Role of corneal sensory nerves in tearing and blinking

Abstract

The cornea is innervated trigeminal neurons classified in three different functional types: polymodal nociceptor, mechano‐nociceptor and cold thermoreceptor neurons. Corneal sensory nerves are responsible of the conscious sensations evoked by corneal stimulation, and are also involved in the trophic maintenance of the tissue. Besides, corneal nerve activity is also involved in the regulation of the protective functions of the ocular surface such as tearing and blinking.Regulation of reflex and basal tearing is integrated at the brainstem and, in both cases, under the influence of the sensory inflow provided by several types of corneal sensory nerves. While reflex tearing is induced selectively by the stimulation of corneal polymodal nociceptors, basal tearing rate depends on the spontaneous activity of cold thermoreceptors. Thus, when cold thermoreceptors are silenced by environmental warming, basal tearing is compromised.Voluntary, reflex and spontaneous (basal) blinking present differences in their characteristics and regulation mechanisms. Voluntary blinking depends on the direct instructions generated at the brain cortex, acting over the motor neurons located at the facial nucleus. Reflex blinking is integrated at the brainstem, being induced by the stimulation of the cornea and periocular skin, and even by loud sounds. Spontaneous blinking is claimed to be controlled by a CNS centre that established a blinking pattern. This pattern is modified under different conditions such as cognitive activity, being for example reduced during visual attentional tasks in order not to lose relevant visual information. When the activity of corneal cold thermoreceptors is abolished or reduced by ocular instillation of local anaesthetics, the frequency and characteristics of the spontaneous blinking is not modified. However, when the activity of cold thermoreceptors is increased for prolonged periods (for instance after topical instillation of F6H8 drops, which reduces the ocular surface temperature), blinking pattern become more regular and an increased number of blinks of larger the amplitude and shorter duration are induced. This increased blinking frequency produced when the activity of cold thermoreceptors is increased is sufficient to even counteract the reduction of blinking frequency induced during visual tasks performance. The increase in blink frequency driven by corneal cold thermoreceptors' activation may constitute a protective mechanism to prevent excessive desiccation of the ocular surface.(Supported by PID2020‐115934RB‐I00 from DOI: MCIN/AEI/10.13039/50110001103, and CIPROM/2021/48 from the Generalitat Valenciana, Spain)