Two studies reveal insight into how the spinal cord and brainstem contribute to tactile sensation

The sense of touch is essential to almost everything we do, from performing routine tasks at home to navigating unfamiliar terrain that may hide hazards. Scientists have long been interested in understanding exactly how tactile information from our hands and other parts of our body travels to our brains to create the sensations we feel.

Still, important aspects of touch — such as how the spinal cord and brainstem are involved in receiving, processing, and transmitting signals. I wasn’t sure.

Two papers by Harvard Medical School scientists now reveal important new insights into how the spinal cord and brain stem contribute to the sense of touch.

Specifically, we show that the spinal cord and brainstem, previously thought to be mere relay centers for tactile information, are actively involved in processing tactile signals as they travel to higher regions of the brain. .

One study published Nov. 4 cell, We show that specialized neurons in the spinal cord form a complex network that processes light touch. Think of a hand brush or a cheek peck. It then sends this information to the brainstem.

Another study was published on November 23rd. Natureresearchers established that direct and indirect touch pathways work together and converge in the brainstem to shape how touch is processed.

These studies spotlight the spinal cord and brainstem as where touch information is integrated and processed to convey different types of touch. How these regions contribute to the representation of vibration, pressure, and other features of tactile stimuli in the brain was previously poorly understood. ”

David Ginty, Edward R. and Anne G. Leffler Professor of Neurobiology at the Blavatnik Institute at HMS and senior author of both papers

Although the research was done in mice, the mechanisms of touch are largely conserved across species, including humans. In short, the basics of touch processing may be useful for scientists studying human conditions such as neuropathic pain, which are characterized by touch dysfunction.

“This detailed understanding of touch—the feeling of the world through contact with the skin—is critical in understanding how disease, disability, and injury affect our ability to interact with the environment around us. It could have deep implications,” said James Gunat, program director at the National Institute of Neurological Disorders and Stroke (NINDS), who provided some of the research funding.

overlooked and underrated

The historical view of touch is that sensory neurons in the skin encounter contact stimuli, such as pressure or vibration, and send this information in the form of electrical impulses that travel directly from the skin to the brainstem. There, other neurons relay tactile information to the brain’s primary somatosensory cortex. The top level of the touch hierarchy – where it is processed into sensations.

However, Ginty and his team wondered whether and how the spinal cord and brain stem are involved in processing contact information. These regions occupy the lowest level of the contact hierarchy and combine to form more indirect contact pathways to the brain.

“People in the field thought that tactile diversity and richness came from sensory neurons in the skin, but that idea bypassed the spinal cord and brainstem,” said Ginty, a postdoctoral fellow in the lab. Nature paper.

Many neuroscientists are not familiar with spinal cord neurons called post-synaptic dorsal column (PSDC) neurons that project from the spinal cord to the brainstem. Also, textbooks tend to exclude PSDC neurons from diagrams that show tactile details, Turecek explained.

For Ginty, the overlooked way the spinal cord and brainstem come into contact is reminiscent of early work on the visual system. Initially, scientists studying vision thought that all processing took place in the visual cortex of the brain. However, it turns out that the retina, which receives visual information long before it reaches the cerebral cortex, is heavily involved in processing this information.

“Similar to the studies of the visual system, these two papers show how contact information from the skin is processed in the spinal cord and brain stem before traveling up the contact hierarchy to more complex brain regions. We are dealing with it,” said Ginty.

connect the dots

in the cell In the paper, the researchers used a technique they developed to simultaneously record the activity of many different neurons in the spinal cord as mice experienced different types of touch. They found that more than 90% of her neurons in the dorsal horn were negative. Sensory processing areas of the spinal cord -; responds to light touch.

“This was surprising, because classically, dorsal horn neurons in the superficial layer of the spinal cord were thought to respond primarily to temperature and painful stimuli. How light touch information is distributed in the spinal cord. I didn’t understand how,” says Anda Chirila. Ginty Lab Research is co-first author with her fellow and graduate student Genelle Rankin.

Moreover, these responses to light touch have been found to differ considerably among genetically distinct populations of neurons in the dorsal horn, forming complex, highly interconnected neural networks. Alterations in this response resulted in diverse contact information being transported from the dorsal horn to the brainstem by PSDC neurons. Indeed, when researchers silenced various dorsal horn neurons, they found that the diversity of light touch information transmitted by PSDC neurons decreased.

“We believe that this information about how contacts are encoded in the spinal cord, the first part of the contact hierarchy, is important for understanding fundamental aspects of contact processing,” Chirila said. I’m here.

In their other research, Nature, scientists turned to the next step in the touch hierarchy, the brainstem. They investigated the relationship between direct pathways from sensory neurons in the skin to the brainstem and indirect pathways that send tactile information through the spinal cord. cell paper.

“Brainstem neurons receive both direct and indirect input, and we were very interested in the aspect of contact that each pathway brings to the brainstem,” Turecek said. .

To analyze this question, the researchers alternately silenced each pathway and recorded the responses of neurons in the brainstem of mice. Experiments have shown that direct pathways are important for transmitting high-frequency vibrations, while indirect pathways are necessary for encoding the strength of pressure on the skin.

“The idea is that these two pathways converge in neurons and the brainstem that can encode both vibration and intensity, so we can shape the response of these neurons based on the amount of direct and indirect input.” explained Turecek. In other words, when brainstem neurons receive more direct than indirect input, they transmit more vibration than intensity, and vice versa.

Additionally, the team found that both pathways can convey contact information from the same small area of ​​skin. Information about intensity bypasses the spinal cord and then combines information about vibration that travels directly to the brainstem. Thus, direct and indirect pathways work together to enable the brainstem to form spatial representations of different types of contact stimuli from the same region.

finally on the map

Until now, “most people viewed the brainstem as a tactile relay station and didn’t even have a spinal cord on the map,” Ginty said. For him, the new research “shows that an enormous amount of information processing takes place in the spinal cord and brain stem, and this processing is critical to the way the brain represents the tactile world.”

Such processing likely contributes to the complexity and diversity of tactile information that the brainstem sends to the somatosensory cortex, he added.

Next, Ginty and team plan to repeat the experiment in awake and behaving mice to test the results under more natural conditions. They also want to expand their experiments to include more types of real-world touch stimuli, such as textures and movement.

Researchers are also interested in how information from the brain is transmitted–for example, about an animal’s level of stress, hunger, or fatigue. Affects how contact information is processed in the spinal cord and brain stem. Given that contact mechanisms appear to be conserved across species, such information may be particularly relevant to human conditions such as autism spectrum disorders and neuropathic pain. .

“These studies have built the basic building blocks of how these circuits work and what their significance is,” Rankin said. We have the tools to analyze it and understand if the circuit is working properly and what is changing when something goes wrong.”