RESUMEN
Studies on living turtles have demonstrated that shells are involved in the resistance to hypoxia during apnea via bone acidosis buffering; a process which is complemented with cutaneous respiration, transpharyngeal and cloacal gas exchanges in the soft-shell turtles. Bone acidosis buffering during apnea has also been identified in crocodylian osteoderms, which are also known to employ heat transfer when basking. Although diverse, many of these functions rely on one common trait: the vascularization of the dermal shield. Here, we test whether the above ecophysiological functions played an adaptive role in the evolutionary transitions between land and aquatic environments in both Pseudosuchia and Testudinata. To do so, we measured the bone porosity as a proxy for vascular density in a set of dermal plates before performing phylogenetic comparative analyses. For both lineages, the dermal plate porosity obviously varies depending on the animal lifestyle, but these variations prove to be highly driven by phylogenetic relationships. We argue that the complexity of multi-functional roles of the post-cranial dermal skeleton in both Pseudosuchia and Testudinata probably is the reason for a lack of obvious physiological signal, and we discuss the role of the dermal shield vascularization in the evolution of these groups. This article is part of the theme issue 'Vertebrate palaeophysiology'.
Asunto(s)
Adaptación Biológica , Evolución Biológica , Huesos/fisiología , Reptiles/fisiología , Animales , Huesos/anatomía & histología , Fósiles/anatomía & histología , Reptiles/anatomía & histología , Tortugas/anatomía & histología , Tortugas/fisiologíaRESUMEN
Two successive mechanisms have been described in perichondral ossification: (1) in static osteogenesis, mesenchymal cells differentiate into stationary osteoblasts oriented randomly, which differentiate into osteocytes in the same site; (2) in dynamic osteogenesis, mesenchymal cells differentiate into osteoblasts that are all oriented in the same direction and move back as they secrete collagen fibers. This study is aimed at testing the hypothesis that the ontogenetic sequence static then dynamic osteogenesis observed in the chicken and in the rabbit is homologous and was acquired by the last common ancestor of amniotes or at a more inclusive node. For this we analyze the developmental patterns of Pleurodeles (Caudata, Amphibia) and those of the lizard Pogona (Squamata, Lepidosauria). We processed Pleurodeles larvae and Pogona embryos, prepared thin and ultrathin sections of appendicular bones, and analyzed them using light and transmission electron microscopy. We show that static osteogenesis does not precede dynamic osteogenesis in periosteal ossification of Pleurodeles and Pogona. Therefore, the null hypothesis is rejected and according to the parsimony method the ontogenetic sequence observed in the chicken and in the rabbit are convergent. In Pleurodeles and Pogona dynamic osteogenesis occur without a previous rigid mineralized framework, whereas in the chicken and in the rabbit dynamic osteogenesis seems to take place over a mineralized support whether bone (in perichondral ossification) or calcified cartilage (in endochondral ossification). Interestingly, in typical dynamic osteogenesis, osteoblasts show an axis (basal nucleus-distal endoplasmic reticulum) perpendicular to the front of secreted unmineralized bone matrix, whereas in Pleurodeles and Pogona this axis is parallel to the bone matrix.
Asunto(s)
Osteogénesis/fisiología , Pleurodeles/fisiología , Reptiles/fisiología , Animales , Calcificación Fisiológica/fisiología , ConejosRESUMEN
Bone ornamentation, in the form of rounded pits framed by a network of ridges, is a frequent feature among a great diversity of gnathostome taxa. However, the basic osteogenic processes controlling the differentiation and development of these reliefs remain controversial. The present study is a broad comparative survey of this question with the classical methods used in hard tissue histology and paleohistology. Distinct processes, unevenly distributed among taxa, are involved in the creation and growth of pits and ridges. The simplest one is mere differential growth between pit bottom (slow growth) and ridge top (faster growth). The involvement of several complex remodeling processes, with the local succession of resorption and reconstruction cycles, is frequent and occurs in all major gnathostome clades. Some broad, inclusive clades (e.g., Temnospondyli) display consistency in the mechanisms controlling ornamentation, whereas other clades (e.g., Actinopterygii) are characterized by the diversity of the mechanisms involved. If osteogenic mechanisms are taken into account, bone ornamentation should be considered as a character extremely prone to homoplasy. Maximum likelihood (ML) optimizations reveal that the plesiomorphic mechanism creating ornamentation is differential apposition rate over pits (slow growth) and ridges (faster growth). In some taxas e.g., temnospondyls vs lissamphibians or pseudosuchians, bone ornamentation is likely to be a homoplastic feature due to a convergence process driven by similar selective pressures. ML models of character evolution suggest that the presence of resorption in the development of ornamentation may be selectively advantageous, although support for this conclusion is only moderate.
Asunto(s)
Evolución Biológica , Desarrollo Óseo/fisiología , Cordados/anatomía & histología , Cordados/crecimiento & desarrollo , Morfogénesis/fisiología , Animales , HuesosRESUMEN
Bone ornamentation, in the form of highly repetitive motives created by pits and ridges, is a frequent feature on vertebrate skull roofs and osteoderms. The functional significance of this character remains a matter of controversy and speculation. The many diverging hypotheses proposed to explain it all share a common logical prerequisite: bone ornamentation should increase significantly the surface area of the bones that bear it. In order to test this assumption in the Crocodylia, we developed a method for quantifying the gain in area due to ornamentation using a three-dimensional-surface scanner. On crocodylian osteoderms, the gain in area can be up to 40%, and on the cranial table, it ranges between 10 and 32% in adult specimens (in both cases, it shows substantial differences between the adults of the various species included in the sample). Area gain on the snout is lesser (0-20% in adults), and more variable between species. In general, bone ornamentation is less pronounced, and results in fewer area gains in juvenile specimens. The main morphometric results yielded by this study are discussed in reference to the few comparative data available hitherto, and to the functional interpretations proposed by previous authors.
Asunto(s)
Caimanes y Cocodrilos/crecimiento & desarrollo , Desarrollo Óseo , Caimanes y Cocodrilos/anatomía & histología , AnimalesRESUMEN
This review addresses the history since antiquity of studies on the anatomical and functional relations between nerves and muscles, and the progressive use of newer approaches to this topic. By the Hippocratic era (almost 2500 years ago) the digestive, circulatory and nervous systems were thought to participate in the production of animal spirits. This concept had strong support for nervous conduction, even after the dawn of electrophysiology in the late 18th C. The idea that these spirits explained the nature of the motor command to muscles continued to prevail until work in the mid-to-late 19th C dispelled the concept of "fluid/spirit" transmission by measurements of nerve "action currents" and conduction velocity. In parallel with this work, the functional relations between nerves and muscles were studied with the use of curare, which continued well into the 20th C. In the late 19th C the debate was formalized about whether transmission at the motor endplate was electrical or chemical, which continued as the "soup" vs. "sparks" battle until, surprisingly, the late 1960s. The concept of the motor unit was introduced in the 1920s, this being defined as a motor neuron in the spinal cord connecting to a specific set of muscle fibers. This development accelerated work on two-way trophic relations between nerve and muscles and their essential plasticity in response to the demands of usage and disease. Thus, the relation between nerves and muscles has been on the forefront of neuroscience since antiquity.
Asunto(s)
Neuronas Motoras/fisiología , Músculo Esquelético/fisiología , Neurociencias/historia , Historia del Siglo XVIII , Historia del Siglo XIX , Historia del Siglo XX , Historia Antigua , Historia Medieval , HumanosRESUMEN
This article addresses the emergence of the "motoneuron concept," i.e., the idea that this cell had properties of particular advantage for its control of muscle activation. The motor function of the ventral roots was established early in the 19th C and the term "motor cell," (or "motor nerve cell") was introduced shortly thereafter by Albrecht von Kölliker and some other histologists. They knew that motor cells were among the neurons with the largest soma in vertebrates and for this reason they were, and remained for many decades, the best and most studied neuronal model. The work of clinicians like Guillaume Duchenne de Boulogne and Jean-Martin Charcot on motor degenerative syndromes began before a clear description of motor cells was available, because it was initially more difficult to establish whether the deficits of paralysis and muscle weakness were due to neuronal or muscular lesions. Next, the pioneering physiologist, Charles Sherrington, who was influenced greatly by the anatomical contributions and speculations of Santiago Ramón y Cajal, used the term, "motor neuron," rather than motor cell for the neuron that he considered was functionally "the final common path" for providing command signals to the musculature. In the early 20th C he proposed that activation of a motor neuron resulted from the sum of its various excitatory and inhibitory CNS inputs. The contraction of motor neuron to "motoneuron(e)" was put into common usage by John Fulton (among possibly others) in 1926. The motoneuron concept is still evolving with new discoveries on the horizon.
Asunto(s)
Neuronas Motoras/fisiología , Neurociencias/historia , Animales , Historia del Siglo XIX , Historia del Siglo XX , HumanosRESUMEN
The Middle Ages saw the development of numerous universities in the different provinces that later became the kingdom of France. In 1794, Napoleon I established 3 medical schools in Paris, Montpellier and Strasbourg, which were transformed into medical faculties in 1808. France had always been a highly centralized country, but during the 19th century, this trend started to change with the creation of medical faculties in Nancy (1872), Lille (1877), Lyon (1878), Bordeaux (1879), Toulouse (1891), Algiers (1910) and Marseille (1930). Following the creation of the 12 foundation courses, specialized chairs were progressively established in Paris, but for a long time this remained restricted to the French capital. However, with the emergence of medicine as an academic discipline in several towns outside Paris, came the development of neurology. This was greatly influenced by former students of Jean-Martin Charcot, local personalities, and the interactions between the two. Leading figures included Albert Pitres in Bordeaux, Léon Ingelrans in Lille, Eugène Devic and Jules Froment in Lyon, Lucien Cornil in Marseille, Joseph Grasset in Montpellier, and Marcel Riser in Toulouse. The interaction between French and Germanic medical communities also developed at this turbulent time under the influence of several great physicians such as Wilhelm Waldeyer, Adolf Kussmaul, and later Jean Alexandre Barré in Strasbourg, and Hippolyte Bernheim in Nancy. There are a number of other university towns outside Paris in which the development of neurology was probably influenced by the same interactions with psychiatry. It would be worth carrying out a thorough analysis of these towns in order to present an exhaustive overview of the development of neurology in France.
Asunto(s)
Hospitales/historia , Neurología/historia , Educación Médica/historia , Francia , Historia del Siglo XVIII , Historia del Siglo XIX , Historia del Siglo XX , HumanosRESUMEN
Many important notions relative to breathing, walking, and chewing originated from early concepts developed in the nineteenth century. We will consider successively: The measurements of the parameters of these behaviors done by great experts on movement recordings. It corresponded to the first elements of biomechanics. The physiology of central motor activities in locomotion and in respiration completed by the analysis of motor neuropathologies where specific deficits have highlighted the role of some crucial motor structures. The studies of different reflexes, a predominant concept at that time, that have characterized some particular pathways between the sensory receptors and the different motor output. The goal of this review is to show how true pioneers, often using some crude and simple methods of investigation, have addressed important issues relative to control of these three rhythmic motor behaviors.
Asunto(s)
Masticación/fisiología , Neurociencias/historia , Respiración , Caminata/fisiología , Animales , Fenómenos Biomecánicos , Encéfalo/anatomía & histología , Encéfalo/patología , Historia del Siglo XVI , Historia del Siglo XVII , Historia del Siglo XVIII , Historia del Siglo XIX , Historia del Siglo XX , Historia del Siglo XXI , Humanos , Actividad Motora/fisiología , Periodicidad , Postura , InvestigadoresRESUMEN
The history of neurology in France is characterized by the very high degree of centralization in that country where "everything seems to happen in Paris," and yet the considerable degree of autonomous diversity in the evolution of some other medical schools such as Montpellier and Strasbourg. It could be argued that France saw the birth of clinical neurology as a separate discipline since Jean Martin Charcot at the Salpêtrière Hospital obtained a chair of diseases of the nervous system in 1892, a first in the history of the academic world. The chapter shows, however, that the work of Charcot was preceded by a long evolution in medical thinking, which culminated with the introduction of experimental medicine developed by Claude Bernard and François Magendie, and by the study of aphasia by Paul Broca and its localization of language in a specific area of the brain. Many of the great neurologists of France like Duchenne de Boulogne, Gilles de la Tourette, Joseph Babinski and Pierre Marie gravitated around Charcot while others like Charles-Edward Brown-Sequard and Jules Dejerine developed their talents independently. The history of Sainte-Anne Hospital further illustrates this independence. It also shows the relation between neurology and psychiatry with Henri Ey, Jean Delay and Pierre Deniker, who collaborated with Henri Laborit in the clinical development of chlorpromazine. Sainte Anne also saw the birth of modern neuropsychology with Henry Hécaen. Jean Talairach and his group developed human stereotaxic neurosurgery and a 3-dimensional brain atlas that is used around the world. The chapter also mentions institutions (the CNRS and INSERM) that have contributed to developments partially independently from medical schools. It concludes with a presentation of schools located outside of Paris that have played a significant role in the development of neurology. Six of the most important ones are described: Montpellier, Toulouse, Bordeaux, Strasbourg, Lyon, and Marseilles.
Asunto(s)
Enfermedades del Sistema Nervioso/historia , Neurología/historia , Academias e Institutos/historia , Encéfalo/fisiología , Francia , Historia del Siglo XVIII , Historia del Siglo XIX , Historia del Siglo XX , Historia del Siglo XXI , Humanos , Enfermedades del Sistema Nervioso/terapia , Neurología/métodos , ParisRESUMEN
Many currently accepted notions of motor control originate from a few seminal concepts developed in the latter half of the 19th century (see Bennett and Hacker, 2002). The goal of this review is to retrace some current ideas about motor control back to the thought of three French neurologists of Hospital of the Salpetrière hospital in Paris during the latter half of the 19th century and early 20th century (Fig. 1): Guillaume Duchenne de Boulogne (1806-1875), Jean-Martin Charcot (1825-1893), and Joseph Babinski (1857-1932). A common theoretical and methodological thread unites these three men as Charcot was taught neurology by Duchenne, and Babinski was trained by Charcot. The influential concepts developed by these pioneering French neurologists have been neglected for nearly a century and only rediscovered recently. We intend to highlight how these astute clinicians used their meticulous clinical observations of patients to reveal novel and original perspectives of motor co-ordination. Between 1850 and 1930, all three men played a major role in developing and shaping the entire field of normal and pathological motor control in addition to making important contributions to three major scientific issues; the centralist view of muscle sense, the emerging concept of muscle synergy in voluntary movements and in locomotion and finally the specific role of the cerebellum in muscle synergy. The important contributions of these men will be considered in the context of other significant schools of neurology from other countries. Finally, the concept of cerebellar asynergy as proposed by Babinski anticipated the development of the internal models which much later were able to provide a theoretical basis for understanding the mechanism of learned motor co-ordination involving the cerebellum.
Asunto(s)
Neurología/historia , Historia del Siglo XIX , Historia del Siglo XX , Movimiento , ParisRESUMEN
This article reviews the scientific contributions of Jacques Paillard (1920-2006), who strengthened substantially the role of physiological psychology in the field of movement neuroscience. His research began in 1947 under the direction of the French neurophysiologist, Alfred Fessard (1900-1982), with whom he then collaborated for 9 years while an undergraduate and then graduate student and junior faculty member in psychology at the University of Paris (the Sorbonne). Paillard moved to the University of Marseille in 1957 as a Professor of Psychophysiology. In parallel, he became a founding member and administrator of the Institute of Neurophysiology and Psychophysiology, which began in 1963 on the Marseille campus of the National Center of Scientific Research (CNRS). Paillard retired from his university and CNRS positions in 1991 but he continued seminal research until his demise. Paillard advanced understanding of higher brain influences on human spinal motor mechanisms and the functional role of proprioception as revealed in patients deprived of such sensibility. He remains best known, however, for his work on human motor cognition. He reasoned that brain "maps" of the external world are constructed by the body's own movements and the central effects of their resulting central and peripheral feedback. He proposed two levels of interactive brain processing for the planning and/or execution of a reaching movement: 1) a sensorimotor level, using body posture as a key reference; and 2) a "higher" cognitive level for accurate movement performance, using learned representations of the position and shape of the environmental components, including the body, itself.
Asunto(s)
Encéfalo/fisiología , Movimiento/fisiología , Desempeño Psicomotor/fisiología , Percepción Espacial/fisiología , Cognición/fisiología , Francia , Historia del Siglo XX , Humanos , Actividad Motora/fisiología , Propiocepción/fisiologíaRESUMEN
Thought on the neural control of locomotion dates back to antiquity. In this article, however, the focus is more recent by starting with some major 17th century concepts, which were developed by René Descartes, a French philosopher; Thomas Willis, an English anatomist; and Giovanni Borelli, an Italian physiologist and physicist. Each relied on his personal expertise to theorize on the organization and control of movements. The 18th and early 19th centuries saw work on both the central and peripheral control of movement: the former most notably by Johann Unzer, Marie Jean-Pierre Flourens and Julien-Jean-César Legallois, and the latter by Unzer, Jirí Procháska and many others. Next in the 19th century, neurologists used human locomotion as a precise tool for characterizing motor pathologies: e.g., Guillaume Duchenne de Boulogne's description of locomotor ataxia. Jean-Martin Charcot considered motor control to be organized at two levels of the central nervous system: the cerebral cortex and the spinal cord. Maurice Philippson's defined the dog's step cycle and considered that locomotion used both central and reflex mechanisms. Charles Sherrington explained that locomotor control was usually thought to consist of a succession of peripheral reflexes (e.g., the stepping reflexes). Thomas Graham Brown's then contemporary evidence for the spinal origin of locomotor rhythmicity languished in obscurity until the early 1960s. By then the stage was set for an international assault on the neural control of locomotion, which featured research conducted on both invertebrate and vertebrate animal models. These contributions have progressively became more integrated and interactive, with current work emphasizing that locomotor control involves a seamless integration between central locomotor networks and peripheral feedback.
Asunto(s)
Locomoción/fisiología , Fenómenos Fisiológicos del Sistema Nervioso , Neurología/historia , Animales , Sistema Nervioso Central/fisiología , Vías Eferentes/fisiología , Historia del Siglo XIX , Historia del Siglo XX , Humanos , Sistema Nervioso Periférico/fisiologíaRESUMEN
This review summarized the contribution to neurobiology achieved through the use of invertebrate preparations in the second half of the 20th century. This fascinating period was preceded by pioneers who explored a wide variety of invertebrate phyla and developed various preparations appropriate for electrophysiological studies. Their work advanced general knowledge about neuronal properties (dendritic, somatic, and axonal excitability; pre- and postsynaptic mechanisms). The study of invertebrates made it possible to identify cell bodies in different ganglia, and monitor their operation in the course of behavior. In the 1970s, the details of central neural circuits in worms, molluscs, insects, and crustaceans were characterized for the first time and well before equivalent findings were made in vertebrate preparations. The concept and nature of a central pattern generator (CPG) have been studied in detail, and the stomatogastric nervous system (STNS) is a fine example, having led to many major developments since it was first examined. The final part of the review is a discussion of recent neuroethological studies that have addressed simple cognitive functions and confirmed the utility of invertebrate models. After presenting our invertebrate "mice," the worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster, our conclusion, based on arguments very different from those used fifty years ago, is that invertebrate models are still essential for acquiring insight into the complexity of the brain.
Asunto(s)
Sistema Nervioso Central/fisiología , Invertebrados/fisiología , Vías Nerviosas/fisiología , Neurobiología/historia , Neuronas/fisiología , Animales , Encéfalo/citología , Encéfalo/fisiología , Sistema Nervioso Central/citología , Ganglios de Invertebrados/citología , Ganglios de Invertebrados/fisiología , Historia del Siglo XX , Invertebrados/anatomía & histología , Modelos Animales , Neurobiología/tendenciasRESUMEN
Motor behaviors of some species, such as the rat and the human baby, are quite immature at birth. Here we review recent data on some of the mechanisms underlying the postnatal maturation of posture in the rat, in particular the development of pathways descending from the brain stem and projecting onto the lumbar enlargement of the spinal cord. A short-lasting depletion in serotonin affects both posture and the excitability of motoneurons. Here we try to extrapolate to human development and suggest that the abnormalities in motor control observed in childhood--e.g. deficits in motor coordination--might have their roots in the prenatal period, in particular serotonin depletion due to exposure to several environmental and toxicological factors during pregnancy.
Asunto(s)
Tronco Encefálico/crecimiento & desarrollo , Vías Eferentes/crecimiento & desarrollo , Movimiento/fisiología , Equilibrio Postural/fisiología , Médula Espinal/crecimiento & desarrollo , Animales , Tronco Encefálico/anatomía & histología , Diferenciación Celular/fisiología , Vías Eferentes/anatomía & histología , Humanos , Lactante , Recién Nacido , Neuronas Motoras/fisiología , Núcleos del Rafe/anatomía & histología , Núcleos del Rafe/crecimiento & desarrollo , Ratas , Serotonina/metabolismo , Médula Espinal/anatomía & histologíaRESUMEN
In both vertebrates and invertebrates, the elaboration of locomotion, and its neural control by the central nervous system, are extremely flexible. This is due not only to the network properties of relevant sets of central neurons, but also to the active participation of mutually co-operative central and peripheral loops of neural projections and activity. In this chapter, we describe experiments in which the above concepts have been advanced by comparing locomotor properties in the adult vs. neonatal rat preparation. Data obtained from the in vivo vs. in vitro preparation, and swimming vs. walking behavior, suggest that the locomotor pattern progressively exhibited after birth corresponds to successive steps in the maturation of locomotor networks. Our work emphasises that during the late pre- and early postnatal period, three distinct neural entities--segmental sensory input, descending pathways, and motoneurons--play a key role in the maturation of locomotion and its neural control. We propose that the neonatal rat preparation is an excellent model for studying the conversion from immature to adult locomotion. Some neural controls are more clearly demonstrable in the developing animal preparation than in the adult because the latter exhibits an array of complex and redundant adaptive mechanisms.
Asunto(s)
Envejecimiento/fisiología , Animales Recién Nacidos/fisiología , Actividad Motora/fisiología , Animales , Animales Recién Nacidos/crecimiento & desarrollo , Vías Eferentes/fisiología , Neuronas Motoras/fisiología , Ratas/fisiología , Sensación/fisiología , Médula Espinal/citología , Médula Espinal/fisiologíaRESUMEN
The central pattern generators (CPGs) for locomotion, located in the lumbar spinal cord, are functional at birth in the rat. Their maturation occurs during the last few days preceding birth, a period during which the first projections from the brainstem start to reach the lumbar enlargement of the spinal cord. The goal of the present study was to investigate the effect of suppressing inputs from supraspinal structures on the CPGs, shortly after their formation. The spinal cord was transected at the thoracic level at birth [postnatal day 0 (P0)]. We examined during the first postnatal week the capacity of the CPGs to produce rhythmic motor activity in two complementary experimental conditions. Left and right ankle extensor muscles were recorded in vivo during airstepping, and lumbar ventral roots were recorded in vitro during pharmacologically evoked fictive locomotion. Mechanical stimulation of the tail elicited long-lasting sequences of airstepping in the spinal neonates and only a few steps in sham-operated rats. In vitro experiments made simultaneously on spinal and sham animals confirmed the increased excitability of the CPGs after spinalization. A left-right alternating locomotor pattern was observed at P1-P3. Both types of experiments showed that the pattern was disorganized at P6-P7, and that the left-right alternation was lost. Alternation was restored after the activation of serotonergic 5-HT(2) receptors in vivo. These results suggest that descending pathways, in particular serotonergic projections, control the strength of reciprocal inhibition and therefore shape the locomotor pattern in the neonatal rat.
Asunto(s)
Indofenol/análogos & derivados , Actividad Motora/fisiología , Médula Espinal/fisiología , Animales , Animales Recién Nacidos , Axotomía , Electromiografía/efectos de los fármacos , Miembro Posterior/inervación , Miembro Posterior/fisiología , Técnicas In Vitro , Indofenol/farmacología , Región Lumbosacra , Actividad Motora/efectos de los fármacos , N-Metilaspartato/farmacología , Inhibición Neural/efectos de los fármacos , Inhibición Neural/fisiología , Periodicidad , Estimulación Física , Ratas , Ratas Wistar , Receptores de Serotonina/metabolismo , Serotonina/farmacología , Agonistas de Receptores de Serotonina/farmacología , Médula Espinal/efectos de los fármacosRESUMEN
Serotonin (5-HT) plays an important role both in the development and in the recovery of locomotion after spinalization in vertebrates. We investigated the contribution of the serotonergic system to the maturation of the lumbar motoneurons and networks in the neonatal rat. A 5-HT synthesis inhibitor, p-chlorophenylalanine (PCPA), was administered daily from the first postnatal day (P0) onward. This protocol depleted serotonin in the spinal cord within 3-4 d, as demonstrated by immunohistochemistry. PCPA-treated rats exhibited postural changes characterized by lesser flexion at the knee and ankle levels and lesser extension of the hip. Posture was asymmetric, suggesting possible deficits in the interlimb coordination. Intracellular recordings were made at P3-5 from motoneurons innervating different hindlimb muscles, using the in vitro brainstem-spinal cord-nerve-attached preparation. In PCPA-treated rats, the conduction velocity of motoneurons was increased, and their excitability was decreased (because of higher rehobase and input conductance) compared with sham animals. In accordance with postural observations, changes were more pronounced in hip extensor/knee flexor than in ankle extensor motoneurons. The maturation of repetitive firing properties was stopped by PCPA treatment, although PCPA, applied in vitro, had no effect on membrane properties. The spontaneous endogenously generated activity, which is a characteristic of immature networks, was increased in PCPA-treated rats, suggesting that developing lumbar networks are sensitive to 5-HT levels. Serotonin may play a critical role during development in regulating the balance between the excitability of motoneurons and that of interneurons. Interneuronal excitability is crucial for the activity-dependent development of spinal cord networks.
Asunto(s)
Actividad Motora , Neuronas Motoras/fisiología , Postura , Serotonina/fisiología , Médula Espinal/crecimiento & desarrollo , Animales , Animales Recién Nacidos , Articulación del Tobillo/fisiología , Conducta Animal , Inhibidores Enzimáticos/farmacología , Fenclonina/farmacología , Cadera/fisiología , Cinética , Rodilla/fisiología , Vértebras Lumbares , Conducción Nerviosa , Ratas , Ratas Wistar , Médula Espinal/anatomía & histología , Médula Espinal/fisiología , Triptófano Hidroxilasa/antagonistas & inhibidoresRESUMEN
Bicuculline is the most commonly used GABA(A) receptor antagonist to investigate the contribution of these receptors in motor control. However, this compound has been shown recently to potentiate the burst firing of neurons in various brain regions by blocking a calcium-activated potassium current underlying the spike after-hyperpolarization (AHP). This effect may distort our understanding of the role of GABA(A) receptors at the network level. In vitro brainstem-spinal cord preparations isolated from neonatal rats were used to compare the effects of bicuculline methiodide (bicuculline-M) and picrotoxin (PTX), another GABA(A) receptor antagonist, on the AHP of lumbar motoneurons as well as on spontaneous and locomotor-like motor activities. Intracellular recordings of lumbar motoneurons showed that bicuculline-M (20 microM) reduced the AHP to 57% of control whereas PTX (20-60 microM) had no significant effect. Bath-application of increasing concentrations of PTX caused an increase in spontaneous ventral root activity, which further increased significantly when bicuculline-M was added. The effects of both antagonists were tested on fictive locomotion. The left-right alternation was disrupted in the presence of bicuculline-M. A slow synchronous bursting activity of large amplitude also appeared in the presence of PTX. This slow rhythm was superimposed on a faster rhythm which still exhibited some degree of left-right alternation. These data demonstrate that bicuculline-M may not reveal accurately the contribution of GABA(A) receptors in motor control and the intrinsic properties of disinhibited networks.