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The nervous system’s circuits
include mechanisms by which the efficiency
of all the synapses in a group (and hence the relative
weight of each component in a neural network) can be adjusted
continuously.
These mechanisms are also used to place
all the neurons in a group of neurons “on the same
wave length.” They can then all be activated simultaneously,
which gives them more impact on each other than on neurons
outside the group. In this way, the brain is believed to
form original, temporary configurations of neurons, known
as neuronal assemblies. |
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The operation of the circuits of the cortex
is often (but not always) schematized in terms of three processes: information input,
synthesis and comparison, and output and action.
Information input to the nervous
system comes through receptors that are sensitive either
to variations in the outside world that are perceived by the
sensory organs or to variations within the body, such as changes
in body position.
A receptor may itself be the first neuron
in this process (as in the olfactory bulb), or it may simply be
in close contact with the first neuron (as is the case for the
photoreceptors in the retina). Before the nerve fibres emerging
from a given sensory organ reach the primary cortical area where
the inputs that they provide are processed, almost all of them
make at least one connection in the brain’s subcortical centres,
of which the main ones are the thalamic nuclei. In fact, each of
the other cortical areas, whether motor or associative, receives
nerve fibres from a thalamic nucleus dedicated to that particular
area.
| Another important cortical input consists
of fibres from the cortex itself, from either the same hemisphere
or the opposite one. These fibres associate several areas with
one another and are therefore called associative fibres. |
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Once sensory signals arrive in their primary
cortical area, they quickly diverge into various local circuits
responsible for information processing. All of
these cortical microcircuits comprise the same types of cells distributed
in the same six layers of the cortex. The function of a cortical
area is thus determined more by its inputs and outputs than by
the intrinsic organization of its local circuits.
The results of the “computations” performed
by these microcircuits ultimately converge at pyramidal neurons
whose axons are the only output pathways from the cortex.
A high proportion of the axons that leave
the cortex return to it, either on the same side of the brain or
on the opposite side. But other axons emerging from the cortex
terminate in subcortical centres such as the thalamic nuclei, where
they come into contact with the sensory fibres that send their
axons to the cortex. “Loop” circuits that return
signals to the cortex can thus be formed at this location.
This is a fundamental characteristic of the
way that the brain processes information. At every stage, some
of the fibres and connections loop back to the preceding stage
to provide it with feedback that helps to control it. For instance,
such feedback loops enable the brain’s motor control centres
to correct and adjust their signals to the muscles, right up to
the moment these signals are sent. It is feedback loops like these
that let you, for example, keep your balance while walking against
sudden gusts of wind. This same phenomenon of feedback loops is
also found in bodily reflexes, such as the leg withdrawal reflex.
In this reflex, the sensory neuron
that detects the painful stimulus sends this information
to various interneurons, some of which excite motor neurons,
and others of which inhibit them.
One interneuron excites the motor neurons for the flexor
muscle in the stimulated leg, causing this muscle to contract.
At the same time, another interneuron inhibits the motor
neurons for the extensor muscle in this same leg, causing
this muscle to relax. The net result: the leg that receives
the painful stimulus reflexively withdraws from it and lifts
off the ground.
Meanwhile, the opposite leg will also be influenced by this
sensory input, by means of a “cross-reflex.” In
this case, the effects are reversed. On this leg, the motor
neurons of the extensor muscle are excited, while those of
the flexor muscle are inhibited. This opposite leg therefore
straightens out and becomes more rigid and stable, to support
the additional weight placed on it when the other leg withdraws
from the ground.
Short, simple reflex circuits like these let the body make
quick, simple reactions to protect itself. In contrast, a
complex task such as playing a piano involves highly complex
connections, because it requires the pianist to contract
and relax so many different muscles simultaneously. |
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| The following diagram gives an idea
of the complexity of the cortical circuits that are known
to be involved in processing visual information. |
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Credit: Felleman
and Van Essen's Circuit Diagram of the Macaque Brain
as of December 1990 |
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Dissection of the left hemisphere,
showing the arcuate fasciculus (the network of long associative
fibres that connects the auditory and motor associative
cortexes while going around the end of the fissure of
Sylvius.
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Credit: The
Digital Anatomist |
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