Mechanisms of electrical stimulation with neural prostheses.

Individual electric and geometric characteristics of neural substructures can have surprising effects on artificially controlled neural signaling. A rule of thumb approved for the stimulation of long peripheral axons may not hold when the central nervous system is involved. This is demonstrated here with a comparison of results from the electrically stimulated cochlea, retina, and spinal cord. A generalized form of the activating function together with accurate modeling of the neural membrane dynamics are the tools to analyze the excitation mechanisms initiated by neural prostheses. Analysis is sometimes possible with a linear theory, in other cases, simulation of internal calcium concentration or ion channel current fluctuations is needed to see irregularities in spike trains. Spike initiation site can easily change within a single target neuron under constant stimulation conditions of a cochlear implant. Poor myelinization in the soma region of the human cochlear neurons causes firing characteristics different from any animal data. Retinal ganglion cells also generate propagating spikes within the dendritic tree. Bipolar cells in the retina are expected to respond with neurotransmitter release before a spike is generated in the ganglion cell, even when they are far away from the electrode. Epidural stimulation of the lumbar spinal cord predominantly stimulates large sensory axons in the dorsal roots which induce muscle reflex responses. Analysis with the generalized activating function, computer simulations of the nonlinear neural membrane behavior together with experimental and clinical data analysis enlighten our understanding of artificial firing patterns influenced by neural prostheses.

Investigation of charge variants of rViscumin by two-dimensional gel electrophoresis and mass spectrometry.

A method for the analysis of the rViscumin heterodimer (recombinant mistletoe lectin) based on two-dimensional (2-D) polyacrylamide gel electrophoresis and mass spectrometry was developed and used for quality control concerning purity and homogeneity of the recombinant protein processed under Good Manufacturing Practice (GMP) conditions. A series of spots with different pI-values in the pH-gradient of both rViscumin A- and B-chain were observed independently from the experimental conditions like urea concentration, heat treatment or the use of cysteine alkylating agents. Comparative studies of the major spots using matrix assisted laser desorption/ionization-mass spectrometry (MALDI-MS), liquid chromatography-electrospray ionization (LC-ESI)-MS and LC-ESI-tandem MS (MS/MS) after tryptic in-gel digestion resulted in a sequence coverage of 92% for the A-chain and 95% for the B-chain. No molecular differences like common chemical or post-translational modifications or nonenzymatic deamidation were found to cause the different charge values of the separated spots. Therefore, these protein spots were extracted from the 2-D gel and separated again by 2-D gel electrophoresis (termed Re-2-DE). Each of the single spots tested in the Re-2-DE experiment split up in the same heterogeneous pattern concerning the pI-values. We suggest that the observed charge variants of rViscumin are the result of conformational protein variants, existing in an equilibrium during sample preparation and/or isoelectric focusing and are not caused from microheterogeneity in the primary structure of rViscumin.

A model of the electrically excited human cochlear neuron. I. Contribution of neural substructures to the generation and propagation of spikes.

Differences in neural geometry and the fact that the soma of the human cochlear neuron typically is not myelinated are reasons for disagreements between single fiber recordings in animals and the neural code evoked in cochlear implant patients. We introduce a compartment model of the human cochlear neuron to study the excitation and propagation process of action potentials. The model can be used to predict (i) the points of spike generation, (ii) the time difference between stimulation and the arrival of a spike at the proximal end of the central axon, (iii) the vanishing of peripherally evoked spikes at the soma region under specific conditions, (iv) the influence of electrode positions on spiking behavior, and (v) consequences of the loss of the peripheral axon. Every subunit of the cochlear neuron is separately modeled. Ion channel dynamics are described by a modified Hodgkin--Huxley model. Influence of membrane noise is taken into account. Additionally, the generalized activating function is introduced as a tool to give an envision of the origin of spikes in the peripheral and in the central axon without any knowledge of the gating processes in the active membranes. Comparing the reactions of a human and cat cochlear neuron, we find differences in spiking behavior, e.g. peripherally and centrally evoked spikes arrive with a time difference of about 400 mus in man and 200 mus in cat.