Nervous system electric currents

Demyelination. The role of myelin in the nervous system is to aid in signal transduction. Myelin acts like an electrical insulator by preventing loss of ion current, and intact myelin is critical for the fast saltatory nerve conduction discussed above. Neurotoxicants that target the synthesis or integrity of PNS myelin may cause muscle weakness, poor coordination, and paralysis. In the brain, white matter tracts that connect neurons within and between hemispheres may be destroyed, in a syndrome known as toxic leukoencephalopathy. A multifocal distribution of brain lesions is reflected in mental deterioration, vision loss, speech disturbances, ataxia (inability to coordinate movements), and paralysis. 

Cavendish, a century before, had attempted to compare quantitatively the electrical conductivity of rain water with various salt solutions. Possessing no galvanometer to register the strength of the currents, he had bravely converted his own nervous system into one. As he discharged Leyden jars through the different liquids he compared the electric shocks which he received. With this crude, heroic method he obtained a number of surprisingly accurate results. 

Electrochemical detection involves the induction of a change in redox state (electrolysis) by application of an electrical potential to an electrode (71). Compounds that can be readily detected by this means are termed electroactive. Under physiological conditions, these compounds tend to be in their reduced state in the nervous system because of the rich level of antioxidants (e.g., ascorbic acid) and, thus, can be oxidized by application of a positive potential to the electrode. The evolved electrons are detected at the electrode in the form of electrical current. This current is proportional to the number of electroactive molecules at the surface of the electrode, and therefore it is proportional to their concentration in the bulk solution. By implanting an electrode in the extracellular space close to the release site and detecting changes in the local (extracellular) concentration of the neurotransmitter, neurotransmitter release can be monitored. The key advantage of this approach is the high temporal resolution that can be in the millisecond domain. Neurotransmitters that can be detected this way include dopamine, norepinephrine, epinephrine, serotonin, and melatonin. 

In the myocardium, automaticity is the ability of the cardiac muscle to depolarize spontaneously (i.e., without external electrical stimulation from the autonomic nervous system). This spontaneous depolarization is due to the plasma membrane within the heart that has reduced permeability to potassium (K+) but still allows passive transfer of calcium ions, allowing a net charge to build. Automaticity is most often demonstrated in the sinoatrial (SA) node, the so-called pacemaker cells. Abnormalities in automaticity result in rhythm changes. The mechanism of automaticity involves the pacemaker channels of the HCN (Hyperpolarization-activated, Cyclic Nucleotide-gated) family14 (e.g., If, "funny" current). These poorly selective cation channels conduct more current as the membrane potential becomes more negative, or hyperpolarized. They conduct both potassium and sodium ions. The activity of these channels in the SA node cells causes the membrane potential to slowly become more positive (depolarized) until, eventually, calcium channels are activated and an action potential is initiated. 

The electroconvulsive therapy performed in the past is a far cry from the process used today in the treatment of major depressive disorder. In ECT a generalized central-nervous-system seizure is induced by means of an electric current. The objective is to achieve the full seizure threshold until the full therapeutic gains can be established. The exact process by which ECT works is unknown however, the shock results in an increase in different neurotransmitter responses at the cell membrane. Four to twelve treatments are generally given until therapeutic results are noted (Sachs, 1996). 

Functional electrical stimulation (FES) is a rehabilitation technique for the restoration of lost neurological function, resulting from conditions such as stroke, spinal cord injury, cerebral palsy, head injuries, and multiple sclerosis. FES utilizes low-level electrical current applied in programmed patterns to different nerves or reflex centers in the central nervous system to produce functional movements. The stimulation may be triggered by a single switch (open-loop) or from sensor(s) or neuronal activity (closed-loop). 

All electric currents are accompanied by magnetic fields. With the invention of the Superconducting Quantum Interference Device as an extremely low magnetic field (10 T) detector, it has become possible to measure the magnetic fields from the small endogenic currents of the body, even from the small sources in the heart and the central nervous system. This has opened up a whole new field of measurements analogous to their electric counterparts MKG (EKG), MEG (EEG), and so on. However, these interesting subjects are outside the scope of this book, and the reader is referred to the book by Malmivuo and Plonsey (1995). 

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