In article <2jec03$qu4 at gap.caltech.edu>, rawlings at cco.caltech.edu (Jeffrey Rawlings) writes:
|>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 ?
|>|> -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.
|> >what exactly is meant ?
|> --
|> Jeff (rawlings at cco.caltech.edu)
|> --------------------------------------------------------------------
Re: M. Merzenich's work, It is not known if synapses are formed or broken
inthe course of map changes. It *is* clear that the anatomical borders as
defined by dendritic arborization spread do not correspond to the edges of
maps. Therefore, all that is necessary is for existing synapses to become
more or less active or in some manner relevant to the output of the
system. Merzenich emphasizes activity dependent changes in cortical maps
across standard anatomical boundaries.
Clear cases of change in synpatic number with changes in sensory input are
rather scarce. A fairly recent one is in the peripheral endorgans in
the in rat vestibular system. The utricle and sacule are the two linear
accelerometers in the inner ear. When rats were sent into space (zero
gravity) on the space shuttle, the number of synapses within hair cells
(the receptor cells) decreased. Opposite results were observed in rats
exposed to hypergravity. See Ross, M.D. Morphological changes in rat
vestibular system following wieghtlessness. J. Vestibular Res. 1993,
3:241-251.
Thomas
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