Electrical stimulation of excitable tissue

Electrical stimulation of excitable tissue has been used for over a century and important discoveries have been made concerning the mechanisms underlying the interactions between the apphed fields with the... 

Merrill D.R., Bikson M., and Jefferys J.G. Electrical stimulational of excitable tissue design of efficacious and safe protocols. /. Neuroscience methods, 141 171-198,2005. 

The goal of electrical stimulation of excitable tissue is often the triggering of action potentials in axons, which requires the artificial depolarization of some portion of the axon membrane to threshold. In the process of extracellular stimulation, the extracellular region is driven to relatively more negative potentials, equivalent to driving the intracellular compartment of a cell to relatively more positive potentials. Charge is transferred across the membrane due to both passive (capacitive and resistive) membrane properties as well as through active ion channels [95]. The process... 

Meyer RD, Cogan SE, Nguyen TH et al. (2001) Electrodeposited iridium oxide for neural stimulation and recording electrodes. IEEE Trans Neural Sys Rehab 9 2 Durand DM (1999) Electrical stimulation of excitable tissue. Biomedical Engineering Handbook. CRC, Boca Raton, FL... 

Field forces due to the induced dipole moment of the field have been listed as evidence of nonthermal action of electric fields on biologic systems. However, the effects require fairly large field strengths, frequently above those that give rise to heating or stimulation of excitable tissues. The field forces also depend on the electric properties of the particle considered and its environment. 

Cardioversion or defibrillation is the electrical termination of arrhythmias using field stimulation. Unlike pacing, in which cardiac excitation is initiated in and propagates from a small region of tissue near the electrode, cardioversion must arrest electrical activity by simultaneous stimulation of most of the heart. In practice, this means establishing a critical field across a critical mass of cardiac tissue. This requires a compromise between the electrical response of the tissue and the electrical capabilities of the device. The electrical response of cardiac cells is complex, but stimulation mostly depends on the first-order properties of the membrane [6]. Theoretical and experimental studies have shown that the optimum voltage waveform for stimulation of cardiac tissue is a waveform with a characteristic rise time comparable to the cell membrane time constant [7,8]. 

When a wave of excitation starts at a specified point on the fibre this potential difference is abolished and reversed and the surface becomes electronegative with regard to the unexcited portions of the fibre (fig. 16). ( Depolarization coincides with a ohange in the surface of the membrane of the cell which first allows Na+ ions from the tissue fluids to pass into the cell and K+ ions to pass outwards. In the undisturbed state, the cell membrane is relatively impermeable to the Na+ ions which are kept outside and the concentration of K+ ions inside the cell is greater than in the external fluids.) This induced negativity at the excited spot causes local electrical circuits to arise and so new points of excitation are caused (Fig. lc). The passage of electrical disturbance is shown in both directions. In the body, however, the fibres are stimulated at only one end, and hence induction is in one direction. 

As mentioned in section 4.3.3, there are two kinds of a receptor in brain and peripheral tissues. The crucial experiments have shown that brain tissue prelabeled with pH]NE will release neurotransmitter upon electrical stimulation or exposure to K+. The release is reduced by the a agonist clonidine (4.42) and stimulated by the a antagonist yohimbine (4.43). Since the adrenoreceptor involved in this latter experiment plays a vital role in modulating neurotransmitter release, it must be presynaptic and located on the nerve-ending membrane. A similar selectivity has also been shown by peripheral tissues (heart, uterus), leading to the distinction of aj (postsynaptic) and (presynaptic) adrenergic receptors. There are also presynaptic [3 receptors, which show a feedback regulation opposite to that of the ttj receptors that is, their excitation by a neurotransmitter increases NE release. 

Functional Electrical Stimulation Technology for Delivering Stimulation Pulses to Excitable Tissue Stimulation Parameters Implantable Neuromuscular Stimulators Packaging of Implantable Electronics Leads and Electrodes Safety Issues of Implantable Stimulators Implantable Stimulators in Clinical Use Future oflmplantable Electrical Stimulators Summary Defining Terms References Further Information... 

Electrodes are the direct interface between the biological structures (auditory neurons) and the electronic system in the CFs. Stimulation electrodes inject charge into the tissue to functionally excite the nerves by electrical stimulation. In other words, electrodes measure the electric potential for charge transfer between solid metal state and electrolyte solution in liquid state inside the cochlea. For better stability implants electrode properties must be evaluated with respect to a biocompatible application for optimum stability, efficacy and life time with a minimum of toxicity. From the material point of view the requirements of an ideal electrode [8-10] might be summarized as follows ... 

Neuromuscular Stimulation. Based on a method that has remained unchanged for decades, electrodes are placed within the excitable tissue that provide current to activate certain pathways. This supplements or replaces lost motor or autonomic functions in patients with paralysis. An example is application of electrical pulses to peripheral motor nerves in patients with spinal cord injuries. These pulses lead to action potentials that propagate across neuromuscular junctions and lead to muscle contraction. Coordinating the elicited muscle contractions ultimately reconstitutes function. 

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