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From the simple to the complex
Function by Level of Organization

Help Link Module : Jean-Pierre Changeux, L'homme neuronal, Fayard, Paris, 1983. (379 p.) Linked Module: Brain circuitry Link: Brain Anatomy
Link : Association cortex (animations) Link : Animation : Reflexe Arcs Linked Module: Le Cerveau !  Sciences & Découverte (Arte)
History Module : Cognitive functions : phrenology vs History Module : Le cerveau à travers les âges

We Are All Complex Networks

To process the information that it needs in order to function, the brain employs many structures simultaneously. In other words, it operates on parallel circuits. The same piece of information may be processed simultaneously in several "centres" of the brain. This redundancy explains the brain’s ability to recover from injuries.

The brain’s centres must therefore be designed more like strategic crossroads or points through which certain data must pass in order to be processed, rather than like the sole sites where certain functions are executed.

The proportion of white matter (axons) to grey matter (neural cell bodies and dendrites) is far higher in the human brain than in the brain of a rat, for example. Human neurons make many interconnections with one another, which involves more axons and hence more white matter.

The connections that make the brain a “wired” organ are made possible by the extensions of its neurons, which are called axons. These axons can be distributed diffusely or concentrated in bundles that form the brain’s white matter.

No nerve impulse ever encounters a “dead end” in the brain. The point where it arrives in any part of the brain is always a potential point of departure toward other neurons. This assemblage of billions of circuits that loop back on themselves makes it very difficult to have “entirely rational thoughts” or “purely emotional reactions”.

Some researchers have even posited that each neuron in the brain is separated by only a few synapses from every other, just as it is often said that each person on Earth is an average of only six degrees of separation from every other. However, none of us really communicates with more than a few hundred individuals over the course of a lifetime. Likewise, the brain’s neurons make significant connections only with certain other, very specific neurons.

Thus, using various methods, researchers have identified some very specific circuits by which certain regions of the brain interact with certain others.

Tool Module: Identifying Pathways in the Brain

The simplicity of the knee-jerk reflex, which is involved in maintaining a standing position, allows the three basic parts of a neural circuit to be readily distinguished: the sensory input, the information processing, and the motor output.

Each of these three parts of the circuit corresponds to a different kind of neuron. First, the sensory neuron detects the stretching of the muscle. Since this reflex involves two opposing muscles, several connections are needed to process the information (which is why this is classified as a polysynaptic reflex).

An excitatory synapse directly stimulates a motor neuron responsible for contracting the extensor muscle. But in addition, another branch of the sensory axon stimulates an inhibitory interneuron that reduces the activity of the motor neuron for the flexor muscle.

This circuit comprises only a few synapses and is very short, which is why it acts so quickly. It also functions very well with no intervention by the conscious brain, which has better things to do than constantly remind the body to stay upright.


Now let us look at another example of a neural circuit: this time, the one involved in reading something out loud.

After making a first connection in the thalamus (not shown in the diagram here), the sensory stimulus reaches the primary visual area, where it is decoded.

The resulting information is then sent to Wernicke’s area (for understanding words) and Broca’s area (for analyzing syntax), where it is compared with similar information already stored in memory.

This new signal will be taken up by the motor cortex, which then coordinates the harmonious muscle contractions involved in pronouncing sounds.

Thus, to even hope to understand complex functions such as language, a characterization of the circuits of the cortex is essential.

This second diagram shows the physical route of the arcuate fasciculus, the bundle of nerve fibres that connects Wernicke’s area (green) to Broca’s area (yellow).


Link : Vincent, Jean-Didier. Biologie des passions. Odile Jacob/Seuil. Paris; 1986 (352 p.) Link : "Three Topographical Arrangements" Model of Mind Link : Propagation of the action potential

Neuroendocrinology is the discipline that seeks to overcome the outmoded image of the body as having two separate communication systems–a low-speed system that uses hormones to maintain the body’s general equilibrium and a nervous system that manages immediate adaptations, movements, and sensations.

On the contrary, as well as acting on the body rapidly via the nerves, the brain greatly influences the body’s glands and their hormones. But at the same time, these hormones can modify the brain’s own operation through the neurons’ receptors for these hormones. The body-brain system thus depends on the state of its hormones, which in turn depend on the brain, which itself reacts to its external environment.

Tool Module: Cybernetics


The neurons of the hormonal brain differ from those of the wired brain in several ways. The hormonal neurons are concentrated mainly in the brainstem and the central region of the brain. They form small masses of thousands of cells, but these cells project their axons into large areas of the forebrain and the midbrain.

Just one of these neurons can therefore influence over 100,000 others through the neuromodulators that it secretes into the brain’s extracellular space (rather than into a synaptic gap).

The effects of these neuromodulators take longer to become established and last longer than those of the neurotransmitters in the circuits of the wired brain. One reason for these differences is that groups of neurons called “second messengers” are also involved in the neuromodulators’ action mechanism.

Each of these groups of neurons projects its axons into large areas of the central nervous system and thus modulates numerous behaviours.

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