The Optic Nerve

The axons of retinal ganglion cells (RGCs) in each eye collect into a nerve fiber as they exit the optic cavity in which the eyeball is housed. This fiber is the optic nerve, or cranial nerve II. Though it might look like a dull string, it's far from homogeneous, since the RGC axons that make it up form distinct information circuits. There are 9 primary visual nuclei within the brain that these circuits make use of. The information systems so formed can be visual, like those involved in perceiving color or motion. They can also be non-visual, like those involved in our reflexes or circadian rhythms.

The optic nerve is pretty short, as far as nerves go: its main part formally ends at the optic chiasm, just in front of the midbrain. After this landmark it diverges into branches that each have their own names. Below, I've given a brief list of the neural circuits that these branches take part in.

The Visual Pathways

1. The Central Visual Pathway: This pathway takes up most of the RGC axons. It is mainly concerned with reconstructing the visual world, and as such it encompasses the sensations we associate with color, movement, shape and depth. I go into the weeds of how this happens in the next few sections.

The Non-Visual Pathways

Non-visual information pathways consist of the kinds that don't produce the image of the world we generally associate with sight. It turns out that there are crucial functions our eyes perform for us outside of creating the wonderful images we see.

The first clue that the retina also transmitted non-visual information was given to us way back in 1923, when the eye-pupils of rodless and coneless mice exposed to light were nonetheless seen to contract. This observation implied the existence of neural circuits which originated in the retina but diverged from the central visual pathway at some point in the brain. Below is a list of those we currently know about.

1. The light reflex: controls the diameter of the pupil in changing light intensities. The neural pathway that achieves these changes is made up of three channels. One channel is sensory; it's made up of the special class of RGCs which I introduced in the previous section as intrinsically photosensitive (ipRGCs). They gather information about the intensity of light falling on the retina, and project it to the olivary pretectal nucleus of the midbrain.

The olivary pretectal nucleus sends out a motor channel of neurons to the nearby Edinger-Westphal (EW) nucleus. This nucleus is one of the group of nuclei from which the third cranial nerve, the oculomotor, originates. One branch of this nerve connects to the ciliary ganglion, a bundle of cell bodies located just behind the eye. From there, small post-ganglionic ciliary nerves branch off to innervate the sphincter muscle of the iris.

When this sphincter muscle contracts, the pupil constricts. This circuit is used to prevent damage to the cells of the retina, which is important because they don't seem able to regenerate.

2. The accommodation reflex: this reflex is used when the eye needs to change its focus as it comes to rest on objects positioned different distances away from it. When an object is close to the eye, the pupil must constrict, so that sharply diverging rays of light don't blur the image created on the retina. This constriction is achieved by the pathway described in the light reflex above, which employs fibers from the oculomotor nerves emerging from the Edinger-Westphal nucleus.

The lens must also swell in order to become more curved and so be able to refract light more sharply. Once again, a branch of the optic nerve interacts with the oculomotor nerve at the EW nucleus, instructing it to constrict the ciliary muscles which ring the eye and keep the naturally-curved lens taut. When these ciliary muscles constrict, they stop pulling on the lens and let it curve to (hopefully) the appropriate degree.

3. The circadian rhythms: The word 'circadian' is derived from the Latin circa diem, which means 'about a day'. In humans, the phrase 'circadian rhythm' refers to a 24-hour clock which is kept within our brains and governs cyclic changes in our behaviors and physiologies. It persists even when the external environment doesn't change; for example, when the lights are on constantly. However, external signals like light cues are necessary for circadian rhythms to become properly aligned. This is what's going on when you experience jet lag - your internal clock takes some time to get used to unusually shifted periodicities of light and darkness.

The change that you experience over time when you adapt to a new time zone isn't a continuous change. Instead, it's brought about by a series of discrete events. This means that when your circadian cycle is interrupted by light intruding at a different time than normal, the system responsible for aligning the cycle to the environment is triggered. When you're exposed to light early on in the part of the cycle in which you should be asleep, you experience a 'phase delay', which means that levels of hormones and other physiological factors peak at a later time than normal. Light exposure at the end of the night phase leads to a 'phase advance', in which physiological peaks occur at earlier times.

This has interesting implications for decisions you make each day. When you stay up too late, with lights on around you, you delay the periodicity of some of your natural cycles, and so they'll peak at a later time than you're used to the next day. On the other hand, when you wake up earlier than normal, your body sets things into motion faster. For example, you may have experienced your digestion working faster on days you wake up early, or your appetite peaking later after nights on which you stayed up late.

It actually appears that circadian rhythms are set at a cellular level via genetic mechanisms, called the 'core clock'. This means that tissues like your lungs or liver can regulate their own circadian activity even when isolated. Nonetheless, throughout a range of different organisms, there exists a main pacemaker - which organizes the rhythms of all those tissues that make up your body so that they can work synchronously - which seems to be located within small, specific structures. In humans, it resides in the suprachiasmatic nuclei (SCN), a cluster of cell bodies found in the anterior hypothalamus. This small nucleus acts as a 'master oscillator' which controls the timings of 'slave oscillators' specific to particular physiological cycles.

The SCN receives input from the ipRGCs I mentioned above. ipRGCs arrive as part of the retinohypothalamic pathway, going straight to the SCN after crossing the optic chiasm. The SCN exhibits rhythmical rates of electrical firing which peak at midday and fall during the night, and we know it to be connected to the pineal gland, which secretes the hormone thought key in sleep, melatonin. How these activities cause rhythmical cycles in digestion, alertness, and even alcohol preference, among many others, is on the whole yet unknown.

What is known is that ipRGCs have an absorption peak of light at around 480 nm, which falls under blue light in the color spectrum. An understanding of this has recently led people into using blue-light filtering glasses, especially when exposed to electronic devices into the night.

As a final note, it's useful to keep in mind that although light is the main component in the control of circadian rhythms, other factors play a role too. Temperature, social interactions, and exercise are part of the many forces whose effects cause the internal pacemaker to constantly re-calibrate.