• brain stem
• neurale
• selettività
• stimolazione

When the function of the auditory nerve (VIIIth cranial) is disabled an Audi-
tory Brain stem Implant (ABI) could restore the sense of hearing in patients.
The most common cause (other than temporal bone fracture or severe ossi-
cation) of this loss is neurobromatosis 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 connec-
tion 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 ap-
plied 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 elec-trically stimulates the auditory pathways by exciting the rst 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 contribu-
tions 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; compu-
tational 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 mod-
els 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 or-
der 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 dierent biophys-
ical 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 aerent branch of the auditory path-
ways. These branches reach several major nuclei before entering the auditory
cortex, projections arising from well distinguished, highly specialized popula-
tions, none projecting to all nuclei.