Fine structure of the brain and brain nerves ofOikopleura dioica (Urochordata,Appendicularia)

Summary Each second brain nerve consists of only one single fibre terminating at two different types of touch receptors in the oral region. The two nerves are the dendrites of two perikarya in the forebrain and are the master neurons for ciliary reversal in the stigmata, which is a two-neuron reflex. By axoaxonal synapses they control one motor neuron in the midbrain, i.e. the command neuron for ciliary reversal in both rings. This cell sends one axon branch in each third nerve to the cilia cells. In the left nerve this fibre is closely associated with a coarsely granulated accessory fibre, which apparently regulates the ciliary beat. The third nerves also contain one fibre each from another motor neuron in the hindbrain. These fibres make synaptic contacts at some specialized epidermal cells in the lateral trunk behind the ciliary rings. A few previously unknown nerves in the dorsal forebrain innervate epidermal cells. It is likely that the complicated epidermal motor innervation regulates the secretory activity of the oikoplasts or of the epidermal cells in constructing a new house, including the necessary complicated filters and food trapping mechanisms.     

Substance P-immunoreactive nerve fibres in the anterior segment of the rabbit eye

Summary Substance P-immunoreactive nerve terminals were found in several locations in the anterior segment of the rabbit eye. In the iris they occurred in the sphincter muscle and were randomly distributed in the iris stroma with some fibres running close to the dilator muscle. In the ciliary body these immunoreactive elements were few and occurred within bundles of nerve fibres, while in the ciliary processes they were more numerous with a predominantly subepithelial location. Blood vessels in the anterior uvea were often surrounded by substance P-immunoreactive fibres. No substance P-fibres were found in the cornea, while the sclera contained very few such elements.Using conventional in vitro techniques it was found that the sphincter pupillae muscle of the iris responded to electrical stimulation with a contraction that was resistant to cholinergic and adrenergic blockade, but was inhibited by the neuronal blocker tetrodotoxin. This indicates the existence of a non-cholinergic, non-adrenergic neuronal mediator of the contractile response. Exogenously applied substance P produced a long-lasting contraction of the spincter muscle, an observation compatible with the view that substance P is the noncholinergic, non-adrenergic neurotransmitter involved.     

Quantitative electron-microscopy analysis of cranial nerves III, IV, and VI and of the extrinsic eye muscles in fog.

A comparative study of the quantitative data of the frog extraocular muscles and the cranial nerves that innervate them was performed. Oculorotatory muscles contain muscle fibres of at least 4 types which are arranged in heterogeneous layers. The zonal arrangement of the muscles does not occur on the cross-sections in the vicinity of muscle insertions. In these regions only two muscle fibre types are present and the total number of fibres is smaller by 70% than in the central region of the muscle. Most numerous are muscle fibres in the rectus inferior muscle, while the smallest number of fibres is found in rectus interior muscle. Three distinct types of nerve fibres are distinguished according to the following criteria: occurrence and thickness of myelin sheath, fibre diameter and ratio "g". The fibres with thin myelin sheaths indicate small diameters (1-5--6- mum) and their ratio "g" equals 0-82 +/- 0-08. They constitute about 30% of the myelinated fibres in the nerve supply of the oculorotatory muscles and about 14% in the supply of the retractor bulbi muscle. Both the value of the ratio "g" and a greater number of these fibres in the nerve supply of the muscles that contain slow contracting muscle fibres indicate that they are rather slow conducting nerve fibres. The range of the diameters of the fibres with thick myelin sheaths is greater (3-5--13-5 mum) and their "g" equals 0-66 +/- 0-06. These fibres constitute about 70% of the myelinated ones in the nerve supply of the oculorotatory muscles and 86% in the supply of the retractor bulbi muscles. The value of the ratio "g" in these fibres indicates that they are fast contracting ones. The smallest diameters are found in the myelinated fibres (0-5--1-7 mum). These fibres occur frequently in all the examined nerves; they constitute 36--47% of the total number of all the nerve fibres. The frog extraocular muscles are characterized by an abundal nerve supply which is reflected in the low innervation ratio (1:4--1:5). On the distal cross-section of nerves the number of nerve fibres is greater than on the proximal ones. Ganglionic neurons occur sporadically around the nerves; in the nerve III synaptic contacts between two neurons were observed.     

Synapse-specific expression of acetylcholine receptor genes and their products at original synaptic sites in rat soleus muscle fibres regenerating in the absence of innervation.

To test the hypothesis that synaptic basal lamina can induce synapse-specific expression of acetylcholine receptor (AChR) genes, we examined the levels mRNA for the alpha- and epsilon-subunits of the AChR in regenerating rat soleus muscles up to 17 days of regeneration. Following destruction of all muscle fibres and their nuclei by exposure to venom of the Australian tiger snake, new fibres regenerated within the original basal lamina sheaths. Northern blots showed that original mRNA was lost during degeneration. Early in regeneration, both alpha- and epsilon-subunit mRNAs were present throughout the muscle fibres but in situ hybridization showed them to be concentrated primarily at original synaptic sites, even when the nerve was absent during regeneration. A similar concentration was seen in denervated regenerating muscles kept active by electrical stimulation and in muscles frozen 41-44 hours after venom injection to destroy all cells in the synaptic region of the muscle. Acetylcholine-gated ion channels with properties similar to those at normal neuromuscular junctions were concentrated at original synaptic sites on denervated stimulated muscles. Taken together, these findings provide strong evidence that factors that induce the synapse-specific expression of AChR genes are stably bound to synaptic basal lamina.     

Satellite cells in the tail muscles of the urodelan larvae during development

The incidence and ultrastructure of satellite cells in the tail muscles of urodelan larvae were examined during development during which the number of satellite cells is gradually reduced. They are found more frequently in red than in the white fibres in all four stages examined (stage 53, 64, 66+ and juvenile). As development proceeds, intercellular space between satellite cell and muscle fibre is in general gradually extended and is mostly filled with basal lamina. Small muscle cells, satellite fibres, which are situated under the basal lamina of the parent fibre, are morphologically similar to satellite cells but contain a small amount of myofibrils. Three types of satellite fibres are distinguishable on the basis of differences in K2-EDTA-treated ATPase activity, width of Z line, and parent fibre type. Neuromuscular junctions are visible in satellite fibres.     

Adrenergic Nerves in the Avian Eye and Ciliary Ganglion

Summary The distribution of adrenergic fibres to the eye and to the ciliary ganglion was studied in pigeons, chicken and ducks with the aid of the sensitive and highly specific fluorescence method of Falck and Hillarp. In some animals the intensity of the fluorescence was increased by treating the animals with Nialamide and 1-DOPA. The cornea contained no adrenergic fibres except at the limbus, where a plexus of adrenergic varicose fibres was seen, partly associated with vessels. In the chamber angle, adrenergic varicose fibres were common in the loose connective tissue covering the canal of Schlemm. The canal of Schlemm was supplied by only few adrenergic fibres, but such fibres appeared along the intrascleral aqueous drainage vessels. In the iris, adrenergic varicose fibres appeared immediately in front of the posterior layer of pigment cells, strongly indicating the presence of a dilator homologous with that seen in mammals. The frontal third of the stroma contained several adrenergic varicose fibres, many of which seemed to lack association with any vessel. Varicose adrenergic fibres were also sparsely seen in the striated muscle of the iris. The ciliary processes contained many adrenergic varicose fibres, at least part of which seemed to be associated with the ciliary epithelium. The striated muscles of the ciliary body contained adrenergic varicose fibres along the vessels only. The retina contained adrenergic varicose fibres in three layers in the inner plexiform layer. Adrenergic ganglion cells of two sizes were detected in the inner nuclear layer. The retinal vessels had no adrenergic nerve fibres. The pecten was also devoid of adrenergic nerve fibres, except along the vessels close to the papilla. The optic nerve contained adrenergic varicose nerve fibres along vessels only. In the ciliary ganglion, varicose adrenergic fibres appeared at the small ganglion cells, often forming baskets of synaptic character.Acknowledgements. The work has been supported by the United States Public Health Service (grant NB 06701-01), by the Swedish Medical Research Council (project B 67-12 X-712-02 A) and by the Faculty of Medicine, University of Lund, Sweden.     

Nerves containing substance P,vasoactive intestinal polypeptide,enkephalin or somatostatin in the guinea-pig taenia coli

Summary The guinea-pig taenia coli is rich in peptide-containing nerves. Nerve fibres containing substance P (SP), vasoactive intestinal peptide (VIP), or enkephalin, were numerous in the smooth muscle while somatostatin fibres were very few. Nerve fibres displaying SP or VIP immunoreactivity were numerous in the myenteric plexus. Enkephalin nerve fibres were fairly numerous in the plexus while somatostatin nerve fibres were sparse. Nerve cell bodies containing immunoreactive SP or VIP were regularly seen in the plexus. Delicate varicose elements of the different types of nerve fibres were found to ramify around nerve cell bodies in a manner suggestive of innervation.In the electron microscope the various peptide-storing nerve fibres (i.e., elements containing SP, VIP or enkephalin) were found to contain a varying number of fairly large, electron-opaque vesicles in the varicose swellings. These vesicles represent the storage site of the neuropeptides.The isolated taenia coli responded to electrical nerve stimulation with a contraction. After cholinergic and adrenergic blockade the contractile response was replaced by a relaxation followed by a contraction upon cessation of stimulation. SP contracted the taenia while VIP caused a relaxation. The enkephalins raised the resting tension slightly while somatostatin had no effect. These observations are compatible with a role for SP as an excitatory neurotransmitter and for VIP as an inhibitory one, and with the view that both SP neurones and VIP neurones act as motor neurones. In preparations contracted by SP the electrically induced contractions were reduced in amplitude while the electrically induced relaxations seen after adrenergic and cholinergic blockade were enhanced in amplitude. In preparations relaxed by VIP there was an increased contractile response to electrical stimulation, while in the atropine + guanethidine-treated preparation the electrically induce relaxations were reduced in amplitude. The enkephalins reduced the contractile response to electrical stimulation, while somatostatin induced a very small reduction in the amplitude of such responses. These observations suggest that SP neurones and VIP neurones may play additional roles as interneurones. Somatostatin neurones probably act as interneurones. Enkephalin-containing fibres may serve to modify the release of transmitter from other nerves in the smooth muscle, perhaps through axo-axonal arrangements. Alternatively, the enkephalin nerve fibres in the smooth muscle are afferent elements involved in mediating sensory impulses to the myenteric plexus.     

The structure of the nerve fibre layer and neurocord in the enteropneusts

Summary The nerve fibre layer and the neurocord of the Enteropneusts Saccoglossus horsti, Harrimania kupfferi and Ptychodera flava have been examined with the electron microscope. The nerve fibres vary in diameter between 0.15 to 10 m. The majority of the fibres are of the smaller diameters. The nerve fibre layer is intraepidermal, and is divided by processes running radially from the epithelial cells to the basement membrane that separates the nerve fibre layer from the muscle cells.The cells of origin of these nerve fibres are situated mainly in the innermost layers of the epidermal cells. The nerve fibre profiles contain numerous vesicles of very varied diameter and contents, together with larger granular inclusions that are also found in the nerve cell bodies.Morphologically recognisable synapses are rare, but the majority of fibres are in intimate contact with one another. Sometimes the mass of fibres is divided into bundles by the epithelial cell processes. The majority of giant fibres are situated near to the basement membrane of the neurocord. The giant fibres also have a varied content of vesicles as well as neurofilaments and neurotubules.The central canal in Ptychodera flava and the remnants of the central canal in Saccoglossus horsti are both lined by columnar cells that bear microvilli as well as cilia with the typical 9 + 2 pattern of tubules. Scattered amongst these cells are mucus secreting cells which open into the cavity of the canal.I (P.N.D.) should like to thank Professor J. Z. Young, F. B. S. for much advice and encouragement. Dr. R. Bellairs generously provided the electron microscope facilities, and Dr. R. Newell kindly collected and identified the Saccoglossus specimens. Mr. R. Moss, Mrs. J. Hamilton and Mr. A. Aldrich gave excellent technical and photographic assistance.     

CORRELATION OF FINE STRUCTURE AND PHYSIOLOGY OF THE INNERVATION OF SMOOTH MUSCLE IN THE GUINEA PIG VAS DEFERENS

An electron microscope study of the innervation of smooth muscle of the guinea pig vas deferens was undertaken in order to find a structural basis for recent electrophysiological observations. The external longitudinal muscle coat was examined in transverse section. Large areas of the surfaces of adjacent muscle cells were 500 to 800 A apart. Closer contacts were rare. A special type of close contact suggested cytoplasmic transfer between neighbouring cells. Groups of non-myelinated axons from ganglia at the distal end of the hypogastric nerve ramified throughout the muscle. Some small axon bundles and single axons lay in narrow fissures within closely packed muscle masses. Many axons contained "synaptic vesicles." About 25 per cent of the muscle fibres in the plane of section were within 0.25 µ of a partly naked axon; of these 15 per cent were within 500 A of the axon, and about 1 per cent made close contact (200 A) with a naked axon. It is unlikely that every muscle fibre is in close contact with an axon, and it is not possible for every fibre to have many such contacts. Muscle fibres are probably activated by both diffusion of transmitter from naked portions of axons a fraction of a micron distant, and electrotonic spread of activity from neighbouring cells.     

Effects of KCl-EGTA solution on the isolated Mytilus smooth muscle: a method for denervation

The ABRM of M. edulis was immersed in KCl-EGTA solution (540 mM KCl + 5 mM EGTA) for 30 min. Then the muscle was returned to normal ASW, effects of the KCl-EGTA solution being examined on the ultrastructure of the neuromuscular junctions and also on the mechanical responses of the muscle to several kinds of stimuli. By the KCl-EGTA treatment of the muscle, synaptic vesicles in the nerve terminals at the junctions were found to be markedly deformed in shape and materially reduced in number. In most of the muscles tested, the contractile response to ACh and catch-relaxing responses to serotonin and dopamine were not depressed by the present treatment, though in some other muscles those responses were depressed a little. Contractile and catch-relaxing responses to repetitive electrical pulses of stimulation were markedly depressed or almost blocked by the treatment. All of the catch-relaxing responses to hexylamine (10(-3)M), phenylethylamine (10(-4)M), Na+-free ASW and 8-bromo-cyclic GMP (10(-3)M) were also markedly depressed or almost completely blocked. These results indicate that the function of intramuscular nerve fibres in the ABRM is markedly impaired by treating the muscle with KCl-EGTA solution while that of muscle fibres is little impaired. The treatment seems to be a useful method for denervation in the isolated ABRM.     

An electron microscopic radioautographic study of the uptake of tritiated serotonin by nerve fibres in the posterior salivary duct and gland of cephalopods

In the posterior salivary duct and gland of Octopus vulgaris and of Eledone cirrhosa, the duct secretory nerve trunks and their ramifications in the gland tubules include many fibres that incorporate labelled serotonin. However, there are also unlabelled secretory fibres, which cannot be discriminated from incorporating fibres on morphological grounds. Neuroglandular junctions are not apparently established by incorporating fibres. In the duct, the motor nerve trunks contain a small number of labelled fibres, and nerve bundles supplying the duct muscle contain, in variable proportions, serotonin incorporating fibres. Both labelled and unlabelled nerve fibres reach the duct muscle fibres, but neuromuscular junctions involve only unlabelled presynaptic fibres. The nerve fibres which join the gland muscle are usually unlabelled, and the small quota of incorporating fibres in the motor trunks apparently supply only duct tissues. Both secretory and motor trunks, originating from different ganglia, can be considered to contain heterogeneous fibres, releasing different neurotransmitters at the terminals. Certain of these fibres could be serotoninergic.     

Recovery of acetylcholine release and choline acetyltransferase activity after freezing-induced degeneration of rat soleus muscle

In order to study the effect of synaptic contact on the amounts of choline acetyltransferase (ChAT) and acetylcholine (ACh) in the nerve terminals and on their ability to release ACh, a freeze—thaw procedure was developed as a means to induce long lasting degeneration of rat soleus muscle. It was found that 4 days after the freeze—thaw procedure the preparation did not contract upon direct electric stimulation and the level of creatine kinase (CK) was below detection. The preparation contained about 15% of the ChAT activity and 15% of the ACh content of the controls. The ACh release evoked by 50 mM KCl was 25% of controls, but it was, when expressed as a fraction of the ACh content, about twice as high as that in control muscles. At day 12, the preparation still did not contract and the level of CK was less than 5% of controls. The ChAT activity and the ACh content were 40%) and 20% of controls, respectively. However, no release of ACh could be evoked by 50 mM KCl. At days 28 and 58 the preparation contracted upon stimulation of the nerve; the CK activity had recovered to about 20% and the ACh content to 40%, while the ChAT activity did not increase above 40%. The KCl–evoked ACh release had recovered to 20—30% of controls. The results indicate that freezing destroyed muscle cells and most intramuscular nerve branches. Subsequent regeneration of muscle fibres was slow, probably because freezing had killed many satellite cells in the muscle. Because the ChAT activity at day 12 had recovered when CK was almost absent and the preparation failed to contract, we conclude that there was expression of ChAT activity in ‘nerve terminals’ which do not make contact with regenerated muscle cells, although little if any ACh was released from these sites. ©1998 Elsevier Science Ltd. All rights reserved.     

The effect of reinnervation on the distribution of muscle fibre types in the tibialis anterior muscle of the mouse

The distribution of fibre types in the tibialis anterior (TA) muscle of adult mice was examined by means of an immunohistochemical approach, using monoclonal antibodies that recognize different myosin heavy chain isoforms. As has been reported previously, the superficial portion of TA contains almost exclusively type IIB fibres and is almost entirely glycolytic in nature. Following section of the lateral popliteal nerve and rotation of the proximal stump to prevent rematching, it was found that the original pattern was virtually restored within 2 months. One possible explanation for this observation is that the activity pattern of peripheral and deep muscle fibres differs and that this aids in specification of muscle fibre type. Alternatively, the muscle fibres of the superficial portion of TA may be inherently resistant to an alteration of their phenotype with regard to expression of myosin heavy chain.     

Asymmetric Wavefront Aberrations and Pupillary Shapes Induced by Electrical Stimulation of Ciliary Nerve in Cats Measured with Compact Wavefront Aberrometer

To investigate the changes in the wavefront aberrations and pupillary shape in response to electrical stimulation of the branches of the ciliary nerves in cats. Seven eyes of seven cats were studied under general anesthesia. Trains of monophasic pulses (current, 0.1 to 1.0 mA; duration, 0.5 ms/phase; frequency, 5 to 40 Hz) were applied to the lateral or medial branch of the short ciliary nerve near the posterior pole of the eye. A pair of electrodes was hooked onto one or both branch of the short ciliary nerve. The electrodes were placed about 5 mm from the scleral surface. The wavefront aberrations were recorded continuously for 2 seconds before, 8 seconds during, and for 20 seconds after the electrical stimulation. The pupillary images were simultaneously recorded during the stimulation period. Both the wavefront aberrations and the pupillary images were obtained 10 times/sec with a custom-built wavefront aberrometer. The maximum accommodative amplitude was 1.19 diopters (D) produced by electrical stimulation of the short ciliary nerves. The latency of the accommodative changes was very short, and the accommodative level gradually increased up to 4 seconds and reached a plateau. When only one branch of the ciliary nerve was stimulated, the pupil dilated asymmetrically, and the oblique astigmatism and one of the asymmetrical wavefront terms was also altered. Our results showed that the wavefront aberrations and pupillary dilations can be measured simultaneously and serially with a compact wavefront aberrometer. The asymmetric pupil dilation and asymmetric changes of the wavefront aberrations suggest that each branch of the ciliary nerve innervates specific segments of the ciliary muscle and dilator muscle of the pupil.     

A histochemical and biometric comparison of the long peroneal muscle and its innervation in Gallus gallus and Coturnix c. japonica]

Following a previous comparison of the peroneus longus muscle of the quail and the starling, the present paper deals with a comparative study of this muscle in two birds of the order Gallinaceae, the quail and the bantam, bearing in mind certain data found in the starling. The study deals with the types of muscle fibres, their frequency in various parts of the muscle, their lipid content and their innervation. In the quail, two types of fibres are found, with a low and high lipid content respectively, while the bantam and starling have also a third, intermediate type. These intermediate fibres have a characteristically intermediate lipid content, peripherally situated nuclei and areas with a myofibrillar structure. The proportion of the two of three types of fibres varies with the species and in the distal and proximal parts of the muscle. The innervation of the peroneus longus muscle is different in the three species. In the quail, the two types of fibres have only one arboriform motor end-plate per fibre. In the bantam, the two types of homologous fibres also have only one motor end-plate, but with fewer arborifications. The intermediate fibre type, on the other hand, is innervated by several small nerve endings for each muscle fibre. This type of multiple innervation is also found in the starling. The peroneus longus muscle is thus functionally different in the two birds of the order Gallinaceae, whereas the rapid and slow system of innervation is found in the bantam and the starling. In the quail and the bantam, there is a strong positive correlation between the diameter of the muscle fibre and the longitudinal extent of the motor end-plate. This correlation is not marked in the starling. The characteristics of the innervation revealed by the cholinesterase activity concentrated in the synaptic grooves were confirmed by a direct study of the nerve fibres, using the Bielschowsky-Gros method. In the quail only 'en plaque' endings are found, while in the bantam and the starling both 'en plaque' and 'en grappe' endings are present. A parallel is drawn between the differences in function of the peroneus longus muscle and the characteristic features of its histology and innervation in the three species.     

The ultrastructure and innervation of muscles controlling chromatophore expansion in the squid,Loligo vulgaris

Squid chromatophores are organs of colour change, consisting of a pigment sac opened by contraction of 10–24 radial muscle fibres. The ultrastructure and innervation of these muscle fibres were examined by electron microscopy and diagramatic reconstructions made on the basis of serial ultra-thin sections. At the proximal end of the fibre, nearest the pigment sac a cortical myofilament zone surrounds 2 cores containing mitochrondria; further along the fibre these merge to form one central core. The myofilament zone forms a groove containing a nerve bundle consisting of 2 to 4 axons per muscle fibre. The axons are surrounded by glial cell processes, and either originate from a neighbouring fibre, or join the fibre at some point along its length. Axons twist around each other, forming a series of synapses with the muscle fibre. As many as 6–37 synapses exist along the length of each muscle fibre; the mean synapse interval is 9.05 m, but the largest may be 123 m. At the distal end of the muscles, the nerve is located towards the middle of the fibre, which it penetrates as the muscle splits up. Electron-lucent vesicles are present in all synaptic regions, but electron-dense vesicles are only found towards the distal end of the fibre. There is thus a possibility that more than one neurotransmitter is present in the nerves innervating chromatophores. Electron-lucent and dense-cored vesicles are not colocalised.This work was carried out during the tenure of a BBSRC CASE studentship     

Immunocytochemical localization of taurine in different muscle cell types of the dog and rat

The presence and distribution of the amino acid taurine in different muscle cell types of the dog and rat was examined by immunocytochemical methods. The light microscope study revealed that smooth muscle cells were similarly immunoreactive for taurine, whereas skeletal muscle fibres showed wide differences in taurine immunoreactivity among individual cells. Some skeletal fibres were strongly immunoreactive whereas others did not display immunolabelling. Mononucleated satellite cells, found adjacent to skeletal fibres in a quiescent stage, were also immunostained. Other myoid cells, such as testicular peritubular cells showed a cytoplasmic and a nuclear pool of taurine. By means of electron microscope immunolabelling, the subcellular localization of taurine was studied in vascular and visceral smooth muscle cells. Taurine was present in most subcellular compartments and frequently appeared randomly distributed. Taurine was localized on myofilaments, dense bodies, mitochondria, the plasma membrane and the cell nucleus. Moreover, the labelling density within individual smooth muscle cells was variable and depended on the state of contraction of each single fibre. Contracted cells showed a higher density of gold particles than relaxed cells. Unmyelinated nerve fibres, found adjacent to smooth muscle cells from the muscularis mucosae and the lamina propria of the stomach, were unstained or poorly stained.     

Effects of denervation and direct electrical stimulation upon the post-hatching differentiation of posterior latissimus dorsi muscle in chicken

The influences of denervation and of direct electrical stimulation of denervated muscle upon the post-hatching differentiation of fibre types in the fast avian muscle posterior latissimus dorsi have been investigated. Denervation inhibits the normal decrease in number of muscle fibres exhibiting acid-stable myofibrillar ATPase activity and leads to weak oxidative activity in all the fibres. Direct stimulation at a low rhythm of denervated muscle induces the normal decrease of fibres exhibiting acid-stable myofibrillar ATPase but does not allow the occurrence of normal oxidative activity pattern. The results emphasize the role of muscular activity upon the differentiation of fibre types in a developing muscle.     

Development of the myoneural junction in the rat

Summary The structure of the myoneural junction in the striated muscle of rat embryos and postnatal rats was studied by electron microscopy in order to assess at ultrastructural level the roles of neuronal and muscular elements and the sequence of events resulting in the formation of a functionally mature synaptic organization.From the observations it is concluded that the axon terminals enveloped by Schwann cells contain vesicles prior to apposition of the prospective synaptic membranes. Subsequently, subsarcolemmal thickening of the postsynaptic membrane takes place after the synaptic gap has been formed by disappearance of the teloglial cell from between the synaptic membranes but before the primary synaptic cleft in the strict sense is formed. Secondary synaptic clefts are formed later, when the primary synaptic cleft is regular in width, by local finger-like invaginations of the postsynaptic membrane, which thereafter expand basally, in a plane transverse to the axis of the axon terminal, to resemble flattened flasks. The junction is formed between multinucleated muscle cells and multiple axons, which at first lie side by side and later, when formation of adult-type secondary synaptic clefts is in progress, become separated by folds of the sarcoplasm and the teloglia. In extraocular muscles of adult rats the sarcoplasmic reticulum is closely associated with the postjunctional sarcoplasm.In the light of earlier observations on the development of contractibility after nerve stimulation, cholinesterase histochemistry and muscle fibre physiology, these observations are interpreted to indicate that functional differentiation of the myoneural synapse results from induction by the motor axon and that the association of the sarcoplasmic reticulum with the postjunctional sarcoplasm in adult extraocular muscles is related to modified fibre physiology.The author wishes to thank Prof. Antti Telkkä, M.D., Head of the Electron Microscope Laboratory, University of Helsinki, for placing the electron microscopic facilities at his disposal.     

Dedifferentiation and remodeling of motor endplates following experimental localized lesions of muscle fibers

Local anaesthetics, cardiotoxin and mechanical injuries may cause necrosis of muscle fibres while leaving the motor nerve fibres and their terminals intact. With local injuries to mouse muscles carried out by freezing or cutting we made a point of preserving both the nerve terminals and the muscle fibre portions on which these terminals were located. It was thus possible to follow the changes induced at endplates by these lesions. Within two or three days of the freezing or cutting, the muscle fibres underwent very different degrees of regression of the contractile material and T-system. The neuromuscular junctions also underwent changes, principally affecting their postsynaptic portion, in particular the folds of the subneural apparatus. After dedifferentiation of subsynaptic areas, we observed sprouting of the nerve terminal on muscle fibres which survived the amputation of one end and formed actively new myofibrils. This sprouting restored synaptic connections at the original sites, but with new structural features and correlative changes in the distribution of cholinergic receptors and cholinesterases. It is probable that after a phase of involution followed by a phase of recovery, the injured muscle fibres triggered off the nerve terminal sprouting which led to the remodelling of the endplates.