While I’m sitting on the exam table with my legs dangling, the neurologist reaches for a metal object. He’s already poked me several times with the safety pin so I’m wondering what he plans to do with this larger pointy thing. He grasps my foot and scraps the object on the bottom of my foot along the outermost side and under the toes. My big toe jumps and the other toes look like they are trying to get away from the torture device. I have just demonstrated a positive Babinski sign.
During a standard neurological exam, the doctor will test many reflexes, or involuntary responses to stimuli. Much of that is done with a rubber mallet as the doctor taps various tendons and measures the response. However not all reflex tests involve the rubber mallet. One very important reflex test involves scraping the bottom of the foot.
“When the doctor scraps the bottom of my feet, what is he looking for?”
A normal response in anybody older than two years of age would be the big toe flexing downward (toward the sole of the foot) or nothing. If your big toe jumps upward or extends (even if for a very brief second) and/or your toes fan out, this would be a “positive” extensor plantar response or the Babinski sign.
“What causes the big toe to jump up?”
In newborns and infants up to age two, the central nervous system is not fully developed with portions of the corticospinal tract still unmyelinated. If communication between the brain and the foot is disrupted, the stimulus of scraping the bottom of the foot causes the muscle on top of the foot to flex and the big toe to extend. The Babinski sign can vanish as early as 12 months of age or as late as three years of age as the corticospinal tract matures. In a person with a fully-functioning central nervous system, the Babinski sign does not return.
“What is the corticospinal tract?”
Very long nerve axons which originate in the part of the brain called the cerebral cortex travel through the brainstem, cross over at the top of the cervical spine and travel down each side of the spinal cord. This path is the corticospinal tract which is sometimes called the pyramidal tract since the area where the crossover of nerves occurs has a pyramid-like shape.
Corticospinal tract neurons are referred to as “upper motor neurons” but they do not control muscles directly. Neurons in the ventral horn that directly innervate (or stimulate) muscle are called lower motor neurons. It is damage in lower motor neurons which causes atrophy of muscle, while damage in upper motor neurons does not.
A positive Babinski sign indicates damage or lesions along the corticospinal tract.
“If I have a positive Babinski reflex, what symptoms might I experience?”
Often a person (older than an infant) who has a Babinski's reflex will also have incoordination, weakness, and difficulty with muscle control.