Abstract
Functional electrical stimulation (FES) techniques may be used to restore motor function lost or impaired through spinal cord injury. In order to use these techniques to restore complex tasks such as walking, it is necessary to provide sensory feedback to regulate the output of the FES controller. It has been suggested that multi-microelectrode probes (microprobes) implanted into the peripheral nervous system can be used to detect signals originating from the body's own sensors. These signals could be decoded and used to regulate the output of the FES controller. Prior to the present work, however, microprobes had primary been used to study neural activity in the brain, not peripheral nerves. In the present work, locust peripheral nerve has been used as an animal model for experimental and computer modelling work. The experimental work was directed at discerning the detail of information that can be obtained using microprobes to record from peripheral nerves (ie, the selectivity of the probes). In the computer modelling work, the effects of filtering the recorded signal were studied using an electrical circuit simulator programme (SPICE). Finite element analysis software (ANSYS) was used to model the electrical potential distribution in the nerve trunk, and to determine the effects of the probe substrate on the recorded signal. The results of the experimental work indicated that it may be possible to achieve higher selectivity in recording with microprobes than predicted by some models. It is concluded that future models need to represent the situation in greater detail in order to make more realistic predictions regarding the practical work. This will require further data on the electrical properties of the structures modelled within the nerve trunk. The SPICE modelling work successfully predicted the shape of the neural signals that would be recorded in the practical work. The partial differentiating effect of high pass filtering neural signals was also demonstrated. The results of the finite element modelling work demonstrated that the probe substrate would be expected to amplify signals from fibres directly in front of it, and attenuate signals from fibres behind it. This was shown to be significant for probe substrates with dimensions much smaller than the longitudinal spread of the action potential along the fibre. It was also found that these effects can be influenced by the position of the microprobe substrate relative to other structures within the nerve trunk; not just relative to the fibre. The significance of these results as they relate to mammalian nerve is discussed. Improved experimentation techniques and models are outlined, based on the results of this work. These include the requirement for improved facilities to determine the limits of selectivity in recording from peripheral nerves, and also the inclusion of inhomogeneities in models of the nerve trunk to make more realistic predictions regarding practical work. Finally, the development of active probes is discussed, including requirements for particularly novel circuitry, and the integration of many devices into a system to control FES.