smooth muscle structure and function

Question 1

Provide examples of smooth muscles and their function in different organs

Answer 1

Phasic: rhythmic; seen in sphincters, esophagus, urinary blader

Tonic: continuous activity; respiratory airways and blood vessels

Question 2

Describe single vs mutli unit smooth muscle

Answer 2

Multi-unit: muscle cells behave as individuals. These are usually involved in finer movements, such as lens and ciliary function in the eye, and in arrector pili of the skin

Single unit: muscle cells act as a group; this relies on gap junctions! This allows for coordinated contraction. This is seen in the smooth muscle of the GI tract in peristaltic contractions

Question 3

Contraction is regulated by ___ filaments

Answer 3

thick

Question 4

Describe caveoli

Answer 4

Invaginations of the sarcolemma that are open to the extracellular space; they’re thought to allow for expansion of the cell membrane.

Question 5

What thin filaments are present in smooth muscle?

Answer 5

Actin and tropomyosin, no troponin

Question 6

Describe the organization of thick and thin filaments in smooth muscle

Answer 6

Small groups of thick filaments are surrounded by many thin filaments to form a contractile unit.

Question 7

What are dense bodies? Where are they found? What connects them to each other?

Answer 7

Sites of attachment for thin filaments; they’re found in the sarcolemma and in the cytoplasm; intermediate filaments such as desmin and vimentin

Question 8

What junction type mechanically connects muscle cells

Answer 8

Adherens junctions

Question 9

Describe the role of different pathways and factors in the initiation and regulation of
smooth muscle contraction

Answer 9

Can be controlled by autonomic control, circling hormones, electronic coupling, locally generated signaling molecules, or intrinsic electrical activity

Question 10

Describe the steps leading to cross-bridge cycling in smooth muscle

Answer 10

-Calcium binds to calmodulin

-Calcium-calmodulin activates MLCK

-MLCK phosphorylates myosin

-Myosin can attach to actin

-ADP AND PI dissociate from the myosin head and contraction can happen

-ATP binds the myosin head, releasing actin and relaxing the muscle

Question 11

Describe myosin phosphatase

Answer 11

Myosin phosphatase removes the phosphate from the myosin, releasing actin filaments and stopping contraction

Question 12

Describe pharmacomechanical coupling and its physiological significance

Answer 12

Impacts contraction without changing membrane potential; this is usually receptor specific and may or may not use calcium

Question 13

Give examples of ligands capable of calcium mediated pharamcomechanical coupling

Answer 13

Examples capable of this include norepinephrine, angiotensin, and vasopressin

Question 14

Describe calcium mediated pharmacomechanical coupling

Answer 14

Activation of phospholipase C, forming IP3 and DAG. IP3 increases calcium cytosolic concentration, which can increase contraction. Store-activate channels will be activated to bring extracellular calcium into the sarcoplasmic reticulum, and SERCA will remove calcium from the cytosol to maintain the decrease and increase of calcium needed for contraction.

Question 15

Describe how cAMP regulates pharmacomechanical coupling

Answer 15

-Lowering MLCLK activity: cAMP is decreased, lowering MLCK phosphorylation and thus its activity. This leads to muscle relaxation. Ligand examples include norepinephrine, adenosine, and b2 agonists

Question 16

Describe how NO regulates pharmacomechanical coupling

Answer 16

Raising MP activity: release of NO activates guanylate cyclase which increases cGMP. The increase of cGMP increases MP activity, which leads to relaxation of the muscle. Viagra blocks this.

Question 17

less myosin phosphatase indicates ___ contraction

Answer 17

more

Question 18

NO leads to ___ of muscle

Answer 18

relaxation

Question 19

cAMP leads to ___ of muscle

Answer 19

contraction