Biology Reference
In-Depth Information
axotomized neurons change to a regrowing mode, the distal stump debris are
cleared by Wallerian degeneration, and the cut axons regenerate along the
distal nerve stump. However, axon reconnection is far from optimal and tar-
get reinnervation is not selective at all after complete nerve transection
(
Allodi, Udina, & Navarro, 2012
). Following injuries in proximal sites of
the limbs, the slow rate of axonal regeneration implies chronic denervation
and atrophy of target organs. Furthermore, if reconnection of proximal
and distal stumps cannot be achieved, by direct suture or interposition of
a graft, the cut axons are not able to elongate distally and aberrantly grow
forming a proximal neuroma. The same situation is obvious after limb
amputation. Moreover, regenerated or amputated nerves show an increase
in excitability, causing undesired complications
such as hyperreflexia,
hyperalgesia, and pain.
3. TYPES OF NERVE ELECTRODES: AN OVERVIEW
Different types of electrodes have been developed to interface the PNS
in order to record electrical activity from and/or to stimulate the nerve fibers
for different biomedical applications (
Navarro et al., 2005; Schultz &Kuiken,
2011
). Most interfaces are implanted around or within a peripheral nerve or
spinal root in order to have low tissue resistance, therefore reducing the inten-
sity needed for stimulation and enhancing the signal-to-noise ratio for record-
ings. Nerve electrodes can be classified into three main classes depending on
nerve invasiveness: extraneural, intraneural, and regenerative (
Fig. 2.1
).With
increasing invasiveness of the implant, higher selectivity of stimulation of
individual nerve fibers may be reached and lower intensity is needed as the
distance from the electrode to individual axons is reduced; likewise, reducing
this distance has a positive influence on the quality of the recorded signals.
However, selectivity comes with a price as the more invasive is the electrode,
the more potential damage to the nerve is prone to be done. For example,
extraneural electrodes, such as cuff and epineurial ones, provide simultaneous
interface with many axons in the nerve resulting in poor selectivity but with
very little nerve damage, whereas intraneural electrodes inserted in the nerve
may interface discrete groups of axons within a fascicle reaching very good
selectivity but with higher risk of nerve damage.
3.1. Extraneural electrodes
Cuff electrodes
(
Fig. 2.1
A) are made of a cylindrical sheath that is wrapped
longitudinally around the nerve with two or more electrode sites in the
