ICL 15.0: Physiology of Labor

Question 1

what is the correct order of how the fetus moves through the pelvis?

Answer 1

1. descent
2. flexion of head
3. internal rotation
4. extension
5. external rotation
6. expulsion

so the fetus is looking to the side once it’s descended through the pelvic inlet and then it rotates its head to the center and extend it to make it through the pelvic inlet – then it externally rotates its head to the side once the baby is outside the pelvis and then the baby is expelled

Question 2

what is engagement?

Answer 2

the descent of the fetal head below the pelvic inlet during delivery

clinically, this needs to be assessed during a cervical exam to make sure the fetal head is below the ischial spin of the pelvis – you can feel the ischial spine during a cervical exam!

the ischial spines are point zero!!

Question 3

what is the composition of the cervix?

Answer 3

1. smooth muscle
2. collagen (majority of cervix)
3. glycosaminoglycans –> dermatan (hydrophobic)

Question 4

how does the cervix change in order for the cervix to dilate and have vaginal delivery?

Answer 4

the collagen making up the cervix has to break down!!

collagen is a triple helix of collagen chains with crosslinks so it’s super strong and wouldn’t allow for dilation oft cervix

during delivery, the collagen is broken down and the hydrophobic glycosaminoglycans become hydrophilic and this is how the cervix dilates 10 cm to allow for vaginal delivery!

Question 5

what happens to the myometrium during labor for vaginal delivery?

Answer 5

contraction!

smooth muscle cells are arranged in spinal bundles ( thick and thin filaments with gap junctions between them)

when muscles contract you need calcium***, ATP, and actin-myosin cross linking

increased intracellular calcium is the bottom line when it comes to muscle contractions!!

Question 6

how do contractions work?

Answer 6

calcium enters the smooth muscle via L-type voltage batted Ca+2 channels

it binds to calmodulin and that complex goes and activates MLCK and MLCK goes and phosphorylates myosin so that it can bind to actin and the crosslink happens and you get a contraction!

MLCP then dephosphorylates myosin which allows for smooth muscle relaxation

Question 7

which agents inhibit muscle contraction?

Answer 7

1. NO which deactivates myosin light chain phosphatase (MLCP)
2. cAMP promotes uterine relaxation

Question 8

how do the muscle fibers of the myometrium change during pregnancy?

Answer 8

early in pregnancy there is no connection between smooth muscle cells and they’re arranged in bundles

gap junctions dont appear till the end of pregnancy! the gap junctions are needed for coordinated uterine contractions!

when Ca+2 enters the cell, part of what it does it activate gap junctions for contractions

Question 9

which nerve innervates the myometrium to contract?

Answer 9

there isn’t one!!

once Ca+2 enters the cells, smooth muscle cells connect with each other and through the gap junctions they coordinate to do contractions!

there’s no nerve that is responsible for myometrial contractions during delivery! this is why contractions dont happen till the end of pregnancy because that’s when gap junctions form

Question 10

what starts labor?

Answer 10

a removal of mechanisms that maintain the uterus in a relaxes stated – hormones from the fetal membranes/placenta coordinate change form irregular to regular contractions

so you have to:
1. recruit calcium to get labor to start!

2. increase prostaglandin synthesis
3. increase myometrial gap junction formation
4. up regulation of myometrial oxytocin receptors

Question 11

what factors/hormones initiate labor?

Answer 11

it’s hormones from the placenta!!!!

1. corticotrophin releasing hormone (CRH)
2. progesterone decreases
3. estrogen increases
4. oxytocin receptors increase

Question 12

what organs and hormones are involved in initiation of labor?

Answer 12

fetal brain, fetal adrenal gland, placenta, uterus

CRH from placenta feeds back to fetal adrenal gland and cause cortisol production which is involved in positive feedback loop that stimulates more CRH and hence, more cortisol

the CRH is also needed to go to the fetal brain to stimulate ACTH release which goes to fetal adrenal gland and also causes cortisol production and hence more CRH which THEN goes and stimulates the brain to produce more ACTH

ACTH also causes the fetal adrenal gland to make estrogen which then acts directly on uterus to promote myometrial contractions

CRH being produced by placenta also acts on membranes surrounding fetus to make prostaglandins which also stimulate uterus to contract

Question 13

how does progesterone inhibit labor?

Answer 13

progesterone keeps the uterus in the relaxed state by:

1. limiting production of prostaglandins
2. inhibiting genes for oxytocin receptors and gap junctions
3. inhibits production and activity of inflammatory mediations that can cause labor

there is a functional loss of progesterone in the uterus during labor that allows the uterus to contract! so if we’re trying to inhibit labor we give progesterone inhections

Question 14

what is oxytocin?

Answer 14

peptide that is synthesize in hypothalamus and released from posterior pituitary

short 1/2 life of only 3 minutes

Question 15

how is oxytocin degraded?

Answer 15

placental oxytocinase

Question 16

how does oxytocin effect the uterus?

Answer 16

1. oxytocin binds to myometrial receptors and causes uterus to contract

there’s more oxytocin receptors when labor gets closer/they’re upregulated

2. stimulates the amnion and decidua to produce prostaglandins which lead to uterine contractions also
3. promotes gap junction formation **