This Post Demands a Knee-Jerk Reaction

August 22, 2014 8:19 PM

Human infants are born with a series of reflexes that help them survive -- or that helped their primate or mammalian ancestors survive, whose persistence marks their importance to primate survival. Reflexes are typically not sent to the brain; they occur along a much shorter pathway called a "reflex arc" (your doctor triggers a simple reflex arc by hitting your kneecap with a hammer; you can't control your leg kicking because the signal doesn't go to your brain but rather hits a reflex arc that is much shorter and only goes to your spinal cord) and "primitive" reflexes in infants are those that disappear and are "integrated" and overwritten by the growing nervous system. You can keep your adult knee-jerk reactions -- infants have much cooler reflexes.

Rooting Reflex -- a baby's gotta eat. When you stroke an infant's cheek, the rooting reflex causes them to turn toward the stimulus in the hopes that it's a nipple bearing milk. (This is why babies try to nurse on anyone who snuggles them.) Disappears at 4 months.

Sucking Reflex -- triggered when the roof of the mouth is poked, by a nipple, finger, or pacifier. Babies don't suck politely on the tip of the nipple; they want it jammed back in there so the nipple triggers their sucking reflex, far enough back on the roof of the mouth to gag an adult.

Moro Reflex -- also known as the startle reflex; if you drop an infant so he feels he has lost control of his (very floppy) head, he will throw his arms out wide, then bring them in towards his body, clench his fists, and start to cry. A sudden drop in temperature (losing mom's body heat?) or loud noise can also trigger it. This is thought to be a primate reflex that allows a falling baby monkey to reach out for mom, grab hard, and alert her that there's a problem. This is believed to be the only universally unlearned fear in humans, and accounts for your difficulty in putting your infant to sleep in the bassinet instead of against you. Disappears at 2 months.

Fencing Posture -- the Asymetrical Tonic Neck Reflex (or ATNR) causes a baby to take on a fencing-like posture. It is thought to prepare the infant for voluntary reaching and hand/eye coordination; it may also help an infant "corkscrew" through the birth canal. Disappears at 6 months and makes for many adorable baby pictures before then.

Palmar Grasp Reflex -- causes an infant to grasp anything that touches or strokes his palm; Disappears around 6 months.

Galant Reflex -- when the spine is stroked on one side, the infant will contract towards that side. This is thought to help the infant wiggle out of the birth canal. Disappears at 4-6 months.

This is not a comprehensive list of Primitive Reflexes, which can be found here.

P.S., infants poop when you feed them for the same reason you (literate mefite) feel the urge to poop after a big breakfast: the gastrocolic reflex, which triggers the intestines to move feces to the exit when you introduce new food to the stomach.

posted by Eyebrows McGee (31 comments total)

One day, while working in a newborn nursery, I was really bored and decided to test the Galant of every kid we had admitted. It is by far my favorite.
posted by midmarch snowman at 8:25 PM on August 22, 2014 [8 favorites]

Geez, you have a baby and suddenly everything you talk about is "baby this" and "baby that"

this is an extremely cool post
posted by DoctorFedora at 8:26 PM on August 22, 2014 [3 favorites]

Awesome (2a) and terrific (2) post, thank you.
posted by the man of twists and turns at 8:30 PM on August 22, 2014 [2 favorites]

Sword fighting babies! Oh man, so much fun in the NICU, testing for reflexes because she was a preemie so there were reflexes that disappear and appear near term like Babkin - she didn't root until one day suddenly neurons clicked and she was rooting.

There was one I haven't seen described but noticed in her when she was asleep that seemed like a reflex - sticking her legs up in the air at 90 degrees, knees locked, feet flat. You could push the legs down and then she would pop them right back up. The nurses found it amusing, so we didn't worry.
posted by viggorlijah at 8:35 PM on August 22, 2014

Question about reflexes: a comment to this remarkable video ascribes this three-year-old's grip to a reflex. Does anyone know if that's true and if so, what the name of reflex is?
posted by longdaysjourney at 8:42 PM on August 22, 2014

I've been told that the reason for hiccups is that it happens to babies in the womb and helps exercise their diaphragms, so that they're strong enough to breath when they're born.
posted by Chocolate Pickle at 9:02 PM on August 22, 2014 [2 favorites]

Hey! I have a picture of my son in the fencing pose! We joked that he was getting ready for the Heisman trophy.
posted by KathrynT at 9:44 PM on August 22, 2014 [2 favorites]

Question about reflexes: a comment to this remarkable video yt ascribes this three-year-old's grip to a reflex. Does anyone know if that's true and if so, what the name of reflex is?

It's not a reflex.

The grasp reflex ("palmar grasp reflex" in the wikipedia article linked above) disappears by 5-6 months. Not coincidentally, that's around the age that babies start to be able to voluntarily grasp objects -- using a pincer grip to eat Cheerios and stick every object within reach in their mouths. As long as the grasp reflex is present, the hand is much less useful for voluntary activities.

The disappearance of other infantile reflexes is similarly tied to attaining developmental milestones. For instance, the stepping reflex (which you can use to get even very young infants to "walk" by dangling them so their feet touch the top of a table or other hard surface) needs to disappear before the child is able to balance on two feet and then start to walk.

For the linked video:

This looks like a developmentally normal 3 year old, so her grasp reflex would be long gone.

Many kids that age are strong enough to support their body weight with 2 hands (as they'll be happy to display at a playground). That's what she's doing when she's upside down in the tube. As soon as she gets 3/4 of the way around and starts to lose her grip, she opens her hands (and therefore, no grasp reflex, which would prevent this) and falls.

The grasp reflex can return in adults, usually in the presence of significant neuropathology: large frontal brain tumours, advanced neurodegenerative dementias, etc. It's considered a frontal release sign.
posted by Plasmon at 9:51 PM on August 22, 2014 [8 favorites]

I knew an adult who hadn't completely lost his rooting reflex.

As a kid, people had teased him about it, and tried to touch his face in a way that would trigger it (kind of stroking along the top of the 'moustache line' at the sides worked best), so he'd turned it into also snapping his teeth in a,
"I'm going to bite your finger off if you don't stop messing with me"-reflex.
posted by Elysum at 11:20 PM on August 22, 2014 [1 favorite]

What's the reflex where the baby sticks their arms in the air over their head while sleeping like a mummy and then glowers in their sleep like a mummy. Is that the Mummy reflex or is my baby the second coming of Brendan Fraser or what
posted by Potomac Avenue at 12:53 AM on August 23, 2014 [5 favorites]

My wife still has the Moro reflex, which I verified as many times as she'd let me.
posted by breath at 2:01 AM on August 23, 2014 [2 favorites]

What is the reflex called when you blow on an infants face or the winds blows on their face and they inhale sharply and quite audibly?
posted by JujuB at 2:13 AM on August 23, 2014 [1 favorite]

Puppies have a useful but less adorbs reflex of pooping if the underside of the base of their tail is stroked. As discovered by my cousin whilst snorgling a litter of Cairn pups: "Aww look at the widdle tails! (twirls tiny fuzz-tails) Aww lookit ... PFFLABTHHT! Aargh!"
posted by scruss at 2:47 AM on August 23, 2014 [1 favorite]

My Moro reflex story:

When Josie Penguin was just born, I was carrying her around without a shirt on and playing with her by pretending to drop her, which she thought was hilarious (I want to note that she got good as holding her head up early). One time I must have startled her too much because she reached out and grabbed my armpit hair with both hands and would not let go. I couldn't get her hands off myself (because then she'd be holding herself up by just my armpit hair which hurt like hell) so I had to yell for Mrs. Penguin to save me, after she stopped laughing at my predicament.
posted by Elementary Penguin at 2:59 AM on August 23, 2014 [13 favorites]

Moro Reflex -- also known as the startle reflex

I think my daughter lost this one a bit late - throwing her hands & arms out wide was a startle reflex to anything startling - not just motion - until at least a year old. We went for some family studio portraits when she was about 6 months & the best one is where her hands are out wide like this, and somehow, one on the folds on her pink dress makes it appear that she has six fingers on one hand.
posted by Devils Rancher at 3:03 AM on August 23, 2014 [1 favorite]

Huh. Asymmetrical Tonic Neck Reflex, hey? MrTaff used to call it ""Free Tibet!" when, then, BabyTaff did it. When her little sister came along two years later and thrust both hands up , MrTaff called that pose "Free Tibet AND Burma!"

I prefer MrTaff's version.
posted by taff at 4:40 AM on August 23, 2014

I think someone needs to do some research into whether there is actually any such thing as a Calming Reflex. It sounds allll-most plausible to me, but just a little too complicated to invoke.

The thing that freaked me out when my oldest daughter was born was an occasional fast, all-over tremor that we feared was a seizure. She did it when I was pregnant too, frequently, and it was the weirdest feeling.

The doctors told us it was normal and just a sign of an immature nervous system. (My OB-GYN said the same thing during pregnancy). She had stopped doing it by the time we left the hospital, and never did it again. My younger daughter never did it at all, during or after pregnancy. I still wish knew how common it really is, though oldest daughter is almost four and completely fine, now.
posted by OnceUponATime at 5:07 AM on August 23, 2014

Very nice post.

Watching baby lambs feed from their mother the other day reminded me of how urgently furious a baby's suckling is. With an incredibly strong pull. Rip the skin right off your nipples, it can. Putting a finger in a young baby's mouth will demonstrate this.
disclosure: having watched three young women in succession cry with pain at the beginning of breast-feeding. I think the advice must have changed, I don't remember that happening in my day.
posted by glasseyes at 6:11 AM on August 23, 2014

"so I had to yell for Mrs. Penguin to save me, after she stopped laughing at my predicament."

My husband liked to play with the rooting reflex and would stroke our first baby's cheek to watch his head turn, all the time. He was nuzzling the baby with his nose doing this one day, and the baby whips his head around, opens his mouth, and latches on to my husband's (rather large) nose, and starts going to town.

Of course I didn't free him (he didn't know how to break the seal) until after I laughed hysterically and took a picture or six.

Husband's comment was "First, ow. Second, you let him do that ON YOUR BOOB?" I was glad he appreciated how hardcore breastfeeding is. :)
posted by Eyebrows McGee at 7:59 AM on August 23, 2014 [16 favorites]

What is the reflex called when you blow on an infants face or the winds blows on their face and they inhale sharply and quite audibly?
posted by JujuB

Nice observation!

I'd never heard of this, but I've often wondered what signal babies could rely upon to know that the time has come to try to take their first breath and inflate their lungs -- which would be disastrous in the womb -- and this seems like a good candidate.

And of course those lungs would have to be furled pretty tightly to keep fluid from being in the lungs at birth, and since I found out that hiccups occur in the womb (from an answer in AskMe), I've wondered whether hiccups could be a pump which clears fluid from the lungs and bronchi when it does happen to get in there -- a view I feel is somewhat supported by the fact that long term hiccups are associated with pneumonia, pleurisy, and asthma.
posted by jamjam at 11:00 AM on August 23, 2014

Babies breathe in the womb! Well, sort of. Their lungs are filled with amniotic fluid-- without it, the alveoli can't develop-- and at the end if pregnancy, they practice inhaling and exhaling. There is one theory that the presence of surfactant in the amniotic fluid is what triggers labor to begin, because it indicates that the lungs are mature and the baby knows how to breathe.
posted by KathrynT at 11:05 AM on August 23, 2014

I've wondered about that surfactant as a possible signal myself, and I've heard that the fetus does breathe fluid in the womb, but I think that fluid would have to be all out of there as much as possible at birth to maximize the baby's chances of survival.
posted by jamjam at 11:15 AM on August 23, 2014

As witness the problem of Transient tachypnea of the newborn:

Transient tachypnea of the newborn (TTN, TTNB, or "transitory tachypnea of newborn") is a respiratory problem that can be seen in the newborn shortly after delivery. Amongst causes of respiratory distress in term neonates, it is the most common.[1][2] It consists of a period of rapid breathing (higher than the normal range of 40-60 times per minute). It is likely due to retained lung fluid, and is most often seen in 35+ week gestation babies who are delivered by caesarian section without labor. Usually, this condition resolves over 24–48 hours. Treatment is supportive and may include supplemental oxygen and antibiotics. The chest X-Ray shows hyperinflation of the lungs including prominent pulmonary vascular markings, flattening of the diaphragm, and fluid in the horizontal fissure of the right lung.

Note that it happens most often in "35+ week" babies delivered by caesarean, implying that the baby gets rid of the fluid when it 'knows' birth impends.
posted by jamjam at 11:29 AM on August 23, 2014

I think the squeezing through the vagina squeezes the fluid out of the baby's lungs -- I seem to remember learning that in childbirth class. Also see here.
posted by KathrynT at 12:58 PM on August 23, 2014 [1 favorite]

My daughter has a brachial plexus injury in her left arm. Right after she was born, she flunked the Moro reflex test on the left side - which got her checked out for a broken collarbone (nope), and then diagnosed with the nerve injury. She began occupational therapy two weeks after birth, and started improving pretty quickly. One of the first ways I could tell she was improving, was when both of her arms would do the Moro reflex.

Anyway, long story short: Six months have gone by, and I still keep mistyping it as the "moron reflex".
posted by Coatlicue at 1:55 PM on August 23, 2014 [2 favorites]

I think the squeezing through the vagina squeezes the fluid out of the baby's lungs -- I seem to remember learning that in childbirth class. Also see here.

That is the putative mechanism, all right, but I think a little reflection makes it look pretty handwavey, and beset with many potential pitfalls, such as the likelihood of squeezing fluid into the lungs from some other source such as the stomach if the passage to the lungs is open for squeezing things out, and especially the fact that the squeezing from the constriction of the vaginal canal, as opposed to the waves of uterine contraction which push the baby out, goes exactly in the wrong direction for squeezing anything out of the lungs, since the the baby passes through the constriction of the vaginal canal head first, which would seem to tend to press fluid toward the bottom of the lungs rather than toward the head and out.

I couldn't find much about vaginal contractions during childbirth, beyond a few mentions in discussions of "orgasmic birth."

Not to mention the inefficiencies inherent in squeezing a large, lumpy, elastic, and internally structured (i.e. ribs) sack of fluid in order to express fluid from a much smaller inner sack in the best of circumstances, and the potential stress it could put on a lung filled with fluid if it were to be squeezed unevenly or the outlet was blocked for any reason.

Here, for example, is an account from a mother who gave birth to a 12 pound baby with fluid in it's lungs and did not tear or have an episiotomy.
posted by jamjam at 2:51 PM on August 23, 2014 [1 favorite]

...the likelihood of squeezing fluid into the lungs from some other source such as the stomach if the passage to the lungs is open for squeezing things out, and especially the fact that the squeezing from the constriction of the vaginal canal, as opposed to the waves of uterine contraction which push the baby out, goes exactly in the wrong direction for squeezing anything out of the lungs, since the the baby passes through the constriction of the vaginal canal head first, which would seem to tend to press fluid toward the bottom of the lungs rather than toward the head and out....

...squeezing a large, lumpy, elastic, and internally structured (i.e. ribs) sack of fluid in order to express fluid from a much smaller inner sack in the best of circumstances, and the potential stress it could put on a lung filled with fluid if it were to be squeezed unevenly or the outlet was blocked for any reason.

I'm not a physiologist (I'm a neurologist), but I believe the idea is that the fluid is resorbed from the alveoli (tiny sacs in the lung where gas exchange takes place once respiration starts) into the capillaries, and thereby into the bloodstream. Increased intrathoracic pressure from external compression in the birth canal would cause decreased venous return into the thorax and thereby increase the rate at which alveolar fluid was resorbed (because of differential pressures in the endovascular and extravascular spaces -- the opposite of this situation, increased endovascular pressure, is roughly what leads to pulmonary edema with increased intracardiac pressures in heart failure).

It's not that the lungs are a bag of fluid being cleared in one direction through an outlet by some kind of peristalsis (directional contraction), like toothpaste being squeezed out of the end of a tube. So the direction of contraction, "passage to the lungs", etc, don't really apply.
posted by Plasmon at 4:58 PM on August 23, 2014 [2 favorites]

Increased intrathoracic pressure from external compression in the birth canal would cause decreased venous return into the thorax and thereby increase the rate at which alveolar fluid was resorbed (because of differential pressures in the endovascular and extravascular spaces ...

That's an interesting point, Plasmon, but if you take a look at the excerpt I quoted from the linked Wikipedia article on transient tachypnea of the newborn, which is caused by fluid in the lungs at birth, you'll see that this condition typically resolves itself in 24-48 hours, which implies that it takes the baby's circulatory system about that long to reabsorb the excess fluid, and that means that your postulated and hypothetical mechanism would have to be about one or even two orders of magnitude more efficient than that in a case of uneventful labor and birth, which I think is exceedingly unlikely.
posted by jamjam at 5:40 PM on August 23, 2014 [1 favorite]

What is the reflex called when you blow on an infants face or the winds blows on their face and they inhale sharply and quite audibly?
Hmmm, when I do this, they don't inhale, they swallow. Same as when you blow on a cats face.
Might depend if the mouth is open or closed?

I didn't really need to give my nephew pills (unlike cats), but the swallow reflex was occasionally useful.
He loved being fed with a bottle, but it was clearly a toy, he hadn't made the connection between it and soothing 'hungry' feelings, while he was quite sure boobs were what fixed that (he wasn't born yesterday, he'd been born, what, a few months ago? He knew the deal. Not). So, if I was babysitting, and he got *too* hungry, he'd refuse the bottle, in the same way he'd refuse a rattle etc. If I did get the bottle in his mouth, he'd sometimes refuse to swallow a mouthful of milk, (clearly I was just being a big meanie pants, where the hell were the boobs?!). Usually I just kept bottle in his mouth and he'd eventually start drinking, until he got a bit older, and really, really, stubborn.
Anyway, at that point, I'd blow on his face, he'd swallow. Much struggling, and stubborness, and I'd get a couple more mouthfuls in him.
At that point, MYSTERIOUSLY, he'd start to feel slightly less starving, and decide, magnanimously, that maybe if he wasn't that hungry, I could continue distracting him until boob arrived. How about that rattle? No? You still want to play with the bottle? Fine!
And I'd finally be able to feed him.
Each time I'd be like, one day, you are going to make the correlation between bottle and boob, and this is going to be much easier. Especially annoying as he'd figured out Mum + Makeup/Jewellery + me there, equaled Mum would be leaving soon (he LOVED those things by themselves). Which. Really? How did you figure that out?
Unfortunately, it continued to elude him for most of his first year. I even gave him milk formula in a shot glass at about 6 months, because he was having a bottle-hunger-tantrum, but a cup like GROWN UPS USE was clearly fascinating (Look at me! I'm a grownup!).
Anyway, watching babies get their logical deduction on is awesome.
posted by Elysum at 1:39 AM on August 24, 2014 [1 favorite]

Looking it up in the literature, the other thing that happens late in pregnancy and during labor and delivery is increased expression of ion channels in the alveolar membrane that allow sodium concentrations to change (and thereby create an osmotic gradient which draws water across the membrane).

Increased catecholamine secretion (basically, adrenaline release) increases the production of these ion channels.

So the stress of labour over (usually) several hours would give both a better physiological mechanism for fluid reabsorption and more time for that mechanism to do its work.

Contraction in the birth canal would change intrathoracic pressures as I described, and that would promote fluid reabsorption, but that's seemingly less important, or at least less studied in the neonatal physiology literature.
posted by Plasmon at 10:58 AM on August 24, 2014 [2 favorites]

"What is the reflex called when you blow on an infants face or the winds blows on their face and they inhale sharply and quite audibly?"

I don't know what it's called, but in seminary they taught us that if you're full-immersion baptizing an infant, the trick is to blow in their face hard so they gasp, startle, and don't breathe for a second, and dunk them in that second where they're paralyzed with startlement. That way they don't swallow water when being dunked. I learned many such extremely limited but highly practical tips. :)

(Also that if you are full-immersion baptizing adults in a river, as some sects prefer, you point the feet DOWNSTREAM because otherwise they get waterboarded by the current going up their nose and -- more awkwardly -- their dunking gowns go up over their heads and show their naked bits to the gathered congregration.)
posted by Eyebrows McGee at 11:39 AM on August 24, 2014 [9 favorites]