Morphometric parameters of peripheral nerves in calves correlated with conduction velocity

Abstract

Background: Peripheral nerve injuries are the most frequent neurologic disorder in cattle. So far, no physiologic values have been established for the motor nerve conduction velocity (mNCV) in this precocial species.

Objectives: The electrophysiologic and morphometric reference values of peripheral nerves in calves were determined. It was hypothesized that these parameters would correlate to the high degree of maturity in the first days of life in this species compared to other species.

Animals: Twenty-six healthy calves were used in this study.

Methods: The mNCV of the radial and the sciatic/common peroneal nerve was measured in all 26 calves. Nerve biopsies from a group of 6 calves were taken to correlate the obtained electrophysiologic data with morphological parameters.

Results: The mean mNCV of the radial nerve was 48.3 ± 10.6 m/s, whereas the mean mNCV of the sciatic/peroneal nerve was with 83.8 ± 5.9 m/s significantly faster (P < .0001). The average fiber diameter was 8.40 ± 2.80 μm (range, 1.98-17.90 μm) and the average g-ratio was 0.61 ± 0.04 SD.

Conclusion and clinical importance: The established reference values for mNCV in calves correlate well with the evaluated morphometric parameters. Attributable to their comparably fast mNCV and high fiber diameters, juvenile calves appear to be much more mature individuals than other mammals. Electrophysiologic characterization of peripheral nerve injury now is feasible in this species.

Keywords: Myelination; Precocial species; g-ratio.

Figure 1

(A) Photograph demonstrating position of electrodes during motor nerve conduction testing of the radial nerve showing proximal stimulation electrodes (=PS, angle between the head of the humerus and the ventral part of the scapula), distal stimulation electrodes (=DS, distal third of the humerus between the brachial muscle and the lateral head of the brachial triceps muscle), ground electrode (=), and bipolar concentric needle electrode (=Rec.) for recording the compound muscle action potential in the extensor carpi radialis muscle. Anatomic landmarks are S, shoulder joint; OP, olecranon process. (B) Photograph demonstrating position of electrodes during motor nerve conduction testing of the sciatic/peroneal nerve showing proximal stimulation electrodes (=PS, trochanteric fossa between the trochanter major and the tuber ischiadicum), distal stimulation electrodes (=DS, in the popliteal fossa), ground electrode (=), and bipolar concentric needle electrode (=Rec.) for recording the compound muscle action potential in the fibularis tertius muscle. Anatomic landmarks are T, major trochanter; St, stifle.

Figure 2

Box plotting of the motor nerve conduction velocity in 20 calves. mNCV, motor nerve conduction velocity; ┬, maximum; ┴, minimum; ─, median; O, outlier; n, number of measurements per nerve. The box includes 50% of the values.

Figure 3

(A) Correlation between age and motor nerve conduction velocity. mNCV, motor nerve conduction velocity; r ², correlation coefficient. (B) Repeated measurements of the motor nerve conduction velocity of sciatic/peroneal nerve of 6 calves on 5 consecutive days. mNCV, motor nerve conduction velocity, the age of the calves is shown in brackets; d, day.

Figure 4

Transverse section of a typical fascicle from peroneal nerve, paraphenylendiamine staining of a biopsy specimen of peroneal nerve (calf E), 400× magnification.

Figure 5

Correlation of g‐ratio with the motor nerve conduction velocity in 6 calves. r ², correlation coefficient. A correlation of 40.44% of the g‐ratio to the motor nerve conduction velocity is demonstrated by the correlation line, indicating that a good myelination of the peripheral nerve (low g‐ratio) correlates positively with an increasing conduction velocity. mNCV, motor nerve conduction velocity.

Figure 6

(A) Frequency distribution of the axon diameter (n = 6 calves), the axon diameters range from 0.95 to 14.91 μm; the mean value is 5.17 ± 2.35 μm. Histogram indicating a bimodal distribution with a lower peak at 3.0 μm and an upper peak at 5.5 μm. (B) Frequency distribution of the fiber diameter (n = 6 calves), the fiber diameters ranging from 1.98 to 17.90 μm; the mean value is 8.40 ± 2.80 μm. Histogram indicating a slight bimodal distribution with a lower peak at 7.0 μm and an upper peak at 10.0 μm. The included bars indicate the maximum fiber diameter recognized in the mentioned species. (C) Frequency distribution of the axon area (n = 6 calves), the axon areas ranging from 0.71 to 174.46 μm² (in 3 observations the axon area was greater than 146 μm², this being represented by the last bar on the right); the mean value is 25.29 ± 22.62 μm². Histogram indicating a monopolar distribution with a peak at 6 μm². (D) Frequency distribution of the fiber area (n = 6 calves) the fiber area ranging from 3.09 to 251.44 μm²; the mean value is 61.57 ± 39.62 μm². Histogram indicating a monopolar distribution with a peak at 41.0 μm². (E) Frequency distribution of the myelin thickness (n = 6 calves), myelin thickness ranging from 0.38 to 3.20 μm; the mean value is 1.62 ± 0.64 μm. Histogram indicating a bipolar distribution with a lower peak at 0.8 μm and an upper peak at 2.0 μm.