This is the 2007 version. Click here for the 2017 chapter 10 table of contents.

Nerve Growth in Old Age

Studies in the 1990s suggested that old people given regular doses of human growth hormone (HGF) could "look and feel 20 years younger." This line of research was inspired by the observation that production of HGF normally tapers off around age 60, right when the most severe signs of aging begin to appear. However, by 2007 studies of people who began taking human growth hormone in the 1990s showed no reduction in physical signs of aging. Bathing cells in hormones typical of a younger person is not enough to maintain eternal youth.

Did HGF slow aging? How do the neurons from old rats differ from those of younger rats?

Nerve growth factors (NGF) make neurons grow. Normally, neurons grow more slowly in older people. Scheff and colleagues (1978) noted a decrease in axon sprouting after injury in old rats. Buell and Coleman (1979) found the same phenomenon in humans. Patients with senile dementia—mental confusion due to Alzheimer's Syndrome and similar disorders—had neurons that could no longer grow dendrites. However, in normal, healthy aging humans, some neurons continue to grow.

What two populations of neurons did Buell & Coleman discover?

...There are two populations of neurons in normal aging cortex, one a group of dying neurons with shrinking dendritic trees, the other a group of surviving neurons with expanding dendritic trees. In normal aging the latter population prevails. (Buell & Coleman, 1979, p.856)

Before a person enters his or her 90s, the continued growth of remaining neurons is enough to compensate for the loss of other neurons. Dendritic trees make up 95 percent of the surface of nerve cells in the cortex, so if individual dendritic trees grow, the brain as a whole stays the same size, even though the number of cells decreases.

Marian Diamond of the University of California at Berkeley found that continued stimulation was necessary to retain large dendritic trees in the brains of aging rats. When rats were kept in standard barren metal laboratory cages, instead of enriched environments, their brains decreased in size with age. The decrease in brain size was not due to nerve death; it was due to the dendritic trees on existing neurons getting smaller. Diamond commented:

How does the phrase "use it or lose it" relate to the findings of Diamond and Scheibel?

When I lecture, I show my hand-my palm is the cell body and my fingers are the dendrites. With use, you can keep those dendrites out there, extended, but without stimulation, they shrink down. It's quite simple: You use it or lose it. (Hopson & Diamond, 1984)

Dr. Arnold Scheibel of UCLA, Diamond's husband and co-researcher, added, "The dendritic projections are like muscle tissue. They grow more the more they're used." (Goleman, 1985) This helps explain why continuing an active lifestyle after retirement seems to help older people delay the negative effects of old age.


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