PROBLEMS OF REINNERVATION OF MUSCLE

Dual innervation may occur in a partially denervated muscle

INTRODUCTION

With a view to simplifying surgical techniques for selective laryngeal reinnervation, we addressed the question of whether it is feasible to receive additional innervation by a partially denervated muscle using an infrahyoid muscle model.

METHODS

In 90 rats (6 groups of 15), phrenic nerve transfer was used to reinnervate the sternothyroid muscle. In some cases, residual innervation by the original nerve was present. Three months later we performed electromyographic studies, contraction strength measurements, histologic assessment, and retrograde labeling.

RESULTS

Muscles reinnervated by the phrenic nerve had a greater "dual-response" rate (in terms of nerve latency, contraction strength, and retrograde labeling) than muscles in the control groups.

DISCUSSION

The phrenic nerve can impart its inspiratory properties to an initially denervated strap muscle-even when residual innervation is present. The preservation of contractile potential confirmed the feasibility of dual innervation in a previously injured muscle. Muscle Nerve 59:108-115, 2019.

Laryngeal reinnervation in the horse

Left laryngeal hemiplegia is a frustrating condition for the equine athlete and equine veterinarian. Treatment for the past 30 years has centered on the prosthetic laryngoplasty ("tie-back") with or without ventriculectomy. Laryngeal reinnervation has been used successfully in people and has been shown experimentally to benefit affected horses. This article reviews equine laryngeal reinnervation using the nerve muscle pedicle graft and describes the surgical technique, its complications, and the follow-up in 146 cases treated over the past 10 years. Also discussed is ongoing research into stimulation studies to improve the success of equine laryngeal reinnervation.

Long-term excitability and fine tuning of nerve pedicles reinnervating strap muscles in the dog

Contraction of paralyzed striated muscles has been restored by stimulating reinnervating pedicles with currents of low intensity. In order to allow clinical application, stable, long-term excitability must emulate the parameters necessary for the stimulation of normal motor nerves. In 6 dogs, the ansa hypoglossi nerve was implanted into the contralateral denervated sternohyoid muscle and surrounded with a bipolar cuff electrode. Three of the reinnervating pedicles were chronically paced with a Medtronic Itrel II Multiprogrammable Pulse Generator (0.5 V, 0.2 second on [30 pulses per second, 0.21-millisecond pulse width], 2.9 seconds off). At reexploration after 8 months (6 months for 1 dog), frank contraction confirmed by electromyography tracings occurred in all animals with currents in the range of 0.1 to 0.5 mA. Muscle force was further manipulated by selective release of blocking currents (600 Hz, 1.7 to 0.4 mA) superimposed over regular stimulation (50 Hz, 0.3 to 1.7 mA). Nerve and muscle vitality were histologically confirmed. Long-term, low-intensity conduction capabilities, fine tuning, good tolerance of implanted electrodes, and lack of fatigue suggest that reinnervating pedicles may be successfully used for pacing when clinically indicated.

Reinnervation of skeletal muscle: a comparison of nerve implantation with neuromuscular pedicle transfer in an animal model

In recent years there has been increased interest in the possibility of reinnervating paralyzed muscles, particularly the muscles of the face and of the larynx. There has been dispute over the relative effectiveness of different approaches. This study was undertaken to compare direct nerve implantation with the transfer of a neuromuscular pedicle using denervated strap muscles in a rabbit model. Reinnervation was carried out both immediately after denervation and after a delay period. Evidence of return of function was obtained by a method of long-term electromyographic monitoring and was confirmed electrically by evoked electromyography and muscle tension measurement and histologically by brain stem uptake of horseradish peroxidase. Both methods of nerve transfer produced consistently functional neuromuscular units, with physiological activity and muscle strength comparable with those in normally innervated controls. Return of function was apparent within 6 weeks of both nerve implantation and neuromuscular transfer; with this model, neither method showed a clear advantage over the other.

The formation of synapses in reinnervated mammalian striated muscle

1. The hemidiaphragm of the adult rabbit has a single band of end-plates running around the middle of the muscle. A study has been made of the formation of synapses during spontaneous reinnervation of this muscle, using histological, ultrastructural and electrophysiological techniques.2. Following spontaneous reinnervation, silver-stained nerve terminals were found in association with cholinesterase-stained end-plates only in the region of the muscle corresponding to the original innervation band.3. The regenerated nerve terminals were observed with the electronmicroscope in positions overlying or adjacent to the old synaptic folds.4. Spontaneous miniature end-plate potentials and evoked synaptic potentials were recorded only in the middle of the muscle fibres after reinnervation.5. The growth of the regenerating axons was not oriented towards the end-plate zone but followed muscle fibres and blood vessels in random directions.6. It is concluded that, in adult mammalian striated muscle, the old end-plate region is preferentially reinnervated as a consequence of some special property of the muscle fibre at this site.

Nerve implants in botulinum poisoned mammalian muscle

1. In albino rats the botulinum poisoned gastrocnemius muscle was supplied with an accessory motor nerve in order to investigate whether muscle fibres with structurally intact but non-transmitting synapses would accept additional innervation. As a control similar operations were made in unpoisoned rats.2. One to three months after nerve implantation the muscles were examined histologically for the presence of a new end-plate zone. In botulinum treated muscles 1662 +/- 165 (mean +/- S.D.) of new end-plates were found. In the control animals only a few (90 +/- 13) were observed in the immediate vicinity of the implanted nerve trunk.3. Following recovery from the paralysing action of botulinum toxin electrical stimulation of both the implanted and the original motor nerve evoked strong mechanical twitches in the gastrocnemius muscle.4. When the nerves were stimulated simultaneously little or no summation of tension occurred, indicating that presumably many of the muscle fibres with new end-plates also had functionally intact junctions from the original nerve. The presence of two end-plates in a muscle fibre was confirmed in a few experiments on single curarized fibres by intracellular recording of end-plate potentials on stimulation of each nerve.