Single-Neuron Projectome Reveals Organization of Somatosensory Ascending Pathways in the Mouse Brain

Neurology Medicine
single-neuron projectome somatosensory ascending pathways mouse brain neuron subtypes neural circuits

Single-neuron Projectome Reveals Organization of Somatosensory Ascending Pathways in the Mouse Brain

Academic Background

The somatosensory system plays a crucial role in processing diverse sensory signals such as mechanical, thermal, pain, and itch sensations. These signals are transmitted through various types of peripheral afferents to the spinal cord, where they are extensively processed and integrated before being relayed to the brain by spinal projection neurons (SPNs). SPNs are key nodes in the somatosensory system, but their projection patterns in the brain and specific functions remain incompletely understood. In particular, the projection patterns of individual SPNs and how they form complex neural circuits in the brain remain a significant challenge in neuroscience.

To gain a deeper understanding of the neural circuits of the somatosensory system, researchers need to map the projections of SPNs in the brain at high resolution. Previous studies have primarily relied on bulk labeling techniques, which lack single-cell resolution and fail to reveal the diversity of projections from individual neurons. Therefore, this study aims to construct a high-resolution neural circuit map of somatosensory ascending pathways in the mouse brain through single-neuron projectome analysis, revealing the projection patterns and functions of SPNs and central relay neurons.

Source of the Paper

This study was conducted by multiple research teams from the Institute of Neuroscience at the Chinese Academy of Sciences, Huazhong University of Science and Technology, Fudan University, and other institutions. The main authors include Wen-Qun Ding, Wei Song, Xiaoxue Shi, and others. The paper was published on July 9, 2025, in the journal Neuron, titled “Single-neuron projectome reveals organization of somatosensory ascending pathways in the mouse brain.”

Research Process

1. Construction of the Single-neuron Projectome

The study first used retrograde labeling techniques to sparsely label 785 SPNs and 1,464 central relay neurons in the cervical spinal cord of mice. The specific steps are as follows:

  • Virus Injection: The scretro-AAV-Cre virus was injected into multiple target brain regions (e.g., thalamus, hypothalamus, midbrain), followed by injection of the AAV-DIO-EYFP virus into the dorsal horn of the cervical spinal cord to label SPNs.
  • Imaging and Reconstruction: High-resolution imaging of the brain and spinal cord was performed using fluorescence micro-optical sectioning tomography (FMOST), and the complete morphology of neurons was reconstructed using the semi-automatic tracing algorithm “Fast Neurite Tracer.”
  • Data Registration: The reconstructed neurons were registered to the Allen Mouse Brain Common Coordinate Framework (CCFv3) and a custom-built cervical spinal cord template.

2. Analysis of SPN Projection Patterns

The researchers analyzed the projection patterns of SPNs and found that they widely project to regions such as the hypothalamus, thalamus, midbrain, pons, and medulla. Through clustering analysis, SPNs were classified into 19 projectome-defined subtypes, which exhibited diverse projection patterns. For example, some subtypes primarily project to the thalamus, while others project to the brainstem or midbrain. Additionally, the study discovered that some SPNs directly project to the cerebral cortex, a phenomenon not previously reported.

3. Analysis of Central Relay Neuron Projection Patterns

Central relay neurons are responsible for transmitting somatosensory signals to downstream brain regions. The study labeled relay neurons in the thalamus, pons, and other regions by injecting the AAV2/1-CMV-Cre virus and reconstructed their complete morphology. The results showed that thalamic relay neurons primarily project to sensory, motor, and higher-order cortices, with significant differences in projection patterns among different subtypes. For example, some subtypes mainly project to the primary sensory cortex, while others project to the motor cortex or prefrontal cortex.

4. Analysis of Parallel and Divergent/Convergent Projection Patterns

The study found that the transmission of somatosensory information in the brain involves both parallel pathways and divergent/convergent projection patterns. For instance, SPNs transmit information to the thalamus and brainstem through different pathways, while thalamic relay neurons transmit information to different cortical regions through multiple parallel pathways. Additionally, the study identified new parallel pathways, such as the spinal-superior colliculus-brainstem pathway, which may be involved in orienting and defensive behaviors.

Main Findings

  1. Diversity of SPN Projections: The study revealed that SPNs exhibit highly diverse projection patterns, with individual neurons projecting to multiple brain regions. Through clustering analysis, SPNs were classified into 19 subtypes, which showed significant differences in their distribution and projection patterns in the spinal cord.

  2. Projection Patterns of Central Relay Neurons: Central relay neurons also exhibit diverse projection patterns, with significant differences in cortical projection patterns among different subtypes. For example, thalamic relay neurons primarily project to sensory, motor, and higher-order cortices.

  3. Parallel and Divergent/Convergent Projection Patterns: The study found that the transmission of somatosensory information in the brain involves both parallel pathways and divergent/convergent projection patterns. For instance, SPNs transmit information to the thalamus and brainstem through different pathways, while thalamic relay neurons transmit information to different cortical regions through multiple parallel pathways.

Conclusions and Significance

This study, through single-neuron projectome analysis, has for the first time constructed a high-resolution neural circuit map of somatosensory ascending pathways in the mouse brain. The research revealed the diversity of projections from SPNs and central relay neurons and identified new parallel pathways and projection patterns. These findings provide an important structural framework for understanding the neural circuits of the somatosensory system and lay the foundation for future research on the neural mechanisms of somatosensory information processing.

Research Highlights

  1. Single-neuron Resolution: This study is the first to reveal the projection patterns of SPNs and central relay neurons at the single-neuron level, filling a gap in previous research.

  2. New Parallel Pathways: The study identified new parallel pathways, such as the spinal-superior colliculus-brainstem pathway, which may be involved in orienting and defensive behaviors.

  3. High-resolution Neural Circuit Map: Using high-resolution imaging and reconstruction techniques, the study constructed a high-resolution neural circuit map of somatosensory ascending pathways, providing an important reference for future research.

Additional Valuable Information

All data and code from this study have been made publicly available. Researchers can access these resources through the relevant website to further conduct related studies.