Eric Wassermann (ewass at helix.nih.gov) wrote:
: In article <2jec03$qu4 at gap.caltech.edu>, rawlings at cco.caltech.edu (Jeffrey
: Rawlings) wrote:
: > exb0405 at csdvax.csd.unsw.edu.au (BARRY MANOR, NO DOUBT.) writes:
: > >Something I'm curious about. During the course of the adult lifetime, say even
: > >from minute to minute, does synaptic topography change dynamically ? Are
: > >synapses 'broken' and formed as part of normal ongoing brain function ?
: > I would say "probably." A couple of relevant lines of research:
: > -In developing chick optic tectum, Scott Fraser and
: > colleagues have imaged individual axonal arbors on a time scale
: > of an hour (maybe finer, now). The results: there are different
: > classes of branch points with different lifetimes, and you can
: > get very significant remodeling of entire arbours over fairly short
: > time spans. There is of course the assumption that these branches
: > actually contain functional synapses, and this is in developing, not
: > adult, chick.
: > -In adult monkey, Mike Merzenich has demonstrated large changes
: > in the receptive fields and maps in somatosensory cortex following
: > training of the monkey. These changes occur (I think) on the order of
: > weeks-months, although some may be faster. Presumably, synapses
: > would have to be formed and broken on a large scale to get these changes.
: Experiments in our lab with transcranial magnetic stimulation showed
: expension of the cortical representation of muscles proximal to an ischemic
: nerve block in the arm within minutes of cuff inflation. Kind of like
: instant Merzenich and too fast for anything but loss of inhibition from the
: deafferented part of the limb representation (see papers from Brasil-Neto
: et. al: Neurology 42, Brain 116).
: As far as I know,
: Eric Wassermann The opinions expressed are not
: Human Motor Control Section those of the Federal Government,
: NINDS, NIH the U.S. Public Health Service
: or the National Institutes of Health
There is also some interesting work published by Dale Purves and
colleagues on autonomic ganglia that show that dendrites (in the superior
cervical ganglion) and synaptic boutons (on submandibular ganglion
neurons) undergo significant reorganization over periods of a few
weeks.There is also some suggestion that under certain circumstances
dendritic remodelling can occur as rapidly as within a few hours, but this
is quite controversial still.