Dr.Hanno Sebastian Meyer

The human brain constantly performs a series of complex functions such as learning, memorizing and decision making. A research team at the Max Planck Florida Institute led by Nobel laureate Dr. Bert Sakmann and Dr. Hanno-Sebastian Meyer is conducting basic research that provides new insight into how these higher brain functions come about at a mechanistic level. To do this they are investigating the structure and function of neuronal circuits — networks of interconnected nerve cells — in the cerebral cortex, the part of the brain responsible for such higher functions.

Sakmann and Meyer’s research team uses the whisker system of rodents as their model. (For our earlier story of the study, go here.) Rodents use their whiskers as primary sensory organs to explore their environment in a way that is comparable to the way humans use their fingertips, or in more general terms, comparable to other human sensory systems such as vision, hearing, and sense of smell.  Information from a rodent’s whiskers is transformed into electrical signals and conveyed to nerve cell networks in the cerebral cortex that are organized in cortical columns (vertical) and layers (horizontal). Input picked up by one whisker is sufficient to trigger a decision, such as whether to make a particular jump based on the width of a gap an animal encounters in pursuit of food.

“It’s a one-to-one relationship; one cortical column processes the information from one particular whisker. So if you understand what is going on in the nerve cell network that comprises a column, you understand how the animal processes whisker input and, thus, the basis of decisions such as to jump or not to jump” Meyer said, adding since columnar and horizontal organization are hallmark features of cortical organization across species, understanding the mechanisms behind the whisker system in rodents could greatly increase our understanding of how the brain works in general.

In the present study, the MPFI researchers were able to locate and obtain 3D positions of all the inhibitory neurons in complete cortical columns (about 2,200 of the 19,100 cells in a column) for the first time and to analyze their distribution.

This research fills in an important gap in the current body of knowledge because while much is known about the structure of excitatory, or activating, nerve cells in the cortical columns processing sensory information, and their activity during whisker touch, much less is known about inhibitory neurons. These are the neurons that prevent activation and potentially have a substantial influence on the activity of excitatory nerve cells and that may be responsible for the inactivity of some of the excitatory neurons.

The research team found, for example, that inhibitory neurons are most dense in layers 2 and 5 of the cortical columns and that these parts of the column that have high inhibitory neuron density are those where excitatory neurons are surprisingly “silent” or inactive during whisker input; suggesting that this phenomenon is due to the abundance of inhibitory neurons in these layers. This data, Meyer noted, is fundamental to any model of cortical column function and signal processing and key to increasing our understanding such complex brain functions as decision making.