Clearing A Path Through the Brain
MADRID – Our brains are like a dense forest – a complex, seemingly impenetrable terrain of interacting neurons that mediates cognition and behavior. The great challenge is to uncover its mysteries, that is, to find out how the neurons are structured and mutually connected. How close are we to that goal?
In general, the exchange of information between the billions of neurons that make up the neuronal forest takes place through two types of highly specialized structures: chemical synapses (the majority) and so-called gap junctions (a substrate of one class of electrical synapse). Chemical synaptic transmission involves the release of specific molecules, neurotransmitters, which diffuse through the intercellular space and interact with specific receptors located on an adjacent neuron. In the electrical transmission mediated by gap junctions, the plasma membranes of adjacent neurons are separated by a gap of about two nanometers (two-billionths of a meter), but contain small channels (the gap junctions) that connect the cytoplasm of the adjoining neurons, permitting the diffusion of small molecules and the flow of electric current.
The major problem when analyzing the brain is the extreme complexity of its synaptic connections. A very dense network of processes occupies the space between the cell bodies of the neurons, neuroglia (cells that support and protect neurons), and blood vessels. This space (the neuropil) represents 90-98% of the volume of the human cerebral cortex, with an estimated one billion synapses per cubic millimeter of neuropil.