The effect of the external tissue resistivity to the threshold level of a myelinated nerve fiber

The rise of the threshold of electric current stimulation to generate compound action potential of nerve conduction study is considered to have a relationship to malfunctions of the nerve. The effect of the decrease of the resistivity of the external tissue and the thickness of myelin sheaths was investigated by computer simulation. A myelinated human nerve fiber dipped in the homogeneous conductor was stimulated with a monopolar cathode located outside the axon. As a result, the rise of the threshold by demyelination was comparable to the effect of the decrease of the resistivity of the external tissue by a few ωm when the external resistivity is about 10 ωm. Actually the reduction of the thickness of the myelin sheaths also reduces the resistivity of the external tissue. Therefore the contribution of both effects in case of demyelination was estimated. As a result, the contributions of each effect were antagonized. As one of the causes of the rise of the threshold of nerve activation, the decrease of the resistivity of the external tissue is considerable.