What does the Babinski reflex indicate when the sole of the foot is stroked in infants?

The Babinski reflex is a crucial neurological response observed in infants, typically up to about 2 years of age. When the sole of the foot is stroked, the normal response in infants is for the big toe to bend upward while the other toes fan out. This reaction is an indicator of healthy neurological development and indicates that the corticospinal pathway—which is responsible for voluntary motor function—is still maturing in the infant's brain and spinal cord.

This reflex is notable because it usually disappears as the nervous system matures and is replaced by the adult response, in which the toes curl downwards when the sole is stroked. Therefore, the upward movement of the big toe and the fanning of the other toes signify that the pathways are functioning properly in young infants.

In adults or older children, the presence of the Babinski reflex can indicate neurological damage or dysfunction, as it should not normally be present. Thus, the upward movement of the big toe and fanning of the toes in infants is a normal sign indicating appropriate neurological development.