Extending the lifespan: problems and questions

William Calvin wcalvin at hardy.u.washington.edu
Thu Apr 30 00:20:36 EST 1992

sarima at tdatirv.UUCP (Stanley Friesen) writes:

>birth that William mentioned.  There entire axons wither and disappear,
>presumably along with the originating neuron.  This appears to be the final
>stage in establishing the primary wiring diagram of the brain.  It seems
>that the genetic program can carry the wiring only so far without environmental
>inputs to select the correct subset of the avialable wiring.

>Thus I doubt that the conclusion William made really follows.  Even if the
>trend is mainly downward in adulthood, this may just mean that learning
>is progressive.

Here is how I described the synapse creation and destruction problem in

           So there seem to be a variety of ways of editing brains: 
      killing whole cells (as happens in prenatal development and senile
      dementia, and possibly in songbirds that learn a new song every
      year), disconnecting some interconnections between selected cells
      (as happens during the tuning-up to the environment seen in
      childhood), and simple increases and decreases in synaptic
      strengths (as certainly happens in short-term memory throughout
      childhood and adult life).  Long-term memories, of the multitrial
      varieties we call schemas, likely involve both altered synaptic
      strengths -- and sometimes the creation of additional synapses by
      an existing cell budding off a new axon branch and attaching to
      another cell.
           New synapses?  All that the childhood halving of synaptic
      numbers in cerebral cortex means is that there is a difference
      between the rate at which new connections are being made and
      the rate at which old synapses are being disconnected.  Up to
      eight months after birth in humans, the creation rate exceeds the
      destruction rate; afterward, slightly more are disconnected than
      new ones are formed.  But no one knows how many synapses are
      being destroyed in the average week:  All we know is the
      cumulative difference between creation and destruction rates,
      which yields a 35-50 percent loss during childhood.  We have no
      way of tracking individual cortical synapses over time in a given
      animal, though brain imaging techniques that visualize proteins
      involved in making new synapses should eventually give us a
      clue about creation rates.  Sprouting to make brand-new
      connections gives us an additional process to modify for
      memory's sake:  We have little idea of how frequently this
      happens in adult life, or if there are favored sites for sprouting,
      or how it might be regulated.

    William H. Calvin   WCalvin at U.Washington.edu
    University of Washington  NJ-15
    Seattle, Washington 98195 FAX:1-206-720-1989

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