Reaction on Motoneurons and Their Microenvironment to Axotomy. In: Gilad GM, Gorio A, Kreutzberg GW, editors. Processes of Recovery from Neural Trauma. Berlin: Springer Berlin Heidelberg; 1986. pp. 3-8. [DOI: 10.1007/978-3-642-70699-8_1]

Facial nerve palsy and hemifacial spasm

Facial nerve lesions are usually benign conditions even though patients may present with emotional distress. Facial palsy usually resolves in 3-6 weeks, but if axonal degeneration takes place, it is likely that the patient will end up with a postparalytic facial syndrome featuring synkinesis, myokymic discharges, and hemifacial mass contractions after abnormal reinnervation. Essential hemifacial spasm is one form of facial hyperactivity that must be distinguished from synkinesis after facial palsy and also from other forms of facial dyskinesias. In this condition, there can be ectopic discharges, ephaptic transmission, and lateral spread of excitation among nerve fibers, giving rise to involuntary muscle twitching and spasms. Electrodiagnostic assessment is of relevance for the diagnosis and prognosis of peripheral facial palsy and hemifacial spasm. In this chapter the most relevant clinical and electrodiagnostic aspects of the two disorders are reviewed, with emphasis on the various stages of facial palsy after axonal degeneration, the pathophysiological mechanisms underlying the various features of hemifacial spasm, and the cues for differential diagnosis between the two entities.

Clinical consequences of reinnervation disorders after focal peripheral nerve lesions

Axonal regeneration and organ reinnervation are the necessary steps for functional recovery after a nerve lesion. However, these processes are frequently accompanied by collateral events that may not be beneficial, such as: (1) Uncontrolled branching of growing axons at the lesion site. (2) Misdirection of axons and target organ reinnervation errors, (3) Enhancement of excitability of the parent neuron, and (4) Compensatory activity in non-damaged nerves. Each one of those possible problems or a combination of them can be the underlying pathophysiological mechanism for some clinical conditions seen as a consequence of a nerve lesion. Reinnervation-related motor disorders are more likely to occur with lesions affecting nerves which innervate muscles with antagonistic functions, such as the facial, the laryngeal and the ulnar nerves. Motor disorders are better demonstrated than sensory disturbances, which might follow similar patterns. In some instances, the available examination methods give only scarce evidence for the positive diagnosis of reinnervation-related disorders in humans and the diagnosis of such condition can only be based on clinical observation. Whatever the lesion, though, the restitution of complex functions such as fine motor control and sensory discrimination would require not only a successful regeneration process but also a central nervous system reorganization in order to integrate the newly formed peripheral nerve structure into the prepared motor programs and sensory patterns.

Agmatine treatment and vein graft reconstruction enhance recovery after experimental facial nerve injury

The rate of nerve regeneration is a critical determinant of the degree of functional recovery after injury. Here, we sought to determine whether treatment with the neuroprotective compound, agmatine, with or without nerve reconstruction utilizing a regional autogenous vein graft would accelerate the rate of facial nerve regeneration. Experiments compared the following seven groups of adult male rats: (A) Intact untreated controls. (B) Sham operation with interruption of the nerve blood supply (controls). (C) Transection of the mandibular branch of the facial nerve (generating a gap of 3 mm) followed by saline treatment. (D) Nerve transection with unsutured autogenous vein (external jugular) graft reconstruction plus saline treatment. (E) Nerve transection with sutured vein graft approximation (coaptation of the proximal and distal nerve stumps) plus saline. (F) Nerve transection with sutured vein graft followed by agmatine treatment (four daily intraperitoneal injections of 100 mg/kg agmatine sulfate). (G) Nerve transection with unsutured vein graft followed by agmatine treatment. Functional recovery, as assessed by grading vibrissae movements and by recording nerve conduction velocity and numbers of regenerated axons, indicated that either vein reconstruction or agmatine treatment resulted in accelerated and more complete recovery as compared with controls. But best results were observed in animals that underwent combined treatment, i.e., vein reconstruction plus agmatine injection. We conclude that agmatine treatment can accelerate facial nerve regeneration and that agmatine treatment together with autogenous vein graft offers an advantageous alternative to other facial nerve reconstruction procedures.

Movement disorders in patients with peripheral facial palsy

Acute unilateral facial paralysis is usually a benign neurological condition that resolves in a few weeks. However, it can also be the source of a transient or long-lasting severe motor dysfunction, featuring disorders of automatic and voluntary movement. This review is organized according to the two most easily recognizable phases in the evolution of facial paralysis: (1). Just after presentation of facial palsy, patients may exhibit an increase in their spontaneous blinking rate as well as a sustained low-level contraction of the muscles of the nonparalyzed side, occasionally leading to blepharospasm-like muscle activity. This finding may be due to an increase in the excitability of facial motoneurons and brainstem interneurons mediating trigeminofacial reflexes. (2). If axonal damage has occurred, axonal regeneration beginning at approximately 3 months after the lesion leads inevitably to clinically evident or subclinical hyperactivity of the previously paralyzed hemifacial muscles. The full-blown postparalytic facial syndrome consists of synkinesis, myokymia, and unwanted hemifacial mass contractions accompanying normal facial movements. The syndrome has probably multiple pathophysiological mechanisms, including abnormal axonal branching after aberrant axonal regeneration and enhanced facial motoneuronal excitability. Although the syndrome is relieved with local injections of botulinum toxin, fear of such uncomfortable contractions may lead the patients to avoid certain facial movements, with the implications that this behavior might have on their emotional expressions.

Amyotrophic lateral sclerosis is a distal axonopathy: evidence in mice and man

The SOD1 mutant mouse is the most widely used model of human amyotrophic lateral sclerosis (ALS). To determine where and when the pathological changes of motor neuron disease begins, we performed a comprehensive spatiotemporal analysis of disease progression in SOD1(G93A) mice. Quantitative pathological analysis was performed in the same mice at multiple ages at neuromuscular junctions (NMJ), ventral roots, and spinal cord. In addition, a patient with sporadic ALS who died unexpectedly was examined at autopsy. Mice became clinically weak at 80 days and died at 131 +/- 5 days. At 47 days, 40% of end-plates were denervated whereas there was no evidence of ventral root or cell body loss. At 80 days, 60% of ventral root axons were lost but there was no loss of motor neurons. Motor neuron loss was well underway by 100 days. Microglial and astrocytic activation around motor neurons was not identified until after the onset of distal axon degeneration. Autopsy of the ALS patient demonstrated denervation and reinnervation changes in muscle but normal appearing motor neurons. We conclude that in this widely studied animal model of human ALS, and in this single human case, motor neuron pathology begins at the distal axon and proceeds in a "dying back" pattern.

Reflex excitability of facial motoneurons at onset of muscle reinnervation after facial nerve palsy

We studied 18 patients with complete unilateral denervation of the facial muscles after idiopathic facial nerve palsy to determine whether motoneuronal excitability is enhanced in the few motor units that are active at onset of muscle reinnervation. The study was carried out between 75 and 90 days after the facial nerve lesion. We used two needle electrodes to record simultaneously the spontaneous and voluntary activity of the orbicularis oris (OOris) and orbicularis oculi (OOculi) muscles, as well as the responses to ipsilateral and contralateral facial and supraorbital nerve stimuli. All patients showed involuntary firing of motor unit action potentials (MUAPs) in at least one of the muscles. Synkinetic activation of motor units in the OOris was induced by spontaneous blinking in all patients, and by inhalation and swallowing in some. Electrical stimulation of the ipsilateral facial nerve induced a direct M response in only 4 patients. In contrast, long-latency reflex responses were induced in both muscles by electrical stimulation of ipsilateral and contralateral facial and supraorbital nerves in all patients, at latencies ranging between 44 and 132 ms. The shape of such MUAP reflex responses was the same as that of the MUAPs seen to fire at rest. These findings provide evidence of enhanced excitability of facial motoneurons in our patients. Such hyperexcitability may be partly responsible for the postparalytic motor dysfunction syndrome that occurs after facial palsy with severe axonal damage.

3-Acetylpyridine-induced degeneration in the dorsal root ganglia: involvement of small diameter neurons and influence of axotomy

3-Acetylpyridine (3-AP), an analogue of nicotinamide, produces highly selective CNS lesions, the severity of which may be influenced by prior alterations in the metabolic activity of the affected neurons. The present study was undertaken to determine whether prior axotomy modified the response of dorsal root ganglia (DRG) and anterior horn (AH) neurons to 3-AP. A single administration (50 or 80 mg/kg i.p.) of 3-AP to adult rats resulted in degeneration of primarily small-dark DRG neurons by 24 h. The AH neurons were not affected by either dose of 3-AP. Light and electron microscopy of the DRG revealed a spectrum of damage ranging from loss of Nissl substance and cytoplasmic degradation to frank necrosis with neuronophagia. Frequently, injured neurons exhibited perinuclear aggregation of cytoplasmic organelles with dissolution of Nissl substance, clearing of the peripheral cytoplasm, and formation of large peripheral vacuoles. Occasionally, a second pattern of 3-AP injury was observed in which the nuclear chromatin of the neurons was condensed and there was formation of small vacuoles throughout the cytoplasm without peripheral clearing or perinuclear aggregation of cytoplasmic organelles. Axotomy induced typical axon reactions in both large-pale and small-dark DRG neurons. The combination of axotomy followed by 3-AP 4 days later produced morphological features characteristic of both axotomy and 3-AP exposure, but did not appear to alter the incidence of neuronal cell death. The almost exclusive vulnerability of the small dorsal root ganglion neurons to 3-AP neurotoxicity make this model potentially useful for the study of small fibre neuropathies.

Regeneration and soma size changes following axotomy of the trochlear nerve

The effects of CNS and PNS axotomy of the IVth nerve on cell death, soma size, axon size, and axon number were investigated. In adult cats, the IVth nerve was axotomised by using four surgical paradigms: (1) peripheral IVth nerve crush, (2) peripheral IVth nerve cut, (3) peripheral IVth nerve resection, and (4) a CNS IVth nerve cut in the velum. The extent of cell death resulting from each surgical paradigm was determined. Following axotomy distal to the decussation of the IVth nerves, cell death was least after nerve crush, intermediate after nerve cut, and maximal after resection of 5-7 mm of the nerve. Following axotomy at the decussation--a CNS lesion--most cells died but some successful regeneration was observed. Soma size measurements following a short-term survival (3 days to 4 weeks) before the regenerating axons reached their target muscle revealed that somas of axotomised cells underwent hypotrophy within 1 week of axotomy and then gradually increased in size. They re-attained normal size by 4 weeks postoperative when regenerating axons first reach their target. Following a long-term survival (greater than 2 months), somas were significantly hypertrophied, and the degree of hypertrophy was inversely related to the extent of cell survival up to a limit of 40% soma size increase. Counts and measurements of axons revealed that mean axon diameter of regenerated axons was much smaller than normal 3 months after axotomy, increased during the third to sixth postoperative months, but then showed no subsequent increase and remained below normal. In animals with cell death varying from 10% to 70%, the number of axons in the nerve was maintained constant at approximately 1,000. These data indicate that there is a mechanism for the production and maintenance of the appropriate number of regenerative axonal branches following axotomy. In animals in which cell death exceeded 70%, the number of axons was controlled by a maximum ratio of 3 to 4 axon branches per surviving cell. The results suggest that axon number is strongly influenced by the target muscle and that hypertrophy of regenerated cells is related to the number of axonal sprouts each cell has to produce and support in order to re-establish the preoperative number of axons in the regenerated trochlear nerve.

Changes in contralateral protein metabolism following unilateral sciatic nerve section

Changes in nerve biochemistry, anatomy, and function following injuries to the contralateral nerve have been repeatedly reported, though their significance is unknown. The most likely mechanisms for their development are either substances carried by axoplasmic flow or electrically transmitted signals. This study analyzes which mechanism underlies the development of a contralateral change in protein metabolism. The incorporation of labelled amino acids (AA) into proteins of both sciatic nerves was assessed by liquid scintillation after an unilateral section. AA were offered locally for 30 min to the distal stump of the sectioned nerves and at homologous levels of the intact contralateral nerves. At various times, from 1 to 24 h, both sciatic nerves were removed and the proteins extracted with trichloroacetic acid (TCA). An increase in incorporation was found in both nerves 14-24 h after section. No difference existed between sectioned and intact nerves, which is consistent with the contralateral effect. Lidocaine, but not colchicine, when applied previously to the nerves midway between the sectioning site and the spinal cord, inhibited the contralateral increase in AA incorporation. It is concluded that electrical signals, crossing through the spinal cord, are responsible for the development of the contralateral effect. Both the nature of the proteins and the significance of the contralateral effect are matters for speculation.

Increase of transferrin receptors and iron uptake in regenerating motor neurons

After injury, motor neurons exhibit a number of metabolic and protein changes that are assumed to be part of an inherent neuronal regeneration program, which, when activated, eventually leads to functional restitution. The mechanisms underlying this regeneration are unclear, but it may be expected that factors supporting neuronal growth or survival play an important role in the restoration of neuronal integrity. A number of neuronal growth-associated proteins have been identified, but their functional roles remain unclear. This paper shows that axotomy results in a strong increase in transferrin receptors (TfRs) in regenerating motor neurons and that this phenomenon is functionally associated with an elevated uptake of exogenous iron. The association of TfR expression in regenerating motor neurons with direct uptake of iron into the brain provides evidence that iron uptake into neural tissue may be related to neuronal metabolic activation. We suggest that the enhanced capacity of regenerating motor neurons to bind transferrin and to take up iron plays an important role in neuronal repair.

Neuron-glial relationship during regeneration of motorneurons

Following axonal interruption, structural, metabolic and physiological parameters change in motorneurons. Also, glial cells are involved in this process. Microglia proliferate and express new proteins such as vimentin or MHC antigens. Astrocytes show hypertrophy, increased GFAP synthesis, and formation of lamellae. Both glial cell types participate in deafferentation and insulation of regenerating neurons, a process with significance for post-lesioning functional impairment.

Changes in microglial cell numbers in the spinal cord dorsal horn following brachial plexus transection in the adult rat

The effect of peripheral nerve transection on the size of the microglial cell population in cytoarchitecturally distinct regions of the spinal cord dorsal horn of rats was evaluated at selected intervals 2 through 35 days after unilateral brachial plexotomy. The identification of cells was verified by electron microscopic examination of a representative random sample of cells included in the counts. Microglial cell numbers were increased in laminae I, II as well as the arbitrarily defined deeper laminae 3.5 days after surgery. Although microglial cell numbers in laminae I were within normal range 35 days after axotomy, those of the more ventrally located laminae remained significantly greater than control values for the duration of the experimental period. These findings demonstrate that: 1) microglial cell proliferation in the dorsal horn is an early event in the central changes that are attendant to peripheral nerve injury 2) the time course of the response varies in cytoarchitecturally different regions.