Défricher un chemin à travers le cerveau

MADRID – Nos cerveaux sont comme une forêt dense – un terrain complexe, apparemment impénétrable de neurones en interaction qui déterminent la cognition et le comportement. Le grand défi est de découvrir ses mystères, c'est-à-dire comprendre la façon dont les neurones sont structurés et reliés entre eux. Où en sommes-nous par rapport à cet objectif?

En général, l'échange d'informations entre les milliards de neurones qui composent la forêt neuronale s'effectue à travers deux types de structures hautement spécialisées : les synapses chimiques (qui représentent la majorité) et ce qu'on appelle des jonctions communicantes (un substrat d'une classe de synapse électrique). La transmission synaptique chimique implique la libération de molécules spécifiques, les neurotransmetteurs, qui se diffusent à travers l'espace intercellulaire et interagissent avec des récepteurs spécifiques situés sur un neurone adjacent. Dans la transmission électrique à travers les jonctions communicantes, les membranes plasmiques de neurones adjacents sont séparées par un écart d'environ deux nanomètres (deux milliardièmes de mètre), mais contiennent de petits canaux (les jonctions communicantes) qui relient le cytoplasme des neurones voisins, permettant la diffusion de petites molécules et la circulation du courant électrique.

Le problème majeur lorsque l'on analyse le cerveau est posé par l'extrême complexité de ses connexions synaptiques. Un réseau très dense de processus occupe l'espace entre les corps cellulaires des neurones, des cellules gliales (qui soutiennent et protègent les neurones) et des vaisseaux sanguins. Cet espace (le neuropile) représente de 90 à 98% du volume du cortex cérébral humain, comprenant environ un milliard de synapses par millimètre cube de neuropile.

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