Smooth Muscle

Question 1

Where is smooth muscle located in the body?

Answer 1

• In the blood vessels

- In the walls of hollow organs

Question 2

Is smooth muscle striated or non-striated?

Answer 2

Non-striated

Question 3

Is smooth muscle uni- or multinucleate?

Answer 3

Uninucleate

Question 4

Is smooth muscle branched or unbranched?

Answer 4

Unbranched

Question 5

What is the shape of smooth muscle cells?

Answer 5

Long and tapered at each end

Question 6

Why is smooth muscle non-striated?

Answer 6

It does not contain sarcomeres as it’s functional unit of contraction.

Question 7

What does the smooth muscle contain instead of sarcomeres?

What orientation are the actin filaments arranged in because of this?

Answer 7

They have dense bodies, which are present throughout the cell and anchor the actin filaments.

The actin filaments are therefore arranged obliquely.

Question 8

So smooth muscle cells contain gap junctions?

Answer 8

Yes.

Question 9

What different things can regulate the contraction of smooth muscle?

Answer 9

• The ANS
• Central hormone release
• Local hormone/autocoid release
• Pacemaker cells/Intrinsic and rhythmic oscillations in membrane potential

Question 10

Which layer of elastic tissue separates the tunica intima from the tunica media in vessels?

Which layer of elastic tissue separates the tunica intima from the tunica media in vessels?

Answer 10

The internal elastic lamina.

The external elastic lamina.

Question 11

In terms of smooth muscle contraction via the Gq/pip2 pathway, what does the breakdown product ip3 do within the smooth muscle cell?

Answer 11

It causes release of calcium from sarcoplasmic stores which causes the smooth muscle cell to contract.

Question 12

In terms of smooth muscle contraction via the Gq/pip2 pathway, what does the breakdown product DAG do within the smooth muscle cell?

Answer 12

DAG opens non-selective receptor gated channels (RGC) which allow the passage of Na and Ca into the cell, which depolarise the cell and open voltage gates calcium channels (VGCC) allowing even more calcium into the cell.

Question 13

What happens when the smooth muscle cell stretches due to the influx of all the calcium and sodium ions?

Answer 13

It stimulated stretch receptors which open stretch activated Na channels (SAC) to allow in even more sodium, adding to the depolarisation of the cell and therefore is contraction.

Question 14

Name X2 ways a smooth muscle cell can relax?

Answer 14

1) Passive relaxation if the stimulus to contract is removed

2) active relaxation via hormones/autocoids such as nitric oxide

Question 15

In active smooth muscle cell relaxation due to N.O., in what ways does the cell relax and by what pathway is this achieved?

Answer 15

This is via N.O. Diffusing into the cell and increasing the concentration of cGMP from GTP. The increase in cGMP causes K channels to open and K to leave, hyperpolarising the cell. This in turn causes the VGCC which are open in contraction to close which stops calcium entering the cell and therefore stops contraction. It also causes the activation of SERCA and a plasma membrane calcium ATPase pump to remove Ca from the cell. Lastly it causes calcium desensitisation occurs.

Question 16

What happens to the smooth muscle cell in relation to its sensitivity to calcium during contraction?

Answer 16

It becomes sensitised to calcium due to Rho Kinase.

Question 17

How does active smooth muscle cell relaxation occur in response to adrenaline?

What receptors are triggered in this process?

Answer 17

The same process as the one which occurs for N.O. regulated vasodilation, but using the Gs/cAMP second messenger pathway instead.

Adrenaline will trigger beta-2 adrenergic receptors in this process.

Question 18

How does cross-bridge regulation differ in smooth muscle to the other two types of striated muscle?

Answer 18

It does not involve calcium binding to troponin C of the troponin-tropomyosin complex.

Question 19

What does calcium bind to instead of troponin C in smooth muscle cells to initiate contraction?

Answer 19

Calmodulin

Question 20

What is the structure of calmodulin in terms of calcium binding?

Answer 20

Calmodulin has X4 calcium binding sites, all 4 of which must be occupied for the calmodulin to become active.

Question 21

Which area of the myosin is affected in smooth muscle cell contraction?

Answer 21

Regulatory light chains.

Question 22

What does activated calmodulin do in the next stages of smooth muscle cell contraction?

Answer 22

It binds to myosin light chain kinase, forming an activated complex which phosphorylates regulatory light chains, freeing the myosin to form a cross-bridge.

Question 23

Which enzyme dephosphorylates myosin regulatory light chains when it comes to smooth muscle relaxation?

Answer 23

Myosin phosphotase.

Question 24

How does calcium sensitivity work via rho kinase?

Answer 24

Smooth muscle contractors (agonists) activate rho kinase, which inhibits myosin phosphotase. This means calcium sensitivity increases as there is no backwards reaction occurring.

Question 25

How does calcium desensitivity work?

Which relaxation agonists cause desensitivity to calcium and which do not?

Answer 25

The opposite of sensitisation, as more myosin phosphotase is activated meaning the backwards reaction is increased.

• cGMP does (therefore N.O.)
• Adrenaline does NOT

Question 26

What are latch bridges?

When do they occur?

Answer 26

Latch bridges occur when myosin regulatory light chains are dephosphorylated whilst myosin is still attached to actin. This means the number of phosphorylated myosin molecules decreases yet force still remains.

Question 27

Which cells are responsible for the peristaltic waves of the GIT?

Answer 27

The interstitial cells of Cajal.

Question 28

Which ion is responsible for depolarisation in smooth muscle cells?

Which channel does this ion enter the cell through?

Answer 28

Extracellular calcium via VGCC’s

Question 29

Do smooth muscle cells obey the all or none principle?

Answer 29

No.