The plantar reflex is an automatic reflex triggered when the bottom of the foot is stimulated by a blunt object; such as a thumbnail, pen lid, or handle of a typical reflex instrument.
In a healthy adult, the reflex will cause the toes to flex downwards, away from the shin.
In a healthy newborn, the reflex will cause the toes to flex upwards, towards the shin.
This upwards motion of the toes towards the shin is called the Babinski sign, named after 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 towards the toes, a message is sent up the tibial nerve, through the sciatic nerve to the nerve roots of L5 and S1. It is here that the motor response is elicited, causing innervation of the toe flexor muscles through the deep peroneal nerve, moving the toes away from the shin. This all happens automatically in a normal adult.
In an infant, this is not the case. The corticospinal tract, a part of the spinal cord that runs from the brain downwards, is not fully developed in the newborn. It is lacking the thin myelin sheath that will develop later on in the first year of life. Without myelin (such as in multiple sclerosis), the messages being sent through the nerves are altered, and the reflex circuit cannot be inhibited, as it would normally be.
Until the myelin sheath develops, most infants will show an extensor response, with the toes pulling upwards (this movement is called dorsiflexion).
So why is this test so important?
If the Babinski sign remains in the baby past one year of life, or is present in any adult, this can identify disease of the spinal cord and brain, specifically to the corticospinal tract. Sometimes, a pathological plantar reflex is the first finding to indicate a disease process because it is quick and easy for any health care provider to do.
If the Babinski sign is not present in a newborn, this is possibly suggestive of neurological abnormality, and the infant should be monitored for spasticity or other neurological findings.
As a practicing chiropractor working with many newborns, assessment of the plantar reflex, and observation of the Babinski sign is an important part of a newborn analysis. By continuing to observe the regression of this sign during their chiropractic visits throughout 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.