Muscle pyruvate oxidation following denervation and reinnervation

Muscle pyruvate metabolism was studied in rats following sciatic nerve crush. Control studies showed high pyruvate dehydrogenase and lipoamide dehydrogenase enzyme activity in muscle with type 1 fiber predominance (soleus) and low activities in muscle with type 2 fiber predominance (extensor digitorum longus), whereas lactate dehydrogenase was much higher in the latter. Following denervation, both muscles showed a significant reduction in pyruvate dehydrogenase enzyme activity. During reinnervation, muscle with type 2 predominance developed significantly elevated pyruvate oxidation enzyme activities.     

Muscle hypertrophy after partial denervation: a human case.

While undergoing long-term physiotherapy, a 41 year old woman with a chronic S1 radiculopathy developed progressive, painless enlargement of the weak calf. Gastrocnemius muscle biopsy disclosed changes of partial denervation and reinnervation, with small groups of type I and type II atrophic muscle fibres and abundant hypertrophic fibres of both types but mostly type II. It is postulated that, in addition to comprensatory work-induced type II muscle fibre hypertrophy, there was an element of (type I) stretched-induced hypertrophy of denervated fibres, a condition well recognised experimentally but not documented in man.     

Muscle magnetic resonance imaging of denervation and reinnervation: correlation with electrophysiology and histology

A signal increase in denervated muscle on magnetic resonance imaging (MRI) has been described in several clinical and experimental studies. Here, we studied the time course of T2-relaxation time changes in denervation and subsequent reinnervation in a rat model and correlated the findings with electrophysiology and quantitative histology. A prolongation of the T2 relaxation time in muscles was present 48 h after denervation, which was paralleled by spontaneous activity on electromyography (EMG). Histologically, there was a marked enlargement of the capillaries at that time point, indicating increased blood volume. The relaxation time changes peaked 3 weeks after beginning of nerve regeneration identified by EMG. Subsequently, the T2 prolongation normalized until 10 weeks after beginning of regeneration which was associated with a histological regression of the capillary enlargement. MRI closely mirrors the electrophysiological changes following denervation and reinnervation and may thus be used as adjunct to electrophysiology. The pathophysiological basis for the MR relaxation time changes is predominantly the enlargement of the capillary bed.     

Myoglobin in rat hind limb muscles after denervation and during reinnervation

Radioimmunoassay of myoglobin (Mb) was performed in rat hind limb muscles after surgical denervation and during reinnervation following cryolesion of the sciatic nerve. Muscles of the contralateral leg served as controls. After resection of the sciatic nerve, decreased Mb concentrations were noted on the fourth day in the tibialis anterior, peroneus longus, and extensor digitorum longus (EDL) muscles. Thereafter, the levels increased up to the last observation on day 32. The increases in Mb levels in the tibialis anterior and EDL muscles were considerably more pronounced (305% and 324%, respectively) than in the peroneus longus and soleus muscles (148% and 137%, respectively). After cryolesion of the sciatic nerve, the Mb concentrations in the tibialis anterior, peroneus longus, and EDL muscles increased, reaching maximal values on days 16-21. The levels then decreased and normal values were observed 2 months postoperatively. The normalization of the Mb levels during reinnervation corresponded fairly well in time with the clinical recovery and neurophysiological findings observed in a previous study.     

Muscle fiber conduction velocity in situ (MFCV) in denervation, reinnervation and disuse atrophy

OBJECTIVES: Muscle fiber conduction velocity (MFCV) was performed in disuse atrophy, in denervated muscle and during reinnervation as a possible index of muscle atrophy, and to clarify the evolution of the fiber size. MATERIAL AND METHODS: MFCV was performed in 12 patients with complete denervation of biceps brachii muscle and during various stages of reinnervation. Twenty-one patients with disuse quadriceps atrophy were also tested. Invasive MFCV was performed according to the method reported elsewhere (2). RESULTS: MFCV decreased significantly in denervated muscles. Reduction of MFCV was found during the first weeks and was progressive. Peak frequency in histograms decreased and the normal Gaussian distribution was lost. MFCV increased progressively after reinnervation with coexistence of slow and significant increase of faster MFCV. MFCV decreased significantly also during the first weeks after immobilization and improved by rehabilitation therapy. CONCLUSION: MFCV is a reliable method to test the muscle fiber size after denervation and immobilization, and its evolution by reinnervation and therapy.     

Responses of cat muscle spindle afferents after short periods of denervation and reinnervation

The functional recovery of cat muscle spindles has been examined following short periods of denervation and reinnervation. The restoration of function was more rapid than after longer periods of denervation though the firing rates showed a similar reduction except after the shortest recovery periods when a number of highly dynamic afferents were recorded.     

Single fiber EMG evaluation in denervation and reinnervation

SFEMG can be used to follow the course of reinnervation. The parameters of fiber density, mean jitter and percent blocking must each be followed and related to the type of injury to determine the stage of reinnervation. The few reported cases in the literature of SFEMG recordings followed over time after nerve compromise are reviewed and an additional case is presented by the author. Mean jitter values can return to normal approximately 1 1/2 years postonset of reinnervation. Some individual recordings may remain abnormal permanently.     

Muscle reinnervation in hypothyroid rats

Reinnervation of extensor digitorum longus muscle following crush of sciatic nerve was studied in rats made hypothyroid after weaning. In vitro intracellular recordings of muscle cell postsynaptic potentials were carried out; moreover twitch and tetanus following direct muscle stimulation and nerve stimulation were recorded. Frequency of miniature end-plate potentials (mepps) may be regarded as an index of presynaptic mechanism in regenerated nerve endings: when regenerating axons reach the muscle, the frequency of spontaneous acetylcholine quantal release is very low and increases in subsequent weeks. No significant differences were noted in miniature end-plate potential frequency between muscles of normal and hypothyroid rats at the same time from denervation; mepp amplitude was higher in hypothyroids, in accordance with the smaller muscle fibre diameters. Regenerating nerve fibres entering the muscle extensively sprout, giving rise to a number of nerve endings which exceeding the number of muscle cells, are subsequently withdrawn; correspondingly, muscle cells are transiently polyinnervated and the number of polyinnervated muscle cells peaks decreases subsequently approximating zero. The percentage of polyinnervated cells peaked sooner in hypothyroid rats than in controls and afterwards decreased; a tail of polyinnervation was found at long term. Tension recording experiments showed a shorter time of reinnervation of muscles in hypothyroid rats, but no difference in regeneration rate could be argued. These findings suggest an influence of thyroid hormones in the stabilization of motor innervation of reinnervated muscle, but not in nerve regeneration process.     

Collateral reinnervation of taste buds after chronic sensory denervation: a morphological study

Peripheral transganglionic transport of horseradish peroxidase (HRP) was used to label afferent fibers in the taste buds and lingual epithelium 2-12 weeks after chronic chorda tympani or combined chorda tympani-lingual nerve lesions. From 4-12 weeks after a chronic chorda tympani lesion, taste buds could be found. These were innervated by fibers from the ipsilateral lingual nerve. From 8-12 weeks after a chronic chorda tympani-lingual nerve lesion, nerve fibers from the contralateral lingual nerve could be found in a few taste buds on the denervated side of the tongue. Thus, collateral sprouting took place over the midline in this instance. These findings indicate that intact gustatory axons do not sprout into denervated taste buds, but trigeminal fibers in the lingual nerve do have this ability.     

Loss and renewal of thick myofilaments in glucocorticoid-treated rat soleus after denervation and reinnervation.

Denervation of rat soleus muscle and simultaneous administration of high doses of corticosteroids for 7 days caused marked muscle fiber atrophy and selective loss of thick myofilaments from many muscle fibers by light and electron microscopy. Myosin heavy chain/actin ratios were greatly reduced on polyacrylamide gel electrophoresis. Nerve crush instead of cut permitted reinnervation after 2 weeks and demonstrated the reversibility of the muscle changes within a week after reinnervation. There was formation of new thick filaments and their reintegration into myofibrils without further breakdown, although large areas of Z-disc streaming appeared. The mechanism of A-band breakdown remains obscure, but it presumably starts with limited proteolysis and continues with disaggregation of myosin molecules. This is consistent with our observation that the muscle fibers retain a relatively good reactivity to antibodies against myosin heavy chain 1 week after denervation and corticosteroid administration. A syndrome recalling these experiments is seen in severely asthmatic patients receiving corticosteroids and pharmacologically paralyzed for mechanical respiration.     

Sensory reinnervation after partial removal of the olfactory bulb.

In this study, in order to provide the anatomical basis for future behavioral and electrophysiological experiments, we describe the effects of unilateral bulbar lesion on the peripheral sensory neurons and the parameters of reinnervation of the damaged olfactory bulb. Neonatal mice and rats were subjected to removal of portions of the olfactory bulb. At survival times from 2 to 6 months, the animals were killed by transcardial perfusion and processed for light (histological, immunohistochemical, autoradiographic) and electron microscopic observations. As a result of this surgery, in the basal layer of the olfactory neuroepithelium the rate of mitotic activity increased while the number of mature olfactory neurons was greatly reduced. The regrowing olfactory axons, by forming ectopic glomerular structures in the damaged target, profoundly influenced its reorganization. The typical layered morphology of the olfactory bulb was often disrupted in the bulbar remnant; the large dendrites of the deafferented mitral cells bent toward the ectopically located glomerular structures establishing numerous synaptic contacts. The results from this study indicate that the olfactory input plays an important role in the reorganization of the damaged olfactory bulb. Behavioral experiments in partially bulbectomized animals should provide essential information about the importance of a topological map in the processing of olfactory cues.     

Myosin light and heavy chains in rat gastrocnemius and diaphragm muscles after chronic denervation or reinnervation

Myosin of chronically denervated and reinnervated rat muscles was studied. The two-dimensional gel electrophoretic pattern of light chains and the tryptic mapping of the native molecule show that almost only fast-type myosin was present in 6-month denervated hemidiaphragm and gastrocnemius muscles, whereas reinnervated myosins were mixed in the respective controls. The chymotryptic peptide mapping under denaturing conditions of electrophoretically purified myosin heavy chains confirmed that the observed changes concerned not only light chains but also the heavy subunits of the myosin. The results of reinnervation experiments allow one to conclude that the selective maintenance of fast-type myosin in chronically denervated muscles cannot be attributed to minimal reinnervation events.     

Recovery and loss of muscle force of rat plantaris after partial denervation

The isometric contractile characteristics of rat plantaris muscles were assessed 1, 3, 6, 9, and 12 months after resection of the L4 radicular nerve. After 1 month of partial denervation, twitch and tetanus were significantly lower (26.1 and 22.1%, respectively) than those of sham-operated rats. Plantaris muscle weight was reduced (22.5%), but twitch-to-tetanus ratio (TW/TT), time-to-peak, and one-half relaxation time were not changed significantly. At 3 months, average twitch force was normal but average tetanic force was significantly lower (27.1%) than control value. Muscle weight was reduced (28.9%), but TW/TT was increased by 31%. After 6 months, twitch, tetanus, and all other variables were similar to those of control rats. Normal twitch at 3 months indicates that all muscle fibers have been reinnervated by sprouting from L5 motor axons; however, the new synaptic contacts may not support the tetanic response. At 9 months, muscle force was again reduced and remained at about the same level at the 12-month interval. These results are consistent with the recovery and loss of function seen in poliomyelitis and the postpolio syndrome.     

Long-term reorganization of respiratory pathways after partial cervical spinal cord injury

High cervical spinal cord injury (SCI) interrupts bulbospinal respiratory pathways innervating phrenic motoneurons, and induces an inactivation of phrenic nerves (PN) and diaphragm. We have previously shown that the ipsilateral (ipsi) PN was inactivated following a lateral C2 SCI, but was spontaneously partially reactivated 7 days post - SCI. This phrenic reactivation depended on contralateral (contra) descending pathways, located laterally, that cross the spinal midline. We analysed here whether long-term post-lesional changes may occur in the respiratory network. We showed that ipsi PN reactivation was greater at 3 months compared with 7 days post-SCI, and that it was enhanced after acute contra phrenicotomy (Phx), which also induced a substantial reactivation of the ipsi diaphragm (not detected at 7 days post-SCI). At 3 months post-SCI (compared with 7 days post-SCI), ipsi PN activity was only moderately affected by ipsi Phx or by gallamine treatment, a nicotinic neuromuscular blocking agent, indicating that it was less dependent on ipsi sensory phrenic afferents. After an additional acute contra SCI (C1) performed laterally, ipsi PN activity was abolished in rats 7 days post-SCI, but persisted in rats 3 months post-SCI. This activity thus depended on new functional descending pathways located medially rather than laterally. These may not involve newly recruited neurons as retrograde labelling showed that ipsi phrenic motoneurons were innervated by a similar number of medullary respiratory neurons after a short and long post-lesional time. These results show that after a long post-lesional time, phrenic reactivation is reinforced by an anatomo-functional reorganization of spinal respiratory pathways.     

Fetal-type creatine kinase in rat fast and slow muscles during denervation and reinnervation

Creatine kinase (CK) has three forms of isozymes; CK-BB, CK-MB, and CK-MM. In adult rats they show a specific tissue distribution: the BB form in the brain, the MB form in the heart, and the MM form in skeletal muscle. In embryonic skeletal muscles only the BB and MB forms are found. Adult slow-twitch muscles contain more fetal type creatine kinase (CK-B) than do fast-twitch muscles. In the present experiment the effect of denervation and reinnervation on the CK-B concentration was investigated in rat fast (extensor digitorum longus)- and slow (soleus)-twitch muscles by a highly sensitive immunoassay. Denervation of these muscles produced a progressive increase in CK-B concentration in both muscles. When the sciatic nerve was cut and immediately sutured, the CK-B concentration in both muscles showed a gradual reduction after an initial increase. By the 34th postoperative week the CK-B concentration in the soleus was about one-half that of the contralateral control, whereas that in the extensor digitorum longus was nearly normal. After cross union of the nerves innervating the muscles, the CK-B concentration in the soleus was reduced at 35 weeks to about one-half normal, but that in the extensor digitorum longus was always higher than the control value. After self-reunion of the nerves, the CK-B concentration at the 20th week was approximately normal in the extensor digitorum longus and significantly increased in the soleus. We suggest that the motoneurons normally innervating the extensor digitorum longus have a greater capability in suppressing the production of CK-B than do the soleus motoneurons.     

Spontaneous and drug-induced locomotor activity after partial dopamine denervation of the ventral striatum

Spontaneous and drug-induced locomotor behavior was investigated in rats subjected to lesions of the ventral striatum, using the neurotoxin 6-hydroxydopamine to produce selective depletions of dopamine. Locomotor activity changed with time after lesion. At 2 weeks postoperative less spontaneous rearings were observed compared to controls, a reduced response to 1.0 mg/kg amphetamine and an increased response to 0.1 mg/kg apomorphine. These changes were not observed 9 weeks postoperative; that is, spontaneous locomotor activity and the response to amphetamine were not different from those of controls, and the rearing response to apomorphine was now reduced. The neurochemical assays of the lesioned ventral striate showed equivalent dopamine depletions of about 48% in both lesion groups relative to controls.     

A specific force deficit exists in skeletal muscle after partial denervation.

Skeletal muscle demonstrates a specific force deficit after repair of injured peripheral nerves, microneurovascular muscle transfer, and normal aging. Because atrophy cannot account for deficits in specific force, other, unknown, mechanisms are responsible for the resulting muscle contractile dysfunction under these circumstances. We tested the hypothesis that a subpopulation of denervated fibers is partially or completely responsible for the specific force deficit after partial denervation of the rat extensor digitorum longus muscle (EDL). Adult Fisher rats underwent either sham exposure or partial transection of 80% of the cross-sectional area of the left deep peroneal nerve. After a 2-week recovery period, maximum isometric force (F(0)) was measured in situ and maximum specific force (sF(0)) was calculated for EDL from both control (n = 8) and partial denervation (n = 7) groups. Innervated fiber cross-sectional area (CSA(inn)) was measured directly from whole EDL cross sections after immunohistochemical labeling for neural cell adhesion molecule (NCAM), a marker of muscle fiber denervation. A corrected specific force value (sF(0-inn)) was calculated by normalizing F(0) to CSA(inn). Partial skeletal muscle denervation resulted in significant reductions in muscle mass, F(0), and sF(0). The percentage of muscle fibers expressing NCAM in the extrajunctional sarcolemma increased from 1.0 +/- 0.8% in control to 49 +/- 15% in partially denervated EDL muscles. A 62.7% deficit in EDL specific force was observed after partial denervation. Denervated muscle fibers accounted for 59.3% of this deficit, but sF(0-inn) still differed significantly between control and partially denervated muscles, with a 25.5% difference between groups. In partially denervated muscles, the specific force deficit is partially but not fully explained by a subpopulation of noncontractile, denervated fibers.