220 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”in the synaptic connections, while long-term memories require, in addi-
tion, gene activation and protein synthesis, along with permanent struc-
tural alterations in the synapses.
From the early work of the Spanish anatomist Santiago Ramon y
Cajal and subsequently the research of the British physiologist Charles
Sherrington, the importance of synaptic connections as “switches” for
the passage of information had been recognized. In 1949, the Canadian
psychologist Donald Hebb proposed the “synaptic theory” of learning,
stating that repeated firing across a synapse facilitates the future fir-
ing across the same synapse. However, demonstrating this process is a
formidable task. Since each neuron is equipped with thousands of syn-
apses, and there are 100 billion (1 x 10^11 ) neurons in the human brain,
the total number of synapses per brain is astronomical, exceeding the
number of stars in our galaxy. The brain, in a sense, is an enormously
complex information-processing machine, with multiple parallel compu-
tations and re-entrant processing, along with endless feedback and feed-
forward loops. The only way to catch a glimpse of this picture is to seek a
simple system, where interaction between single neurons can be demon-
strated. Such a system was found in the invertebrate Aplysia califor-
nica (sea slug), a giant marine snail capable of exhibiting simple reflexes
such as gill retraction as a defensive response when the body is touched
(Fig. 10.4). Largely through the work of Eric Kandel and his associ-
ates, much cellular and molecular underpinnings of memory have been
obtained by studying this model.^11
The abdominal ganglion of Aplysia contains about 2,000 cells, a
small number compared to the mammalian nervous system, many of
which are large and identifiable. Those involved in gill withdrawal reflex,
a defense response, are amenable to behavioral manipulations, including
habituation, sensitization, and classical conditioning. Normally, lightly
touching the siphon leads to gill withdrawal. Repeated touching (a
harmless stimulus) leads to habituation (ignoring the stimulus). Elec-
trical shock (noxious stimulus) applied to the tail followed by touching
of the siphon facilitates the response to touch (called sensitization).