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NeuroScience Decoded

A plain-language encyclopedia of the brain — history and pioneers, neurons and synapses, brain chemistry and anatomy, senses, memory, emotion, sleep and consciousness, disorders and treatments, brain-computer interfaces and AI. Every entry with a plain-English summary, everyday examples and links to the concepts you need first.

Movement & Motor Control Beginner

Reflexes

The Editors February 11, 2026 3 min read
Reflexes

The knee jerk, pulling back from a flame, a baby's grasp: fast, automatic responses wired into the spinal cord and brainstem — how they work and why doctors test them.

In plain English

A reflex is an automatic, very fast response to a stimulus — like your leg kicking when a doctor taps your knee. Many reflexes are handled by the spinal cord without waiting for the brain. Doctors test them to check that nerves and the spinal cord are working.

Beginner level: No background needed. Everyday language, everyday examples — the place to start.

Concepts to know first

Beginner The Spinal CordBeginner How the Brain Moves the Body

Go deeper next

Intermediate Sherrington and the SynapseIntermediate Balance and Proprioception

A reflex is a rapid, automatic, stereotyped response to a specific stimulus. Reflexes protect the body and form building blocks for more complex movement.

The knee-jerk reflex arc: a stretch sensor, a sensory neuron, a single synapse in the spinal cord, and a motor neuron back to the muscle.
The knee-jerk reflex arc: a stretch sensor, a sensory neuron, a single synapse in the spinal cord, and a motor neuron back to the muscle. (tap to enlarge)

The knee jerk

When a doctor taps the tendon below your kneecap, it briefly stretches the thigh muscle. Muscle spindles in the muscle detect the stretch and send signals along sensory neurons to the spinal cord, where they connect directly with motor neurons to the same muscle — a single synapse. The muscle contracts and your leg kicks. At the same time, interneurons inhibit the opposing hamstring muscles. This stretch reflex is monosynaptic and takes only a few tens of milliseconds. It normally helps maintain posture.

Withdrawal and crossed extension

Touch something hot and you pull your hand back before you feel the pain. This withdrawal reflex is polysynaptic — signals pass through interneurons in the spinal cord, which activate muscles to pull the limb away. If you step on a tack, the crossed extensor reflex stiffens your other leg to keep you upright.

Why doctors test reflexes

Reflexes test the whole loop: sensory nerve, spinal cord segment, motor nerve and muscle. Absent or weak reflexes suggest a problem in the peripheral nerves or the specific spinal segment. Exaggerated reflexes suggest damage higher up, in the brain or upper spinal cord, which normally dampens them. The Babinski sign — the big toe moving upward when the sole is stroked — is normal in infants but signals upper motor neuron damage in adults.

Primitive reflexes in babies

Newborns show reflexes that fade as the brain matures: the grasp reflex (fingers curl around anything in the palm), the rooting reflex (turning toward a touch on the cheek to find food), and the Moro reflex (flinging the arms out when startled). Their timely disappearance is a sign of healthy development.

Make it simple

Everyday examples & things to try

Try it

The pupillary reflex

In a dim room, look into a mirror and switch on a light — or close your eyes for 20 seconds and open them. Watch your pupils shrink quickly. A brainstem reflex just adjusted your eyes' aperture, with no conscious decision.

Analogy

A fire alarm wired to sprinklers

A reflex is like a smoke detector wired straight to the sprinklers, rather than calling head office first. The local response is faster — and head office (the brain) gets the report a moment later.

Key takeaways

  • Reflexes are fast, automatic responses organised in the spinal cord or brainstem.
  • The knee jerk is monosynaptic; withdrawal reflexes are polysynaptic.
  • Reflex testing localises nerve and spinal problems; primitive reflexes fade with development.
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