>What about the lenght of time that a mom is BF?  Why would she
>feel/have MERS in the beginning and maybe not later?  What about when a mom
>loses her supply (but did have MERs) then relactates and has none?  Why
>would this happen?

Jay-- the basics are this: When a baby suckles at breast,
tactile-sensitive (stretch) nerve receptors are stimulated to send
afferent messages that are routed up to and through the hypothalamus. The
hypothalamus is a major control center of several autonomic functions as
well as a regulator of hormones emotions, and directs the release of
oxytocin from the posterior pituitary upon the appropriate stimulation.

According to Lawrence, women in general have more sensitive nipples
during ovulation, menstruation,  pregnancy and lactation; apparently
drops in related hormones may also lead to decreased sensitivity over
time, and may account for that on-again-off-again sensation of MER that
women experience in the later stages of lactation when babies nurse more
erratically. It would make sense to me that when the milk supply is
higher, it is because of more stimulation resulting in more circulating
hormones overall.

Another interesting issue is that of oxytocin receptors.  Lawrence states
that oxytocin receptors in the mammary tissue multiply ten-fold during
pregnancy (40-fold in the uterus hours before labor).  This implies that
the receptors will gradually diminish over time, perhaps in a manner
similar to involution in general. Like milk production issues, I do
wonder if there comes a point of "no return" when too many receptors have
died off and the response to suckling is limited.  I also wonder what, if
any, other factors might affect the development and maintenance of
oxytocin receptors, as I have recently learned that prolactin receptors
can be "down-regulated" by excess male hormones.

Because of the major role of the hypothalamus in the release of oxytocin
and in light of the role the hypothalamus also plays in regulating
emotions, it is easy to see why strong negative emotions can inhibit a
let-down response, and why visual, olfactory and auditory stimuli can
actually trigger it in the absence of any mechanical stimulation.  There
is truth to the roots of some cultural myths which warn not to scare or
upset a mother because she might "lose her milk"--- they probably noticed
the effect it sometimes has on mother's MERs and came to that simplistic
conclusion.

>>What is it in nipple stim that causes the alveoli to contract?

Nervous stimulation triggers oxytocin release, and oxytocin in the blood
then binds to receptor sites in the target organ (the breast), which in
turn trigger the contraction of myoepithelial (smooth muscle, which is
non-volutary) cells of the alveoli. This squeezes the milk and fat out of
the alveoli and into the milk ducts, where it is further pushed forward
to the lactiferous sinuses for baby.

Does this help, Jay, or did I just make things more confusing?

-Lisa Marasco, BA, IBCLC