which act on IP3 ,ediated calcium release channels in SR

Question 11

Ca2+ ATPase in surface membrane, Ca@+/Mg2+ exchangers, SERCA pumps, Na+/Ca2+ exchangers all important in ___

Answer 11

Ca2+ extrusion

Question 12

Initiation of contraction step 1 (smooth muscle) : Calcium ions enter the cytosol from the ECF via __

Answer 12

voltage-dependent or voltage-independent Ca2+ channels, or from the scant SR.

Question 13

Initiation of contraction (smooth muscle) step 2: Ca2+ binds to activates:

Answer 13

Calmodulin, in smooth muscle the regulatory protein is calmodulin.

Question 14

Initiation of contraction (smooth muscle) step 3:

Answer 14

3. The activated calmodulin then activates myosin light chain kinase MLCK, an enzyme

Question 15

Activation of MLCK:

Answer 15

Regulation is myosin, not actin, based.
Myosin does not hydrolyse ATP unless it is first phosphorylated.
-Phosphorylates the light chain, in the presence of the activated calmodulin

Question 16

Step 4: MLCK activates the myosin by ___, which

Answer 16

phosphorylating it, which in turn activates the myosin ATPases.

Question 17

Step 5:

Answer 17

Activated myosin forms cross bridges with actin of the thin filaments and shortening begins in the usual fashion

Question 18

Initiation of contraction (smooth muscle) summary:

Answer 18

1. Increased intracellular Ca++
2. Calmodulin (CaM) activation
3. Activation of calmodulin dependent protein kinase (myosin light chain kinase :MLCK)
4. Phosphorylisation of myosin
5. Action/myosin interaction resulting in contraction
   - MLCK is active only in the presence of small Ca-binding protein, calmodulin (And only when it has Ca bound)

Question 19

Smooth muscle relaxation:

Answer 19

Contraction ends when a myosin light chain phosphotase dephosphorylates the myosin light chain.

• Ca-ATPase is the primary mechaism for reducing intracellular Ca++
• Cmooth muscle contraction is enzyme regulated - slow but efficient

Question 20

Innervation of smooth muscle: ____ in the terminal axons contain neurotransmitter

Answer 20

Varicosities

Question 21

Innervation of smooth muscle: Neurotransmitter is secreted into

Answer 21

the matrix coating and diffuses into the muscle cells

Question 22

Innervation of smooth muscle: Autonomic nerve fibers:

Answer 22

branch and form diffuse junctions with underlying smooth muscle fibers.