Sensory experience influences the shape of neurons

Víctor Borrell has revealed the cellular mechanism that controls the development and differentiation of stellate neurons in layer 4 of the cerebral cortex. The results of this project, obtained in collaboration with Edward M. Callaway of the Salk Institute for Biological Studies in California, show that the development of these neurons occurs in two phases and are largely dependent on neuronal activity. This discovery has been published in the journal The Journal of Neuroscience.

The results of this research show the cellular mechanisms involved in the acquisition of the different morphologies of neurons in the cerebral cortex and demonstrate for the first time the active participation of sensory experience in this process, a critical step for the proper functioning of the brain.

The researcher Víctor Borrell explains that 'for most neurons in the cerebral cortex, the developmental process is relatively simple: to grow and increase in complexity. However, in the case of stellate neurons, the process is different and consists of two completely opposite phases. In a first stage, these neurons increase in size and complexity, becoming similar to the typical morphology of pyramidal neurons. In a second stage, these neurons begin a process of regression, during which they suffer a reduction in the size and complexity of their main dendrite (apical dendrite), and finally reach the typical star shape. This type of development had only been observed in a very particular small population of cortical neurons, but this study shows that the same applies in most layer 4 neurons.'

The cerebral cortex is the most complex structure of the mammalian brain and, without a doubt, the most expanded part of the human brain. The information we receive from the outside world through our senses travels through the nervous system to the cerebral cortex, where this information is processed, integrated and combined with past memories and feelings, leading to our particular perception of the world around us. The cerebral cortex contains a unique repertoire of types of neurons, which are distinguished by their characteristic shape each defined by the extension and arborisation of their dendrites. Most of the excitatory neurons of the cerebral cortex are characterized by a long apical dendrite that predominates over several shorter basal dendrites, giving these neurons a pyramidal appearance. In layer 4, however, the predominant type of neuron has a short apical dendrite similar to the basal dendrites, so these neurons have a very characteristic asterisk or star shape.

Several previous studies have shown that during development of the cerebral cortex pyramidal neurons undergo a remarkable growth and arborisation of all dendrites, which eventually ends up giving the typical pyramidal shape of these neurons in the adult brain. It has also been proposed that the final size and shape of these pyramidal neurons are the result of a combination of intrinsic genetic factors and local environmental influences, including the electrical activity of neurons themselves. In contrast to pyramidal neurons, the mechanisms responsible for leading the development of dendrites towards a star shape, like neurons in layer 4 of the cerebral cortex, and the factors influencing this process were entirely unknown until now.

The importance of senses

Once understood the process by which stellate neurons acquire their final form in two phases, researchers Victor Borrell and Edward Callaway began searching for the factors that regulate this process. They found that sensory activity seems to play a central role. They found that in situations of visual deprivation, where the cerebral cortex does not receive electrical impulses from the retina, neurons in layer 4 only completed successfully the first phase of development: growth and increase of complexity. However, when it was time to start the second phase of development, the retraction of the apical dendrite, most neurons were unable to make that change and remained in a growth phase, maintaining their pyramidal shape. Therefore, sensory perception, and in this case visual perception, plays a fundamental role in the process of brain development because it determines the shape of many of the neurons of the cerebral cortex.