How do reflexes work?

Reflexes are one of the many ways that your body protects you from the world. If you accidentally touch something hot, your body knows to pull away without taking the time to send the information to your brain, process it, and send an action back to the muscles. So, if reflexes don’t involve using your brain, how do they work?

What is a reflex?

Reflexes are immediate, involuntary responses to stimuli. They’re designed to be faster than a typical reaction to protect you from harm. Imagine you’re out in the park on a nice day. A breeze picks up and a leaf flies toward your face. You don't really see it coming, but somehow, your eyes are shut by the time it reaches you.

In this instance, a reflex triggered a reaction in a fraction of a second or the “blink of an eye,” so the leaf wouldn’t be able to hurt you. If your eyes had waited for your brain to process the information and make a decision, you’d already be headed to the doctor’s office. Instead, your eyes shut, the leaf blew away, and you went happily about your day.

How do they work?

Man at doctor with rubber mallet placed near the top of spinal cord
Credit: EsfilPla/ Shutterstock

Unlike the rest of the nervous system, reflexes aren’t controlled by the brain. Instead, the spinal cord is responsible for the speed of your reflexes. When your body senses an immediate stimulus, a signal is sent from the sensor to your spinal cord. Instead of sending the information “upstairs” to your brain, the spinal cord sends a message through a motor neuron to the appropriate muscles.

Think of the spinal cord as an assistant manager to your body. The brain is the boss, but when a decision needs to be made quickly, you don’t have time to go to the boss. The assistant manager can step in and make the necessary decision.

Monosynaptic reflexes

Simple reflexes are called monosynaptic or reflex arcs. The message is sent from the receptor — the part that sensed the stimulus — to the spinal cord. To change the signal from a sensory neuron to a motor neuron, your body uses synapses. Reflexes that involve only one movement — or one synapse — are called monosynaptic.

Jerking your knee at the doctor’s office is an example of a monosynaptic reflex. Your touch receptors in your knee sense the impact of the hammer, the information travels to your spinal cord using sensory neurons, the synapse converts the signal to the motor neuron attached to your leg, and your leg kicks. Shutting your eyes when a wind gust throws sand at your face at the beach is another example of a monosynaptic reflex.

Polysynaptic reflexes

Woman at doctor appointment, showing rubber mallet hit against knee
Credit: Robert Przybysz / Shutterstock

Polysynaptic reflexes work the same way that monosynaptic reflexes do except they have more moving parts. When the sensory neuron sends the information to your spine, it uses multiple synapses to send the information to several muscles at once instead of sending the action response back to a single muscle.

These types of reflexes are helpful when your entire body needs to react. When someone scares you by jumping around a corner, you experience what’s called the startle reflex. In a fraction of a second, your eyes and ears recognize the immediate threat and you gasp a deep breath, widen your eyes, bend your knees to lower your center of gravity, widen your stance, and contract your muscles. Now, you’re ready for action. Each movement that was triggered during the reflex has its own synapse that was fired. Multiple moving parts means a polysynaptic reflex.

Autonomic reflex

Knee-jerk and startle reactions are easy to see, but there are still many reflexes that occur in your body that you can’t see. Some reflexes help your internal organs operate at peak efficiency. You don’t ever have to think about dilating your pupils when you walk into a dark room or salivating when it’s almost time to eat.

These are called autonomic reflexes. They work in a similar way as physical reflexes except instead of targeting skeletal muscles to make you move, they target smooth muscles that control your organs.