Neurobiology
Brain cortex structure
The brain cortex is the main residence of consciousness. All sensory stimuli are
realized only when they reach the brain cortex and not before that! Nieuwenhuys
(1994) outlines the origin and evolutionary development of the neocortex. A
cortical formation is lacking in amphibians, but a simple three-layered cortex is
present throughout the pallium of reptiles. In mammals, two three-layered cortical
structures, i.e. the prepiriform cortex and the hippocampus, are separated from
each other by a six-layered neocortex. Still small in marsupials and insectivores,
this "new" structure attains amazing dimensions in anthropoids and cetaceans.
Neocortical neurons can be allocated to one of two basic categories: pyramidal
and nonpyramidal cells. The pyramidal neurons form the principal elements in
neocortical circuitry, accounting for at least 70% of the total neocortical
population. The evolutionary development of the pyramidal neurons can be
traced from simple, "extraverted" neurons in the amphibian pallium, via pyramid-
like neurons in the reptilian cortex to the fully developed neocortical elements
designated by Cajal as "psychic cells".
Typical mammalian pyramidal neurons have the following eight features in
common: (1) spiny dendrites, (2) a stout radially oriented apical dendrite, forming
(3) a terminal bouquet in the most superficial cortical layer, (4) a set of basal
dendrites, (5) an axon descending to the subcortical white matter, (6) a number
of intracortical axon collaterals, (7) terminals establishing synaptic contacts of the
round vesicle/asymmetric variety, and (8) the use of the excitatory aminoacids
glutamate and/or aspartate as their neurotransmitter. The pyramidal neurons
constitute the sole output and the largest input system of the neocortex. They
form the principal targets of the axon collaterals of other pyramidal neurons, as
well as of the endings of the main axons of cortico-cortical neurons.