To perform some of the body’s most primitive actions, there just isn’t time for a signal to make it all the way to your brain and back. If you put your finger in boiling water, for instance, your hand jerks away even before pain consciously registers in your mind. For these responses, called reflexes, signals travel to and from the spinal cord instead—shortening the time needed for your body to react.

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Reflex pathways typically involve information flowing from a sensory neuron (in your fingertip, for example) that connects to a motor neuron in the spinal cord. Reflexes are involuntary, meaning that we don’t typically control them or decide to do them consciously. They include blinking or jumping in response to a sudden noise and sneezing or coughing when irritants enter the nose or mouth. Reflexes are also the reason you gag when something threatens to block your throat and airway and why your pupils get smaller when a bright light comes on.

One commonly known reflex is the knee-jerk response, also known as the patellar or deep tendon reflex. When a doctor taps an area under the knee with a rubber hammer, the patellar tendon—and a connected muscle in the thigh—stretches slightly. A message is sent to the spinal cord, which instructs the muscle to contract, causing the lower leg to kick outward. The entire process takes about 50 milliseconds and is important for maintaining balance and standing upright. Deep tendon reflexes can also be checked along the outsides of the elbows, in the crooks of the arms, and at the wrists and ankles.

In this process, the brain acts as an interested observer of reflex responses, collecting information about what is happening in the body. That way, you can learn from these involuntary reactions and better protect yourself from threats (like that boiling water) in the future.

ADDITIONAL FACTS

  1. People typically blink about 15 times per minute—or 14,400 times a day if they spend 16 hours awake.
  2. Doctors aren’t sure why, but some people sneeze when they look at a bright light or walk outside into the sunlight.
  3. Babies are born knowing how to suck milk from a breast (the rooting reflex), how to tightly grab onto objects placed in their hands (the grasping reflex), and how to move their extremities and cry out when startled by a loud noise (the Moro reflex).