Reflexes and the reflex arc

In the knee-jerk reflex the sensory neuron talks straight to the motor neuron, with no brain in the loop.

Fast answers that skip the cortex

A reflex is an automatic response to a stimulus. Simple somatic reflexes do not involve the higher centres of the brain that handle conscious, voluntary movement, which is why they are quick and why they look the same every time. Some reflexes run through the spinal cord and are called spinal reflexes; others run through the brain stem and cranial nerves and are called cranial reflexes. In the 1890s Charles Sherrington showed how muscle contractions are followed by relaxation, and how different reflexes work together as the spinal cord and brain turn incoming impulses into new ones for muscles and organs.

Five parts of a reflex arc

The path a reflex takes is the reflex arc, and it has five parts. A receptor detects the stimulus. A sensory neuron carries the signal into the spinal cord through the dorsal root; its cell body sits in the dorsal root ganglion just outside the cord. In the gray matter the signal is passed on, either directly or through one or more interneurons. A motor neuron, with its cell body in the anterior horn, sends the command out through the ventral root. Finally the effector, a muscle or a gland, responds. In the model these five steps light up one after another.

The knee jerk: a stretch reflex

When a doctor taps the patellar ligament just below the kneecap with a rubber hammer, the tap stretches the big muscle on the front of the thigh. Muscle spindles inside that muscle sense the stretch. Their sensory axons enter the spinal cord and connect directly with the motor neurons of the same muscle, which contracts and makes the lower leg kick forward. A branch of the sensory fibre also acts through an interneuron to inhibit the motor neurons of the opposing muscles at the back of the thigh, so they do not pull against the kick. In daily life the stretch reflex helps hold muscles at a steady length.

Pulling away: the withdrawal reflex

Touch a hot stove and pain receptors in the skin fire. In the dorsal horn of the spinal cord the sensory neuron excites an interneuron, which drives the motor neurons of the biceps, and the elbow bends to pull the hand away. At the same time another interneuron inhibits the motor neurons of the triceps. Without that opposing contraction the withdrawal is faster, which limits the damage. Step on a sharp stone and a similar arc lifts the foot: interneurons cancel the push-down movement of the calf muscles while the muscle at the front of the shin lifts the foot. A copy of the signal also climbs the spinal cord to the brain. That is how you come to feel the pain and remember the stove, but the reflex does not wait for it.

Reflexes in the head, and in babies

The corneal reflex protects the eye. When something touches the cornea, or in a related reflex when bright light hits it, you blink. This arc runs through cranial nerves and the brain stem, and doctors often check it with a gentle puff of air or a light touch with a cotton tip. Newborns have a set of reflexes of their own, which are thought to have helped survival before the modern age and fade as the baby grows. In the Babinski reflex, stroking the sole makes the big toe extend upward and the other toes fan out.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.