Eyes Reflex
Accommodation reflex
When you look at something close, three things happen at once. The eyes turn inward, the pupils shrink, and the lens in each eye gets fatter. You do not decide any of it.
Everyone
Reflex
Automatic responses that run without asking you.
17 oddities.
Nerves Reflex
Babinski reflex
Stroke the sole of a baby's foot and the big toe lifts while the other toes fan out. Do the same to an adult and the toes curl down. The baby's response is normal until about two years. In an adult it is a warning sign.
All healthy infants
Skin Reflex
Blushing
When you are embarrassed, blood vessels in the face open wide and the skin turns red. You cannot stop it. Darwin called it the most human of all expressions, and no other animal seems to do it.
Everyone
Organs Reflex
Cold diuresis
Being cold makes you need to urinate. The body pulls blood away from the skin into the core, the heart reads this as too much blood, and the kidneys are told to dump water.
Everyone
Brain Reflex
Contagious yawning
Seeing, hearing, or even reading about a yawn makes about half of adults yawn. The effect appears in early childhood, is linked to empathy, and crosses over to dogs.
About 50–60% of adults
Nerves Reflex
Cutaneous rabbit illusion
A few quick taps at the wrist followed by taps at the elbow feel like a small animal hopping up the arm. The brain fills in taps between the two spots that never happened.
Works on most people tested
Skin Reflex
Dermatographia
In a few people, a firm stroke on the skin raises a red welt within minutes, clear enough to write words with a fingernail. It is the skin's hive response set on a hair trigger.
About 2–5% of people
Heart Reflex
Diving reflex
Cold water on the face makes the heart slow down, sends blood to the brain, and holds the breath. Every mammal has it. You can trigger it with a bowl of cold water.
Everyone
Eyes Reflex
Emotional tears
Humans are the only animals known to weep from feeling. Emotional tears have a different chemistry from the tears that wash the eye, and babies cannot make them until a few months after birth.
Everyone
Mouth Reflex
Gag reflex
Touch the back of the tongue or the soft palate and the throat clamps shut. It keeps things out of the airway. Up to a third of healthy people barely have it, and that is normal.
Present in about 2 of 3 people, weak or absent in 10–37%
Skin Reflex
Goosebumps
Tiny muscles at the base of every hair pull the hair upright when you are cold, scared, or moved. In a furry animal this traps warm air or makes it look bigger. In a human it does almost nothing.
Everyone
Nerves Reflex
Knee-jerk reflex
A tap on the tendon below the kneecap makes the lower leg kick. The signal goes to the spinal cord and straight back through one connection. It is the simplest reflex in the body.
Everyone
Nerves Reflex
Palmar grasp reflex
Put a finger in a newborn's palm and the fingers close around it, hard. The grip is strong enough to hold the baby's weight for a moment. It fades by about six months and is replaced by grasping on purpose.
All healthy newborns
Nerves Reflex
Photic sneeze reflex
About one person in four sneezes when they step into bright sunlight. It runs in families, and it is probably a wiring mix-up between the nerve for the eyes and the nerve for the nose.
About 18–35% of people
Eyes Reflex
Pupillary light reflex
Shine a light into one eye and both pupils shrink, even the eye in the dark. The wiring crosses in the midbrain so that the two eyes always match. You can watch it in a mirror.
Everyone
Nerves Reflex
Referred pain
Pain from an internal organ often shows up somewhere else, like the left arm during a heart attack or the shoulder from a gallbladder. The nerves from the organ and from that patch of skin share a line into the spinal cord.
Everyone
Heart Reflex
Vasovagal syncope
The most common reason people faint. A trigger like blood, pain, heat, or standing too long makes the vagus nerve slow the heart and drop blood pressure. The brain briefly runs short of blood and you go down.
About 4 in 10 people faint at least once