By: Michelle Paxson
Luteinizing hormone
Production of a surge of luteinizing hormone (LH) in pro-estrus is essential to the process of ovulation. In recent years, evidence has accumulated suggesting that OT contributes to the mechanisms
modulating LH surge. Concentrations of OT in the portal blood system are higher at pro-estrus than at other times in the ovarian cycle. Additionally, OT in peripheral plasma is also highest at that
time. An increase of LH release in response to OT was observed both in animals models and in dispersed cells in culture. The effect of OT was enhanced by estrogens and inhibited by
progesterone. Most demonstrative is a recent study conducted on healthy volunteer women, showing that intravenous administration of OT at preovulatory stage of the menstrual cycle markedly
advances LH surge and ovulation as compared to infusion of saline. Such an effect was not observed in women in a state with low estrogen or high progesterone levels, nor in males. The site(s)
and mechanism(s) of action of OT have yet to be definitely established. Evidence available so far suggest both a direct effect of OT on LH secretion and an indirect effect by sensitization of
luteotrophs to GnRH. Source: http://www.gfmer.ch/Endo/Lectures_10/Oxytocin.htm
An example of positive feedback can be found in childbirth. The hormone oxytocin stimulates and enhances labor contractions. As the baby moves
toward the vagina (birth canal), pressure receptors within the cervix (muscular outlet of uterus) send messages to the brain to produce oxytocin.
Oxytocin travels to the uterus through the bloodstream, stimulating the muscles in the uterine wall to contract stronger (increase of ideal normal
value). The contractions intensify and increase until the baby is outside the birth canal. When the stimulus to the pressure receptors ends, oxytocin
production stops and labor contractions cease Source: http://www.besthealth.com/besthealth/bodyguide/reftext/html/endo_sys_fin.html
Pregnancy and Childbirth
PROCESS
• The steps by which oxytocin
stimulates uterine contractions
during childbirth
• The head of the baby
stretches the cervix of
the uterus
• Nerve impulses are
sent to the
hypothalamus
• The hypothalamus
signals the posterior
pituitary to release
oxytocin
• The posterior pituitary
releases oxytocin which
travels in the blood to
the muscles of the
uterus.
•The uterus responds to
oxytocin by
contractions
•Once childbirth occurs
the positive feedback
cycle is broken.
Oxytocin - A Positive Feedback Mechanism
Nerve cells in the hypothalamus
produce the hormone oxytocin
which causes contractions in the
uterus.
WHAT IS THE ROLE OF THE HYPOTHALAMUS?