The plantar reflex is an automatic reflex that is activated when the sole of the foot is stimulated by a blunt object; such as a miniature, the cap of a ballpoint pen, or the handle of a typical reflex instrument.
In a healthy adult, the reflex will cause the toes to flex downward, away from the shin.
In a healthy newborn, the reflex will cause the toes to flex upward toward the shin.
This upward movement of the toes toward the shin is called the Babinski sign, named after the French neurologist Joseph Babinski. The Babinski sign is considered a normal finding for a newborn, but when a child begins to walk, the Babinski sign should be inhibited and disappear.
When the sole of the foot is stroked from the heel to the toes, a message is sent up the tibial nerve, through the sciatic nerve, to the L5 and S1 nerve roots. This is where the motor response is triggered, causing innervation of the finger flexor muscles through the deep peroneal nerve, drawing the fingers away from the shin. All of this happens automatically in a normal adult.
In a baby, this is not the case. The corticospinal tract, a part of the spinal cord that runs from the brain downward, is not fully developed in the newborn. It lacks the fine myelin sheath that will develop later in the first year of life. Without myelin (as in multiple sclerosis), the messages that are sent through the nerves are disrupted and the reflex circuit cannot be inhibited, as it would normally be.
Until the myelin sheath develops, most babies will show an extensor response, with the toes pulling up (this movement is called dorsiflexion).
So why is this test so important?
If the Babinski sign remains in the baby after one year of life, or is present in any adult, this can identify a disease of the spinal cord and brain, specifically in the corticospinal tract. Sometimes a pathological plantar reflex is the first finding indicating a disease process because it is quick and easy for any healthcare provider to perform.
If Babinski’s sign is not present in a newborn, this possibly suggests a neurological abnormality, and the baby should be monitored for spasticity or other neurological findings.
As a practicing chiropractor who works with many newborns, evaluating the plantar reflex and observing the Babinski sign is an important part of newborn screening. As you continue to observe the regression of this sign during their visits to the chiropractor during the first year of life, you begin to understand that that baby’s body is progressing closer to having a fully developed corticospinal tract, something that is required for them to walk normally. .