The brainstem is the part of the brain that descends just in front of the cerebellum. It drops down from the brain to meet and meld with the spinal cord rising from the body.

The spinal nerves form a tight bundle at the core of the brainstem, whilst nine of the twelve pairs of cranial nerves - nerves which do not use the spinal cord - are wrapped round the outside, splitting from the brainstem at varying heights.

Above image from Gray's Anatomy, released public domain

The issue is, how do we go about hijacking the brainstem, to splice a virtual body, or artificial body parts onto it?

Further down in the body, it is possible to hijack and re-route nerves, tapping into them directly with electrode arrays, or moving them to a new location to replace those lost in a severed limb. This allows reuse of existing nerve fibres, in many cases without interrupting their normal tasks. That works fine on the periphery of the nervous system, but what do we do in this case? In order to access nerves in the middle of this clump, do we slice our way through it? Which nerves do we choose to damage? What bodily functions are we happy with losing, for the sake of accessing others?

The answer is, of course, none. It is not in any way sane to deliberately sever perfectly healthy nerve function, leaving it severed, just so as to be able to access other nerves. So what solutions are there?

Well, the first and most obvious solution is the jack. To help understand what a jack is, consider the following excerpt from a botany page:

"Put oat, barley or wheat grains in a flat dish containing tap water. The next day, sow the grains in a pot. When the seedlings are 3 cm high, cut off 10 mm from the tips of the two-thirds of the shoots. Leave one third of the seedlings not treated as a control. Dissolve 1 g of gelatine, with heating, in 20 mL demineralized water. Use this solution to stick the shoot tips back on half of the cut seedlings. Note any further growth. The seedlings without a shoot tip stop growing. The seedlings with the shoot tips stuck on again continue to grow almost as much as the control seedlings. The growth substance, auxin, diffuses out of the replaced tips through the gelatine into the cut end, and allow the plant to continue to grow." http://www.uq.edu.au/_School_Science_Lessons/UNBiol1a.html

A jack, or neural jack, is a device which in theory at least allows 'jacking in' or the direct connection of your nervous system to a fully immersive virtual reality.

To do this, it requires complete access to your full range of natural movement - all the nerves in your brainstem.

If we use the same basic principle as the auxin block, save with delicate and precise nerves rather than capillary tubes, we sever the brainstem, inserting an artificial substrate into the gap, which links severed nerve end above, with severed nerve end below, binding each as though they have never been severed, but at the same time allowing the signals to be re-routed through a computing device intermediary, and, perhaps, just perhaps, even blocked from proceeding all the way down the brainstem, from brain to body, so the body does not react badly, when the mind thinks it is undergoing trauma.

If we have such a system in place, then we have a neural jack.

The image to the left, shows such a system in very crude concept. The black mass is the brainstem, winding it's way down from the brain. The purple band, is the jack, with conduits leading outside the body for further processing.

The height of the jack here, is not coincidental. It has been placed before the highest of the cerebral nerves, the oculomotor nerve branches off. This is so that it could catch all the cranial nerves (save olfactory and optic, which do not traverse the brainstem) in one fell swoop, along with the main spinal nerves. The danger here, is we have actually cut off parts of the brain's own processing regions, the colliculi. These two regions - the upper and lower colliculi are not really brain processing regions as such, more areas where incoming nerve data is sorted and collated, like a network node before being sent on to the brain.

It may well not be in our favour to jack below this busy exchange, where much of the work of collating signals is handled for us if we choose to utilise it. This forces sensory data to pass through the brain's natural sorting system, both to and from the VR or augments. An example of such a system is shown here, to the right.

The problem we face, is that if the grafting is done below the colliculi, then many of the cranial nerves have already branched off from the brainstem, and would need to be tapped into individually, resulting in a much more complex neural jacking (assuming a jack across millions of electrical nerve signal generators in a flat cross-section could be called simple).

The additional jacking points for different nerves are again marked in purple, and this image only shows one side of the brainstem, with the same arrangement of nerves mirrored in symmetry round on the opposing side. These cranial nerves each flow to facial muscles, teeth, and muscles in the throat.

Technologically, we are of course still a long way from such neural jacks being feasable. Reconnecting severed nerves is a monumentous task, and one which current technology can only handle in small numbers - hundreds at a time. However, neural jacks are possible in concept, and highly likely to be practical within just a handful of decades, given the current pace of advancement in both neuroscience and sensory prosthetics development.

We truly do need to be thinking about these issues now, as they will be upon us, well within a single, natural lifespan.