Abstract
Purpose :
Activation of sensory nerves innervating the ocular surface (OS) by mechanical and thermal stimuli does not generates tactile and pure thermal sensations but rather evokes sensations referred as dryness, freshness, gritty eyes, discomfort or pain. Central circuits involved in OS sensitivity are still largely unknown. Here we describe the activity of thalamic and somatosensory cortex neurons responding to OS stimulation, analyzing the processing of OS sensory input at higher levels of the CNS.
Methods :
In the anesthetized rat, the spontaneous and stimulus-evoked activity of 49 trigeminal ganglion (TG) neurons, 69 somatosensory thalamus (Th) neurons and 101 primary somatosensory cortex (S1) neurons conveying OS information was recorded extracellularly using tungsten electrodes. Stimulation was performed by topical application of saline drops at different temperatures, producing five types of thermal stimulus: intense cooling, mild cooling, neutral, warming, and noxious heating. Neurons were classified into various functional types depending on their response to one or more of the different types of applied stimuli.
Results :
Neurons responding to OS stimulation were found in precise locations of Th and S1, spatially grouped depending on their functional type. They encoded the type and intensity of the applied stimulus as the magnitude and temporal components of the firing response. Neurons responding to only one or to two types of stimulus (modality-specific neurons) were found in the TG while Th and S1 neurons responded to multiple types of stimulus (multimodal neurons). Furthermore, in Th and S1 there were more functional types of neurons than in TG. In S1 and Th neurons, noxious and innocuous stimulation evoked responses with different time course, the noxious ones being long lasting.
Conclusions :
Thalamic and cortical representation of the OS is functionally reported. While ocular trigeminal neurons are quite selective, being activated most of them by 1-2 types of stimulus, Th and S1 OS neurons process information originated by multiple types of stimulus, suggesting divergence and convergence of ocular sensory information along the somatosensory pathway. This central convergence of information on different types of stimulus does not occur in other territories innervated by the trigeminal nerve, and would explain the diverse and unique sensations evoked at the ocular surface.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.