Jay--
Re mechanisms of letdown--
1. Suckling on the areola, as well as just thinking about infant,
hearing infant cry, seeing infant or picture etc, causes
2. nerve message to mother's posterior pituitary causing
3. release of oxytocin from brain into bloodstream.
4. Oxytocin reaching the breast then works to cause the myoepithelial
cells (microscopic muscle fibers) that surround each little alveolus and
well as the little ductules and ducts, to contract.
5. This combined contraction of all of these little alveoli all at once
then squeezes the milk out, into the lactiferous sinuses, which is the
"milk ejection reflex" MER, or letdown.
6. The lactiferous sinuses distend, stretching sensory fibers. [My
conjecture: If the lactiferous sinuses are already pretty big, then the
milk filling may not distend the sinuses, but just fill them, so the
sensory fibers will not be stretched. The stretched sensory fibers
probably signal the sensation of "letdown." See below for further
discussion of this]
7. The milk is now available for the infant to remove from the sinuses
by suckling. Infant thus can get a larger bolus of milk in mouth per
tongue movement; suckling slows, and visible and audible suckling is
seen ("nutritive suckling").

(Certain brain neurotransmitters esp those assoc with stress, can
interfere with this suckling-oxytocin-MER process.  Thus, the more the
mother worries about her supply, etc., the more this process of
suckling-oxytocin-MER can be interfered with. I say "can" because it
doesn't always, and I'm sure there are many many many other factors that
enter the picture. It can't be simply stress per se.  I've seen too many
moms with all sorts of major stress in their lives (and certainly isn't
just having a baby a biggie for us all!) who have no trouble with let
down, it seems to actually be the folks who distrust themselves and
distrust the process and are particularly worried about milk supply or
whether than can nurse who have the most trouble with this and seem to
create the self fulfilling prophecy, but can reverse it when they start
trusting themselves and the process.)

As for why certain people never feel their "letdowns", or, for those
that do, why they feel them sometimes and not later, and what is the
process by which we feel it:

My hunch is that it has to do with the relative change in the duct
volume-- the stretching of the lactiferous sinuses as they fill-- I
guess this because at least in most of our other organs (e.g. stomach,
bladder, bowel), it is the relative change that signals the sensation.
This is because our sensory nerves are designed to notice a change in
their stretch, but if they are constantly stretched out, as ducts that
are always filled, or ducts that are so large that it takes a lot to
stretch them, then you don't sense anything until there is a larger
volume to stretch it out more.  This is why the formula fed infant whose
stomach has been stretched doesn't feel full til he reaches 8 ounces,
while we know breastfed infants feel full with much smaller volumes.  Or
why, when you've been sick, and your stomach "shrinks" it takes less to
make you feel full-- it takes less to make those fibers stretch and
signal fullness. (It's my hunch that that's why some older (> 1 mo) BF
infants can go so long between normal stools, their colons are bigger,
but the volume of stool made per feeding is so tiny that it just takes a
lot more of them before that stretch is finally signalled, and the baby
feels the urge to go.)

That is, my hunch is that it is not so much the volume per se, but the
change in volume-- at the beginning it may be only the bigger letdowns
that are felt, later if letdowns were always larger than they were in
those first weeks, then the ducts get stretched out, and it takes more
volume to signal a change, so you might only feel very very big
letdowns. On the other hand, if the mom gets many small letdowns, she
may only feel the occasional medium sized one. Again this is just my
hunch, based on how our other distensible organs signal sensations to
us. Does anyone have more definitive info?

Also, again, my opinion only, I believe moms' breasts work well in a
wide variety of ways, some people probably make a significant amount of
milk between feeds, especially those early postpartum moms who have high
baseline prolactins, while others make almost all milk on the spot with
the prolactin stimulation of suckling, and the removal of the milk
suppressor peptides from the alveoli. That is the switch from endocrine
to autocrine control.

Anyway, while it is obvious that relative milk production between feeds
vs during feeds will be different between stages of lactation as
baseline prolactin levels drop, and switch from endocrine to autocrine
control, I also believe there is probably a wide variation in this from
mother to mother. I also think different mothers probably have different
"storage capacities" and different alveolar baseline production
capacities, but I also believe all of these apparently have a huge
excess capacity, so that whatever combination you get, it works,
particularly because no matter what your capacity, no matter what your
prolactin levels, or prolactin bursts, no matter how many or few
alveolar clusters, if there is no milk sitting next to the alveoli to
suppress the speed of production then the alveoli just speed up
production. And this ability of the alveoli to keep going at nearly
unlimited capacity (like the wetnurse with 6 mother's children) is
probably what allows women with a wide variety of anatomies to all make
enough milk for their infants.

At least this is how I'm able to visualize the process as it works for
all the various women I've met with such varied nursing patterns. (And
I'm not talking about moms who approach nursing with a plan, but rather
the great variety of "successful" patterns that have developed with moms
who have been willing to just let their breasts and their babies be
their guides.)

[Paradoxically, it seems that that interference with oxytocin and MER --
worry, distrust, etc--can actually interfere with milk production. I say
"paradoxically" because while oxytocin and epinephrine and
norepinephrine technically have nothing to do with production, but only
to do with MER and permitting or not permitting the availability of the
milk in the sinuses, if MER is interfered with, then the milk stays in
the alveoli, instead of getting regularly to the sinuses, and when it
sits too long and too often in the alveoli, those old milk suppressor
peptides to their thing.]

Anyway, back to the subject, my guess is that some women have lots of
little letdowns, and hence have very little change each time in duct
volume, so feel little in the way of MER, others have a few bigger ones,
so that the duct volume changes more from "empty" to "filled", and many
women in between have a variety of sizes in their letdowns so they feel
the larger ones. But you can feed your baby just as well with lots of
little ones as with a few big ones.

Also, if ducts get "stretched" all the time by the generally large
volumes of most of your letdowns, then even a pretty big letdown might
not be felt because the sensory fibers were already stretched pretty
well even when "empty", so no change is sensed when going from "empty"
to "filled" (relative terms, Kathy D.), so even folks with a few big
letdowns may only feel the biggest.

Do you see how this reasoning could answer some of Jay's Q's about who
feels which letdowns and why? As the ducts get generally stretched out
over time, it will take more and more stretch to make you feel a
letdown. To apply this conjecture to guess an answer to just one of
Jay's questions, the hardest, a mom who relactates: The first time she
lactated, immediatley postpartum, she had tons of prolactin on board and
was making big volumes all at once which when letdown she could feel--
stretching the newly used ducts, etc. After weaning, and then
relactating, she still has big stretched out ducts, but has lower
prolactin levels, makes less at any one time, but does make it steadily
and consistently all the time her baby is suckling, but no sudden large
volumes are hitting those ducts that have already been stretched by the
earlier lactation experience. So no sudden stretching is sensed, altho
the ducts are clearly being filled, and milk is being pushed into the
ducts, may even be enough to totally fill those stretched out ducts, but
just not enough to stretch those sensory fibers the extra little bit
necessary, since they were already stretched out the first time around.

Please remember that this is all conjecture on my part, and I'd love to
know what other people actually know.

Tina Smillie, MD IBCLC

> Subject:
>         Re: MER
>   Date:
>         Tue, 14 Oct 1997 11:30:26 -0700
>   From:
>         Janet Simpson <[log in to unmask]>
>
>
> Hi All,
> I guess that when it comes to MERs I need to be a bit more specific in my
> querys.
> I know that as far as the intensity of "feeling" an MER it varies from one
> woman to the next.  So, wondering if a mom needs to feel one to have one is
> not really my question.
> I also know that if a mom is not feeling an MER that we need to watch the
> baby for signs of MER and if we see the appropriate signs (good hydration,
> weight gain, appropriate wet and poopy drawers, etc) then we don't worry.
>
> I am wanting to know the mechanical workings of the inner structure of the
> breast in relation to MERs and how they influence the breastfeeding success
> or failure of a dyad.  Also,  how does the baby's suckling at the breast
> affect the internal strcutures of the breast to produce an MER (or not to
> for some women).  WHat is it in nipple stim that causes the alveoli to
> contract?  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