Electrical characteristics of rat skeletal muscle in immaturity, adulthood and after sciatic nerve injury, and their relation to muscle fiber size

Abstract

Localized impedance methods can provide useful approaches for assessing neuromuscular disease. The mechanism of these impedance changes remains, however, uncertain. In order to begin to understand the relation of muscle pathology to surface impedance values, 8 immature rats, 12 mature rats and 8 mature rats that had undergone sciatic crush were killed. Measurement was made on tissue from the gastrocnemius muscle from each animal in an impedance cell, and the conductivity and relative permittivity of the tissue were calculated in both the longitudinal and transverse directions for frequencies of 2 kHz to 1 MHz. In addition, quantitative histological analysis was performed on the tissue. Significant elevations in transverse conductivity and transverse relative permittivity were found with animal growth, but longitudinal values showed no difference. After sciatic crush, both transverse and longitudinal conductivity increased significantly, with no change in the relative permittivity in either direction. The frequency dependence of the values also changed after nerve injury. In the healthy animals, there was a strong linear relation between measured conductivity and relative permittivity with cell area, but not for the sciatic crush animals. These results provide a first step toward developing a comprehensive understanding of how the electrical properties of muscle alter in neuromuscular disease states.

Figure 1

Conductivity and permittivity values (± standard error at selected frequencies) for the 3 groups of animals (immature, mature, and sciatic crush). A. Longitudinal conductivity from 2kHz to 1MHz. B. Transverse conductivity from 2 kHz to 1 MHz (note the different scales between 1A and 1B). C. Log(Longitudinal permittivity) from 2kHz to 1MHz. D. Log(Transverse permittivity) from 2 kHz to 1MHz

Figure 2

A. Gastrocnemius muscle from immature rat (40X), Hematoxylin and Eosin. B. Gastrocnemius muscle from mature rat (40X), Hematoxylin and Eosin C. Gastrocnemius muscle from mature rat 2 weeks after sciatic crush (40X), Hematoxylin and Eosin. Bar = 50 μm

Figure 3

Scatterplots comparing the four electrical parameters to myocyte size. Each point represents the value from an individual animal. Note the prominent near-linear relation between transverse measurements and myocyte size in the immature and mature healthy animals and the lack of a relation in the longitudinal direction. In addition, crush animals show an elevation in conductivity in both longitudinal and transverse directions.