A video demonstrating the Plantar reflex in newborns.
Plantar reflex: Definition
The plantar reflex is a reflex elicited when the sole of the foot is stimulated with a blunt instrument. In normal adults the plantar reflex causes a downward response. In newborn babies, the response is upwards, also known as Babinski's sign.
A baby's smaller toes will fan out and their big toe will dorsiflex slowly. This happens because the corticospinal pathways that run from the brain down the spinal cord are not fully myelinated at this age, so the reflex is not inhibited by the cerebral cortex. The extensor response disappears and gives way to the flexor response around 12 to 24 months of age.
In adults, the presence of Babinski's sign can identify disease of the spinal cord and brain in adults.
Definition source: http://en.wikipedia.org/wiki/Plantar_reflex