Sensory protection of rat muscle spindles following peripheral nerve injury and reinnervation

Abstract

Background: Skeletal muscle structure and function are dependent on intact innervation. Prolonged muscle denervation results in irreversible muscle fiber atrophy, connective tissue hyperplasia, and deterioration of muscle spindles, specialized sensory receptors necessary for proper skeletal muscle function. The protective effect of temporary sensory innervation on denervated muscle, before motor nerve repair, has been shown in the rat. Sensory-protected muscles exhibit less fiber atrophy and connective tissue hyperplasia and maintain greater functional capacity than denervated muscles. The purpose of this study was to determine whether temporary sensory innervation also protects muscle spindles from degeneration.

Methods: Rat tibial nerve was transected and repaired with either the saphenous or the original transected nerve. Negative controls remained denervated. After 3 to 6 months, the electrophysiologic response of the nerve to stretch in the rat gastrocnemius muscle was measured (n = 3 per group). After the animals were euthanized, the gastrocnemius muscle was removed, sectioned, stained, and examined for spindle number (n = 3 per group) and morphology (one rat per group). Immunohistochemical assessment of muscle spindle innervation was examined in four additional animals.

Results: Significant deterioration of muscle spindles was seen in denervated muscle, whereas in muscle reinnervated with the tibial or the saphenous nerve, spindle number and morphology were improved. Histologic and functional evidence of spindle reinnervation by the sensory nerve was obtained.

Conclusion: These findings add to the known means by which motor or sensory nerves exert protective effects on denervated muscle, and further promote the use of sensory protection for improving the outcome after peripheral nerve injury.

Fig. 1

Drawings depicting the experimental groups. The first group of animals (above, left) was denervated and not repaired; the second group (above, right) had sensory protection with saphenous nerve transfer from its normal target, the skin, to the distal stump of the tibial nerve; the third group (below, left) had immediate division and microsuture of the tibial nerve. (Above, left and above, right) The tibial nerve was sutured to the biceps femoris muscle to prevent reinnervation of the gastrocnemius muscle. The contralateral, unoperated leg served as a control in each experimental cohort (below, right).

Fig. 2

Micrographs of representative muscle spindles. Unoperated control (above, left), immediate repair (above, right), sensory-protected (below, left), and denervated (below, right) sections. These 10-μm transverse serial sections through the equatorial region were stained with hematoxylin and eosin (original magnification, ×400). Scale bar = 10 μm.

Fig. 3

Muscle spindle innervation in unoperated control (above, left), immediate repair (above, right), sensory-protected (below, left), and denervated (below, right) groups. Ten-micron transverse and 20-μm longitudinal sections of muscle spindles from the lateral gastrocnemius muscle were stained with antibodies against slow-tonic myosin heavy chain (red). Nerve afferents were identified using antisera against neuro-filament heavy chain (green). Transverse sections, original magnification, ×630; scale bar = 10 μm. Longitudinal sections, original magnification, ×200; not shown to scale.

Fig. 4

Electrophysiologic response of the nerve to stretch in the rat gastrocnemius muscle 3 months after denervation and various treatments. Mean amplitude of the response (in microvolts) ± SEM; n = 3 per group except n = 6 for the control group. C, control; IR, immediate repair; SP, sensory-protected; D, denervated. Significant differences between denervated and all other groups and between control and all other groups, but no differences between immediate repair and sensory-protected groups (analysis of variance followed by post hoc t test; *p < 0.05; **p < 0.005).