There are twelve cranial nerve pairings (making 24 nerves in total) which split out from the brain, and move to cover the needs of the cranium and face, rather than make their way down through the central spinal cord. These nerves are important to consider, as most are of critical importance to sensory data, yet do not pass through the central cord, and so cannot be intercepted at the same juncture.
They are traditionally ordered from I to XII, based on the height at which they split out from the brain - I is highest.
I. Olfactory Nerve
The olfactory nerve is actually the highest nerve, above the optic nerves. Splitting into two nerves, one runs to each nostril. It has specialized smell receptor neurons which stick out in the upper nasal cavity.
The sense of smell which we enjoy comes from the stimulation of these nerve receptors by gas molecules when we breathe.
The olfactory nerve is the shortest of all the twelve cranial nerves and only one of two cranial nerves (the other being the optic nerve) that do not join with the brainstem.
II. Optic Nerve
The optic nerve is the nerve that transmits visual information from the retina of the eye to the brain.
The optic nerve contains 1.2 million nerve fibres. This number is low compared to the roughly 130 million receptors in the retina, and implies that substantial pre-processing takes place in the retina before the signals are sent to the brain through the optic nerve. Each eye transmits roughly 8,000,000 bits of information per second.
The eye's blind spot is a result of the absence of retina where the optic nerve leaves the eye. This is because there are no photoreceptors in this area.
III. Oculomotor Nerve
The oculomotor nerve is another eye nerve, often overlooked in favour of the optic nerve. Whilst the optic nerve transmits all visual data back from the retina, something has to actually tell the eye to move. The oculomotor nerves pair out below the optic nerves and control most of the eye movements, constriction of the pupil, and holding the eyelid open.
IV. Trochlear Nerve
The trochlear nerve controls the function of the superior oblique muscle, which rotates the eye away from the nose and also moves the eye downward.
V. Trigeminal Nerve
The trigeminal nerve is responsible for sensation in the face. Haptic feeling from forehead, cheekbones, jaw, nose tissue, lips, all pass through here. Sensory information from the face and body is processed by parallel pathways in the central nervous system, so it might be possible to adjust this information with the rest of the haptics from the body.
The nerve is primarily a sensory nerve, but it is also responsible for the motor functions biting, chewing and swallowing. This makes it a key nerve for taste senses.
VI. Abducent Nerve
The abdulent nerve is the last nerve to control the eyes, and has only one function - to move the eyes away from the midline.
VII. Facial Nerve
The facial nerve works with the trigeminal nerve above, to control the rest of the facial muscles. It also operates the middle ear's muscles, and controls the rear two thirds of the tongue, making it important for both taste and to a minor extent, hearing.
VIII. Vestibulocochlear Nerve
The vestibulocochlear nerve, sometimes called the auditory nerve is the main nerve for hearing, the same as its counterpart, the optic nerve is for sight. It splits into two nerves as it enters the ear; the cochlear nerve, carries information about hearing; the vestibular nerve carries information about balance.
IX. Glossopharyngeal Nerve
This nerve is predominantly a mouth nerve. It receives sensory fibres from the front third of the tongue, completing sensory taste information, also taking data from the tonsils, the pharynx, the middle ear and monitors blood pressure and the composition of the blood for the brain.
X. Vagus Nerve
The vagus nerve is the only nerve that starts in the brainstem and extends, through the jugular foramen, down below the head, to the abdomen.
It is also called the pneumogastric nerve since it innervates both the lungs and the stomach.In addition, it supplies parasympathetic nerves to the whole body (controls the subconscious immune system). This makes it a very unlikely candidate for use by either VR or AR, and any modification is likely to be pure cell replacement.
XI. Accessory Nerve
The accessory nerve controls neck muscles, and sensory (haptic) feedback from the neck.
XII. Hypoglossal Nerve
The hypoglossal nerve controls the tongue muscle, and tongue movement. It is the last muscle involved in the use of the mouth, responsible for eating, and part of speech.
All other nerves pass down through the spinal sheath, and can theoretically be tackled en-masse at the hindbrain or cerebellum. These twelve, that control the head, and various parts within, are the only ones you cannot tap by intersecting there.