Brain Structure - Brain Theory from a Circuits and Systems Perspective - page 11

Sensory Inputs
Sight, sound, touch, taste, and odor send over a million parallel signals via appro-
priate nerves to respective parts of the brain. Sensory signals are encoded into a useful
set of logical signals termed attributes. The 1.2million axon fibers in each eye's optic
nerve, for example, are first processed in the thalamus, then in the amygdala for
emotional analysis, and then within credit card-sized areas in the occipital lobes.
Additional processing occurs in the cerebral cortex. As a result, the existence of
numerous attributes such as edges and textures become available.
Each attribute construed from the senses has its own unique cells and its own
physical region. In a distributed model, each memory element, for instance, is
dedicated to a particular attribute such that no other attribute would ever enter it.
Memory like this has been termed distributed memory [ 2 ] where each attribute has
its own respective memory path. It is biologically efficient, but cannot be used in
man-made machines since it results in excessive wiring.
The process of converting analog sensory inputs into attributes, each true or
false, denoting the presence or the absence of a given feature, is termed encoding.
The brain continuously encodes sensory information into attributes, which surely is
a major undertaking. Sensory encoding is beyond the scope of this topic.
Another form of encoding that is completely unrelated to sensory encoding is the
creative generation of mnemonics as an aid in remembering random facts.
Man-made microprocessors employ encoding so that a small number of wires
can be shared and made to send a large amount of information (for example, n wires
carry 1 of 2 n binary codes). In a brain, however, binary codes are not generally
advantageous, since there is an abundance of neurons for parallel connections.
The Amazing Neuron
A neuron can be schematically portrayed as in Fig. 1.5 . Neural signals generally
move from dendrites to soma, to axon, with many synaptic inputs, and many
outputs of a given result at the tips of the axon. Although there is only one axon
per neuron, there can be a large fan-out.
Boutons, Receptors, Spines
There are about 500 trillion synapses in the adult brain, and twice as many in a
child. A synapse includes a presynaptic region, or a bouton that can release
excitatory neurotransmitters from its vesicles as a result of an incoming pulse
burst. Once released these are sensed by postsynaptic receptors, quite numerous,
located mainly on spines. Synapses involve a narrow gap of about 20 nm between a