RESUMEN
Populations of dendritic spines can change in number and shape quite rapidly as a result of synaptic activity. Here, we explore the consequences of such changes on the input-output properties of a dendritic branch. We consider two models: one for activity-dependent spine densities and the other for calcium-mediated spine-stem restructuring. In the activity-dependent density model we find that for repetitive synaptic input to passive spines, changes in spine density remain local to the input site. For excitable spines, the spine density increases both inside and outside the input region. When the spine stem resistances are relatively high, the transition to higher dendritic output is abrupt; when low, the rate of increase is gradual and resembles long-term potentiation. In the second model, spine density is held constant, but the stem dimensions are allowed to change as a result of stimulation-induced calcium influxes. The model is formulated so that a moderate amount of synaptic activation results in spine stem elongation, whereas high levels of activation result in stem shortening. Under these conditions, passive spines receiving modest stimulation progressively increase their spine stem resistance and head potentials, but little change occurs in the dendritic output. For excitable spines, modest stimulation frequencies cause a lengthening of both stimulated and neighboring spines and the stimulus eventually propagates. High-frequency stimulation that causes spines to shorten in the stimulated region decreases the amplitude of the dendritic output slightly or drastically, depending on initial spine densities and stem resistances.
Asunto(s)
Forma de la Célula/fisiología , Espinas Dendríticas/fisiología , Modelos Neurológicos , Recuento de Células/métodos , Simulación por Computador , Transmisión Sináptica/fisiología , Factores de TiempoRESUMEN
To identify genes involved in synaptic functions, we screened lethal enhancer trap lines by monitoring synaptic activities at the neuromuscular junction in Drosophila embryos. It was found that MY7919, thus isolated, has moderate defects in both pre- and postsynaptic functions. The mean amplitudes of spontaneous as well as evoked synaptic currents were smaller than those in wild-type. The failure rate was higher than normal at any given concentration of external Ca(2+), indicating that presynaptic functions were impaired. In addition, the mean amplitude of miniature synaptic currents was smaller, and the unitary current amplitudes of junctional glutamate receptor channels were slightly but significantly smaller. Thus, postsynaptic functions were also altered. The gene was cloned and found to be identical to the previously reported apontic (=tracheae defective) locus, which is believed to be a transcription factor expressed in the central nervous system (CNS) as well as in the head, tracheae, and heart. Immunohistochemical analysis using an antiapontic antibody revealed that the protein is localized to nuclei. Null alleles of the apontic locus were obtained by imprecise excision of the enhancer trap vector. Synaptic activities in null mutants were not different from those of the original allele, even though null homozygotes had uncontracted ventral nerve cords and more severe behavioral phenotypes. The morphology of the neuromuscular junction of the null mutant was qualitatively similar to that of wild-type, with the presence of typical pre- and postsynaptic specializations, but with some suggestions of quantitative differences. This strategy for screening mutants with synaptic defects will reveal more genes directly or indirectly affecting synaptic transmission.
Asunto(s)
Proteínas de Unión al ADN , Proteínas de Drosophila , Regulación del Desarrollo de la Expresión Génica , Pruebas Genéticas/métodos , Proteínas de Insectos/genética , Unión Neuromuscular/fisiología , Terminales Presinápticos/fisiología , Factores de Transcripción , Secuencia de Aminoácidos , Animales , Secuencia de Bases , Calcio/farmacología , Drosophila , Electrofisiología , Elementos de Facilitación Genéticos/genética , Genes Letales , Homocigoto , Microscopía Electrónica de Rastreo , Datos de Secuencia Molecular , Neuronas Motoras/fisiología , Fibras Musculares Esqueléticas/fisiología , Mutación/fisiología , Unión Neuromuscular/ultraestructura , Plasticidad Neuronal/genética , Fenotipo , Terminales Presinápticos/ultraestructura , Transmisión Sináptica/efectos de los fármacos , Transmisión Sináptica/genética , Tetrodotoxina/farmacologíaRESUMEN
As a motor axon grows from the CNS to its target muscle, the terminal has the form of a flattened growth cone with a planar central region, lamellipodia, and filopodia. A mature terminal usually has a stereotyped shape that may be elongated with varicosities, as in several invertebrate species, or have short branches with boutons, as in mammals. We examined in Drosophila the developmental changes between growth cone and mature terminal using ultrastructural and immunocytochemical methods. The transition period, which occurs 2-3 hr after the first growth cone reaches its target muscle, is marked by the formation of "prevaricosities," smoothly contoured enlargements of the axons at the point where the nerve trunk first contacts the muscle fiber (MF). There is a 15-30 min ventral-to-dorsal gradient in the formation of prevaricosities on the individual abdominal MFs. Multineuronal innervation of each MF has occurred by this time, and two or more different axons undergo prevaricosity formation while they are intimately intertwined at the nerve entry point (NEP). Presynaptic active zones, both nerve-nerve and nerve-muscle, occur within the prevaricosities along broad contact regions. Synaptotagmin immunoreactive clusters form concurrently. The first varicosities then develop as a result of constrictions of the larger prevaricosities rather than as enlargement of discrete portions of the filopodia or neurites. The prevaricosity stage therefore may include the key steps that lead to the differentiation of functional differences in terminal subtypes as well as those leading to the formation of a stable neuromuscular junction.
Asunto(s)
Axones/fisiología , Proteínas de Unión al Calcio , Drosophila melanogaster/embriología , Neuronas Motoras/fisiología , Músculos Abdominales/embriología , Músculos Abdominales/inervación , Animales , Axones/ultraestructura , Biomarcadores , Drosophila melanogaster/crecimiento & desarrollo , Desarrollo Embrionario , Femenino , Larva , Masculino , Glicoproteínas de Membrana/análisis , Microscopía Confocal , Microscopía Electrónica de Rastreo , Morfogénesis , Terminaciones Nerviosas/ultraestructura , Proteínas del Tejido Nervioso/análisis , Sinapsis/ultraestructura , SinaptotagminasRESUMEN
In previous studies, 2,4-dithiobiuret (DTB) caused a delayed onset neuromuscular weakness in rats which was associated with decreased quantal content, alterations in postsynaptic ion channel properties, and abnormalities in nerve terminal ultrastructure. The latter include features typical of degenerating or diseased motor endplates as well as a marked proliferation of smooth endoplasmic reticulum (SER), swelling of mitochondria and evidence for a decreased in intraterminal calcium concentrations at early stages of intoxication (Jones, 1989, Acta Neuropathol. 78:72). These in vivo studies do not allow us to distinguish between the initial effects of DTB on the nerve terminal and those evolving as a result of disuse or secondary to its action on the muscle fiber or Schwann cells. To begin to distinguish between primary and secondary effects of DTB, we examined DTB-treated rat PC12 cells for comparable changes. The direct effects of DTB on PC12 cells included signs of general toxicity. Cell death in sparsely- plated cultures increased from 8-9% in controls to 13.7% at 10 microM for 24 hr exposure, and continued to increase in a concentration-dependent fashion to 25% mortality at 25 microM. However, between 25 and 100 microM there was little additional increase in mortality. 10 to 40 microM DTB slightly decreased the ability of both differentiated and undifferentiated cells to adhere to a substrate. This effect was independent of cell mortality. In moderately-differ-entiated cells having processes up to 10 cell diameters and several varicosities, concentrations of DTB as high as those invoking increased cell mortality and comparable to those affecting the rat neuromuscular junction did not cause abnormalities in the structure of the SER. No masses of tubulovesicular profiles were seen with transmission electron microscopy, and large changes in the quantity or distribution were not detected at the light microscope with the fluorescent stains DiOC6 or rhodamine B. Other signs of neuronal degeneration (blebbing of the plasmalemma, large intracellular droplets, mitochondrial abnormalities) preceded or accompanied any evidence for abnormalities in the SER. Thus the effect of DTB on the SER at the rat motor nerve terminal may occur secondary to a more general toxic action on other cell types, or may be dependent on a level of neuronal activity not achieved in sparsely- plated cultures, or may require a greater degree of differentiation of the neuronal cells than provided by the PC12 cell model used in this study.
Asunto(s)
Neuronas/efectos de los fármacos , Neurotoxinas/toxicidad , Tiourea/análogos & derivados , Animales , Adhesión Celular , Diferenciación Celular/efectos de los fármacos , Supervivencia Celular/efectos de los fármacos , Retículo Endoplásmico Liso/efectos de los fármacos , Vidrio , Modelos Neurológicos , Factores de Crecimiento Nervioso/farmacología , Células PC12 , Parálisis/inducido químicamente , Polilisina , Ratas , Tiourea/toxicidadRESUMEN
We characterized a subset of leech sensory afferents, the photoreceptors, in terms of their molecular composition, anatomical distribution, and candidate postsynaptic partners. For reagents, we used an antiserum generated against purified LL35, a 35 kD leech lactose-binding protein (galectin); monoclonal antibody (mAb) Lan3-2, which is specific for a mannose-containing epitope common to the full set of sensory afferents; and dye injections. Photoreceptors differ from other types of sensory afferents by their abundant expression of galectin. However, photoreceptors share in common with other sensory modalities the mannose-containing epitope recognized by mAb Lan3-2. Photoreceptors from a given segment project their axons directly into the CNS ganglion innervating the same segment. They assemble in a target region, the optic neuropil, which is separate from the target regions of other sensory modalities. They also extend their axons as an optic tract into the connective to innervate optic neuropils of other CNS ganglia, thereby providing extensive intersegmental innervation for the 33 CNS ganglia comprising the leech nerve cord. Because of its intimate contact with the optic neuropil, a central neuron, the AP effector cell, is a strong candidate second order visual neuron. In confocal images, the AP cell projects its primary axon for about 100 microns alongside the optic neuropil. In electron micrographs, spines emanating from the axon of the AP cell make contact with vesicle laden nerve terminals of photoreceptors. Leech photoreceptors and their second order visual neurons represent a simple visual system for studying the mechanisms of axonal targeting.
Asunto(s)
Axones/química , Dendritas/química , Lectinas/análisis , Neuronas Aferentes/ultraestructura , Nervio Óptico/química , Células Fotorreceptoras de Invertebrados/química , Animales , Sistema Nervioso Central/anatomía & histología , Ganglios de Invertebrados/ultraestructura , Sanguijuelas/anatomía & histología , Sanguijuelas/metabolismo , Modelos Neurológicos , Neuroglía/química , Nervio Óptico/ultraestructura , Sistema Nervioso Periférico/anatomía & histología , Sinapsis/ultraestructuraRESUMEN
During the degenerative processes that precede and accompany metamorphosis of the larval mesothoracic dorsal longitudinal muscles of Manduca sexta, the motor nerves and neuromuscular junctions undergo a variety of structural changes that are largely secondary to the changing morphologies of their respective glia. In the central region of the main motor nerve, the multiple layers of glial processes surrounding each of the large axons withdraw, leaving them apposed. In the peripheral region of the main motor nerve and in the secondary and tertiary nerve branches supplying the muscle, the outer glial processes of the nerve sheath and those that loosely wrap accompanying small neurosecretory axons all swell. Phagocytic cells and cells of unknown function invade the outer region of the nerve. In the neuromuscular junctions, the glial cells withdraw their processes from a complicated interdigitation with processes from the muscle fiber and from their relationship with the nerve terminal. As degeneration proceeds, this allows a greater area of contact between each nerve terminal and the muscle fiber. Within each junction there is a mixture of both functional and non-functional regions and active zones, as determined by both thin-section and freeze-fracture observations. No correlation was found between the degree of degeneration of a neuromuscular junction and its association with a particular muscle fiber or its position on the fiber relative to the origin or insertion.
Asunto(s)
Mariposas Nocturnas/ultraestructura , Unión Neuromuscular/ultraestructura , Animales , Técnica de Fractura por Congelación , Metamorfosis Biológica , Microscopía Electrónica , Mariposas Nocturnas/crecimiento & desarrollo , Neuronas Motoras/ultraestructura , Degeneración Nerviosa , Unión Neuromuscular/crecimiento & desarrolloRESUMEN
The ultrastructural changes associated with the early stages of degeneration of the larval mesothoracic muscle fibers of Manduca sexta were examined during the prepupal period and on the first day after ecdysis. Over this 5 day period, the muscle fibers decrease in cross-sectional area but increase in apparent surface area compared to the dimensions of early fifth-instar fibers. Large numbers of electron-dense granules or droplets are formed and extruded from the muscle cytoplasm into the hemolymph; this process may account for some of the decrease in muscle fiber mass and may represent a developmental mechanism for recycling nutrients. As the fibers shrink, the thick basal lamina is thrown into folds. Phagocytic hemocytes (granulocytes) congregate in clusters over the surface of the degenerating fibers and appear to remove specifically the basal lamina. The timely removal of the thick larval basal lamina may be essential for subsequent fusion of myoblasts to the residual larval myofibers. The contractile elements within the degenerating muscle fibers become disorganized but are not dysfunctional at the end of the first 12 h after the pupal ecdysis. Tracheoles withdraw from intimate contact with each muscle fiber in its clefts and T-tubules and associate in groups adjacent to it. Mitochondria appear to be degenerating. These structural changes are concurrent with a previously observed decline in resting potential and suggest that a significant change in the electrical properties of the muscle fibers should be expected as well.
Asunto(s)
Mariposas Nocturnas/ultraestructura , Músculos/ultraestructura , Animales , Membrana Basal/ultraestructura , Metamorfosis Biológica , Microscopía Electrónica , Microscopía Electrónica de Rastreo , Mariposas Nocturnas/crecimiento & desarrollo , Desarrollo de MúsculosRESUMEN
In Manduca sexta the decline in neuromuscular function during metamorphic degeneration was compared in two muscles which differed characteristically with regard to pre- and postsynaptic physiological properties. In both muscles, morphological evidence indicated that a significant number of the active zones within the population of neuromuscular junctions on a given fiber were nonfunctional. Nevertheless, the degenerating nerve terminals were able to produce an above-threshold excitatory junction potential (EJP) which was facilitated in a manner characteristic of the muscle being observed. Abnormal findings during the early stages of degeneration included a larger than normal EJP, a decline in EJP amplitude over a 20 min period even with low frequencies of stimulation, an increase in EJP duration, a decline in muscle fiber resting potential amplitude with age, a decrease or disappearance of post-tetanic potentiation and long-term facilitation, and an increased likelihood that the motor nerve would fail to conduct a stimulus. The two muscles were qualitatively similar but quantitatively different with regard to these degenerative changes. It is suggested that this combination of relatively normal function with abnormal properties might be associated with the withdrawal of glial processes from the neuromuscular junctions, changes in the cable properties associated with shrivelling of the muscle fibers, and a decline in the metabolic functions supporting both muscle fiber resting potentials and those underlying transmitter synthesis, mobilization and release.
Asunto(s)
Mariposas Nocturnas/fisiología , Unión Neuromuscular/fisiología , Animales , Estimulación Eléctrica , Potenciales Evocados , Larva/fisiología , Potenciales de la Membrana , Metamorfosis Biológica , Mariposas Nocturnas/crecimiento & desarrollo , Degeneración Nerviosa/fisiología , Neurotransmisores/metabolismoRESUMEN
Individual insect muscle fibers, whose neuromuscular junctions have been stained with a modification of Ranvier's gold chloride method, can be dissected free and mounted whole if the muscle is prefixed in aldehydes. The neuromuscular junctions along the length of the individual fibers are well delineated and can be measured and counted. Effective procedures include fixation with glutaraldehyde buffered to low pH with sodium citrate, or glutaraldehyde and paraformaldehyde combined in phosphate buffer at neutral pH, followed by exposure to citric acid and to gold chloride. The method is convenient, and could be useful for the study of arthropod neuromuscular junctions in general, since their nerve terminals do not release acetylcholine as a transmitter and cannot be stained by the more commonly used cholinesterase methods.
Asunto(s)
Compuestos de Oro , Oro , Lepidópteros/anatomía & histología , Mariposas Nocturnas/anatomía & histología , Músculos/ultraestructura , Unión Neuromuscular/ultraestructura , Coloración y Etiquetado/métodos , Animales , Mariposas Nocturnas/crecimiento & desarrollo , Desarrollo de Músculos , Unión Neuromuscular/fisiologíaRESUMEN
The morphology, ultrastructure, innervation and physiology of the third axillary muscle in Manduca sexta were examined to investigate the role of this muscle in flight. The muscle consists of three parts: the upper bundle, which originates on the episternum, and the middle and lower bundles, which originate on the epimeron; all three parts insert on the tip of a projection from the third axillary sclerite. The middle bundle is composed of tonic fibres, and is innervated by a single slow axon, while the other two bundles consist of intermediate fibres and are each innervated by a single fast axon. The shape and position of the third axillary sclerite within the wing hinge are such that its primary function appears to be remotion of the wing. The length of the third axillary muscle determines the amount of remotion, independency of the degree of elevation or depression of the wing and independently of the amount of remotion of the contralateral wing. Electrophysiological recordings from the three parts of the muscle during tethered flight indicate that they may each function independently of each other and in different ways. The tonic (middle) bundle is capable of maintaining tension to hold the wings in the folded position at rest and is active when the wings are folded at the end of flight. The intermediate (upper and lower) bundles are activated phasically with impulses that may occur with various relationships to the timing of activation of a direct depressor, the subalar, or of several of the elevators. The findings are consistent with the hypothesis that the third axillary muscles on both sides are important in determining the asymmetric degrees of remotion observed in turning flight.
Asunto(s)
Lepidópteros/fisiología , Mariposas Nocturnas/fisiología , Músculos/fisiología , Animales , Vuelo Animal , Técnica de Fractura por Congelación , Microscopía Electrónica , Músculos/anatomía & histología , Músculos/inervaciónRESUMEN
The multiterminal slow and fast neuromuscular junctions of the moth Manduca sexta were compared using scanning, thin-section, and freeze-fracture techniques to see what structural features might underlie their functional differences. Slow neuromuscular junctions, here formed on tonic muscle fibers, produce a facilitating e.j.p. the amplitude of which is 1/5 to 1/3 the size of a fast excitatory junction potential (EJP) and the duration of which is nearly four times longer. A slow junction consists of a single terminal branch that is shorter in length than either of the pair of branches that a fast junction forms close together on the muscle fiber. Within the junction, slow nerve terminals exhibit longer, more frequent constrictions and are very varicose compared with fast. Since fast larval junctions on tonic muscle fibers are also varicose (Schaner and Rheuben, 1985), this is unlikely to represent an intrinsic property of the nerve. However, calculations of the length constants of the varicose versus nonvaricose shapes indicate that the effect of passive cable properties on normal functioning may act to limit the length of the slow terminals more than that of fast. Even though the varicose shape can be predicted to prolong the time course of the EJP, calculations show that, at the measured length, this would not explain the very long EJP that is observed. Within the neuromuscular junctions, the synapses are characterized on the muscle membrane by a patch of densely packed particles on the external leaflet and on the nerve membrane by a single linear active zone. The total number of synapses per slow junction is about 1/3 that of fast junctions. There is a weak correlation between average area of the individual postsynaptic particle patches and cross-sectional area of the muscle fibers that transcends nerve and muscle fiber types. The average lengths of active zones from the two types do not differ significantly. However, the number of particles per active zone in slow junctions is about 55% of the number in fast active zones. Chemically fixed slow nerve terminals have a greater density of synaptic vesicles remaining than do fast. If a proportion of the active zone particles represent structures directly involved in the probability of transmitter release, such as Ca++ channels, then the latter two characteristics may jointly reflect differences in capability to release and mobilize transmitter that would partly explain the different EJP amplitude and facilitation properties.
Asunto(s)
Lepidópteros/ultraestructura , Mariposas Nocturnas/ultraestructura , Unión Neuromuscular/ultraestructura , Membranas Sinápticas/ultraestructura , Vesículas Sinápticas/ultraestructuraRESUMEN
The nerves and nerve terminals to tonic larval muscle fibers in third and fifth instar caterpillars were studied to compare them with those formed by the same motor neurons on phasic flight muscles in adult moths. Scanning micrographs showed a primary nerve branch running the length of each fiber, with secondary nerve branches extending from it at intervals. There was a great deal of variability in both the length of the branches and the distance from the nerve at which the neuromuscular junctions were formed. The rapid increase in muscle fiber size during larval development may be responsible for this variability. The nerves and junctions were often found to be obscure by overlying fibroblasts and tracheoblasts or entering the deep muscle clefts. Those examined were similar in appearance to the adult junctions formed by the same neurons, although some may have formed single branches instead of y-shapes. The membrane specializations of the synapse seen in freeze-fractured specimens were similar to those of the adult junction. However, the overall shape of the nerve terminal within the junction differed. The larval nerve terminals appeared varicose instead of having a uniform diameter. The spacing of the nerve plaques varied, in contrast with the relatively straight alignment and even spacing of plaques found in adult junctions. Such differences could result from an interaction between the motor neuron and the two different types of muscle fiber that it innervates, an intrinsic change in the motor neurons themselves that occurs with metamorphosis, or a plastic functional response that occurs as a result of the different types of motor patterns that are used in the two stages.
Asunto(s)
Lepidópteros/anatomía & histología , Mariposas Nocturnas/anatomía & histología , Sistema Nervioso/ultraestructura , Unión Neuromuscular/ultraestructura , Animales , Técnica de Fractura por Congelación , Larva , Microscopía Electrónica de RastreoRESUMEN
Muscle fibers from fourth and fifth instar caterpillars were examined with scanning and thin section electron microscopy. Scanning micrographs showed that early fifth instar specimens had a population of cells lying beneath the basal lamina over the surface of the muscle fiber and in conjunction with tracheoles and nerves. At least two cell types were present. One type could be categorized as tracheoblasts of their close association with the tracheoles and the presence of taenidia within the tracheoblast cytoplasm in sectioned material. A second cell type, characterized by long filamentous processes, contained extensive rough endoplasmic reticulum and cisternae swollen with an electron-dense substance similar in appearance to the basal lamina. This ultrastructural appearance is characteristic of vertebrate fibroblasts and certain types of insect hemocytes. Early and late fourth instar specimens had few cells on their muscle fiber surfaces. Measurements of the basal lamina thickness were taken from thin sections of nondigested muscle fibers of early fourth, late fourth, and early fifth instar animals. The results showed that the basal lamina underwent a large increase in thickness between the fourth and fifth instars. The proliferation of cells which appeared to be in an actively synthesizing state paralleled the increase in basal lamina thickness. This suggests the hypothesis that these cells are active in connective tissue formation, and contribute to the formation of the basal lamina that lies over both them and the muscle fiber.
Asunto(s)
Células del Tejido Conectivo , Fibroblastos/citología , Músculos/citología , Animales , Membrana Basal/citología , Lepidópteros/citología , Microscopía Electrónica , Microscopía Electrónica de Rastreo , Desarrollo de MúsculosRESUMEN
Immature synapses, developing moth neuromuscular junctions, were studied using electro-physiological and ultrastructural techniques, and were compared with synapses from the flight muscles of adult moths. Neuromuscular junctions, formed by short side branches of the single fast motor axon, were assessed for functional state by stimulating the nerve and recording the endplate potential intracellularly from the muscle fibre. The muscle was then fixed and prepared for scanning, thin-section, and freeze-fracture microscopy. The immature stage differs from the adult by having very small (average 7.8 mV, compared with 20-30 mV), long duration ejp's that fatigue rapidly. The immature junctions are, however, only 13% shorter than those of the adult. Within the junction, the nerve terminal comes into direct contact with the muscle membrane in a series of oval patches separated by glial processes. These regions of apposition or 'plaques' in the immature synapse are about half the diameter of the adult plaques. In freeze-fractured material, the nerve terminal membrane in the plaque region bears an irregular band of particles on the cytoplasmic leaflet; the length of the band is essentially the same in the immature synapse as in the adult. This band marks the location of the active zone, an electron dense bar of the same length in thin section. The apposing external leaflet of the muscle membrane bears a patch of postsynaptic particles; the patch is much smaller than in the adult plaque. These immature patches, presumably representing clusters of receptors, range in size from a dozen particles to a hundred or more. We consider it likely that a lack of postsynaptic receptors may partially explain the very small ejp in the developing synapse, but that other factors may also be limiting. Desmosome-like contacts between glial cells and the muscle fibre were observed. Small wisps of electron dense material appear to bridge the extracellular space between the nerve terminal and the muscle fibre or between the glial processes and the muscle fibre in some locations. They are found in the same regions of the neuromuscular junction as small groups of large particles, suggesting that these two features are different aspects of the same structure. From their location one could hypothesize that they have either a mechanical function of stabilizing the glial invaginations, or a role in communication between the three types of developing cells.
Asunto(s)
Membranas Sinápticas/ultraestructura , Envejecimiento , Animales , Técnica de Fractura por Congelación , Lepidópteros , Microscopía Electrónica de Rastreo , Unión Neuromuscular/crecimiento & desarrollo , Unión Neuromuscular/ultraestructura , Pupa , Membranas Sinápticas/fisiologíaRESUMEN
1. Muscles innervated by an identified set of motor neurones were compared between larval and adult stages. 2. The structure of the larval muscle is typically tonic: long sarcomeres, irregular Z-bands, and 10-12 thin filaments around each thick filament. The structure of the adult muscle is phasic: 3-4 micrometers sarcomeres, regular Z-bands, 6-8 thin filaments around each thick filament, and large mitochondrial volume. 3. The tensions produced by these muscles were correspondingly different. The larval twitch was about 7 times slower and the tetanus/twitch ratio 10 times greater than those of the adult. 4. No structural or physiological differences were observed in the neuromuscular junctions of the two stages. 5. The relatively unchanging functional relationship of a single motor neurone with two different muscle fibre types during two developmental stages is compared with the converse situation in which it has been reported that implantation of a different type of motor nerve into a muscle modifies contractile properties.
Asunto(s)
Lepidópteros/crecimiento & desarrollo , Mariposas Nocturnas/crecimiento & desarrollo , Neuronas Motoras/crecimiento & desarrollo , Músculos/inervación , Animales , Larva , Potenciales de la Membrana , Contracción Muscular , Desarrollo de Músculos , Músculos/ultraestructura , Unión Neuromuscular/fisiología , Unión Neuromuscular/ultraestructura , Pupa , Transmisión SinápticaRESUMEN
1. Frog sartorius muscles, newly denervated and transplanted to the lymph sac of the back, are reinnervated by implanted cholinergic nerves (spinal somatic motor nerves or the preganglionic sympathetic splanchnic nerve), but not by nerves). 2. Foreign somatic motor nerves (s.m.n.s) form synapses that resemble normal sartorius neuromuscular junctions electrophysiologically. 3. Axons of the sympathetic preganglionic splanchnic nerve (s.p.n.) grow throughout the muscle, but only a small percentage of fibres form synapses. Most e.p.p.s are of low quantal content, generally subthreshold. Long onset latencies and multiple post-synaptic responses indicate that innervation is multiple, multi-terminal, and by unmyelinated axons. 4. Spontaneous miniature e.p.p.s at splanchnic junctions occur at an average rate under 0.1/sec. Their average amplitude and time course are about the same as for control muscles, but the variability of amplitudes is greater than for control muscles. 5. The amount of facilitation shown by s.p.n.-evoked e.p.p.s is the same as by s.m.n. e.p.p.s, but the time course is almost twice as long. 6. S.p.n.-reinnervated fibres show dramatic post-tetanic potentiation preceded by depression, following as few as 20--50 stimuli. 7. As judged by standard physiological and histochemical criteria, AChEsterase is absent at s.p.n. junctions. 8. The pharmacological responses of the s.p.n. junctions are similar to those of normal or foreign s.m.n. innervated neuromuscular junctions in their sensitivity to the cholinergic blocking agents D-tubocurarine and hexamethonium. 9 The s.p.n. is capable of restricting ACh sensitivity to the sites of nerve contacts, although this restriction occurs more slowly and less completely than with s.m.n. reinnervation. The loss of extrajunctional ACh sensitivity can be correlated with effectiveness of innervation; but significant restriction occurs even in s.p.n. reinnervated fibres that probably never contract to nerve stimulation.
Asunto(s)
Fibras Autónomas Preganglionares/fisiología , Músculos/inervación , Acetilcolina/fisiología , Animales , Anuros , Axones/fisiología , Colinesterasas/fisiología , Masculino , Contracción Muscular , Desnervación Muscular , Rana catesbeiana , Nervios Esplácnicos/fisiología , Sinapsis/fisiología , Transmisión SinápticaRESUMEN
1. Competition between two foreign nerves innervating frog skeletal muscle has been studied by using pairs of somatic motor nerves (s.m.n.s) or one s.m.n. and the preganglionic splanchnic nerve (s.p.n.) implanted into a denervated sartorius muscle that has been transplanted to the lymph sac of the back. 2. A single s.m.n. implanted into the muscle succeeded in innervating essentially every fibre within 2--3 months; tetanic stimulation of the nerve elicited 9--100% of the maximal direct tetanus tension. Most of the e.p.p.s were suprathreshold, since a single indirect stimulus evoked a twitch 60--100% as large as that to a direct stimulus. 3. If two s.m.n.s were implanted simultaneously, tetanic stimulation of either elicited 80--100% of the maximal tension to direct stimulation. If one nerve was implanted 2--3 months before the other, the second, although usually less effective than the first, normally innervated 50--100% of the fibres, with approximately the same time course of innervation as a single s.m.n. 4. Mutual synaptic repression was seen on examination of twitch tensions. With either simultaneous or staggered innervation, stimulation of each s.m.n. resulted in a twitch of 30--50% of the total direct twitch tension, with little overlap between the fields driven by the two nerves. Intracellular recordings showed that the distribution of subthreshold and spike-producing e.p.p.s reflected the existence of separate twitch fields. Even if one s.m.n. was implanted several months before the other and had time to establish suprathreshold junctions on most muscle fibres, an s.m.n. implanted later was able to reduce sharply the effectiveness of many junctions from the earlier nerve while itself innervating most muscle fibres. 5. The subthreshold e.p.p.s had low quantal content, typically ten or fewer quanta/e.p.p. The min e.p.p. frequency was very low, while min e.p.p. amplitude appeared to be normal. 6. In the vast majority of muscle fibres, junctions from the two nerves were not within recording distance of each other. Hence, we infer that the competitive interaction was mediated somehow via the muscle fibre. 7. The preganglionic splanchnic nerve, which also successfully reinnervated frog skeletal muscle, competed with a foreign s.m.n. in ways which differ qualitatively from the competition by a second s.m.n. In the presence of a s.m.n., synapses of the s.p.n. were almost universally subthreshold. However, if the s.p.n. was implanted 2--3 months before the s.m.n., the s.m.n. was prevented for several months from innervating fibres driven by the s.p.n. This delay in s.m.n. reinnervation was greater than if the first nerve implanted was also an s.m.n. 8. After 6--8 months of dual innervation by s.m.n. and s.p.n., the s.m.n. became almost totally dominant. However, if the s.m.n. was then sectioned, the s.p.n. became as effective, within approximately 1 week, as it would have been in the absence of the s.m.n.