Science explanations for daily phenomenon. A biweekly column by Nicholas G. Norwitz ’18

Onions cause reflex tears

Onions cause reflex tears

Why do we cry when we chop onions? In the immortal words of Donkey, “ogres are like onions… they make you cry.” But unlike Shrek who might cause you to shed fearful emotional tears, onions cause you to cry “reflex tears.” Reflex tears are produced in order to flush away potential irritants. So what irritant does an onion create to stimulate reflex tears? Chopping an onion produces an enzyme called lachrymatory-factor synthase. This enzyme catalyzes the conversion of compounds in the onion, called amino acid sulfoxides, into the aerosolized chemical irritant “syn-Propanethial-S-oxide.” Syn-Propanethial-S-oxide includes several sulfur compounds including sulfuric acid. Incidentally, this is the same acid that is in lead acid batteries. So don’t beat yourself up if you break down next time you chop an onion. There is no shame in tearing up when you effectively get battery juice on your eyeball.

 

Gate theory of pain

Gate theory of pain

Why do we squeeze a sore finger? Have you ever accidently touched a stove or hit your finger with a hammer? If so, chances are that you quickly grabbed your injured digit or shook your hand vigorously in the air. To understand this behavior we first must understand the gate theory of pain. Pain signals arise in sensory receptors in your skin called nociceptors. Nociceptors relay the pain message to a group of neurons in your spinal cord. For the sake of explanation, let’s call these neurons “group A neurons.” The group A neurons then relay the signal further, to a group of neurons that project to your brain, creating the sensation of pain. Pressure and motion activate different sensory receptors in your skin. These receptors send a signal to a different group of neurons in your spinal cord, which we will call “group B neurons.” Group B neurons relay their signal to neurons that project to the brain as well. However, group B neurons also project back and inhibit the group A neurons in the pain pathway. Therefore, when you activate the pressure-motion pathways by squeezing a sore finger, you activate neurons that quiet the pain signal. As it turns out, emotional pain can also be remedied by a form of squeezing. I’ve tried to avoid scientific vocabulary, but perhaps you are familiar with this term, it’s called a “hug.”

 

Wrinkled fingers explained by osmosis

Wrinkled fingers explained by osmosis

Why do we get prune fingers when we are wet? This is a question that is surprisingly far from obvious. If you’ve learned about tonicity, then you might guess that, since your blood fluid is full of solutes, osmotic pressure forces water through your skin causing the skin’s surface area to expand and wrinkle your digits. However, your skin is not permeable and wrinkly fingers are not the result of osmosis. Rather, your fingers wrinkle due to an involuntary nerve response to moisture that causes your finger’s blood vessels to constrict and the upper layers of skin to pucker. It is thought that the channels created by the ridges in your skin help to channel away water. This is evolutionarily adaptive because it allows you to grip objects more effectively when you are wet. Perhaps if I had written this is the bathtub, I would have been able to grasp a halfway decent closing joke.

 

Diagram of human ear

Diagram of human ear

Why do we hate the sound of our own voices? We should all know what our voices sound like, right? They are our own voices after all. However, when we hear a recording of ourselves, what we hear rarely matches our expectations. We dislike the sounds of our own voices because they sound foreign. Unfortunately, that really is how we sound to other people. Speaking necessarily distorts how we perceive our speech. Normally, the process of hearing begins at the outer ear. Sound waves are captured by our pinna and funneled towards our eardrums. As the eardrum vibrates, it jostles the chain of three tiny bones in the middle ear called the ossicles. The ossicles amplify the sound as the last ossicle in the chain taps on the oval window. As the oval window vibrates, it causes the fluid in the inner ear to slosh about, stimulating hair cells on the cochlea and causing the perception of a sound. However, when we speak muscles attached to our ossicles contract. This “attenuation reflex” makes the ossicle chain more rigid and impairs the conduction of sound waves from the air. In addition, sound waves form speech can travel to our inner ears via another route that completely bypasses the outer and middle ear. To prove so, try this little experiment: hum for a moment. Now, cover both your ears with your hands and continue to hum. Did the sound get quieter? It shouldn’t have, though the quality of the sound may have changed. This is because making a sound causes the bones of the head to vibrate, which can directly stimulate the cochlea. So get it through your thick skull, you really do sound like that.