Not only is this such good news if we thought that our nervous systems were in a state of constant decline, but also this update to parts of Book 4 and Book 6 is so important that I have reproduced it here for you. The following text is taken directly from the Bulletin, written by the Course Team.
In Book 4 (p.117) the birth of new neurons (neurogenesis) in the brain of the adult canary is mentioned and the question is posed as to how prevalent is the phenomenon? Well, it now seems that the answer is: rather more prevalent than we, the Course Team, thought when we wrote the book.
In recent years, considerable evidence has accumulated that new neurons do arise in mature brains. You may have seen the article by Alison Motluk in the New Scientist of l2th February 2000 (Volume 2225, pp.25-28), in which the case for this neurogenesis in adult human brains is put. Whilst there remains disagreement over the extent of neurogenesis, both in the numbers of neurons born and in the regions of the brain in which it occurs, the evidence now seems overwhelming that neurogenesis occurs not only in adult fish, amphibians, reptiles and birds, but also in adult rodents, primates and humans.
It seems appropriate then to update the balance of the message in the course from 'neurogenesis does not happen in adult mammals' to 'neurogenesis occurs to a limited extent in adult mammals, including humans, notably in the hippocampus but probably in the cortex and other regions too'. This latter statement should be taken as representing the current SD206 course position.
Because of this we would be pleased if you would amend parts of SD206
Books 4 and 6.
There is however, increasing evidence that new neurons are produced
in vertebrates, and even in the brains of adult birds (see p.117) and mammals.
The lizard's tail is a good example. The end portion of the tail breaks off relatively easily, providing the animal with a means of escape from a predator that grabs it by the tail. Following the loss, a complete new tail, including muscles, vertebrae and spinal cord regenerates from the broken stump. The connections to and from the rest of the central nervous system are also re-established. Another example is the ability of the eyes of many fish and amphibians to regenerate from a small piece of the dark pigmented cells at the back of the eye. In many fish and amphibians, the eye, like the rest of the body, continues to grow throughout life. The eye gets larger by the addition of new retinal cells around the eye margin, and the axons from the added retinal ganglion cells grow to terminate in the brain. Here development continues until death.
In mature mammals, new neurons have been found in the olfactory system, and more recently in the hippocampus and the cortex of rats and primates. Olfactory receptors are modified bipolar neurons, located in the nose in a delicate membrane that is being continuously destroyed. Thus, the olfactory receptors have to be replaced throughout life in all mammals. Their axons have to grow through the mucosal membrane and through the bone of the skull to their targets in the olfactory bulb at all ages. There may be a similar kind of loss and replacement in the dentate gyrus. New neurons in the cortex may well arise from the sub-ventricular zone (where the original neurons were born) and migrate away, but it is not yet clear whether neurogenesis in the cortex is ongoing or only occurs in response to some specific stimulus, e.g. neuronal damage.
The recovery of function following injury will depend very much on the timing of the injury and also on its location and severity. In addition, there is often a large increase in glial cells at the site of injury, growing what is known as a glial scar. Whilst it is clear that some lost neurons will be replaced, the relationship between neurogenesis and recovery of function remains to be uncovered.
The growth or re-growth of damaged axons is considered next.
The spontaneous replacement of cells, as seen in other bodily organs,
is seen in the human brain, though only to a limited extent. The proliferation
of neurons in the human brain by cell division is much diminished soon
after birth, although the complexity of fine structure of the nervous system,
indicated by the number of axon branches, continues to increase over several
years (Book 4, Chapter 3). Teuber's and Kertesz's findings show that there
can be recovery of function even where damage occurs to the adult brain.
The extent to which the brain can rebuild its structures by regenerating
neurons is unknown, and until recently was thought not to occur at all.
The following sections consider mechanisms other than neurogenesis that
can be invoked to explain recovery of function after damage.