Panthenol is the alcohol form of pantothenic acid, more
familiar as Vitamin B5. In a living cell, panthenol is converted
to pantothenic acid, which then becomes an important part of the
compound "Coenzyme A", which is important in cellular metabolism.
In hair, which contains no living cells, it remains as panthenol.
Because it could become a vitamin if it were ever to get to a living
cell, it is marketed as a "provitamin", even though its effects as a
vitamin are never used.
Because panthenol binds well to hair follicles, and attracts moisture
from the air, it is a moisturizing agent used in shampoos and
conditioners. It lubricates the hair without feeling greasy.
It smoothes roughened hair surfaces, making them shiny and easier
Panthenol does not "nourish" hair. It coats it to make it slippery.