Axonal degeneration within the tympanal nerve of Schistocerca gregaria

This study describes time course and ultrastructural changes during axonal degeneration of different neurones within the tympanal nerve of the locust Schistocerca gregaria. The tympanal nerve innervates the tergit and pleurit of the first abdominal segment and contains the axons of both sensory and motor neurones. The majority of axons (approx. 97%) belong to several types of sensory neurones: mechano- and chemosensitive hair sensilla, multipolar neurones, campaniform sensilla and sensory cells of a scolopidial organ, the auditory organ. Axons of campaniform sensilla, of auditory sensory cells and of motor neurones are wrapped by glial cell processes. In contrast, the very small and numerous axons (diameter <1 microm) of multipolar neurones and hair sensilla are not separated individually by glia sheets. Distal parts of sensory and motor axons show different reactions to axotomy: 1 week after separation from their somata, distal parts of motor axons are invaded by glial cell processes. This results in fascicles of small axon bundles. In contrast, distal parts of most sensory axons degenerate rapidly after being lesioned. The time to onset of degeneration depends on distance from the lesion site and on the type of sensory neurone. In axons of auditory sensory neurones, ultrastructural signs of degeneration can be found as soon as 2 days after lesion. After complete lysis of distal parts of axons, glial cell processes invade the space formerly occupied by sensory axons. The rapid degeneration of distal auditory axon parts allows it to be excluded that they provide a structure that leads regenerating axons to their targets. Proximal parts of severed axons do not degenerate.     

The pattern of innervation in serially duplicated axolotl limbs: further evidence for the existence of local pathway cues?

The innervation of the biceps muscle was examined in regenerated and vitamin A-induced serially duplicated axolotl forelimbs using retrograde transport of horseradish peroxidase. The regenerated biceps muscle becomes innervated by motor neurones in the same position in the spinal cord as the normal biceps motor pool. In previous experiments in which the innervation of a second copy of a proximal limb muscle was examined in serially duplicated limbs (Stephens, Holder & Maden, 1985), the duplicate muscle was found to become innervated by motor neurones that would normally have innervated distal muscles. In the present study, the innervation of the second copy of biceps was examined under conditions designed to encourage nerve sprouting from 'correct' biceps axons. Following either partial limb denervation or denervation coupled with removal of the proximal biceps, the second copy of the muscle was still innervated by inappropriate motor neurones, which again would normally innervate distal limb muscles. These results are interpreted as evidence for the necessity for an appropriate local environment for axonal growth to allow reformation of a correct pattern of motor innervation in the regenerated limb.     

Organizational principles of outputs from Dipteran brains

Two major classes of Descending Neurones (DNs) originate in fly cerebral ganglia: (1) uniquely identifiable DNs, most of which arise preorally in duetocerebral neuropil of the supraoesophageal ganglion, the brain proper (2) parallel projecting DNs (PDNs) most originating in the suboesophageal ganglion. Brain DNs receive inputs directly from sensory systems and indirectly via higher center and peptidergic interconnections of the protocerebrum. Direct inputs include primary mechanosensory afferents, first order relay neurones from the olfactory lobes and ocellar receptor cups, and higher order visual neurones that interact with retinotopic inputs from compound eyes. Uniquely identifiable DNs arising in the brain are arranged in uniquely identifiable clusters. Each cluster receives a unique combination of inputs which are shared wholly or in part by the dendritic trees of its constituent DNs. Axons arising from a cluster diverge to different targets in the thoracic ganglia. PDNs form groups of as many as 40 neurones, as determined from outgoing axon bundles. Dendrites of PDNs are thin and diffuse, and arborize amongst collaterals from through-going axons of descending neurones arising in the brain. Axon bundles of PDNs are typically organized in rather simple ladder-like patterns in thoracic ganglion. A third type of uniquely identifiable DN also arises in the suboesophageal ganglion but does not seem to be arranged in clusters.     

Incorporation of neurons of transplants of embryonal rat neocortex in the achievement of sensory function of the cerebral cortex of the recipient

The barrelfield of the adult rats was removed by suction and embryonic tissue of the somatosensory neocortex was transplanted into the cavity. Spontaneous and evoked activity of the grafted neurones was investigated extracellularly 2-3 months after the grafting. The light microscopy of the grafts revealed the presence of normal neuronal cells, but their distribution was diffuse, and they were not organized into barrels as in intact neocortex. The background activity of grafted neurones depended upon the level of the recipient's anaesthesia. The response types of the grafted neurones to vibrissae deflection and to tactile stimulation of the host body surfaces, their latencies and lability did not differ from such of the intact somatosensory cortex, but the receptive fields of the grafted neurones were larger. There was also substantial convergence of inputs from other surfaces upon the grafted neurones. The effectiveness of stimulation of the various skin areas was determined by the proximity of their neocortical representations to the graft.     

The thalamus as a monitor of motor outputs

Many of the ascending pathways to the thalamus have branches involved in movement control. In addition, the recently defined, rich innervation of 'higher' thalamic nuclei (such as the pulvinar) from pyramidal cells in layer five of the neocortex also comes from branches of long descending axons that supply motor structures. For many higher thalamic nuclei the clue to understanding the messages that are relayed to the cortex will depend on knowing the nature of these layer five motor outputs and on defining how messages from groups of functionally distinct output types are combined as inputs to higher cortical areas. Current evidence indicates that many and possibly all thalamic relays to the neocortex are about instructions that cortical and subcortical neurons are contributing to movement control. The perceptual functions of the cortex can thus be seen to represent abstractions from ongoing motor instructions.     

Reversing the outcome of synapse elimination at developing neuromuscular junctions in vivo: evidence for synaptic competition and its mechanism

During mammalian development, neuromuscular junctions and some other postsynaptic cells transition from multiple- to single-innervation as synaptic sites are exchanged between different axons. It is unclear whether one axon invades synaptic sites to drive off other inputs or alternatively axons expand their territory in response to sites vacated by other axons. Here we show that soon-to-be-eliminated axons rapidly reverse fate and grow to occupy vacant sites at a neuromuscular junction after laser removal of a stronger input. This reversal supports the idea that axons take over sites that were previously vacated. Indeed, during normal development we observed withdrawal followed by takeover. The stimulus for axon growth is not postsynaptic cell inactivity because axons grow into unoccupied sites even when target cells are functionally innervated. These results demonstrate competition at the synaptic level and enable us to provide a conceptual framework for understanding this form of synaptic plasticity.     

Structural basis of the intracortical synchronization of epileptic potentials in the sensomotor region of the rat neocortex

In control rats, penicillin-induced epileptiform discharges were completely synchronous in the neocortex sites at a distance of up to 4 mm from each other. Number of the cells decreased by 45.5% during 90 days in isolated cortical slabs and the synchronisation disappeared. The data obtained show that the loss of large pyramidal neurones of the layer V entailed a loss of the spatial synchronisation. The main axonal collaterals of large pyramidal neurones of the layer V could be followed horizontally for a distance of up to 2 mm in the somatosensory cortex. The neuronal network formed by the large pyramidal neurones of the layer V seems to provide a spatial synchronisation in the neocortex.     

Molecular mechanisms of axon guidance

In order to form a functional nervous system, neurones extend axons, often over long distances, to reach their targets. This process is controlled by extracellular receptors and their ligands, several families of which have been identified. These proteins may act to either repel or attract growth cones and a given receptor may transduce either type of signal, depending on the cellular context. In addition to these archetypal axon guidance molecules, it is becoming apparent that molecules previously known for their role in patterning can also direct axonal outgrowth. The growth cone receptors do not act in isolation and combine with members of the same or other families to produce a graded response or even a complete reversal in its polarity. These signals can be further combined and/or modulated by processing of the molecule both directly at the cell surface and by the network of intracellular signalling pathways which are activated. The result is a sophisticated and dynamic set of cues that enable a growth cone to successfully navigate to its destination, modulating its response to changing environmental cues along its pathway.     

Synaptic contacts of neurons of fascia dentata transplants with nonspecific targets in neocortex of recipients

We carried out an electron microscopy study of possible synaptic contacts of the neurons of intracortical transplants of the rat brain fascia dentata with targets in the recipient somatosensory cortex. The axons of fascia dentata granular cell and their synaptic terminals could be easily identified in the neocortex due to their distinct morphological features (mossy fibers), although the fascia dentate cells normally do not interact with the neocortex. Thin nonmyelenized mossy fibers were found in both an intermediate zone between the transplant and brain and in the adjacent brain. Their presynaptic buds, like in situ, had large size and formed characteristic terminal, intraterminal, and en passant multiple synaptic contacts and desmosome-like junctions. The aberrant nerve fibers used perykaryons, dendrites of varying diameter, and dendrite spikes of the somatosensory cortex pyramidal neurons as postsynaptic targets in the neocortex. In addition to vacant spaces that appeared in the brain as a result of transplantation, the ingrowing axons induced the formation of additional contact sites: deep invaginations of the plasmalemma of perykaryons, somatic spikes, terminal branchings of dendrites, and dendritic outgrowths of complex branched shape. These aberrant contacts were characterized by the presence of polyribosomes, endoplasmic reticulum cisternae, and mitochondria in the postsynaptic loci. Osmiophility and extension of desmosome-like junctions were also enhanced in such synapses. Thus, it was shown that mossy fibers ingrowing in the recipient neocortex were capable of forming cell-to-cell contacts with signs of functional synapses to atypical cell targets.     

Synaptic Contacts of Neurons of Fascia Dentata Transplants with Nonspecific Targets in Neocortex of Recipients

We carried out an electron microscopy study of possible synaptic contacts of the neurons of intracortical transplants of the rat brain fascia dentata with targets in the recipient somatosensory cortex. The axons of fascia dentata granular cell and their synaptic terminals could be easily identified in the neocortex due to their distinct morphological features (mossy fibers), although the fascia dentate cells normally do not interact with the neocortex. Thin nonmyelenized mossy fibers were found in both an intermediate zone between the transplant and brain and in the adjacent brain. Their presynaptic buds, like in situ, had large size and formed characteristic terminal, intraterminal, and en passant multiple synaptic contacts and desmosome-like junctions. The aberrant nerve fibers used perykaryons, dendrites of varying diameter, and dendrite spikes of the somatosensory cortex pyramidal neurons as postsynaptic targets in the neocortex. In addition to vacant spaces that appeared in the brain as a result of transplantation, the ingrowing axons induced the formation of additional contact sites: deep invaginations of the plasmalemma of perykaryons, somatic spikes, terminal branchings of dendrites, and dendritic outgrowths of complex branched shape. These aberrant contacts were characterized by the presence of polyribosomes, endoplasmic reticulum cisternae, and mitochondria in the postsynaptic loci. Osmiophility and extension of desmosome-like junctions were also enhanced in such synapses. Thus, it was shown that mossy fibers ingrowing in the recipient neocortex were capable of forming cell-to-cell contacts with signs of functional synapses to atypical cell targets.     

Mechanisms of pain arising from articular tissues

This paper reviews the peripheral and central neural mechanisms underlying pain from articular tissues innervated by spinal and trigeminal afferents. The paper especially addresses trigeminal mechanisms related to pain from the temporomandibular joint and its associated craniofacial musculature. Recent studies have shown the existence of articular nociceptive primary afferents that project to the spinal cord dorsal horn and trigeminal brainstem complex. A particular feature of most neurones receiving these deep nociceptive afferent inputs is their responsivity also to cutaneous nociceptive afferent inputs. This suggests the involvement of these neurones not only in the detection of acute articular pain, but also in the hyperalgesia and poor localization, spread, and referral of pain that characterize many painful conditions of joints and other deep structures. While only limited information is available on related higher brain centre mechanisms, convergence and interaction between cutaneous and deep afferent inputs also seem to be a characteristic of somatosensory neurones in the thalamus and somatosensory cerebral cortex. Muscle and autonomic reflexes may be induced by such deep noxious stimuli, but the functional significance of some of these effects (e.g., in relation to clinical concepts of myofascial dysfunction) requires further study in more appropriate functional settings.     

Participation of puncta adherentia junctions in the formation of synaptic connections between a dentate fascia graft and the host brain

The fetal dentate fascia of Wistar rats on the 20th day of gestation was heterotopically grafted into the somatosensory neocortex of adult rats. Granule cells of a graft projected their axons (mossy fibers) to the host brain and established synaptic contacts with inappropriate targets. The organization of ectopic mossy fiber synapses was studied by electron microscopy. It was shown that ectopic synapses reproduce the structural determinants of hippocampal giant synapses and induce a subcellular reorganization of postsynaptic neocortex dendrites. Using morphometric analysis, a significant increase was found in the number of discrete puncta adherentia junctions and their total length in ectopic synapses as compared with the control group. The data obtained indicate that puncta adherentia contacts participate in the structural and chemical adaptation of neuronal targets to alien axons growing from transplants.     

An anatomical and electrophysiological study of synapse elimination at the developing frog neuromuscular junction

Synapse elimination was examined in the developing frog cutaneous pectoris muscle using histological and electrophysiological techniques. Morphological synapse elimination occurred in two phases. The first phase, which began at the time of metamorphosis and continued until the second to third postmetamorphic week, was characterized by a rapid decline in the number of endplates receiving greater than or equal to 3 synaptic inputs. However, 50% of the muscle fibers still remained dually innervated. This dual innervation decreased with a much slower time course; approximately 20% of the muscle fibers were dually innervated in 1- to 2-year-old frogs. During the first phase of synapse elimination no difference was noted between the distribution of acetylcholine receptors or acetylcholinesterase activity associated with the terminal arborizations formed by separate axons at one synaptic site. However, terminal arborizations formed by small diameter axons and consisting of varicosities separated by thin interconnectives became apparent during this period. Such varicose arborizations responded to nerve stimulation and released acetylcholine in proportion to their terminal length as did the nonvaricose arborizations. In addition, the number of morphological and physiological inputs at one endplate site was well correlated throughout the first phase of synapse elimination.     

The development of peptide-containing neurons within neocortical transplants in adult mice

Transplantation of embryonic neocortex into adult host neocortex leads to the survival of many donor cells, with the subsequent differentiation of the cortical neurons within a loosely laminated cellular pattern. We wanted to know whether peptide-containing neurons that are known to exist in normal neocortex would survive in the transplants, and if so, whether they would differentiate into morphological cell types that normally contain these peptides in cortex. By 30 days after transplantation, the implants were well vascularized and the donor neurons appeared healthy in Nissl-stained preparations. AChE-positive axons grew across the interface and innervated the transplant in moderate densities. Immunocytochemical localization of peptides in the transplant revealed that processes containing the four peptides normally present in cortex also develop in the transplants. These were vasoactive intestinal polypeptide, cholecystokinin, pancreatic polypeptide and somatostatin. Other peptides not yet demonstrated in and presumably not present in neocortex, did not develop in the transplants. These included alpha-melanocyte stimulating hormone, arginine-vasopressin, corticotropin releasing factor, beta-endorphin and substance P. The results demonstrate that peptide-immunoreactive neurons survive in neural transplants, where they develop complicated patterns of axonal arborization. The conditions used in these experiments produced no evidence that peptidergic neurons within the transplant grow out of the transplant and into the host brain within six weeks. Similarly, host peptidergic axons were never seen crossing the interface zone and entering the transplant in any significant numbers.     

Long systems of the dorsolateral fascicle: influence of afferentation from the splanchnic region.

The influence of afferentation from the splanchnic region on the activity of long systems in the dorsolateral fascicle (DLF) was studied in chloralose-anaesthetized cats. Ascending axons (the dorsal spinocerebellar tract, DSCT and the spinocervical tract, SCT) did not respond to splanchnic stimuli. Conditioning from the splanchnic region modified their activity; the effect (mainly inhibitory) was strongest in the case of exteroceptive channels. The reaction of long propriospinal neurones depended on whether they belonged to ascending or descending pathways. No responses to splanchic stimuli were observed in the axons of ascending (proprioceptive, exteroceptive) units and conditioning from the splanchnic region only modified their activity. In decending axons, synaptic discharges with a long latent period (over 20 msec) were generated. The interaction of inputs from the visceral and somatosensory regions resulted in reciprocal inhibition of the tested activity. In axons descending from suprasegmental areas, splanchnic afferentation generated synaptic discharges with a time course comparable to spino-bulbo-spinal activity. Interaction with the responses from somatic nerves resulted inhibition of the tested activity. The results confirmed that the splanchnic region participates in modifying and evoking activity in the long DLF systems.     

Convergence of multisensory inputs in Xenopus tadpole tectum

The integration of multisensory information takes place in the optic tectum where visual and auditory/mechanosensory inputs converge and regulate motor outputs. The circuits that integrate multisensory information are poorly understood. In an effort to identify the basic components of a multisensory integrative circuit, we determined the projections of the mechanosensory input from the periphery to the optic tectum and compared their distribution to the retinotectal inputs in Xenopus laevis tadpoles using dye‐labeling methods. The peripheral ganglia of the lateral line system project to the ipsilateral hindbrain and the axons representing mechanosensory inputs along the anterior/posterior body axis are mapped along the ventrodorsal axis in the axon tract in the dorsal column of the hindbrain. Hindbrain neurons project axons to the contralateral optic tectum. The neurons from anterior and posterior hindbrain regions project axons to the dorsal and ventral tectum, respectively. While the retinotectal axons project to a superficial lamina in the tectal neuropil, the hindbrain axons project to a deep neuropil layer. Calcium imaging showed that multimodal inputs converge on tectal neurons. The layer‐specific projections of the hindbrain and retinal axons suggest a functional segregation of sensory inputs to proximal and distal tectal cell dendrites, respectively. © 2009 Wiley Periodicals, Inc. Develop Neurobiol, 2009     

The structure of the coeliac ganglion of a teleost fish Myoxocephalus scorpius

Summary In order to compare the structure of a teleost sympathetic ganglion with those of other vertebrates, light, fluorescence histochemical and electron microscopy were carried out on the coeliac ganglion of the scorpion fish, Myoxocephalus scorpius. In common with studies on other vertebrates, fluorescence histochemistry distinguished two cell types: a) principal neurones which exhibited low levels of specific catecholamine fluorescence and comprise the majority of neurones in the ganglia, and b) smaller intensely fluorescent cells, some of which had processes tens of micrometers long.With the electron microscope, the principal cells were seen to make axodendritic and axosomatic synapses with axons containing mainly 30 nm agranular vesicles at the synaptic site while in other vertebrates usually only one or other synaptic association is present.Both the somata and the processes of intensely fluorescent cells contain 300–600 nm diameter vesicles many of which have electron dense cores. These cells are also innervated by axons containing 30 nm agranular vesicles.     

Fine structure and innervation of the honey stomach (crop) of the honeybee,Apis mellifera L. (Hymenoptera : Apidae)

The present paper describes the ultrastructure and innervation of the honey stomach (crop) of Apis mellifera (Hymenoptera : Apidae). The crop is innervated by 10–20 μm-thick nerves with numerous axons, running longitudinally, and small nerves (1–3 μm, often containing only one distinct axon), running latitudinally. Crop muscles are striated, often branched and have only few multidirectionally arranged myofilaments; they seem to be of the slow-contracting type. Longitudinal and circular muscles produce projections that connect synaptically to small nerves. At these neuromuscular junctions, a high density of synaptic vesicles is found in the axons. A small number of apparently multipolar neurones, stained with Procion Yellow, are irregularly distributed over the crop. The possibility that these neurones work as stretch receptor organs is discussed.     

Spinal and supraspinal contributions to central sensitization in peripheral neuropathy

We will focus on spinal cord dorsal horn lamina I projection neurones, their supraspinal targets and involvement in pain processing. These spinal cord neurons respond to tonic peripheral inputs by wind-up and other intrinsic mechanisms that cause central hyper-excitability, which in turn can further enhance afferent inputs. We describe here another hierarchy of excitation - as inputs arrive in lamina I, neurones rapidly inform the parabrachial area (PBA) and periaqueductal grey (PAG), areas associated with the affective and autonomic responses to pain. In addition, PBA can connect to areas of the brainstem that send descending projections down to the spinal cord - establishing a loop. The serotonin receptor, 5HT3, in the spinal cord mediates excitatory descending inputs from the brainstem. These descending excitatory inputs are needed for the full coding of polymodal peripheral inputs from spinal neurons and are enhanced after nerve injury. Furthermore, activity in this serotonergic system can determine the actions of gabapentin (GBP) that is widely used in the treatment of neuropathic pain. Thus, a hierarchy of separate, but interacting excitatory systems exist at peripheral, spinal and supraspinal sites that all converge on spinal neurones. The reciprocal relations between pain, fear, anxiety and autonomic responses are likely to be subserved by these spinal-brainstem-spinal pathways we describe here. Understanding these pain pathways is a first step toward elucidating the complex links between pain and emotions.     

Helicostatins: brain-gut peptides of the moth, Helicoverpa armigera (Lepidoptera: Noctuidae)

Gene expression and immunolocalisation studies have determined that the helicostatins are brain-gut peptides in larvae of the lepidopteran, Helicoverpa armigera. Mapping of the distribution of these peptides in the nervous system and alimentary canal has provided evidence for multifunctional regulatory roles. In situ hybridisation studies have shown that the helicostatin precursor gene is expressed in neurones of the central and stomatogastric nervous systems, and endocrine cells of the midgut demonstrating that the helicostatins are true brain-gut peptides. Antisera raised against Leu-callatostatin 3 (ANRYGFGL-NH(2)), a peptide isolated from the blowfly, Calliphora vomitoria was used to map the distribution of allatostatin-like immunoreactive (Ast-ir) material in H. armigera to elucidate possible functions of the helicostatins. In situ hybridisation studies verified that the helicostatin precursor gene is expressed in neurones shown to contain Ast-ir, providing strong evidence that the Ast-ir material is helicostatins. Extensive immunoreactive axonal projections into complex regions of neuropile indicate that the helicostatins may have a neuromodulatory role in the brain and segmental ganglia of the ventral nerve cord. The presence of large amounts of immunoreactive material in axons within the corpora cardiaca (CC) and transverse nerves of the perisympathetic nervous system, two known neurohaemal organs, provides evidence for a neurohormonal role. The corpora allata (CA) were innervated only sparsely by Ast-ir axons suggesting that the CA are not a neurohaemal release site or a target. Thus, it is unlikely that the helicostatins regulate juvenile hormone (JH) biosynthesis or release. Ast-ir axons extended from the frontal ganglion through the recurrent nerve and many branches were closely associated with muscles of the foregut, stomodeal valve, and anterior midgut, implicating helicostatins in regulation of foregut motility. Ast-ir material was also present in nerves associated with muscles of the pyloric valve and rectum, and in endocrine cells of the midgut.