• brain stem
• neurale
• selettività
• stimolazione

When the function of the auditory nerve (VIIIth cranial) is disabled an Auditory Brain stem Implant (ABI) could restore the sense of hearing in patients.
The most common cause (other than temporal bone fracture or severe ossification) of this loss is neurofibromatosis type 2 (NF2). This disease is genetic related (chromosome 22) and causes tumor growths in the Schwann cells that insulate electromagnetically axons exiting the internal auditory meatus. When the tumors are removed the auditory nerve is usually cut, leaving no connection between the cochlea and the brain. For this reason cochlear implants are not an option, and the only solution to restore the sense of hearing are ABIs.
Most surgeons prefer the translabyrintine approach for ABI placement, a clear view of the ventral region of the cochlear nucleus allows neurosurgeons to put in place the stimulating part of the device: an array of electrodes directly applied on the surface of the brain stem. This paddle is connected to a radio receiver/stimulator that is implanted in the temporal bone. Sound is recorded at the level of the pinna and sent to a speech processor; the latter sends the excitatory signal to the receiver thanks to a transmitter coil. This device elecrically stimulates the auditory pathways by exciting the first auditory relay station: the cochlear nucleus. The origins of electrical stimulation and neurons signaling behaviour were put in the classic paper by Hodgkin-Huxley.The activity of the axon, its response to an extracellular applied potential[3], is caracterised by the well known Rattay's "activating function"[4] (later analitically improved[5]). In models of electrical stimulation the potential applied to the organ is calculated using the quasi-static approximation; therefore capacitive, inductive and wave propagation contributions in Maxwell's equations could be neglected, thus simplifying the modelling of the system under analysis. One of the most important goals of neuroscience research is the understanding of how various tasks are computed by neural populations and networks; computational models could really come in help testing new stimulating regimes and electrode designs. Thanks to the massive amount of calculation power of today's processors nonlinear dynamics can be taken into account. State of the art models can be created upon the actual neural dynamics as well as immunochemistry nature of the subject to study [7].
Recently new geometries and material structure of the electrode are being investigated towards an improvement of stimulating frequency spectrum in order to obtain a device that could deliver a better sound experience to patients diminishing noise and current erogated. To get to this a profound understanding of neural networks is essential. Neural networks are based on different biophysical properties of neurons e.g. Octopus, Stellate, Bushy cells) constituting, for instance, the cochlear nuclei; they play a key role in the encoding process of the acoustic signal, what is conveyed to each afferent branch of the auditory pathways. These branches reach several major nuclei before entering the auditory cortex, projections arising from well distinguished, highly specialized populations, none projecting to all nuclei.