Key Questions Answered
Q: What part of the brain does this study focus on?
A: The study focuses on the feedback loop between the thalamus and the somatosensory cortex, specifically how thalamic projections modulate pyramidal neurons.
Q: What is the key discovery about sensory perception?
A: Researchers found a novel modulatory mechanism where thalamic input adjusts neuron excitability, explaining why the same touch can feel different depending on context.
Q: How does this impact our understanding of mental health?
A: This pathway may underlie perceptual shifts seen in disorders like autism and provides a potential target for future interventions.
Summary: Our sense of touch can feel sharp one moment and muted the next. New research reveals this inconsistency may be due to a feedback loop between the thalamus and somatosensory cortex, where thalamic input subtly changes how sensitive cortical neurons are to incoming stimuli.
Rather than triggering immediate action, this pathway primes neurons using an alternative glutamate receptor, fine-tuning perception based on context. The finding reshapes our understanding of sensory processing and could help explain altered perception in neurological conditions like autism.
Key Facts
- New Pathway Identified: A feedback loop from the thalamus modulates neuron excitability in the somatosensory cortex.
- Priming, Not Activation: Glutamate binds to non-traditional receptors, enhancing neuron responsiveness instead of directly triggering activity.
- Clinical Relevance: This mechanism could help explain sensory variability in sleep states and conditions like autism.
Source: University of Geneva
The cerebral cortex processes sensory information via a complex network of neural connections. How are these signals modulated to refine perception?
A team from the University of Geneva (UNIGE) has identified a mechanism by which certain thalamic projections target neurons and modify their excitability.
This work, published in Nature Communications, reveals a previously unknown form of communication between two regions of the brain, the thalamus and the somatosensory cortex.
It could explain why the same sensory stimulus does not always elicit the same sensation and open up new avenues for understanding certain mental disorders.
The same sensory stimulus can be perceived clearly at times, and remain vague at others. This phenomenon can be explained by the way the brain integrates stimuli. For example, touching an object outside our field of vision may be enough to identify it…or not. These perceptual variations remain poorly understood, but may depend on factors such as attention or the disruptive presence of other stimuli.
What is certain, according to neuroscientists, is that when we touch something, sensory signals from receptors in the skin are interpreted by a specialised region called the somatosensory cortex.
On their way to it, the signals pass through a complex network of neurons, including a crucial structure in the brain called the thalamus, which serves as a relay station. However, the process is not one-way.
A significant portion of the thalamus also receives feedback from the cortex, forming a loop of reciprocal communication. But the exact role and functioning of this feedback loop are still unclear. Could it play an active role in how we perceive sensory information?
A new modulatory pathway
To explore this question, neuroscientists at UNIGE studied a region at the top of pyramidal neurons of the somatosensory cortex, rich in dendrites – extensions that receive electrical signals from other neurons.
‘‘Pyramidal neurons have rather strange shapes. They are asymmetrical, both in shape and function. What happens at the top of the neuron is different from what happens at the bottom,’’ explains Anthony Holtmaat, full professor at the Department of Basic Neurosciences (NEUFO) and the Synapsy Centre for Neuroscience Research for Mental Health at UNIGE’s Faculty of Medicine, and director of the study.
His team focused on a pathway in which the top of pyramidal neurons in mice receives projections from a specific part of the thalamus. By stimulating the animal’s whiskers – the equivalent of touch in humans – a precise dialogue between these projections and the dendrites of pyramidal neurons was revealed.
‘‘What is remarkable, unlike the regular thalamic projections known to activate pyramidal neurons, is that the part of the thalamus providing feedback modulates their activity, in particular by making them more sensitive to stimuli,’’ says Ronan Chéreau, senior researcher at NEUFO and co-author of the study.
An unexpected receptor
Using cutting-edge techniques – imaging, optogenetics, pharmacology and, above all, electrophysiology – the research team was able to record the electrical activity of tiny structures such as dendrites. These approaches helped clarify how this modulation works at the synaptic level.
Normally, the neurotransmitter glutamate acts as an activation signal. It helps neurons transmit sensory information by triggering an electrical response in the next neuron.
In this newly discovered mechanism, glutamate released from thalamic projections binds to an alternative receptor located in a specific region of the cortical pyramidal neuron.
Rather than directly exciting the neuron, this interaction alters its state of responsiveness, effectively priming it for future sensory input. The neuron then becomes more easily activated, as if it were being conditioned to better respond to a future sensory stimulus.
‘‘This is a previously unknown pathway for modulation. Usually, the modulation of pyramidal neurons is ensured by the balance between excitatory and inhibitory neurons, not by this type of mechanism,’’ explains Ronan Chéreau.
Implications for perception and disorders
By demonstrating that a specific feedback loop between the somatosensory cortex and the thalamus can modulate the excitability of cortical neurons, the study suggests that thalamic pathways do not simply transmit sensory signals, but also act as selective amplifiers of cortical activity.
‘‘In other words, our perception of touch is not only shaped by incoming sensory data, but also by dynamic interactions within the thalamocortical network,’’ adds Anthony Holtmaat.
This mechanism could also contribute to understanding the perceptual flexibility observed in states of sleep or wakefulness, when sensory thresholds vary. Its alteration could also play a role in certain pathologies, such as autism spectrum disorders.
About this sensory perception and neuroscience research news
Author: Antoine Guenot
Source: University of Geneva
Contact: Antoine Guenot – University of Geneva
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Thalamocortical feedback selectively controls pyramidal neuron excitability” by Anthony Holtmaat et al. Nature Communications
Abstract
Thalamocortical feedback selectively controls pyramidal neuron excitability
The apical dendrites of layer (L) 2/3 pyramidal neurons in the mouse somatosensory cortex integrate synaptic input from long-range projections. Among those, inputs from the higher-order thalamic posteromedial nucleus may facilitate sensory-evoked cortical activity, but it remains elusive how this role emerges.
Here we show using ex vivo dendritic recordings that these projections provide dense synaptic input to broad tufted neurons residing predominantly in L2 and cooperate with other inputs to produce NMDA spikes.
They have the unique capacity to block two-pore domain potassium leak channels via group 1 metabotropic glutamate receptor (mGluRI) signaling, which increases excitability.
Slender tufted L2/3 neurons and other long-range projections fail to invoke these mechanisms. In vivo imaging of calcium signals confirms the presence of mGluRI-dependent modulation of feedback-mediated spiking in L2.
Our results imply that higher-order thalamocortical projections regulate neuronal excitability in a cell type and input-selective manner through fast NMDAR and mGluRI-dependent mechanisms.
