Physiology of Birth

Question 1

What are the 2 requirements for diagnosing labor?

Answer 1

1. Uterine contractions
2. dilation of the cervix

Question 2

How do contractions affect blood supply to the baby?

Answer 2

Contraction of uterus -> compression of spiral arteries

-> decreased blood flow to baby

*Ischemia to the uterus is what causes pain with contractions

Question 3

Name the 4 phases of labor

Answer 3

1. When contractions begin until the cervix is fully dilated
2. Complete dilation of cervix until baby is delivered
3. Delivery of fetus until placenta is delivered
4. First 6 hrs after delivery

Question 4

In the quiescent phase, what hormone prevents myometrial contraction?

Answer 4

Progesterone

Question 5

Name 3 ways the uterus changes as it approaches labor

Answer 5

1. Growth (hyperplasia and hypertrophy)
2. Increased gap junctions
3. Increased oxytocin receptors

Question 6

Describe how AP’s move throughout the uterus during a contraction (2)

Answer 6

AP begins at the fundus

AP moves down uterus via gap junctions

Question 7

Is each hormone a myometrial inhibitor or stimulant?

• Progesterone
• Oxytocin
• PG
• Calcium
• CRH

Answer 7

• Progesterone - inhibitor
• CRH - inhibitor and stimulant
• Oxytocin, prostaglandins, calcium - stimulant

Question 8

Oxytocin released by ___________ causes ____________ (inc/decr) uterine contractions

Answer 8

Oxytocin released by posterior pituitary causes increased uterine contractions

Question 9

Name 2 sources of PG’s

Name 2 functions of PG’s in labor

Answer 9

• Released from myometrium and placental membranes
• Functions
  
  • Stimulate contractions
  • Weaken membranes

Question 10

What structure releases CRH during pregnancy?

How does CRH affect the uterus throughout pregnancy? (2)

Answer 10

Released by placenta

Early pregnancy - causes quiescence (blocks MLCK)

Late pregnancy - causes contractions (activates protein kinase C)

Question 11

How do hormones switch from suppressing myometrial contraction to stimulating it?

Answer 11

Different receptors at different time points

Question 12

Describe the molecular signals used by progesterone and CRH to prevent myometrial contraction

Answer 12

1. High progesterone -> high NO -> cGMP -> decreased MLCK phosphorylation
2. High CRH -> cAMP -> PKA -> inhibition of MLCK

Question 13

Name 4 receptors/channels that increase in expression near term.

Answer 13

Increased expression of

1. Caclium channels
2. Oxytocin receptors
3. PG receptors
4. Gap junctions

Question 14

Name 3 signals that stimulate myometrium to begin contracting

Answer 14

1. Change in CRH receptor -> inhibition of MLCK
2. Inhibition of the enzyme that degrades PG’s -> increased PG level
3. Release of Oxytocin

Question 15

How do progesterone levels change with labor?

Answer 15

Progesterone stays high during labor

(but the receptors are downregulated)

Question 16

Use his slides for practice Q’s