Some aspects of the nerve endings and synapses in the vocalis muscle

Mitochondria-filled nerve endings around the terminal arteriole in the rat posterior cricoarytenoid muscle

We employed light and electron microscopy to examine the innervation of terminal arterioles in the rat posterior cricoarytenoid (PCA) muscle over a three-week period before and after unilateral denervation of the recurrent nerve. Observations showed an occasional single small-sized myelinated nerve fiber around the terminal arteriole in both normal and denervated PCA muscles. One axon after the last node of Ranvier of the small-sized myelinated nerve fiber, together with some nonmyelinated nerve fibers, was enwrapped by the Schwann cell and ran along the terminal arteriole, forming varicose swellings with numerous mitochondria along its course. In one instance, a small-sized myelinated nerve fiber in the denervated PCA muscle ramified into several branches containing numerous mitochondria. These nerve endings mainly existed in the adventitia, consisting of some layers of fibroblasts and collagen fibrils, and some of them were close to the vascular smooth muscle cells with only an intervening basal lamina. These findings suggest that the mitochondria-filled nerve endings may be sensory in nature, and function in response to the expansion and contraction of blood vessels caused by increased and/or decreased blood flow or intravascular pressure.

Edrophonium effectively antagonizes neuromuscular block at the laryngeal adductors induced by rapacuronium, rocuronium and cisatracurium, but not mivacurium

PURPOSE

To examine the efficacy of antagonism of rapacuronium-, mivacurium-, rocuronium- and cisatracurium-induced neuromuscular block at the laryngeal adductors (LA).

METHODS

One hundred four patients were randomly assigned to one of eight study groups. They either received rapacuronium 1.5 mg x kg(-1), mivacurium 0.25 mg x kg(-1), rocuronium 0.9 mg x kg(-1) or cisatracurium 0.15 mg x kg(-1). Patients in each treatment group either received edrophonium (0.5 mg x kg(-1)) at 10% recovery of the first twitch (T1) of train-of-four (TOF) at the LA or were allowed to recover spontaneously from neuromuscular block. The effect of antagonism on speed of recovery of neuromuscular function at the LA was evaluated.

RESULTS

The time to recovery to a TOF ratio of 0.9 at the LA, when compared to the spontaneous recovery group, was significantly shortened by the administration of edrophonium in patients receiving rapacuronium [19.2 +/- 7.8 vs 26.2 +/- 4.9 (mean +/- SD) min], rocuronium (24.7 +/- 14.3 vs 44.4 +/- 13.0 min) and cisatracurium (24.2 +/- 5.7 vs 35.1 +/- 7.6 min). Edrophonium administration did not shorten complete recovery from mivacurium-induced block (15.7 +/- 8.0 vs 17.6 +/- 6.1 min).

CONCLUSION

Recovery from rapacuronium-, rocuronium- or cisatracurium- induced neuromuscular block to a TOF ratio of 0.9 as measured at the LA was shortened by the administration of edrophonium, when compared to spontaneous recovery.

Neuromuscular blockade at the larynx, the diaphragm and the corrugator supercilii muscle: a review

PURPOSE

To review recent findings concerning neuromuscular blockade and monitoring at the larynx, the diaphragm, and the corrugator supercilii muscle.

SOURCE

This narrative review is based on recent publications.

PRINCIPAL FINDINGS

Neuromuscular blockade at the larynx and the diaphragm is less intense than at the adductor pollicis muscle; the onset and offset of neuromuscular blockade is more rapid. The corrugator supercilii muscle reflects better the time course of neuromuscular blockade of the larynx than the adductor pollicis muscle, is better suited to monitor the onset of neuromuscular blockade for intubation, and should give a better reflection of the time course and degree of neuromuscular blockade of the larynx or the diaphragm. Recovery of neuromuscular function at the end of any procedure is best reflected at the adductor pollicis muscle where neuromuscular transmission is last restored. Clinical monitoring of the larynx or the diaphragm is still limited by the absence of a simple method. Acceleromyography of the corrugator supercilii muscle is prone to artifacts that do not occur during monitoring of the adductor pollicis muscle. Phonomyography, a new method of monitoring that is currently being tested, is based on the phenomenon that muscle contraction creates low-frequency sound waves, which can be detected using special microphones to quantify neuromuscular blockade. This method seems promising because it can be easily used on all muscles of interest.

CONCLUSION

Research during the last 15 years has greatly enhanced our knowledge about how muscles react differently to muscle relaxants and has enabled us to achieve better surgical conditions with safer use of muscle relaxants. Interesting technologies have been developed to reliably monitor neuromuscular blockade at the larynx and the diaphragm, but are currently restricted to research settings. Our increased understanding should help us in ongoing efforts to develop the "ideal" muscle relaxant and the "ideal" method of neuromuscular monitoring.

The distribution of ganglion cells in the posterior cricoarytenoid muscle of the normal adult rat. A light and electron microscopic study

We employed by light and electron microscopy to examine the innervation of the posterior cricoarytenoid muscle of the adult rat. The laryngeal nerve was found to often bifurcate into two different bundles. One contained large myelinated (motor) nerve fibers, which were located along the frontal (ventral) muscle surface and entered the muscle at its middle portion to form neuromuscular contacts with individual muscle fibers. The other nerve bundle consisting of clustered ganglion cells (20-30 microm in diameter) and their associated nonmyelinated and small-sized myelinated nerve fibers were mainly found on the dorsal side of the muscle and often ran along the peripheral clefts or depressions of the muscle surface. The nerve bundle often extended side branches, which entered the muscle to be distributed among muscle fibers and near arterioles. Some ganglion cells are considered to enter the muscle, accompanied by branched nerves. Intramuscular ganglion cells and their associated nerve fibers examined by electron microscopy were similar in fine structure to perimuscular ganglion cells and their associated nerve fibers. Nerve fibers contained abundant clear synaptic vesicles which were cholinergic in nature, and often formed synapses with both neighboring axons and the cell body of the ganglion cells. These findings suggest that, in the rat posterior cricoarytenoid muscle, perimuscular and intramuscular ganglion cells exist and may be involved in innervating and contracting smooth muscle cells of the arterioles, thus regulating the blood flow or intravascular pressure.

Postnatal morphodifferentiation of the subneural apparatuses of the posterior cricoarytenoid muscle in rats: a scanning electron microscopy study

The subneural apparatus, i. e., the post-synaptic component of the neuromuscular junction, in the posterior cricoarytenoid muscle of the rat was studied by scanning electron microscopy, with special attention given to its postnatal differentiation along with the functional development of the muscle. Primitive synaptic troughs observed in the first postnatal week consisted of single cup-like depressions 5-6 microm in diameter. On the 7th day, low sarcoplasmic ridges appeared in the trough. In the second postnatal week, muscle fibers could be classified into two groups: large (10-15 microm in diameter) and small (less than 10 microm in diameter). In the large muscle fibers, many low ridges became circular and protruded to transform the single trough into numerous cup-like depressions (2-5 microm in diameter). In contrast, the subneural apparatus in the small muscle fibers consisted of a small number of cup-like depressions. The two types of subneural apparatus differentiated into adult forms by the 28th postnatal day, although they remained smaller in size than those of adults. In the large muscle fibers, the number of pit-like or elongated invaginations increased and gradually transformed into slit-like junctional folds by the 28th postnatal day, while the small muscle fibers still possessed a few pit-like or elongated junctional folds at this point in time. The two types of morphodifferentiation of the subneural apparatus are thought to reflect the two types of muscle fibers in the rat posterior cricoarytenoid muscle.

Ultrastructural and histochemical study of the motor end plates of the intrinsic laryngeal muscles in amyotrophic lateral sclerosis

Motor end plates of the intrinsic laryngeal muscles in amyotrophic lateral sclerosis (ALS) were examined light and electron microscopically. Light microscopically, typical neurogenic changes such as small angulated fibers and grouped atrophy were found in the intrinsic laryngeal muscles. Acetylcholinesterase (AchE) activities of the neuromuscular junctions (NMJ) of many fibers in ALS were decreased as compared with those of the controls. Some end-plate areas on each fiber detected by AchE histochemistry were larger than those of the controls. Ultrastructurally, muscle fibers in ALS specimens showed several changes; increased number of lipofuscin granules and/or nuclei, numerous mitochondria, and disappearance of the myofilaments. The NMJ also showed various degrees of structural changes. Some NMJ appeared almost normal. Others showed the absence of nerve terminals and Schwann cells covering the former junctional sites. Their primary synaptic clefts were flattened, and the secondary synaptic clefts were relatively well preserved. On occasion, several small nerve terminals were seen on the severely distorted postsynaptic folds, suggesting regenerative findings. In severely degenerated muscle fibers, the NMJ could not be found.

Innervation of adult human laryngeal muscle fibers

The innervation of laryngeal muscle fibers was appraised in adult humans. Sixteen intrinsic laryngeal muscles were dissected during the autopsy of 4 adults (41-71 years old). Longitudinal serial frozen sections, 60 microm thick, of the whole muscles were double-stained for cholinesterase activity and axonal visualization. About 945 endplates per muscle were analysed using light microscopy. The neuromuscular junctions were always scattered throughout the whole muscles. Most of the muscle fibers showed a single neuromuscular junction, but multi-innervated fibers were found in all of the muscles. Their number was highest in interarytenoid muscles (21% of all the fibers). The distance between multiple neuromuscular junctions was most frequently less than 150 microm. Two neuromuscular junctions were frequently displayed, opposite one another, particularly in thyroarytenoid muscles, and this unusual feature seems specific for laryngeal muscles. The innervation of all of the muscle fibers was exclusively found to be unineuronal, with multi-innervated fibers being innervated by a single axon. Distal axonal degeneration occurred with aging, resulting in a loss in the number of multi-innervated muscle fibers.

Ultrastructural and histochemical study of the neuromuscular junctions in the denervated intrinsic laryngeal muscle of the cat

Morphological changes and acetylcholinesterase (AchE) activity in the neuromuscular junctions (NMJ) of the normal and denervated posterior cricoarytenoid muscle (PCA muscle) of the cat were studied. Two days after denervation, the nerve terminals at the NMJ had almost disappeared. Six weeks after denervation, intensity of AchE activity at the former junctional site (FJS) was unchanged histochemically. At this stage, primary synaptic clefts were distorted and the Schwann's cells covered the FJS. Fourteen weeks after denervation, AchE activity at the FJS had decreased in contrast to that of the non-affected side. Calcitonin gene-related peptide (CGRP) was also investigated at the NMJ of the normal PCA muscles immunocytochemically. The present study shows that CGRP coexists with Ach in the nerve terminals of the PCA muscles and may be involved in the regulation of the contractile function of the intrinsic laryngeal muscle.

From the pattern of human vocal muscle fibre innervation to functional remarks

About 50% of the human vocal muscle fibres possess several endplates. The direct relation between the number of endplates and rate of contraction, which has been postulated in animals, does not apply to the human vocal muscle. However, it may be that this relation is not indispensable to the particular function this muscle is required to perform and undoubtedly justifies a possible pattern of selective muscle contraction, or better still, of a contraction regulated by a particular, independent mechanism. This mechanism is postulated and discussed.

Neuromuscular junctions of the posterior cricoarytenoid muscle in the human adult, human fetus and cat. Histochemical and electron microscopic study

The motor end plate of the posterior cricoarytenoid muscle (PCA muscle) in the human adult, human fetus and cat was examined by using electron microscopy and histochemical methods. In the present study, we observed the single-type motor end plate and en plaque type neuromuscular junction. At the neuromuscular junction of the fetal PCA muscle, the primary synaptic cleft, the basement membrane and the postsynaptic density could already be observed; however, there was no secondary synaptic cleft. Histochemically, the subneural apparatus was filled with electron-dense products, indicating acetylcholinesterase (AChE) activity. The primary and secondary synaptic clefts in the adult PCA muscle were well developed and intense AChE activity was present. The appearance of the neuromuscular junction and its localization of AChE activity was similar to that in the cat PCA muscle.

Three-dimensional computer reconstruction of the distribution of neuromuscular junctions in the thyroarytenoid muscle

Microinjections of botulinum toxin have recently been shown to be effective in the treatment of strabismus, and it has also been suggested that microinjections of this myoneural blocking agent might be of value in the treatment of spastic dysphonia. The success of such a microinjection technique would rely on a precise knowledge of the distribution of myoneural junctions in the thyroarytenoid muscle. In view of this potential application as well as the need for such information in reinnervation procedures, we have used computer graphics to reconstruct the three-dimensional distribution of motor end-plates in the thyroarytenoid muscle. Three cat and one human (fresh autopsy specimen) larynges were frozen and sectioned on a cryostat. Serial sections were then processed for the histochemical localization of acetylcholinesterase activity to demarcate the neuromuscular junctions. An X-Y digitizer was used to reference the position of the motor end-plates in each serial section, and the three-dimensional distribution of the neuromuscular junctions was reconstructed on a computer graphics terminal. The results are discussed in regard to their applicability to clinical treatment of spastic dysphonia and other disorders of phonation.

Neuromuscular junctions of the posterior cricoarytenoid muscle in the cat

The motor end plate of the cat posterior cricoarytenoid muscle is examined by using electron microscope and histochemical method. Furthermore, we studied the localization of acetylcholine receptors by fluorescent staining of snake neurotoxins (alpha-bungarotoxin and erabutoxin b) and autoradiography of 125I-labeled alpha-bungarotoxin. The fine structure of the motor end plate of this muscle is approximately identical to that of other singly innervated skeletal muscles. The subneural apparatus is activated by cholinesterase (ChE). ChE reaction appears markedly in primary and secondary synaptic clefts. No remarkable difference is found between TMR-labeled neurotoxin and FITC-conjugated neurotoxin. Fluorescent staining is associated with subsynaptic muscle membranes and not with motor nerve terminals. The autoradiogram revealed heavy accumulations of grains at the junctional folds.