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.
Showing 18 oddities.
A reflex is a fixed, automatic response to one specific trigger, running the same way every time. The knee-jerk reflex, the pupillary light reflex, and the gag reflex are the clearest examples: tap the tendon, shine the light, touch the palate, and the same response follows.
What separates a reflex from an involuntary entry is predictability. A reflex has one trigger and one short, repeatable path through the nervous system. An involuntary entry, like a random shiver, has no clean trigger you can point to.
The boundary case is blushing, which is a reflex you cannot stop, but the trigger is embarrassment, a feeling rather than a physical stimulus. It stays a reflex because the path from trigger to response is fixed and automatic. The trigger is just harder to define than a tap on a tendon.
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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.
Most healthy newborns
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 is known to blush from embarrassment.
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 many adults yawn. The effect appears in early childhood, may be linked to empathy, and crosses over to dogs.
About 30–70% 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.
Felt on 90% of trials in one brain-scan study
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 studied so far 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 one or two months after birth.
Almost 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. About a third of healthy people have no gag reflex at all, and that is normal.
Present in about 2 of 3 people, absent in 37% in one study
Skin Reflex
Goosebumps
Tiny muscles at the base of most hairs 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 the raised hair 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 a classic example of a one-synapse reflex.
Everyone
Heart Reflex
Oculocardiac reflex
Pressure on the eyeball, or a pull on an eye muscle, makes the heart slow down. It matters most during eye surgery, especially in children. It is not a safe way to slow your own heart.
Found in 14–90% of eye muscle operations. Its strength varies a lot.
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 the leading idea is 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 pupils react together. 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 blade 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 blood pressure drops. The brain briefly runs short of blood and you go down.
About 1 in 3 people faint at least once