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From the simple to the complex
Anatomy by Level of Organization

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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.


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.

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.

Tool Module: Cybernetics

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.

The following diagram gives an idea of the complexity of the cortical circuits that are known to be involved in processing visual information.
Credit: Felleman and Van Essen's Circuit Diagram of the Macaque Brain as of December 1990

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.

Credit: The Digital Anatomist



Ventral tegmental system: neurobiologyPropagation of the action potential

The neurons of the brain secrete substances that regulate the functions of the glands in the rest of the body. The main pathway through which this regulation occurs passes through the pituitary gland, a small gland that is located at the base of the brain and whose hormones influence practically all of the other glands in the body. The pituitary gland is strongly influenced by the neuromodulators produced by the hypothalamus (which are therefore classified as neurohormones).

For example, by diffusing dopamine into the blood vessels that irrigate the pituitary gland, the hypothalamus inhibits this gland’s production of prolactin, which in turn reduces the stimulation of the mammary glands.

The reverse is also true. Several peptide hormones are present in many central structures, where they in turn influence brain activity.

Tool Module: Cybernetics


Many drugs and medications, in particular those prescribed for affective disorders and schizophrenia, act on the neuromodulators used by the diffuse projection neurons of the brainstem.

For this reason, despite their small numbers, the function and distribution of the projections of these neurons have been the subject of much research.

This research has required the use of various tracing techniques, because the axons of these neurons are not myelinated and so do not form readily identifiable bundles. The results have confirmed how widely diffused these projections in fact are. For example, a single axon from one of these neurons may subdivide and innervate both the cortex and the cerebellum.

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