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From the simple to the complex

Function by Level of Organization

Your brain’s circuits do not just stay the same forever. Instead, the plasticity of their synapses lets these circuits continue to change throughout your lifetime. For example, your brain creates memories that change its internal “wiring” and hence your subsequent way of thinking. It could even be said that the brain’s primary activity is to produce changes in itself!

Like Dr. Jekyll and Mr. Hyde, the human brain has two apparently contradictory aspects: one composed of precisely “wired” neural circuits, and another that is more like a soup of molecules with diffuse effects. But the contradiction is only apparent, because the “wired” brain and the hormonal brain actually complement each other remarkably well.

The brain’s wiring consists of axons: extensions of neurons that make connections with other neurons. By means of these axons, various parts of the brain can keep each other aware of what they are doing.

In this way, the “rational” cortex maintains a constant dialogue with the “emotional” limbic system and the “impulsive” structures of the hypothalamus. This is how the integration between the body’s needs and the mind’s desires is achieved.

The neural circuits constitute a fundamental characteristic of the central nervous system.

Unquestionably the simplest neural circuits in the body are the ones responsible for our reflexes.

Reflexes are fast, automatic behaviours that are very old in evolutionary terms and that do not require any conscious action on our part.

However, every reflex still comprises the same three steps involved in many other neural circuits: sensory input, information processing, and motor output.

The example diagrammed here is the reflex that controls the degree of stretching in the leg muscles to help maintain an upright posture.

This circuit involves only two neurons: the sensory neuron that senses the stretching of the muscle, and the motor neuron that keeps the muscle partly contracted.

Between these two neurons, there is just one synapse that does all the information processing, which in this case comes down to stimulating the following neuron.

A reflex of this kind, which involves only one synapse, is called a monosynaptic reflex. But polysynaptic reflexes exist as well.

Another example of a neural circuit, this time one involved in language, is the one that connects Wernicke’s area to Broca’s area in the brain.

Wernicke’s area, which is involved in understanding words, sends Broca’s area the information that it needs to analyze sentences syntactically.


Linked module: Le fonctionnement du système nerveux expliqué pas à pas : Pourquoi "endocrines"?

The brain’s “hormonal” properties
are not limited to its diffuse-projection neurons. The brain literally acts like any other gland, secreting into the bloodstream molecules that will have effects at distant locations in the body.

Many molecules act simultaneously as neurotransmitters, neuromodulators, and hormones.

Thus, the body’s two major communication systems–the nervous system and the hormonal system–are not independent but actually control each other.

Tool Module: Cybernetics


Compared with the precise circuits of the wired brain, the hormonal brain is like a diffuse soup. But this contrast is only theoretical. In real life, the two complement each other admirably well.

In the sensory and motor systems, information must be communicated rapidly from one specific point to another.

But where phenomena such as attentiveness, pleasure, sleep, and anxiety are concerned, the situation is quite different. These overall moods and states of the brain depend on neurons that project their axons much more diffusely within this organ.

The way that these neurons release their chemical messengers also differs greatly from conventional synaptic transmission. These neurons release these chemicals not into a single synapse, but into far broader areas, where they can influence the synapses of many neurons at once.

This influence of diffuse-projection neurons in the brain is called neuromodulation. Neuromodulation does not change the nature of the connection between two neurons, but instead modifies its intensity and gives it a different coloration.

It is somewhat like the volume and frequency-equalizer controls on a stereo receiver. These controls do not change the song coming in over the air, but they can radically alter its impact on your ear.

The neurons responsible for neuromodulation are located in a very specific region of the brain.

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