Smooth Muscle

Question 1

What are the two types of smooth muscle?

Answer 1

Multi-unit smooth muscle and Unitary Smooth muscle

Question 2

Describe Multi-unit smooth muscle

Answer 2

Fibers operate individually

Each fiber is innervated by a single nerve

Not electrically connected (no gap junctions)

Provide ability for fine motor movements

Examples: ciliary muscles of the eye, iris, piloerector muscles (raise hair follicles)

Question 3

Describe unitary smooth muscle

Answer 3

Muscle cells work as a unit

Cell membranes adhere and contain gap junctions

Examples: visceral smooth muscle; GI tract, bile ducts, urterus

Question 4

What makes a smooth muscle cell different from cardiac and skeletal muscle?

Answer 4

No true sarcomere structures

Actin attaches to Dense Bodies/Adherens Junctions

Myosin heads have bi-directional arrangement

No striations

Question 5

What is special about contraction of smooth muscle compared to the other muscle fibers?

Answer 5

Cycling of myosin cross-bridges is slower

The time that myosin and actin are bound is greater (which allows for greater force production)

ATP demand is lower

When excitation slows, contraction remains - Latch Mechanism

Question 6

Describe the contraction mechanism of smooth muscle cells

Answer 6

Ca2+ enters the cytosol and binds to Calmodulin (CaM)

Ca - CaM complex activates Myosin Light Chain Kinase (MLCK)

MLCK phosphorylates the Myosin heads, allowing them to interact with actin which causes a contraction

Question 7

Describe the relaxation mechanism of smooth muscle cells

Answer 7

Relaxation occurs as a result of removing Ca2+ from the cytosol via pumps

Relaxation also occurs as a result of Myosin Light Chain Phosphatase, which dephosphorylates the myosin heads, preventing them from binding with actin and can no longer cause a contraction.

Question 8

What signals induce produce actions in smooth muscle?

Answer 8

Nerves (neurotransmitters)
Hormones
Stretch
Environmental cues

Question 9

What are the effects of Norepinephrine/Epinephrine on smooth muscle?

Answer 9

Excitatory or inhibitory depending on the target organ

Question 10

What is the effect of Acetylcholine on smooth muscle?

Answer 10

Can be excitatory or inhibitory depending on the target organ

Contraction is typically the direct effect

Relaxation is typically the indirect effect

Question 11

Angiotensin II causes smooth muscle to do what?

Answer 11

Constriction

Question 12

Vasopressin causes smooth muscle to do what?

Answer 12

Constriction

Question 13

Endothelin causes smooth muscle to do what?

Answer 13

Constriction

Question 14

What is Adenosine and what does it cause smooth muscle to do?

Answer 14

It is a metabolite and it causes relaxation

Question 15

Nitric Oxide does what to smooth muscle?

Answer 15

Inhibitory
Causes relaxation
NO is a potent vasodilator

Question 16

Hypoxia causes smooth muscle to do what?

Answer 16

Relaxation

Question 17

Excess CO2 causes smooth muscle to do what?

Answer 17

Relaxation

Question 18

Increased H+ causes smooth muscle to do what?

Answer 18

Relaxation

Question 19

What is the effect of increased K+ on smooth muscle?

Answer 19

Relaxation

Question 20

What does Antidiuretic Hormone (ADH) do to smooth muscle?

Answer 20

Constriction

Question 21

What does Atrial Natriuretic Hormone do to smooth muscle?

Answer 21

Relaxation

Question 22

What does stretching induce smooth muscle to do?

Answer 22

Constriction

Question 23

How do GPCRs influence Ca2+ concentration in the smooth muscle cell?

Answer 23

GPCRs will increase production of IP3, which activate IP3 Receptor channels in the Sarcoplasmic reticulum.

IP3 causes the channels to open, allowing Ca2+ to enter the cytoplasm.