Musculotopic organization of the hypoglossal nucleus in the grass frog, Rana pipiens

Neural circuits underlying tongue movements for the prey-catching behavior in frog: distribution of primary afferent terminals on motoneurons supplying the tongue

The hypoglossal motor nucleus is one of the efferent components of the neural network underlying the tongue prehension behavior of Ranid frogs. Although the appropriate pattern of the motor activity is determined by motor pattern generators, sensory inputs can modify the ongoing motor execution. Combination of fluorescent tracers were applied to investigate whether there are direct contacts between the afferent fibers of the trigeminal, facial, vestibular, glossopharyngeal-vagal, hypoglossal, second cervical spinal nerves and the hypoglossal motoneurons. Using confocal laser scanning microscope, we detected different number of close contacts from various sensory fibers, which were distributed unequally between the motoneurons innervating the protractor, retractor and inner muscles of the tongue. Based on the highest number of contacts and their closest location to the perikaryon, the glossopharyngeal-vagal nerves can exert the strongest effect on hypoglossal motoneurons and in agreement with earlier physiological results, they influence the protraction of the tongue. The second largest number of close appositions was provided by the hypoglossal and second cervical spinal afferents and they were located mostly on the proximal and middle parts of the dendrites of retractor motoneurons. Due to their small number and distal location, the trigeminal and vestibular terminals seem to have minor effects on direct activation of the hypoglossal motoneurons. We concluded that direct contacts between primary afferent terminals and hypoglossal motoneurons provide one of the possible morphological substrates of very quick feedback and feedforward modulation of the motor program during various stages of prey-catching behavior.

Anatomical organization of brainstem circuits mediating feeding motor programs in the marine toad, Bufo marinus

The goal of our research has been to investigate the neuronal integration that coordinates feeding movements in the marine toad (genus Bufo). Using injections of fluorescein dextran amines, combined with activity-dependent uptake of sulforhodamine 101, peripheral hypoglossal and trigeminal nerves involved with tongue and jaw movements were labeled. We identified the rostrocaudal distribution of hypoglossal and trigeminal motor nuclei, and their sensory projections. We also identified the extent of neuronal networks for the medial reticular formation, the raphe nucleus, the glossopharyngeal nuclei, and the Purkinje cell layer of the cerebellum. The sensory fibers of the hypoglossal and trigeminal nerves were found projecting to the Purkinje cell layer of the cerebellum and the trigeminal motor nuclei. The activity-dependent sulforhodamine 101 uptake after the trigeminal and hypoglossal nerves stimulation labeled the bilateral hypoglossal motor nuclei, the trigeminal motor nuclei, the medial reticular formation nuclei, the raphe nuclei, the glossopharyngeal nuclei, and the Purkinje cell layer of the cerebellum, suggesting that all these neurons have the potential to be the components of feeding pathways. Taken together, these data are important for understanding the neuronal integration of extremely rapid jaw-tongue coordination during feeding in the marine toad.

Preservation of segmental hindbrain organization in adult frogs

To test for possible retention of early segmental patterning throughout development, the cranial nerve efferent nuclei in adult ranid frogs were quantitatively mapped and compared with the segmental organization of these nuclei in larvae. Cranial nerve roots IV-X were labeled in larvae with fluorescent dextran amines. Each cranial nerve efferent nucleus resided in a characteristic segmental position within the clearly visible larval hindbrain rhombomeres (r). Trochlear motoneurons were located in r0, trigeminal motoneurons in r2-r3, facial branchiomotor and vestibuloacoustic efferent neurons in r4, abducens and facial parasympathetic neurons in r5, glossopharyngeal motoneurons in r6, and vagal efferent neurons in r7-r8 and rostral spinal cord. In adult frogs, biocytin labeling of cranial nerve roots IV-XII and spinal ventral root 2 in various combinations on both sides of the brain revealed precisely the same rostrocaudal sequence of efferent nuclei relative to each other as observed in larvae. This indicates that no longitudinal migratory rearrangement of hindbrain efferent neurons occurs. Although rhombomeres are not visible in adults, a segmental map of adult cranial nerve efferent nuclei can be inferred from the strict retention of the larval hindbrain pattern. Precise measurements of the borders of adjacent efferent nuclei within a coordinate system based on external landmarks were used to create a quantitative adult segmental map that mirrors the organization of the larval rhombomeric framework. Plotting morphologically and physiologically identified hindbrain neurons onto this map allows the physiological properties of adult hindbrain neurons to be linked with the underlying genetically specified segmental framework.

Genioglossal hypoglossal muscle motoneurons are contacted by nerve terminals containing delta opioid receptor but not mu opioid receptor-like immunoreactivity in the cat: a dual labeling electron microscopic study

This study has investigated (1) the distribution of delta opioid receptor (DOR) or mu opioid receptor (MOR) containing elements in the hypoglossal nucleus of the adult cat; and (2) the association of these processes with retrogradely labeled genioglossus muscle motoneurons. Cholera toxin B conjugated to horseradish peroxidase (CTB-HRP) was injected into the genioglossus muscle on the right side of four isoflurane-anesthetized cats. Forty-four to 52 h later, the animals were sacrificed. Motoneurons containing HRP were labeled with a histochemical reaction utilizing tetramethylbenzidine (TMB) as the chromogen. The tissues were then processed for immunocytochemistry, using an antiserum raised against DOR or MOR using diaminobenzidine (DAB) as the chromogen. At the light microscopic level, retrogradely labeled cells were observed primarily ipsilaterally in ventral and ventrolateral subdivisions of the hypoglossal nucleus. The majority of these labeled cells were observed immediately caudal to obex. DOR-like immunoreactive processes were apparent at the light microscopic level in the hypoglossal nucleus, but MOR-like immunoreactive processes were not. Both DOR and MOR-like immunoreactive processes were observed in other brainstem areas such as the spinal trigeminal nucleus. At the electron microscopic level, DOR-like immunoreactive nerve terminals formed synaptic contacts with retrogradely labeled genioglossus muscle motoneuronal dendrites and perikarya in the hypoglossal nucleus. Nineteen (19) percent of the DOR terminals contacted retrogradely labeled genioglossus muscle motoneurons. DOR-immunoreactive terminals also synapsed on unlabeled dendrites and somata. Few MOR-like immunoreactive terminals were found at the EM level in the hypoglossal nucleus, and none of these terminals contacted retrogradely labeled neuronal profiles from the GG muscle. These are the first ultrastructural studies demonstrating synaptic interactions between functionally identified hypoglossal motoneurons and DOR terminals, and that enkephalins most likely act presynaptically to modulate the release of other neurotransmitters that affect GG motoneuron activity. These studies demonstrate that hypoglossal motoneurons which innervate the major protruder muscle of the tongue, the genioglossus muscle, are modulated by terminals containing DOR, and that enkephalins acting on DOR but not MOR in the hypoglossal nucleus may play a role in the control of tongue protrusion.

Quantitative morphological analysis of the motoneurons innervating muscles involved in tongue movements of the frogRana esculenta

We give an account of an effort to make quantitative morphological distinctions between motoneurons of the frog innervating functionally different groups of muscles involved in the movements of the tongue. The protractor, retractor, and inner muscles of the tongue were considered on the basis of their major action during the prey-catching behavior of the frog. Motoneurons were selectively labeled with cobalt lysin through the nerves of the individual muscles, and dendritic trees of successfully labeled neurons were reconstructed. Each motoneuron was characterized by 15 quantitative morphological parameters describing the size of the soma and dendritic tree and 12 orientation variables related to the shape and orientation of the dendritic field. The variables were subjected to multivariate discriminant analysis to find correlations between form and function of these motoneurons. According to the morphological parameters, the motoneurons were classified into three functionally different groups weighted by the shape of the perikaryon, mean diameter of stem dendrites, and mean length of dendritic segments. The most important orientation variables in the separation of three groups were the ellipses describing the shape of dendritic arborization in the horizontal, frontal, and sagittal planes of the brainstem. These findings indicate that characteristic geometry of the dendritic tree may have a preference for one array of fibers over another.

Distribution of hypoglossal motor neurons innervating the prehensile tongue of the African pig-nosed frog, Hemisus marmoratum

Using retrograde neuronal tracers, a study of the distribution of hypoglossal motor neurons innervating the tongue musculature was performed in the African pig-nosed frog, Hemisus marmoratum. This species is a radically divergent anuran amphibian with a prehensile tongue that can be aimed in three dimensions relative to the head. The results illustrate a unique rostrocaudal distribution of the ventrolateral hypoglossal nucleus and an unusually large number of motor neurons within this cell group. During the evolution of the long, prehensile tongue of Hemisus, the motor neurons innervating the tongue have greatly increased in number and have become more caudally distributed in the brainstem and spinal cord compared to other anurans. These observations have implications for understanding neuronal reconfiguring of motoneurons for novel morphologies requiring new muscle activation patterns.

The functional anatomy and evolution of hypoglossal afferents in the leopard frog, Rana pipiens

Previously, we suggested that afferents are present in the hypoglossal nerve of the leopard frog, Rana pipiens. The basis for this was behavioral data obtained after transection of the hypoglossal nerve. These afferents coordinate the timing of tongue protraction with mouth opening during feeding. The goal of the present study was to define anatomically these hypoglossal afferents in Rana pipiens. Retrograde tracing was performed using horseradish peroxidase, fluorescent dextran amines and neurobiotin. Data show that the cell bodies of hypoglossal afferents are located in the dorsal root ganglion of the third spinal nerve and enter the brainstem through its dorsal root. The afferents ascend in the dorsomedial funiculus and move laterally after they pass the obex. They project in the granular layer of the cerebellum and the medial reticular formation. The cervical afferents that travel in this pathway are known to carry proprioceptive and cutaneous sensory information. We hypothesize that the presence of afferents in the hypoglossal nerve is a derived characteristic of anurans, which has resulted from the re-routing of afferent fibers from the third spinal nerve into the hypoglossal nerve. The appearance of hypoglossal afferents coincides with the morphological acquisition of a highly protrusible tongue.

Subcompartmental organization of the ventral (protrusor) compartment in the hypoglossal nucleus of the rat

The extent and myotopic organization of the ventral (protrusor) compartment of the hypoglossal nucleus (nXII) in the rat is controversial. Of particular concern is the location of motoneurons that innervate the intrinsic (verticalis, transversus) as compared to extrinsic (genioglossus) tongue protrusor muscles. These issues were investigated with retrograde transport, lesion/degeneration/immunocytochemical, and classic Golgi staining techniques. Results from these experiments demonstrate the following: (1) the ventral compartment extends the entire rostrocaudal length of nXII and is organized into three longitudinally oriented subcompartments, one medial and one lateral within the boundaries of nXII, and one outside the confines of nXII, defined as the lateral accessory subcompartment; 2) the medial and lateral subcompartments contain motoneurons that innervate the intrinsic (verticalis, transversus) and extrinsic (genioglossus) tongue protrusor muscles, respectively, while the lateral accessory subcompartment innervates the geniohyoid muscle; (3) ventral subcompartments are unequal in size and vary along the rostrocaudal dimension of nXII. The medial subcompartment is largest caudally and smallest rostrally, while the converse is true for the lateral subcompartment. By contrast, the lateral accessory subcompartment is present only along the caudal one-half of nXII; (4) medial and lateral subcompartments are further organized into smaller subgroups. Medial and centromedial subgroups are discernible within the medial subcompartment, lateral and centrolateral subgroups within the lateral subcompartment. Both medial and lateral subgroups extend throughout the rostrocaudal length of nXII, whereas the centromedial and centrolateral subgroups are present only along the middle two-thirds of nXII where they form a central motoneuron band; (5) there is an inverse myotopic organization within the medial and lateral subcompartments such that proximal and distal portions of intrinsic and extrinsic protrusor muscles receive innervation from rostral and caudal motoneurons, respectively; and (6) there is a correlation between motoneuron morphology (size, shape and dendritic field domains), subcompartment localization, and myotopic specificity. Motoneurons in the medial subcompartment are small (mean = 23.08 microns), round to globular, with dendrites oriented medially, dorsomedially, dorsolaterally, and caudally, whereas lateral subcompartment motoneurons are large (mean = 29.49 microns), round to triangular, with dendrites directed mainly mediolaterally and dorsally. These data are relevant to understanding the functional organization of nXII and the motor control of the tongue. Results are further discussed relative to the convergence of multifunctional afferent systems in the ventromedial subcompartment of nXII.

Neural organization of the ventilatory activity in the frog, Rana catesbeiana. II

The paralyzed, decerebrate frog, Rana catesbeiana, displays "fictive" oropharyngeal and pulmonary ventilations. In order to evaluate the neuronal correlates of these two centrally programmed ventilatory bursting patterns, we have performed intra- and extracellular recordings of bulbar respiratory neurons in this fictively breathing preparation. A total of 123 respiratory neurons were recorded from the caudal medulla. Of 51 antidromically activated neurons, 20 were vagal motoneurons and 31 were hypoglossal motoneurons. Respiratory neurons that depolarized during the lung (L) or non-lung (N) ventilatory phases were classified as L or N neurons, respectively. Phase spanning neurons (S) were active during both L and N phases. Some neurons showed oscillations of membrane potential synchronous with oropharyngeal ventilation. Those active during the buccal elevation phase were exclusively L neurons, whereas those having buccal depressor activity were exclusively N neurons. Synaptic drive potentials were observed in all neurons recorded intracellularly. In some neurons, hyperpolarization was caused by inhibitory postsynaptic potentials, as demonstrated by reversal of membrane potential trajectory after intracellular chloride iontophoresis. Some individual motoneurons and interneurons exhibited both pulmonary and buccal ventilatory activity, indicating that both pattern generators project to a common motor control system.

Musculotopic organization of the hypoglossal nucleus in the cynomolgus monkey, Macaca fascicularis

The movements of the tongue in feeding and vocalization are enabled by a complex system of interdigitated muscle fibers in the tongue body. Because of this complexity, the detailed anatomical connections between individual intrinsic tongue muscles and corresponding motoneurons in the hypoglossal nucleus have not been described for any mammal. In this study we describe the distribution of retrogradely labeled neurons in the hypoglossal nucleus, following injections of wheat-germ agglutinin-horseradish peroxidase into different regions of the tongue of 21 cynomolgus monkeys. These experiments demonstrate a spatial organization of hypoglossal motoneurons that reflects the anatomical and functional organization of tongue body muscles: motoneurons innervating the transversus and verticalis muscles are located in medial hypoglossal nucleus regions, motoneurons innervating the genioglossus are located in intermediate hypoglossal nucleus regions, motoneurons innervating the hyoglossus and inferior longitudinalis are located in ventrolateral hypoglossal nucleus regions, and motoneurons innervating the styloglossus and superior longitudinalis are located in dorsolateral hypoglossal nucleus regions. Motoneurons innervating the suprahyoid muscle, the geniohyoid, are situated in a cell column separated ventrally from the main body of the hypoglossal nucleus. Motoneurons innervating the palatoglossus are located in the nucleus ambiguus and, possibly, in dorsolateral hypoglossal nucleus regions. Motoneurons of the medial divisions of the hypoglossal nucleus innervate tongue muscles that are oriented in planes transverse to the long axis of the tongue whereas motoneurons of the lateral divisions innervate tongue muscles that are oriented parallel to this axis. These results suggest that the segregation of motoneurons corresponds to the functional distinction between tongue protrusion and retrusion.