Your Corneal Nerves

The cornea is the clear tissue covering the front of the eye. It has an exceptionally dense sensory nerve supply.

This makes sense when we consider its job. The cornea needs to remain transparent and relatively smooth for clear vision, but it is also directly exposed to the outside environment. Air, wind, particles, contact lenses, chemicals, changes in temperature and changes in the tear film can all affect it.

The cornea therefore needs a highly sensitive warning and monitoring system. Branches of the trigeminal nerve, the major sensory nerve of the face and ocular surface, enter the cornea and form an extensive network of nerve fibers. Fine nerve endings extend close to and within the corneal epithelium. This allows the ocular surface to detect very small changes in its environment.

It is tempting to imagine the corneal nerves as simple “dryness sensors.” They are much more sophisticated than that.

Different populations of sensory nerve endings respond preferentially to different types of stimulation. They are commonly described as including:

• •mechanoreceptors, which respond particularly to mechanical forces
• •polymodal nociceptors, which can respond to mechanical, chemical and thermal stimuli, especially potentially damaging ones
• •cold thermoreceptors, which respond to changes in ocular-surface temperature and other changes associated with the state of the tear film

Together, these sensory systems provide the nervous system with continuous information about what is happening at the surface of the eye. Some of that information eventually becomes something we consciously feel. Much of it does not.

Most of the time, you are not consciously aware of your corneas. Yet sensory activity from the ocular surface continues in the background.

One particularly interesting group of nerves is the cold thermoreceptors. The ocular surface naturally cools to some extent between blinks. Evaporation of water from the tear film contributes to this cooling, and changes in tear concentration can also influence these receptors. Cold-sensitive nerve endings can respond to these subtle changes.

This sensory information is thought to contribute to the normal regulation of basal tearing and spontaneous blinking.

So the eye does not simply wait until it becomes painfully dry and then react. There is an ongoing feedback system continuously helping to maintain the ocular surface.

This is where the nervous system connects many of the things we discussed in the previous articles.

Sensory nerves in the ocular surface send signals through branches of the trigeminal nerve toward the brainstem. The brainstem processes this sensory information and can activate pathways involved in protective responses.

One of those responses is tear secretion. The main lacrimal gland is strongly regulated by autonomic nerves, particularly parasympathetic pathways. Activity in these pathways can be influenced by sensory input coming from the ocular surface.

So a simplified version of the system looks like this:

ocular surface→sensory nerves→brainstem→autonomic nerves→lacrimal gland

This creates a feedback loop. Changes at the ocular surface can influence neural activity, and neural activity can influence tear secretion.

This is part of what is sometimes called the lacrimal functional unit — a concept describing the ocular surface, tear-producing tissues and the neural connections that allow them to function together as an integrated system.

Tearing is not the only response influenced by sensory information from the ocular surface. Corneal and other ocular-surface nerves also participate in the control of blinking.

If something suddenly touches or strongly irritates the cornea, sensory nerves can trigger a rapid protective reflex blink. But ocular-surface sensory input also appears to contribute to our ordinary spontaneous blinking. Cold thermoreceptors, in particular, are thought to provide part of the ongoing sensory input that helps regulate blink rate.

This connects directly with what we discussed in the previous article. Blinking refreshes and redistributes the tear film. The changing tear film affects sensory input from the ocular surface. That sensory input can influence blinking and tear secretion. And the next blink changes the tear film again.

So rather than separate systems, we have a continuous loop:

tear film→ocular surface→nerves→brain→tearing and blinking→tear film

The same sensory system can produce much stronger protective responses when necessary.

Imagine a particle touching the cornea. Or smoke or another irritant reaching the ocular surface. Instead of the subtle background signaling involved in everyday tear-film regulation, stronger sensory input can activate protective reflexes. You may blink immediately. Your eyes may produce a large amount of tears. You may experience irritation or pain.

These responses are useful. Their purpose is to protect the eye and help remove or avoid potentially damaging stimuli.

So discomfort is not simply an unpleasant side effect of having sensitive eyes. In a normally functioning sensory system, it is also part of the eye’s protective warning system.

Corneal nerves are also closely involved with the health of the tissue they innervate.

The corneal epithelium and its nerves interact continuously. Nerve-derived signals and neuropeptides participate in epithelial maintenance, repair and wound healing, while epithelial and immune cells also influence nerve function.

This is why researchers increasingly describe the ocular surface not as a collection of independent tissues but as an interconnected neuro-epithelial-immune system. The tear film, epithelial cells, immune system and nerves constantly influence one another. That interaction is important for maintaining a healthy ocular surface.

There is one more concept that is useful to understand before we move on.

What you feel is not a direct measurement of how many tears are sitting on the eye. The sensation produced by the ocular surface depends on sensory receptors, nerve activity and the way those signals are processed by the nervous system.

Normally, these systems help the eye detect changes and respond appropriately. But this also explains why sensation and physical measurements of the ocular surface are not necessarily identical things.

A test can measure tear volume, tear-film stability or another physical feature. Your nervous system is doing something different: it is receiving and processing biological signals generated by the ocular surface.

This distinction will become very important later in the guide when we discuss why symptoms and clinical signs do not always match perfectly. For now, the important point is simply this: we do not feel “tear quantity” directly. We feel the signals produced and processed by our sensory nervous system.

If we put everything from this first chapter together, a much more interesting picture of the eye begins to emerge.

The meibomian glands provide meibum that contributes lipids to the tear film. The lacrimal glands provide much of the aqueous component. Mucins and the glycocalyx help create the interface between tears and the ocular surface. Blinking spreads and renews the tear film. And the sensory nerves continuously provide information about what is happening at the ocular surface and participate in reflex pathways that influence tearing and blinking.

None of these systems operates completely independently. They communicate. They respond. And they continuously adjust to changing conditions.

That is how the ocular surface maintains homeostasis — a relatively stable environment despite constant exposure to the outside world.

We now understand how the main parts of a healthy ocular surface are supposed to work. The next question is: what happens when one or more of these systems stops working normally?