Human Reflexes Explained

Human Reflexes: A reflex is an automatic, rapid response to a stimulus that bypasses conscious thought, often routed through the spinal cord or brainstem. Some protect us, some are leftovers from our evolutionary past, and doctors still test them to check whether the nervous system is working.

Charles Osborne: The Iowa man began hiccuping in 1922 while weighing a hog and did not stop until 1990, a record 68 years. He still managed to marry, raise children and work.

ACHOO Syndrome: Short for Autosomal dominant Compelling Helio Ophthalmic Outburst, it makes some people sneeze when they step into bright light. Estimates suggest it affects roughly a fifth to a third of people.

Arrector Pili Muscle: These tiny muscles lift hairs and create goosebumps. A 2020 Harvard study led by Ya Chieh Hsu showed that the same nerves that trigger them also connect to hair follicle stem cells, linking cold to hair growth.

Knee Jerk Reflex: Wilhelm Erb and Carl Westphal described the patellar reflex in 1875. A tap on the tendon stretches the muscle, and a loop through the spinal cord kicks the leg before the brain gets a say.

Moro Reflex: Newborns fling out their arms and then pull them in when startled or when their head drops. It fades by about four to six months, and some scientists see it as a leftover from infant primates clinging to their mothers.

Babinski Sign: Joseph Babinski described it in 1896. Stroking the sole makes an infant's big toe point up and the others fan out, normal in babies but a warning sign of nervous system damage in adults.

Palmar Grasp Reflex: Newborns clamp their fingers around anything placed in their palm with surprising strength. Scientists suspect it traces back to ancestors whose infants needed to hold on to fur.

Mammalian Diving Reflex: Cold water on the face slows the heart, narrows blood vessels in limbs and shifts blood toward vital organs. Per Scholander's work on diving seals and humans in the 1940s helped reveal this ancient survival response.

Pupillary Light Reflex: Pupils shrink in bright light and expand in the dark. Some people with no conscious vision still show it, thanks to light sensing cells in the retina discovered by David Berson in 2002.

Blink Reflex: People blink about 15 times a minute, closing their eyes for a noticeable fraction of waking time. Blink rates drop sharply when staring at screens, which is one reason eyes feel dry.

Vestibulo Ocular Reflex: This reflex moves your eyes the opposite way when your head turns, keeping your vision steady as you walk or run. It works in a fraction of a second, far faster than any conscious correction.

Jendrassik Maneuver: Hungarian physician Ernő Jendrassik showed in 1894 that clenching your hands or pulling your fingers together can make the knee jerk stronger. Distraction seems to turn up the reflex by reducing inhibition from the brain.

Withdrawal Reflex: When you touch something painfully hot, a spinal circuit pulls your hand away before the pain message fully reaches the brain. You often feel the burn after your hand is already moving.

Ivan Pavlov: The Russian physiologist won the 1904 Nobel Prize for work on digestion, and his dog experiments on conditioned reflexes grew from it. He showed that neutral signals like a bell could trigger salivation after being paired with food.

Charles Sherrington: The British scientist coined the word synapse in 1897 and won the 1932 Nobel Prize for decoding how neurons coordinate reflexes. His work showed that when one muscle contracts, its opposing muscle is automatically relaxed.

Myrtle McGraw: In the 1930s the Columbia University researcher studied twin boys known as Johnny and Jimmy to show how early training affects infant reflexes and motor skills. Her work on infant swimming reflexes helped shape child development science.

Cushing Reflex: Harvey Cushing described in 1901 how rising pressure inside the skull makes blood pressure climb while heart rate and breathing slow. It is a dangerous emergency reflex doctors still watch for.

Valsalva Maneuver: Antonio Valsalva described it in 1704 as a way to clear pus from the ear. Today straining against a closed airway is used to equalize ear pressure and sometimes to reset an abnormal heart rhythm.

Brain Freeze: A 2012 Harvard Medical School study by Jorge Serrador linked ice cream headaches to rapid widening of an artery in the brain after cold touches the palate. The sudden dilation seems to trigger pain.

Area Postrema: This tiny brainstem region sits outside the usual blood brain barrier, letting it sample the blood for toxins. When it detects something suspicious, it can trigger vomiting.

Arnold's Nerve: A branch of the vagus nerve supplies part of the ear canal, and in a small percentage of people a cotton swab there can trigger a cough. Doctors call it the ear cough reflex.

Gargalesis: Psychologists Stanley Hall and Arthur Allin split ticklishness into two types in 1897. Knismesis is a light tickle that makes you itch, while gargalesis is the heavy, laughter inducing kind that is hard to do to yourself.

Gordon Gallup: The psychologist proposed that yawning helps cool the brain. Ultrasound shows fetuses yawning at about 11 weeks, long before they need to wake up or cool down.

Neil Shubin: The paleontologist argues in Your Inner Fish that hiccups may be an echo of our amphibian ancestors, whose gill breathing used a similar brainstem circuit. It would explain why a reflex that serves no obvious purpose in adults still exists.