Neuronal analysis of pharyngeal peristalsis in the gastropod Navanax in terms of identified motoneurons innervating identified muscle bands. II. Radial and circumferential motor fields

The neuronal basis of pharyngeal ingestion and peristalsis was studied in the gastropod Navanax inermis. Radially and circumferentially oriented muscles produce expansion and constriction of the pharynx. Motor fields of 11 identified radial motoneurons and 13 identified circumferential motoneurons were determined with respect to circumferential and longitudinal muscle band coordinates by muscle movements, electromyography, antidromic stimulation and axonal anatomy. Activation of these identified motoneurons can account for all the elemental pharyngeal movements observed during feeding. Four motoneurons, each innervating most of radial muscle, can mediate ingestion. Three radial motoneurons with anterior motor fields can mediate anterior expansion during sealing of the pharyngeal lips around prey and during regurgitation. Ten circumferential motoneurons have small arciform motor fields, the distributions of which correspond to the regional specializations in circumferential band organization. Arciform constriction can center eccentric ingested prey within the pharyngeal lumen during peristalsis. Arciform constrictions could combine to form an annular constriction in peristalsis. Small, non-overlapping, circumferential motor fields maximize the number of independent annular units available to produce a fine peristaltic wave. Sphincters have more circumferential motoneurons with smaller motor fields; this innervation permits finer motor control. Radial motoneurons with posterior motor fields can produce expansion caudal to a circumferential constriction during peristalsis. Motor fields of regional radial motoneurons show greater interanimal variability than circumferential motor fields, which is correlated with a less essential role of radial motoneurons in peristalsis. Two circumferential motoneurons with giant posterior pharyngeal motor fields can mediate pharyngeal emptying either in swallowing or in regurgitation.     

Neuronal analysis of pharyngeal peristalsis in the gastropod Navanax in terms of identified motoneurons innervating identified muscle bands. II. Radial and circumferential motor fields

The neuronal basis of pharyngeal ingestion and peristalsis was studied in the gastropod Navanax inermis. Radially and circumferentially oriented muscles produced expansion and constriction of the pharynx. Motor fields of 11 identified radial motoneurons and 13 identified circumferential motoneurons were determined with respect to circumferential and longitudinal muscle band coordinates by muscle movements, electromyography, antidromic stimulation and axonal anatomy. Activation of these identified motoneurons can account for all the elemental pharyngeal movements observed during feeding. Four motoneurons, each innervating most of radial muscle, can mediate ingestion. Three radial motoneurons with anterior motor fields can mediate anterior expansion during sealing of the pharyngeal lips around prey and during regurgitation. Ten circumferential motoneurons have small arciform motor fields, the distributions of which correspond to the regional specializations in circumferential band organization. Arciform constriction can center eccentric ingested prey within the pharyngeal lumen during peristalsis. Arciform constrictions could combine to form an annular constriction in peristalsis. Small, non-overlapping, circumferential motor fields maximize the number of independent annular units available to produce a fine peristaltic wave. Sphincters have more circumferential motoneurons with smaller motor fields; this innervation permits finer motor control. Radial motoneurons with posterior motor fields can produce expansion caudal to a circumferential constriction during peristalsis. Motor fields of regional radial motoneurons show greater interanimal variability than circumferential motor fields, which is correlated with a less essential role of radial motoneurons in peristalsis. Two circumferential motoneurons with giant posterior pharyngeal motor fields can mediate pharyngeal emptying either in swallowing or in regurgitation.     

Innervation of the larynx,pharynx, and upper esophageal sphincter of the rat

We identified a ‘semicircular’ compartment of the rat thyropharyngeus muscle at the pharyngoesophageal junction and used the glycogen depletion method to determine how the fibers of this muscle (as well as all others of the pharynx and larynx) are innervated by different cranial nerve branches. The semicircular compartment appears anatomically homologous to the human cricopharyngeus muscle, an important component of the upper esophageal sphincter. While we found very little overlap in the muscle targets of the pharyngeal, superior laryngeal and recurrent laryngeal nerves within the pharynx and larynx, the semicircular muscle receives a dual, interdigitating innervation from two vagal branches: the pharyngeal nerve and a branch of the superior laryngeal nerve we call the dorsal accessory branch. After applying horseradish peroxidase to either of these two nerves, we compared the distribution and number of cells labeled in the brainstem. The dorsal accessory branch conveys a more heterogeneous set of efferent fibers than does the pharyngeal nerve, including the axons of pharyngeal and esophageal motor neurons and parasympathetic preganglionic neurons. The observed distribution of labeled motor neurons in nucleus ambiguus also leads us to suggest that the semicircular compartment is innervated by two subsets of motor neurons, one of which is displaced ventrolateral to the main pharyngeal motor column. This arrangement raises the possibility of functional differences among semicircular compartment motor neurons correlated with the observed differences in brainstem location of cell bodies. © 1994 Wiley-Liss, Inc.     

Identification of motor neurons that contain a FMRFamidelike peptide and the effects of FMRFamide on longitudinal muscle in the medicinal leech, Hirudo medicinalis

Excitatory motor neurons in the leech are cholinergic. By using a combination of intracellular Lucifer yellow injection and indirect immunofluorescence, we localized FMRFamidelike immunoreactivity to a number of the motor neurons innervating longitudinal and dorsoventral muscle in the leech. All excitatory motor neurons innervating longitudinal muscle (cells 3, 4, 5, 6, 8, L, 106, 107, 108) were labeled with an antiserum to FMRFamide, while the inhibitory motor neurons innervating longitudinal muscle (cells, 1, 2, 7, 9, 102) were not. The excitatory motor neuron innervating medial dorsoventral muscle (cell 117) was labeled, while the excitatory motor neuron innervating lateral dorsoventral muscle (cell 109) was not. The inhibitory motor neuron innervating dorsoventral muscle (cell 101) was also labeled. Nerve terminals along dorsoventral muscle were also labeled with the antiserum. FMRFamide was bath applied to strips of longitudinal muscle while recording tension, and the muscle's response was compared to its response to the previously identified neuromuscular transmitter ACh. Brief applications of FMRFamide caused a contraction approximately one-tenth as large as that caused by an equimolar amount of ACh. The muscle response to FMRFamide was unaffected by curare. During extended exposures, FMRFamide caused a maintained contraction in longitudinal muscle without any apparent desensitization of the FMRFamide receptors and occasionally triggered an irregular myogenic rhythm. This extended exposure to FMRFamide caused a post-exposure potentiation of the longitudinal muscle's response to ACh that shorter applications of FMRFamide did not. Thus FMRFamide may act as a transmitter or modulator in cholinergic motor neurons innervating longitudinal and dorsoventral muscles in the leech.     

Serotonin activates overall feeding by activating two separate neural pathways in Caenorhabditis elegans

Food intake in the nematode Caenorhabditis elegans requires two distinct feeding motions, pharyngeal pumping and isthmus peristalsis. Bacteria, the natural food of C. elegans, activate both feeding motions (Croll, 1978; Horvitz et al., 1982; Chiang et al., 2006). The mechanisms by which bacteria activate the feeding motions are largely unknown. To understand the process, we studied how serotonin, an endogenous pharyngeal pumping activator whose action is triggered by bacteria, activates feeding motions. Here, we show that serotonin, like bacteria, activates overall feeding by activating isthmus peristalsis as well as pharyngeal pumping. During active feeding, the frequencies and the timing of onset of the two motions were distinct, but each isthmus peristalsis was coupled to the preceding pump. We found that serotonin activates the two feeding motions mainly by activating two separate neural pathways in response to bacteria. For activating pumping, the SER-7 serotonin receptor in the MC motor neurons in the feeding organ activated cholinergic transmission from MC to the pharyngeal muscles by activating the Gsα signaling pathway. For activating isthmus peristalsis, SER-7 in the M4 (and possibly M2) motor neuron in the feeding organ activated the G(12)α signaling pathway in a cell-autonomous manner, which presumably activates neurotransmission from M4 to the pharyngeal muscles. Based on our results and previous calcium imaging of pharyngeal muscles (Shimozono et al., 2004), we propose a model that explains how the two feeding motions are separately regulated yet coupled. The feeding organ may have evolved this way to support efficient feeding.     

Synaptic organization of expansion motoneurons ofNavanax inermis

The opisthobranch mollusc, Navanax, feeds by rapid pharyngeal expansion that sucks in prey followed by peristaltic swallowing that moves prey into the esophagus. Several identifiable neurons on the ventral surface of the buccal ganglia control radial musculature within the pharyngeal wall, contraction of which leads to pharyngeal expansion. These are considered expansion motoneurons because their axons run into the muscle and twitches and EMGs occur one for one with action potentials. The motoneurons are electrotonically coupled. Electrotonic PSPs, the components of spread associated with impulses, can summate with subthreshold DC depolarizations to yield synchronous impulses in coupled cells. During a train of responses the later electrotonic PSPs can be facilitated because of increase in amplitude and duration of the presynaptic impulses. Expansion motoneurons are synaptically connected by two apparently interneuronal pathways: a low threshold pathway activated by subthreshold depolarization of the two largest expansion motoneurons (the G-cells) that inhibits the entire population, and a high threshold pathway that is activated by a train of G-cell impulses and produces largely excitatory PSPs in the smaller expansion motoneurons and an EPSP--IPSP sequence in the G-cells. Coupling among expansion motoneurons can be abolished by chemical inhibitory synaptic inputs that are activated by electrical stimulation of the pharyngeal nerve or tactile stimulation of the pharyngeal wall. This uncoupling phenomenon can be explained by a simple equivalent circuit in which inhibitory synapses along the coupling pathway short circuit electrotonic spread. Uncoupling can outlast the evoking stimulus by several seconds. During uncoupling the smaller expansion motoneurones can fire independently while the G-cell is inhibited, and impulses still propagate from somata to the periphery. The expansion motoneuron population receives excitatory input from the mechanoreceptors in protractor muscles. Mechanical stimulation of the pharyngeal wall activates primary sensory neurons in the buccal ganglia that fire during excitation and during inhibition and uncoupling of expansion motoneurons.     

The localization of the motor neurons innervating the extraocular muscles in the oculomotor nuclei of the cat and rabbit,using horseradish peroxidase

The localization of the motor neurons innervating the extraocular muscles in the oculomotor nuclei of adult cats and rabbits was investigated by means of retrograde labelling with horseradish peroxidase (HRP). The groups consisting of the motor neurons innervating an individual muscle lay in the nucleus as elongated columns extending in a longitudinal direction. The position of each group in the transverse section varied according to the rostro-caudal level of the nucleus. In the cat and rabbit, entire contralateral innervation of the superior rectus and entire ipsilateral innervation of three muscles of the inferior rectus, medial rectus and inferior oblique were similarly observed. However, the arrangement of individual motor groups differed considerably in both animals except for the group innervating the inferior rectus which was generally found in the ventral position running through the rostral two-thirds of the oculomotor nucleus. In the case of cats, the central caudal nucleus bilaterally innervated the levator palpebrae superioris. The motor neurons innervating this muscle in the rabbit (which lacks the central caudal nucleus) formed a rostro-caudal club-shaped column close to the group innervating the superior rectus. The aberrant cellular mass in the adjoining medial longitudinal fasciculus which belongs to the medial rectus appears to play an important role in the eye movement, because it commonly appears in various animals.     

Control of the central swallowing program by inputs from the peripheral receptors. A review

Swallowing is a complex motor sequence, usually divided into a buccopharyngeal stage (coordinated contractions of several muscles of the mouth, pharynx and larynx) and an esophageal stage, called primary peristalsis. This motor sequence depends on the activity of medullary interneurons belonging to the swallowing center which program through excitatory and inhibitory connections the sequential excitation of motoneurons and vagal preganglionic neurons responsible for the whole motor sequence. The activity of the medullary swallowing neurons can occur without feedback phenomena: it is truly a central activity indicating that swallowing depends on a central network which may function without afferent support. However, the swallowing neurons receive a strong afferent input suggesting the involvement of sensory feedbacks during swallowing. The swallowing neurons present a short latency activation on electrical stimulation of the peripheral afferent fibers supplying the region of the tract which is under their control. In addition, the neurons are activated by localized distensions of the swallowing tract, this distension having to be done more and more distally when the neuronal discharge occurs later and later during swallowing. Furthermore the swallowing discharge of the central neurons is increased either when a bolus is swallowed or during a slight distension of the corresponding region of the tract. Thus, under physiological conditions, swallowing neurons receive sensory information from pharyngeal and esophageal receptors and the central program may be modified by peripheral afferents that adjust the motor sequence to the size of the swallowed bolus. The inputs from the peripheral receptors can also exert inhibitory effects depending on the central connections between the swallowing neurons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Expression of diverse neuropeptide cotransmitters by identified motor neurons in Aplysia

Neuropeptide synthesis was determined for individual identified ventral-cluster neurons in the buccal ganglia of Aplysia. Each of these cells was shown to be a motor neuron that innervates buccal muscles that generate biting and swallowing movements during feeding. Individual neurons were identified by a battery of physiological criteria and stained with intracellular injection of a vital dye, and the ganglia were incubated in 35S-methionine. Peptide synthesis was determined by measuring labeled peptides in extracts from individually dissected neuronal cell bodies analyzed by HPLC. Previously characterized peptides found to be synthesized included buccalin, FMRFamide, myomodulin, and the 2 small cardioactive peptides (SCPs). Each of these neuropeptides has been shown to modulate buccal muscle responses to motor neuron stimulation. Two other peptides were found to be synthesized in individual motor neurons. One peptide, which was consistently observed in neurons that also synthesized myomodulin, is likely to be the recently sequenced myomodulin B. The other peptide was observed in a subset of the neurons that synthesize FMRFamide. While identified motor neurons consistently synthesized the same peptide(s), neurons that innervate the same muscle often express different peptides. Neurons that synthesized the SCPs also contained SCP-like activity, as determined by snail heart bioassay. Our results indicate that every identified motor neuron synthesizes a subset of these methionine-containing peptides, and that several neurons consistently synthesize peptides that are likely to be processed from multiple precursors.     

The specificity of re-innervation by identified sensory and motor neurons in the leech

Re-innervation of skin and muscle by identified sensory and motor neurons in segmental ganglia of the leech was studied using physiological techniques. After lesions of peripheral nerves, sensory axons which re-innervated the skin always regained sensitivity to their original stimulus modality (touch, pressure or noxious stimuli). Motor neurons invariably re-innervated the appropriate type of body wall muscle, such as longitudinal or circular muscle layers. Both sensory and motor axons usually returned to the appropriate region of the body wall (dorsal, lateral, or ventral) when regenerating after a nerve crush or cut. This capacity was lost, however, when growth along old nerve branches was prevented by evulsing long segments of the nerve. Re-innervation usually occurred by way of growth of new axons all the way the periphery, but in a few cases reconnection with the surviving distal segment of the original axon had taken place. The specificity of reinnervation can be accounted for by a combination of selective growth along appropriate nerve branches and specific interactions with target tissues.     

Sonic/vocal motor pathways in squirrelfish (Teleostei, Holocentridae)

Similar to many teleost fish, squirrelfish (family Holocentridae) produce vocalizations by the contraction of muscles that lead to vibration of the swimbladder. We used biotinylated compounds to identify the position and extent of vocal motor neurons in comparison to additional motor neuron groups, namely those of red and white dorsal epaxial muscle and opercular muscle that are located adjacent to or near the sonic muscle. The sonic motor nucleus (SMN) was located in the caudal medulla and rostral spinal cord in a ventrolateral position with dendrites extending dorsally in a dense bundle along the lateral edge of the medulla and axons exiting via ventral occipital nerve roots. Transneuronal transport of biocytin identified premotor neurons within the SMN and in the medially adjacent reticular formation that projected to the contralateral SMN and more rostrally to the octavolateralis efferent nucleus and nucleus praeeminentialis, suggesting interactions between vocal and octavolateralis systems as seen in other teleosts. Motor neurons innervating the red and white dorsal muscle formed a loose aggregate in the dorsal motor column, adjacent to the medial longitudinal fasciculus, sending fibers bilaterally throughout the spinal cord with axons exiting via ventral spinal nerve roots. Opercular motor neurons were located within the facial motor nucleus. The anatomical characteristics of the SMN of squirrelfish, a representative member of the order Beryciformes, are similar to those of representative members of the closely related order Scorpaeniformes, but diverge from the SMN of more distantly related orders of paracanthopterygiian and ostariophysan teleosts. These results therefore suggest a possible homology among the SMNs of acanthopterygiian fishes.     

Synaptic connections of buccal mechanosensory neurons in the opisthobranch mollusc, Navanax inermis

Mechanical stimulation of various areas of the pharyngeal wall and lips can produce EPSPs and IPSPs, as well as abruptly rising impulses, in primary sensory cells. IPSP fields are generally larger than EPSP fields and these fields are distributed without obvious order around fields from which afferent spikes are evoked. Apparently monosynaptic excitatory and inhibitory contacts are formed between primary sensory neurons. These synapses are blocked by high Mg2+ indicating chemical transmission. IPSPs are inverted by Cl- injection. Excitatory inputs can be electrically far from the soma. Sensory cells form apparently monosynsptic excitatory or inhibitory contacts on motoneurons mediating pharyngeal expansion. Brief sensory excitation can initiate sustained firing within this neuronal population and sustained synaptic activity in motoneurons. Interactions of sensory neurons may be important in information processing and in generating motor paterns. These neurons serve both primary sensory and interneuronal functions.     

Morphology of identified preganglionic neurons in the dorsal motor nucleus of the vagus.

To determine the degree of variation of neuronal morphology both within and between the subnuclei of the dorsal motor nucleus of the vagus (dmnX), structural features of the preganglionic neurons of each of the five primary subnuclei in the rat dmnX were characterized quantitatively. Each of the columnar subnuclei was separately labeled by application of the retrograde tracer fast blue to its corresponding subdiaphragmatic vagal branch. Fixed brain slices of 100 microns thickness were then prepared in coronal, sagittal, and horizontal orientations. Next, randomly selected fast blue labeled neurons (n = 1,256) were injected with Lucifer yellow, drawn with camera lucida, and digitized. For each cell, three features of the perikaryon and twelve of the dendritic tree were measured. Dorsal motor nucleus neurons with up to eight primary dendrites, 30 dendritic segments, and seventh order dendritic branches were observed. Throughout the dmnX, the dendrites of preganglionic neurons were preferentially oriented in the horizontal plane. Consistent with an organizing role for the columnar subnuclei, most dendrites remained within their column of origin. However, between 5 and 30% of the neurons in each of the columns projected dendrites into adjacent dmnX subnuclei or other brainstem nuclei, including the nucleus of the solitary tract (NTS). The cyto- and dendroarchitectural analyses revealed systematic gradations in morphology, although they did not support the idea that the dmnX was composed of multiple distinct preganglionic types. The most parsimonious interpretation of the data is that dmnX motorneurons are variants of a single prototype, with dendrites varying widely in length and degree of ramification. The extent of an individual preganglionic neuron's dendritic field was predicted by three factors: the cell's rostrocaudal position within the dmnX, its location within a transverse plane (i.e., its coronal position within or ectopic to the dmnX), and its subnucleus of origin. Neurons at rostral and midlongitudinal levels of each column had more extensive dendritic arbors than those at caudal levels. Ectopic neurons had more extensive dendritic fields than similar cells in the corresponding columns; in fact, of all vagal preganglionic neurons, ectopics had the most extensive dendritic fields. Somata and dendrites of celiac column neurons were more extensive than those of hepatic and gastric column cells. These differential regional distributions of vagal preganglionics suggest that their structure and function are correlated.     

Organization of motor pools supplying the cervical musculature in a cryptodyran turtle, Pseudemys scripta elegans. II. Medial motor nucleus and muscles supplied by two motor nuclei

In this paper we describe the medial motor nucleus of Pseudemys cervical spinal cord and the motor pools of three neck muscles that exhibit an unusual pattern of innervation. Cells of the medial motor nucleus form a longitudinal column at the dorsomedial gray/white border of the ventral horn from C1 through C8. In Nissl-stained transverse sections they appear fusiform with prominent medially projecting dendrites; in HRP material these dendrites are seen to cross into the contralateral ventral funiculus. Medial nuclear cells vary in size (12-31 micron in diameter) and are often relatively large (greater than 21 micron in diameter). They are significantly larger and more numerous in caudal than in rostral cervical segments. Medial nuclear cells supply three of the cervical muscles examined in this study: mm. retrahens capitis collique (RCCQ), testocervicis, and longus colli. These three muscles differ from other cervical muscles in Pseudemys and from vertebrate limb muscles in that they are supplied in parallel by two populations of motor neurons: the medial and ventral motor nuclei (cf. Yeow and Peterson, '86). Ventral nuclear cells supplying these three muscles are organized into a musculotopic pattern with m. testocervicis motor neurons most medial and m. RCCQ motor neurons lateral; in contrast, the location of medial nuclear motor neurons is invariant with respect to muscle position. HRP-positive medial nuclear cells are sometimes smaller (m. testocervicis) but more often are as large or larger (mm. RCCQ and longus colli) than ventral nuclear cells supplying the same muscles, thus suggesting that they supply extrafusal muscle fibers, perhaps different muscle unit types in the three muscles. Based on the morphology of medial nuclear cells and the probable actions of the muscles they innervate, we hypothesize that the medial motor nucleus may represent a discrete functional system for producing bilaterally synchronous muscle activation, and that this system is accessed by a subset of muscles in the cervical complex.     

Formation of electrical connections between cultured identified neurons and muscle fibers of the snail Helisoma

We studied the formation of connections between identified neurons removed from the buccal ganglion of the snail Helisoma and muscle fibers dissociated from the buccal mass. Three types of identified neurons--B19, B5, and B4--were placed into cell culture and muscle fibers from the supralateral tensor muscle (SLT), normally innervated by B19, were subsequently plated adjacent to the neuronal cell bodies. Growth cones from the neurons contacted the muscle fibers within 6-12 h after isolation. Simultaneous intracellular recordings from the neuronal cell bodies and muscle fibers after 4 days in culture indicated that the neurons had formed electrical connections with the fibers. All 3 types of neurons coupled to the muscle fibers but displayed differing probabilities and strengths of connections. The role of growth cone contact in the formation of these connections was tested by plating muscle fibers onto fields of neurites after neuronal growth had stopped. Under these conditions, neurons still became electrically coupled to the muscle fibers, but the strength of these connections differed from those formed by neurons and fibers that were plated simultaneously. Thus, quantitative characteristics of electrical connections formed between cultured Helisoma neurons and dissociated muscle fibers are influenced by neuronal identity and the timing of neuronal contacts.     

Quantitative Golgi study of anatomically identified subdivisions of motor thalamus in the rat

A Golgi study of neurons in the ventroanterior-ventrolateral complex (VAL) and ventromedial (VM) nucleus in the dorsal thalamus of rats was performed. To facilitate the delineation of subdivisions of these nuclei, some animals received injections of horseradish peroxidase (HRP) into the afferent and efferent fields of VAL and VM, and alternate sections were processed for the histochemical detection of HRP. As an adjunct to subjective observations, a multivariate statistical analysis of morphometric variables was performed to provide an objective assessment of neuronal morphology. All Golgi-stained neurons in VAL and VM were tentatively identified as projection neurons; no cells with morphological features commonly ascribed to thalamic interneurons were impregnated. Four classes of morphologically distinct neurons were identified in VAL. Type 1 neurons, the most commonly impregnated cell, were found throughout the extent of VAL and resembled "tufted" or "multipolar bush" neurons described previously in many thalamic nuclei. The remaining three neuronal types differed in a number of morphometric parameters and were differentially distributed throughout VAL. Type 2 neurons, distinguished in part by dendritic spine morphology and elongated bipolar dendritic fields, were found only in the rostral sector of the dorsal division of VAL (VALD). Type 3 neurons, characterized by a large and evenly distributed dendritic field, were situated in rostral VAL (all subdivisions). Type 4 neurons had small soma and dendritic dimensions and were located in the ventromedial aspect of the ventral division of VAL (VALV) adjacent to VM. In contrast, the vast majority of neurons in VM were considered to be a single morphological class (similar in form to type 4 neurons in VAL), although a rarely impregnated second type of neuron was also observed. The apparent scarcity of interneurons in VAL and VM is consistent with previous evidence that the synaptic organization of motor thalamus in the rat is markedly different from that of higher-order mammals. Speculation about the functional attributes of the neuronal types in VAL and VM is necessarily restricted to considerations of afferent and efferent relations, since "motor modality" functions of neurons in these nuclei have yet to be elucidated.     

Morphological identification of the motor neurons innervating the dorsal longitudinal flight muscle of Drosophila melanogaster

The dorsal longitudinal flight muscle (DLM) of Drosophila is composed of six muscle fibers (DLM1-6 from ventral to dorsal) and innervated by five motor neurons (MNs), DLM1-4 being innervated singly by MN1-4, and DLM5 and 6 being jointly innervated by MN5. This study identifies and describes the five motor neurons that innervate these six muscle fibers. Horseradish peroxidase (HRP) applied intracellularly to single identified DLM fibers resulted in the labeling of the single motor neuron innervating that muscle fiber by retrograde transsynaptic transport. This method allowed positive identification of the motor neuron innervating a particular muscle fiber, since only the innervating neuron was labeled. The axonal pathway, soma, and dendritic distribution of each labeled motor neuron was traced in the thoracic ganglion, and their relative positions were determined. The somata of MN1-4 lie in a cluster located near the lateral surface of the thoracic ganglion at the border of the pro- and mesothoracic regions, ipsilateral to the muscle fibers innervated. The somata of MN1 and 2 lie side by side in a horizontal plane with MN1 in a more anterior position. Those of MN3 and 4 lie ventrally to those of MN1 and 2 in a horizontal plane with MN3 in the more anterior position. The soma of MN5 is located contralaterally to the muscle fiber it innervates, lying in the dorsal outermost layer of the thoracic ganglion next to the midline at the border of the pro- and mesothoracic regions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glutamate-like immunoreactivity in identified neuronal populations of insect nervous systems

Glutamate is considered to be the most likely transmitter candidate at excitatory synapses onto skeletal muscles of insects. We investigated the distribution of glutamate-like immunoreactivity (Glu-LI) in identified motor neurons of glutaraldehyde-fixed metathoracic ganglia of the locust in paraffin serial sections. The presumably glutamatergic fast and slow extensor tibiae motor neurons show Glu-LI, whereas other cells, including the GABAergic common inhibitory motor neurons and the cluster of octopaminergic dorsal unpaired median cells, show rather low levels of staining. Immunoreactivity of the fast extensor tibiae motor neuron is located in soma, neurites, axon, and the terminal arborizations. A double-labeling experiment on sections of the locust metathoracic ganglion showed that antisera against glutamate and GABA discriminate between the presumably glutamatergic and GABAergic motor neurons and that GABA-LI-positive neurons are low in Glu-LI. The results suggest that Glu-LI can be used as a marker for detecting potential glutamatergic neurons in insects under the present conditions. Application of the glutamate antiserum to sections of the honeybee brain revealed Glu-LI in motor neurons but also in certain interneurons. The most prominent populations of Glu-LI-positive cells were the monopolar cells and large ocellar interneurons, which are first-order interneurons of the visual and ocellar system. Several groups of descending interneurons also showed Glu-LI. The distributions of Glu-LI and GABA-LI are complementary in locust and bee ganglia. The high level of Glu-LI in certain interneuronal populations, as well as in identified glutamatergic motor neurons, suggests that insect central nervous systems may contain glutamatergic neuronal pathways.