The questions come up of what, exactly, makes up a Shadowrun Cyberlimb. One of the players put up their own take on the subject (and thus earned a couple of extra karma points for the contribution), which has led to this – rather longer, if similar – explanation of just what the rules for cyber-replacements and additions really mean.
The Direct Neural Interface is pretty basic to a cyberlimb. While current basic muscle- and nerve- activity sensor-and-translation systems actually allow some pretty sophisticated operations, Shadowrun goes a bit beyond the current real-world state of the art. Still, in Shadowrun, a “DNI” normally refers to the kind of system you get with a Datajack – an interface with the cerebral cortex and the “conscious levels” of the user’s brain. Links with the deeper levels of the brain normally fall under Hotwiring and Coldwiring. Links with muscular systems, such as are used by vehicle riggers, high-level skillwire systems, exoborgs, neuroborgs, and riggers, are normally handled with Spinal Interfaces – installed below the reflexive and unconscious body-control levels of the medulla oblongata and the upper end of the spinal cord. Oddly enough, such interfaces tend to be extremely essence-intensive, and become even more essence-intensive as more neural channels are diverted. At least in vertebrates, the hindbrain and upper end of the spinal cord seem to be extensively involved in the mind-body-spirit bond – which may be why critters such as shapechangers cannot regenerate damage to that area. Individual limbs can also be run by Peripheral Interfaces, hooked directly to the remainder of the peripheral nerves of the torso or the limb. Such interfaces are less essence-intensive, but cannot report information to the user’s brain except along the usual neural channels for the various components of the senses of touch, kinematics, and related internal senses. If you want to run complex equipment, or have the number of bullets in your cybergun reported to your conscious mind, you’ll want a datajack-style DNI as well as a peripheral or spinal interface. Similarly, using your conscious mind to control muscle-style actions is impossibly clumsy.
Technically, all of these systems – Datajacks, Hotwires and Coldwires, Spinal Interfaces, and Peripheral Interfaces are “Direct Neural Interfaces”. In practice, the term is usually restricted to datajack-style connections. It makes for less confusion.
While any form of DNI probably should involve a fair amount of circuitry to translate neural signals into electrical or optical control signals for the systems they’re linked to, in Shadowrun this is usually simply subsumed into the interface level regardless of the type of interface installed. At least in Shadowrun, the translation-function is considered minor enough to be installed as a simple transducer implant or as a subcomponent of a datajack or knowsoft link. In practical terms, this is a near-nanite level link structure which serves as a socket and interface for each neural fiber and feeds a minuscule translator-chip.
The Power Supply doesn’t need to be in the limb itself even in a meat-anchored cyberlimb, but it usually is; running a power cable through the body to a power supply located elsewhere is clumsy, pointless, and dangerous. In general, practical cyberlimb power supplies include some small, high-efficiency, rechargeable power cells and a glucose/oxygen power cell fueled by the user’s blood sugar and blood oxygen and a modest network of power conduits which transfer power to the various subsystems which use it. This allows the user’s normal dietary intake to fuel the cyberlimb or limbs at normal levels, provides extra power for high-level operation, and allows the powercells to be slowly recharged when demands are low. In theory, external chargers might be useful. In practice, people being people, this tends to lead to excessive weight gain or loss from changes in the metabolic load, internal damage from people attempting to overdrive their cyberlimbs or from people plugging in the wrong transformers and circuitry, people trying to use their cyberlimb power supplies for other things under various circumstances, overloaded, damaged, and even detonated batteries, and running out of power at bothersome moments. Most designers skip the external power ports and settle for normal metabolism, normal levels of fatigue, and people needing to eat more after heavy exercise. It simply works out better in the long run. Cyberlimbs do notably reduce the average load on the heart, which is good: increasing the mechanical load on the heart is a good way to kill someone.
Cyberlimb systems, such as pairs of legs, or any group of cyberlimbs attached to a cybertorso, may or may not share their power supplies. It depends on whether or not they were designed as a unified system – and, if they were, on how the designers felt about efficiency versus backup options.
While there have been discussions of nuclear, magical, rechargeable electrical, and even fuel-driven fuel cell power supplies for cyberlimbs, none of these are even remotely standard systems. This doesn’t mean that some runner with more money than sense may not want to experiment, but it does mean that such things are definitely special-order if they can be obtained at all.
The Physical Interface, where living tissue meets mechanism, isn’t all that far removed from a classical stump-cap and peg or hook attachment – except that these days it’s surgically attached and anchored in the bones and made of special biologically-inert materials to prevent allergies, rejection, and other complications. Obviously enough, this only matters for attaching cyberlimbs to areas that have not already been cyber-replaced – unless you want such limbs to be easy to remove and either reattach or replace. Enhanced-attribute cyberlimbs involve upgraded physical interfaces, but such costs are fairly negligible compared to the other costs involved. Oddly enough, the essence cost of the Physical Interface is fairly negligible: in itself, it doesn’t have much to do with the central nervous system.
The “Muscles” of a cyberlimb normally bear little resemblance to those of an organic limb. Unlike muscle “augmentation” and “replacement”, cyberlimb “muscles” are unconstrained by the necessities of comparability with an organic body or self-repair capabilities. Cyberlimbs may thus be designed using electro-contractile fibers, memory-metal systems, hydraulics, electrical motors, spring and polymer “ligaments”, or a bewildering variety of other systems. All of them tend to be a bit less energy-efficient than natural muscle, but – without the necessity of supporting a cellular metabolism, self-repair capabilities, growth mechanisms, immune capabilities, toxin processing, and all the other paraphernalia of a living system, are usually capable of a considerably greater power output. This is why, of course, cyberlimbs are incompatible with muscle augmentation and replacement, but exoborgs and neuroborgs receive substantial bonuses to their basic strength.
The Structure of a cyberlimb normally includes the internal struts or “skeleton” and may – at least in mechanical-appearing cyberlimbs – be augmented by the limb casing. More natural-appearing cyberlimbs normally use a flexible, non-supportive casing overlaying electrocontractile fibers to provide that “natural” appearance. In game terms making it look natural is basically just a system that takes up a good deal of the available space in a cyberlimb. In practice, the “muscle” layout and general structure of the “skeleton” mimic normal primate anatomy and jointing fairly closely; several million years of evolution have done a fairly good job coming up with an efficient structural design.
The Casing of mechanical-appearing cyberlimbs is a hard external cover, usually made up of some combination of metals, plastics, and advanced ceramics. While this necessarily involves external jointing to allow movement, such cases may also serve as a part of the limbs structural support – another reason why mechanical-appearing cyberlimbs can mount more equipment and armor than naturally-appearing ones. Naturally-appearing cyberlimbs normally use a polymer cover which flexes over the structural skeleton and contractile-fiber bundles, and can be fairly hard to tell apart from normal limbs. Really expensive cyberlimbs may use cultured and grafted skin, with channels laid in a supporting bioplastic grid for nerves and blood vessels. Such limbs provide natural sensory input, bleed from minor nicks, and can heal from minor damage. On the other hand, they require internal heating systems to prevent “frostbite” in cold weather and can transmit uncomfortable levels of pain. If combined with an internal shielding system, however, they can make cyberlimbs quite difficult to detect, even for a knowledgeable observer.
The Sensor Systems range from the minimal – standard internal system sensors which can relay their output to a standard DNI or diagnostic system, kinesthetic and positioning feedback, and a simple set of stress monitors and overload warnings – on up through a variety of hypersense enhancers and scientific-quality instruments. If you want touch sensors which can accurately measure weights, temperatures, and angles – or, for that matter, radiation levels, chemical structures, and magnetic fields – such things are available at some expense. Standard mechanical-appearing cyberlimbs provide sensitive sensors only at areas like the fingertips, where they’re required to allow the user to operate normally*. Other areas are limited to the basic feedback sensors necessary for control and basic damage sensors. Naturally-appearing limbs, and those with living skin, normally offer full sensation. If the damage-sensors and stress-sensors are routed to a DNI, you can get detailed readouts. If they’re routed through a peripheral or spinal interface things are normally limited to simulated “aches” and “pains” – although the peak output is usually well below the pain level that can result from damage to a normal limb. A persistent “pain” is – as with a natural limb – a signal that repairs are required.
*Otherwise there ought to be penalties for things like picking locks, not being able to tell if something is dangerously hot before touching it, and manipulating small objects in the dark. Since no such penalties are mentioned, at least basic sensors must be provided.