An exploration of the deepest and most-mysterious layer of the cortex in mice by researchers at Michigan State University and Brown University has revealed new circuits that may be central to how two key regions of the brain communicate about sensation.
Neuroscientists have studied neural structures called “mouse whisker barrels” for nearly 50 years because they provide an exemplary physiological model of how the mammalian brain receives sensory information.
A new study published in Cell Reports shows that despite nearly five decades of prior scrutiny, significant discoveries remain to be made. Specifically, Shane Crandall, MSU assistant professor of physiology and a team of scientists, found a previously unknown set of matching barrel structures two layers deeper into the cortex, providing a more complete picture of the circuitry involved in handling sensory information.
While humans obviously don’t have whiskers, the new findings still serve to deepen scientists’ understanding of how two brain regions – the cortex, which processes information, and the thalamus, which feeds that information to the cortex – communicate, said study senior author Barry Connors, professor of neuroscience at Brown.
The barrels discovered in 1970 in the fourth layer of the cortex were presumed to handle much of this cross-talk, and while that may still be true, the finding of parallel “infrabarrels” in the sixth layer suggests that the layer closest to the thalamus may play a greater role than previously realized.
It’s vital to understand these circuits given how important cortex-governed behaviors such as sensory processing and attention are. This has proven difficult because of the immense diversity of cells in cortical layers.
What makes the brain science behind mouse whiskers so compelling to scientists is that it is so well organized, Connors said. Each whisker connects into a distinct circuit that can be cleanly traced through the brainstem, into the thalamus, and into an exact mapping in which each barrel in layer 4 corresponds to single whisker. But more generally in humans and mice alike there is a map of neurons in the cortex for sensations from all over the body. The cells in the somatosensory cortex that process touch in your right pinky, for example, are specific and different than the cells that process sensation in your left elbow.
Meanwhile, just as these cells take in information from the thalamus, many more cells send messages from the cortex back to the thalamus, Connors said, perhaps to manage the cortex’s attention to all this incoming sensory information.
“The cortex is not only receiving information about the world through the thalamus, it is also regulating the information it receives,” Connors said. “The brain is constantly making decisions about what kind of information to pay attention to and what not to pay attention to.”
In the new study, the team sought to understand how cells in layer 6 were organized and whether they had a role in this cross-talk. To do that, they used genetically engineered mice to express different fluorescing proteins in specific cell types. For example, they were able to get “corticothalamic,” or CT, neurons – ones that project from the cortex to the thalamus – in layer 6 to light up yellow.
When they did that, they saw that those CT neurons were tightly grouped into structures that were not only quite similar to layer 4 barrels (which they made to glow red), but also directly below them in columns of circuitry that extend across all the layers of the cortex.
“We turned on the microscope, and voila, there were barrel structures where we didn’t expect them to be. We then duplicated it, looked at different angles and across different ages of mice to confirm the new finding,” said Crandall, a former Brown postdoctoral fellow. “This provides us a framework for understanding the functional organization of layer 6.”
Cutting the samples in a nontraditional fashion also helped confirm the organization of the infrabarrels. When Crandall sliced his samples tangential to the cortical surface, he saw a pattern that still brings a smile to his face.
“The barrel structures in layer 6 were organized in rows nearly identical to those of layer 4. These data strongly suggest that infrabarrels are somatotopically arranged to mimic both the layer 4 barrels and the rows of whiskers on the mouse’s snout,” Crandall said. “It’s discoveries like these, seeing images that no one has seen before, that makes me feel an excitement about my research.”
They continued their investigation by stimulating different cells in the thalamus to see which neurons those cells might excite in layer 6. The team explored the input from thalamus using optogenetics – a technique in which specific neurons can be engineered to become activated or suppressed by pulses of visible light.
They found that while the thalamic neurons had little effect on the CT neurons, they excited many of the neurons between the layer 6 infrabarrels. These neurons communicate within the cortex and are therefore called “corticocortical, or CC, neurons.”
The findings suggest that at least some of the cortex’s communications to the thalamus are relayed by layer 6 CT cells in infrabarrels, Connors said. They also suggest that the thalamus not only sends input for the cortex to process in layer 4, but also to CC neurons between the infrabarrels in layer 6.
Crandall plans to further this research at MSU by investigating to which cortex cells these CC cells may be “talking.”
Saundra Patrick and Scott Cruikshank of Brown University also contributed to this study.
This research was funded by the National Institutes of Health and the National Science Foundation.
