Most modern theories of semipermanent structural change (or _engrams_,
as they are sometimes called) look either to the molecular level or to
the cellular level. Various specific locales for the engram have been
suggested, including (1) modifications of RNA molecular structure,
(2) changes of cell size, synapse area or dendrite extensions, (3)
neuropile modification, and (4) local changes in the cell membrane.
There is, in fact, rather direct evidence of the growth of neurons or
their dendrites with use and the diminution or atrophy of dendrites
with disuse. The apical dendrite of pyramidal neurones becomes thicker
and more twisted with continuing activity, nerve fibers swell when
active, sprout additional branches (at least in the spinal cord) and
presumably increase the size and number of their terminal knobs.
As pointed out by Konorski (11), the morphological conception of
plasticity according to which plastic changes would be related to the
formation and multiplication of new synaptic junctions goes back at
least as far as Ramon y Cajal in 1904. Whatever the substrate of the
memory trace, it is, at least in adults, remarkably immune to extensive
brain damage and as Young (24) has said: “ ... this question of the
nature of the memory trace is one of the most obscure and disputed in
the whole of biology.”