brain weighs three pounds and contain an estimated 100 billion nerve
cells. The cerebral cortex, most recently in our evolutionary history,
contains about 30 billion neurons and one million billion connections,
or synapses. If we counted one synapse per second, we would only be
ready after 32 million years. The number of potential neuronal circuits
is ten, followed by at least one million zeros. A hyper astronomical
amount, given the number of particles in the known universe being
10, followed by 79 zeros.
is a wide variety of types of neurons, but they all have tree-like
branches, called dendrites, which receive synaptic connections. They
also have a longer branch, an axon, which makes synaptic connections
at the dendrites or at the cell bodies of other neurons. There are
probably fifty different types of neurons, but possibly more. An important
feature of neural patterns at the microscopic level is their density
and distribution. The body of a single neuron is about 50 microns
in diameter, which is several thousands of a millimetre. But one axon
may be several meters in length. Especially in the cerebral cortex,
the neuronal tissue is extremely dense, which also demands an extraordinary
blood supply to feed this jungle of cells. The brain is the most metabolically
active organ of the body.
hallmark of neurons from other cells in the body is that they are
able to communicate with each other through the synapses mentioned
above. The majority of synapses in human brains is chemical. The inside
of a neuron is negatively charged relative to the outside. After a
cell is stimulated as a result of ion currents like that of sodium
and potassium, it becomes less negative. The result is an electrical
signal, called an action potential, which moves through the axon to
synapses. That allows for the release of neurotransmitters in the
pre-synaptic neuron. When the neuron is excitatory, the released neurotransmitters
pass the cleft, after which they bind to specific receptors on the
postsynaptic neuron, which then becomes less negative. This process
takes place in a time period of hundreds to tenths of milliseconds.
If the postsynaptic neuron becomes sufficiently less negative after
several such events, it will fire, generating an action potential
itself. Inhibitory neurons work in a similar way, but they change
the electrical charge of the postsynaptic neuron in order to prevent
it firing. This electrical signalling leads to changes in biochemistry
and even the expression of genes in the target neuron.
addition to these micro-anatomical characteristics of the brain at
higher levels we find important organizational principles of which
the following three are essential for their overall performance.
The principle player is the thalamo-cortical system, which consists
of separate but highly integrated networks. Theb thalamus, seated
In the middle of the brain, receives information from the senses and
the rest of the body and is reciprocally connected to the cerebral
cortex at the surface of the brain. This cortex has six layers, each
with a specific set of inputs and outputs. The cortex and thalamus
are subdivided into many areas, each with different functions. This
functional separation can be seen at many different spatial scales.
Thus the back of the thalamo-cortical system is roughly devoted to
perception, while the front is dedicated to action and planning. Most
of these cortical areas are grouped as maps: related neurons from
one area are connected to neighbouring nerve cells in the other. The
integration is striking: most of these groups of neurons are interconnected
in a reciprocal pattern. Neurons within the same group at a particular
location are closely connected, so many of them respond simultaneously
when the appropriate stimulus is presented. Paths from one area to
another are called projections.
Neuronal groups that respond to near positions in the visual field
are more connected than those responding to more distant positions.
In other words, the various separate, functional areas are mutually
connected which makes integration of distributed brain functions possible.
They provide the main basis for reentry, a process of signalling along
reciprocal connections between functionally segregated areas, which
thus can be integrated with each other despite the absence of a central
The organization of the thalamo-cortical network is extremely suitable
for integrating large amounts of specialists in a unified response.
The thalamo-cortical system is considered as an important contributor
to the content of consciousness and forms the basis for my model of
the selfreferenced unified informational field (SUIF) that I consider
as a conceptual bridge between the neural correlates of consciousness
(NCC) and phenomenological (first person) awareness.
second topological arrangement is organized as a series of parallel
unidirectional pathways, which connect the cortex with some 'appendages',
each with a special structure: the cerebellum, the basal ganglia and
cerebellum regulates coordination and synchronicity of movement, but
also plays a role in thought and language.
The basal ganglia are a large number of nuclei deep in the brain that
receive many connections from a large part of the cortex, which then
via a number of synaptic steps project to the thalamus and thence
back to the cortex. The function is to plan and execute complex motor
acts. In neurological diseases like Parkinson's and Huntington's it
is these areas that are dysfunctional.
hippocampus is a structure, extended along the lower part of the inside
of the temporal cortex, at which input from many cortical areas is
funnelled. These synaptic inputs are processed in a number of steps
and returned to the same cortical areas. The hippocampus plays a role
in consolidating short-term memories into long-term memory locations
in the cerebral cortex. Also plays a role in orientation in time and
way these Appendices are organized is the same: long parallel paths
with multiple synapses leave the cerebral cortex and reach the successive
synaptic stations in the appendices and project back, whether or not
via the thalamus, to the cortex. This serial, polysynaptic architecture
is fundamentally different from that of the thalamo-cortical system:
connections are not reciprocal but form long loops and there are relatively
few horizontal interactions between the circuits (except those serving
third system consists of clusters or chains, but is a diffuse set
of connections that resembles a big fan. The origin of the fan is
a relatively small number of neurons in specific nuclei in the brainstem
and the hypothalamus. They release specific substances (neuromodulators)
such as norepinephrine, serotonin, dopamine, acetylcholine and histamine.
Each of these nuclei project diffusely to many parts of the brain
and fire when something important happens, such a loud noise, a flash
or sudden pain. They affect not only the overall excitability of other
neurons but also the plasticity of their compounds as they can change
the strength of synapses, serving their adaptive function.
Edelman speaks of value systems. These also play a role in many neurological
and psychiatric dysfunctions. Small changes can have large effects
on mental functions.
source: G. Edelman - Consciousness