By Ornella Valenti
The cerebral cortex of mammals is the most recent, developed and specialized of the brain regions. Thousands of brain cells, the neurons, support its operation. Cortical neurons can be essentially grouped into two big classes, namely pyramidal cells and GABAergic interneurons. However, while only three types of pyramidal neurons have been characterized, interneurons are highly diverse and, as of today, more than 20 types of these cells have been described.
The primary function of neurons is to capture information from the surrounding environment, integrate all the inputs, and forward a recap message to other cells. Thus, neurons send messages! They communicate with each other and exchange information. This is possible because neurons are organized in networks or in cell assemblies. Neurons are, indeed, connected through filaments (their axons; see image below) that originate from one cell and make contacts with many other cells encountered along their way. Pyramidal neurons are excitatory cells and, in response to the environment, increase the activity of other neurons – we can think of them as acting like the accelerator in a car. Interneurons are inhibitory. Each type of interneurons is highly specialized and makes contact with definite portions (subdomains) of pyramidal cells or other interneurons. As such, interneurons orchestrate the activity of pyramidal cells, providing discrete tuning in space and time. In turn, pyramidal cells are capable of modulating the level of inhibition through feedback mechanisms.
Thus, although different, pyramidal cells and interneurons do not function as independent entities; rather, they are interconnected and structured in dynamic networks to better adjust activity in response to demanding changes of the environment. Neither class of these cells plays a leading role. In fact, the “protagonist” of this operation is reciprocal interaction, the balance of excitation and inhibition. Just like in music, where melodies originate from the precise alternation of pitches and rest…
An effect of the synergy among different neurons is, e. g., the emergence of brain rhythms at specific frequencies, which are recognizable in an electroencephalogram. The mechanism of this phenomenon is currently under investigation. However, it has been suggested that if interneurons are active, they elicit inhibition in several thousands of pyramidals at the same time. Once released from inhibition, pyramidals emit their ‘messages’ in a synchronous manner, i.e. coincidentally. The read-outs of this operation are brain oscillations in the gamma frequency range. Noticeably, increasing evidence indicates that these specific waves constitute a correlate of cortical cognitive functions.
At the end of our journey around the cortex, here is a take-home message: the intensification of a networks’ connections among neurons, or the formation of new ones, is dynamic and evoked in response to changes in the environment. The reciprocal interaction of two distinct entities, pyramidal cells and interneurons, gives rise to something intangible and immaterial such as cortical cognitive processing, i.e. representation and interpretation of the world, thoughts, memories, reasoning, motivation, empathy.