The complex neural circuitry responsible for our sense of smell is the only part of the nervous system that regenerates throughout adult life. It is this ability that scientists have tried to exploit in stimulating repair in the spinal cord. Every time we breathe, molecules carrying different odours in the air come into contact with nerve cells in the nose. These transmit messages to our olfactory bulbs - at the very top of the nasal cavity, sitting at the base of the brain. The nerve cells are being continually damaged and must be replaced.
— Read more in the long but fascinating article Paralysed man walks again after cell transplant at BBC News. Hat tip to Kindcrow!