Morphology of dendrite bundles in the cervical spinal cord of the rat: a light microscopic study

Structural and functional identification of two distinct inspiratory neuronal populations at the level of the phrenic nucleus in the rat cervical spinal cord

The diaphragm is driven by phrenic motoneurons that are located in the cervical spinal cord. Although the anatomical location of the phrenic nucleus and the function of phrenic motoneurons at a single cellular level have been extensively analyzed, the spatiotemporal dynamics of phrenic motoneuron group activity have not been fully elucidated. In the present study, we analyzed the functional and structural characteristics of respiratory neuron population in the cervical spinal cord at the level of the phrenic nucleus by voltage imaging, together with histological analysis of neuronal and astrocytic distribution in the cervical spinal cord. We found spatially distinct two cellular populations that exhibited synchronized inspiratory activity on the transversely cut plane at C4–C5 levels and on the ventral surface of the mid cervical spinal cord in the isolated brainstem–spinal cord preparation of the neonatal rat. Inspiratory activity of one group emerged in the central portion of the ventral horn that corresponded to the central motor column, and the other appeared in the medial portion of the ventral horn that corresponded to the medial motor column. We identified by retrogradely labeling study that the anatomical distributions of phrenic and scalene motoneurons coincided with optically detected central and medial motor regions, respectively. Furthermore, we anatomically demonstrated closely located features of putative motoneurons, interneurons and astrocytes in these regions. Collectively, we report that phrenic and scalene motoneuron populations show synchronized inspiratory activities with distinct anatomical locations in the mid cervical spinal cord.

Dendro-dendritic connections between motoneurons in the rat spinal cord: an electron microscopic investigation

The ultrastructural characteristics of identified dendrite bundles in the rat spinal cord were analyzed following retrograde tracing from the soleus muscles. Dendrite bundles are arranged in networks that are heavily interconnected by means of gap junctions. The bundles are formed by at least 10 crossing dendrites travelling in different focal planes. In between dendrites, elongated gap junctional complexes are frequently found. Dendrite lamellar bodies, recently described to occur in relation with gap junctions in the central nervous system were not observed in the present study.

Ultrastructure and synaptology of the nucleus ambiguus in the rat: the compact formation

The fine structure of the esophagomotor compact formation of the nucleus ambiguus was studied. Esophageal motoneurons are atypical in that they have extensive direct somato-somatic and somato-dendritic appositions without intervening glial processes. A unique feature is the presence of finger- and leaf-like somatic protrusions which partially wrap longitudinally oriented dendrites and, occasionally, small groups of dendrites and axons. The neuropil contains many longitudinally oriented, small-diameter dendrites of relatively uniform size (1.1 +/- 0.4 S.D. micrograms in diameter). Motoneuronal somatic profiles have 0-5 synapses per profile which represents a synaptic density of 10.6 synapses per soma. Axodendritic synapses measure 0.5 x 0.7 microgram in the transverse plane and are up to 3.0 micrograms long in the sagittal plane. Many axon terminals contact both a soma and dendrite in close apposition. Most axon terminals (> 90%) contain round vesicles and form asymmetric junctions with somata and dendrites. Axon terminal degeneration after electrolytic lesions and labelling after injection of wheat germ agglutinin-horseradish peroxidase in the nucleus of the tractus solitarius show that afferent connections to the compact formation form axodendritic synapses. The ultrastructure and synaptology of esophageal motoneurons is characterized by the close apposition of somata and dendrites (somatic-dendritic bundling), and the longitudinal orientation of dendrites (dendritic bundling), axons and axon terminals in the neuropil. These features may be important morphological substrates for synchronization and coordination of esophageal motoneuronal activity and esophageal peristalsis.

Brain stem localization of rodent esophageal premotor neurons revealed by transneuronal passage of pseudorabies virus

BACKGROUND/AIMS

Brain stem premotor neurons control swallowing through contacts with both afferent neurons and motoneurons. The location and connectivity of premotor neurons innervating the esophagus was determined using pseudorabies virus.

METHODS

In 30 rats, viral injections were made into either the cervical or subdiaphragmatic esophagus, cricothyroid muscle, or stomach. After a 48-62-hour survival, brain sections were processed immunocytochemically for the virus.

RESULTS

Neuronal labeling was limited to the compact formation of the nucleus ambiguus for survivals of 48-54 hours. At 57-62-hour survivals, virus-labeled second-order neurons (premotor neurons) were localized to the central subnucleus of nucleus of the solitary tract. Injections in the cricothyroid muscle and stomach resulted in distinct patterns of motoneuronal labeling in the nucleus ambiguus and dorsal motor nucleus and premotor neuronal labeling in the nucleus of the solitary tract.

CONCLUSIONS

Virus-labeled premotor neurons in the nucleus of the solitary tract occurred as a result of retrograde transport of the virus from the nucleus ambiguus because no viral antigen was present in the tractus solitarius. The esophagus is controlled by a central circuit whereby esophageal vagal afferents terminate on premotor neurons in the central subnucleus that in turn innervate esophageal motoneurons in the nucleus ambiguus.

Dendritic architecture of hypoglossal motoneurons projecting to extrinsic tongue musculature in the rat

The tracer, cholera toxin-horseradish peroxidase, was used to determine the dendritic architecture and organization of hypoglossal motoneurons in the rat. In 22 animals, the tracer was injected unilaterally into either the geniohyoid, genioglossus, hyoglossus, or styloglossus muscle. Within the hypoglossal nucleus, motoneurons innervating the extrinsic tongue muscles were functionally organized. Geniohyoid and genioglossus motoneurons were located within the ventrolateral and ventromedial subnuclei, respectively, while hyoglossus and styloglossus motoneurons were located within the dorsal subnucleus. Motoneurons located in all subnuclear divisions were found to have extensive dendrites that extended laterally into the adjacent reticular formation and medially to the ependyma. Less extensive extranuclear dendritic projections were found in the dorsal vagal complex and median raphe. Prominent rostrocaudal and mediolateral dendritic bundling was evident within the ventral subnuclei and dorsal subnucleus, respectively. Dendritic projections were also found extending inter- and intrasubnuclearly with a distinct pattern for each muscle. These data suggest that the varied and extensive dendritic arborizations of hypoglossal motoneurons provide the potential for a wide range of afferent contacts for, and interactions among, motoneurons that could contribute to the modulation of their activity. Specifically, the prominent dendritic bundling may provide an anatomic substrate whereby motoneurons innervating a specific muscle receive and integrate similar afferent input and are thus modulated as a functional unit. In contrast, the extensive intermingling of both inter- and intrasubnuclear dendrites within the hypoglossal nucleus may provide a mechanism for the coordination of different muscles, acting synergistically or antagonistically to produce a tongue movement.

Structural changes of the soleus and the tibialis anterior motoneuron pool during development in the rat

The morphological development of motoneuron pools of two hindlimb muscles of the rat, soleus (SOL) and tibialis anterior (TA), was studied in rats ranging in age between 8 and 30 postnatal days (P8-P30). Motoneurons were retrogradely labelled by injecting a cholera toxin B subunit solution directly into the muscles. This resulted in extensive labelling of motoneurons as well as their dendritic trees. The distribution of cross sectional areas of neuronal somata was determined for both muscles at various ages. Somal size increased considerably between P8 and P12, whereas growth was moderate between P12 and P20. The size distribution of SOL motoneurons was bimodal from P20, whereas the size distribution of TA motoneurons remained largely unimodal. The morphological development of the dendritic tree was studied qualitatively. The development of dendritic arborization within the SOL and the TA motoneuron pool showed major differences. The arborization pattern of dendrites of TA motoneurons was basically multipolar at all ages. In contrast, dendrites of SOL neurons tended to line up with the rostro-caudal axis and became organized in longitudinal bundles from P16 onwards. The relatively late appearance of dendrite bundles in the soleus motoneuron pool suggests that they might be related to the fine-tuning of neuronal activity rather than patterning of motor activity. The occurrence of dendrite bundles in SOL and not in TA motoneuron pools suggests that they may be related to the different afferent organization of this postural muscle or to its tonic activation pattern.

Organizational and morphological features of cat sacrocaudal motoneurons

We have investigated the organizational and morphological features of motoneurons from cat sacrocaudal spinal cord, the portion of the neuraxis that innervates the tail. This information is pertinent for development of a new model of spinal cord injury. An understanding of sacrocaudal circuitry is essential for physiological and behavioral assessment of the effects of sacrocaudal lesions. Observations from Nissl-stained sections corroborated Rexed's cytoarchitectural scheme. Putative motoneurons were located within two regions of the ventral horn: the ventromedial nucleus (lamina IX) and the nucleus commissuralis. To map motoneuron pools, cholera toxin-horseradish peroxidase conjugate was injected into each dorsal tail muscle. The dorsomedial muscle was innervated by ipsilateral nucleus commissuralis motoneurons. The dorsolateral and intertransversarius muscles were innervated by ipsilateral lamina IX and nucleus commissuralis motoneurons. Cell bodies of retrogradely labeled sacrocaudal motoneurons ranged from 22 to 82 microns in diameter; the unimodal distributions peaked between 45 and 50 microns. Dendritic trees of motoneurons, revealed by retrograde labeling or by intracellular injection with horseradish peroxidase, were extensive. Five to eight primary dendrites originated from the cell body. Dendritic branches extended throughout the ipsilateral ventral gray matter, with processes spreading into the surrounding white matter and the base of the dorsal horn. Dendrites from motoneurons with their soma in the lateral portion of lamina IX formed a longitudinal plexus at the gray/white border. Medial dendrites from motoneurons in the nucleus commissuralis formed bundles in the ventral gray commissure and spread throughout the contralateral ventral horn. It is speculated that contralateral dendrites subserve synchronized co-contraction of medial muscles from both sides of the tail.

Dendritic architecture of nucleus ambiguus motoneurons projecting to the upper alimentary tract in the rat

The motor innervation for palatal, pharyngeal, laryngeal, and esophageal muscles originates within the nucleus ambiguus. Although the viscerotopic organization of the upper alimentary tract in the nucleus ambiguus has been extensively studied, little information concerning the dendritic arborization of nucleus ambiguus motoneurons is available. The neural tracer cholera toxin-horseradish peroxidase, which is particularly effective at revealing dendrites of retrogradely labeled neurons, was used to determine the dendritic architecture and organization of nucleus ambiguus motoneurons. In 72 rats, cholera toxin-horseradish peroxidase in volumes of 1.0-18 microliters was directly applied under pressure to the musculature of various sites along the upper alimentary tract. Motoneurons innervating the soft palate, pharynx, cricothyroid muscle, and cervical esophagus were all found to have extensive dendrites that extended into the adjacent reticular formation with a distinct pattern for each muscle group. In contrast, the dendrites of motoneurons innervating the thoracic and subdiaphragmatic esophagus were confined to the compact formation of the nucleus ambiguus. Dendritic bundling within the confines of the nucleus ambiguus was prominent following injection of tracer into the soft palate, pharynx, and esophagus. The bundles were primarily oriented in a rostrocaudal direction. These data suggest that the extensive extranuclear dendritic arborization of motoneurons innervating the soft palate, pharynx, larynx, and cervical esophagus provide a wide ranging target for multiple central afferents that may be involved in the differential control of muscles that participate in multiple complex motor functions. The lack of extensive extranuclear dendrites of motoneurons innervating the distal esophagus suggest that they receive focused central afferents. The prominent bundling of dendrites within the nucleus ambiguus may provide for synchronization of motoneurons innervating a specific muscle and perhaps for synchronization of motoneurons innervating different muscles acting in sequence.

Representation of the cecum in the lateral dorsal motor nucleus of the vagus nerve and commissural subnucleus of the nucleus tractus solitarii in rat

Motor fibers of the accessory celiac and celiac vagal branches are derived from the lateral columns of the dorsal motor nucleus of the vagus nerve. These branches also contain sensory fibers that terminate within the nucleus of the tractus solitarii. This study traces the innervation of the intestines by using the tracer cholera toxin-horseradish peroxidase. In 53 rats, the tracer was injected into either the stomach, duodenum, jejunum, terminal ileum, cecum, or ascending colon. With all cecal injections, prominent retrograde labeling of cell bodies occurred bilaterally in the lateral columns of the dorsal motor nucleus of the vagus nerve above, at, and below the level of the area postrema. Dendrites of laterally positioned neurons projected medially and rostrocaudally within the dorsal motor nucleus of the vagus nerve and dorsomedially into both the medial subnucleus and parts of the commissural subnucleus of the nucleus of the tractus solitarii. Sensory terminal labeling occurred in the dorsolateral commissural subnucleus at the level of the rostral area postrema and the medial commissural subnucleus caudal to the area postrema. Additionally, there was sensory terminal labeling within a small confined area of the dorsomedial zone of the nucleus of the tractus solitarii immediately adjacent to the fourth ventricle at a level just anterior to the area postrema. Stomach injections labeled motoneurons of the medial column of the entire rostrocaudal extent of the dorsal motor nucleus of the vagus nerve and a sensory terminal field primarily in the subnucleus gelatinosus, with less intense labeling extending caudally into the medial and ventral commissural subnuclei. Dendrites of gastric motoneurons project rostrocaudally and mediolaterally within the dorsal motor nucleus of the vagus nerve and dorsolaterally within the nucleus of the tractus solitarii. They are most pronounced at the level of the rostral area postrema where many dendrites course dorsolaterally terminating primarily within the subnucleus gelatinosus. Injections of the duodenum labeled a small number of the cells within the medial aspects of the dorsal motor nucleus of the vagus nerve. Jejunal, ileal, and ascending colon injections labeled cells sparsely within the lateral aspects of the dorsal motor nucleus of the vagus nerve bilaterally. No afferent terminal labeling was evident after injection of these areas of the bowel.(ABSTRACT TRUNCATED AT 400 WORDS)