Ocular surface information seen from the somatosensory thalamus and cortex

Area of Science:

• Neuroscience
• Ophthalmology
• Somatosensory System

Background:

• Ocular surface (OS) somatosensory innervation detects external stimuli, leading to perceptions like pain and dryness, crucial symptoms in OS pathologies.
• Understanding the central nervous system (CNS) circuits for these perceptions and multimodal input integration remains limited.

Purpose of the Study:

• To describe thalamic and cortical activity in response to ocular surface stimulation across different modalities.
• To characterize the anatomical and functional representation of the OS within the thalamocortical pathway.

Main Methods:

• Electrophysiological extracellular recordings were performed in anesthetized rats.
• The ocular surface was stimulated using saline drops at varying temperatures.
• Neural activity was recorded in the trigeminal ganglion (TG), thalamus (VPM-POm), and primary somatosensory cortex (S1).

Main Results:

• Precise locations of thalamic and S1 neurons receiving OS information were identified for the first time.
• All recorded nuclei (TG, Th, S1) encoded stimulus modality at single neuron and population levels.
• Noxious stimulation elicited a distinct activity profile; new multimodal responses emerged in Th and S1, organized spatially in clusters, with higher multimodality correlating with increased spontaneous activity.

Conclusions:

• This study provides the first anatomical and functional characterization of the thalamocortical representation of the ocular surface.
• The findings offer insights into how multimodal sensory information is integrated from the periphery to the central nervous system.
• Results suggest increasing functional complexity along the trigeminal neuroaxis and propose a new perspective on peripheral activity shaping CNS function.