Ectopic neurons and the organization of insect sensory systems

Summary The transplantation of appendages from one place to another on the body of crickets (Acheta domesticus) has been used to study the similarities and differences between the sensory systems of various ganglia. Mesothoracic legs have been transplanted to the abdomen in place of a cercus and cerci have been transplanted to thoracic leg stumps. After the ectopic sensory neurons had time to regenerate into the CNS, they were stained and their axonal arborizations examined. The results, which were concerned primarily with bristle receptors, revealed that bristle afferents on ectopic cerci arborized in ventral neuropil (the ventralmost association center) and leg afferents arborized in a ventral anterior region of the terminal abdominal ganglion. The results support the idea that each ganglion contains only a few distinct regions of neuropil (probably three), each receiving separate subsets of the afferent projection.The ectopic cerci were also shown to excite interneurons in the thoracic ganglia whose dendrites were located in the most ventral neuropil. These neurons normally respond to thoracic bristle afferents. Thus, the segregation of afferent axons has a correlate in the interneurons they excite.     

Transplantation of neurons reveals processing areas and rules for synaptic connectivity in the cricket nervous system

In order to assess the nature of spatial cues in determining the characteristic projection sites of sensory neurons in the CNS, we have transplanted sensory neurons of the cricket Acheta domesticus to ectopic locations. Thoracic campaniform sensilla (CS) function as proprioceptors and project to an intermediate layer of neuropil in thoracic ganglia while cercal CS transduce tactile information and project into a ventral layer in the terminal abdominal ganglion (TAG). When transplanted to ectopic locations, these afferents retain their modality-specific projection in the host ganglion and terminate in the layer of neuropil homologous to that of their ganglion of origin. Thus, thoracic CS neurons project to intermediate neuropil when transplanted to the abdomen and cercal CS neurons project to a ventral layer of neuropil when transplanted to the thorax. We conclude that CS can be separated into two classes based on their characteristic axonal projections within each segmental ganglion. We also found that the sensory neurons innervating tactile hairs project to ventral neuropil in any ganglion they encounter after transplantation. Ectopic sensory neurons can form functional synaptic connections with identified interneurons located within the host ganglia. The new contacts formed by these ectopic sensory neurons can be with normal targets, which arborize within the same layer of neuropil in each segmental ganglion, or with novel targets, which lack dendrites in the normal ganglion and are thus normally unavailable for synaptogenesis. These observations suggest that a limited set of molecular markers are utilized for cell–cell recognition in each segmentally homologous ganglion. Regenerating sensory neurons can recognize novel postsynaptic neurons if they have dendrites in the appropriate layer of neuropil. We suggest that spatial constraints produced by the segmentation and the modality-specific layering of the nervous system have a pivotal role in determining synaptic specificity. © 1993 John Wiley & Sons, Inc.     

The Sense Organs and Ventral Ganglion of Sagitta (Chaetognatha)

This paper describes some features of the chaetognath nervous system from ultrastructural observations and observations on material stained with specific techniques for nervous tissue, and from records of the activity of the locomotor muscles and ventral ganglion. Sensory cells grouped on the ventral surface of the head bear ciliary processes (some with multiple tubules), and are probably in connexion with the central nervous system by their own axons, unlike the sensory cells of the hair fan vibration receptors of head and body. The ventral ganglion is motor to the locomotor muscles of the body, and controls the rhythmic locomotor activity of the animal. Electrical events associated with contraction of these muscles are compound non-overshooting spike-like potentials. The ventral ganglion contains several large nerve fibres constant in position and connexions in different individuals. Some of these arise from cells in the ganglia of the head, and pass to the ventral ganglion, others from cells within the ventral ganglion, and probably supply the ciliary hair fan receptors of the body, whilst the motor axons issuing from the ventral ganglion are smaller in diameter. The ganglion is arranged on a ladder-like plan, and axons of the lateral cell bodies cross the central neuropil transversely before they contribute to the longitudinal tracts or pass out in the radial nerves. Synapses in the neuropil contain 30–40 nm electron lucent vesicles; the transmitter is unknown, but is unlikely to be either acetylcholine or l -glutamate. Occasional larger electron dense vesicles up to 70 nm in diameter are also found within nerve fibres of the neuropil. It is concluded that the arrangement of the peripheral nervous system is unlike that of several groups which have been suggested as related to chaetognaths.     

Projection of sensory neurons from a homeotic mutant appendage,Antennapedia, in Drosophila melanogaster

Central projections of sensory neurons from homeotic mutant appendages (Antennapedia) of Drosophila melanogaster were compared with those of wild-type antennae and wild-type legs by means of degeneration and cobalt backfilling methods. Sensory axons originating from wild-type thoracic legs terminate within the ventral ipsilateral half of the corresponding neuropile segment and do not project to the brain. Sensory fibers from the third antennal segment (AIII) of wild-type animals project into the ipsilateral antennal glomerulus (AG) and to a lesser extent into the contralateral AG, whereas those from the second antennal segment terminate principally within the ipsilateral posterior antennal center. The sensory terminals of femur, tibia, and tarsi of the homeotic leg show a distribution very similar to that of the homologous wild-type antennal segment AIII, differing to a minor degree only in the size and precise localization of terminals within the antennal glomeruli. No degenerating axons were evident in ultrastructural examination of neck connectives after removal of homeotic legs. It is thus very improbable that any sensory fibers of the homeotic leg project to normal leg projection areas in the thoracico-abdominal ganglion. Several alternative explanations are offered for the apparent retention of antennal specificity by axons from the transformed appendage.     

The development of the sensory neuron pattern in the antennal disc of wild-type and mutant (lz3, ssa) Drosophila melanogaster

The development of the sensory neuron pattern in the antennal disc of Drosophila melanogaster was studied with a neuron-specific monoclonal antibody (22C10). In the wild type, the earliest neurons become visible 3 h after pupariation, much later than in other imaginal discs. They lie in the center of the disc and correspond to the neurons of the adult aristal sensillum. Their axons join the larval antennal nerve and seem to establish the first connection towards the brain. Later on, three clusters of neurons appear in the periphery of the disc. Two of them most likely give rise to the Johnston's organ in the second antennal segment. Neurons of the olfactory third antennal segment are formed only after eversion of the antennal disc (clusters t1-t3). The adult pattern of antennal neurons is established at about 27% of metamorphosis. In the mutant lozenge3 (lz3), which lacks basiconic antennal sensilla, cluster t3 fails to develop. This indicates that, in the wild type, a homogeneous group of basiconic sensilla is formed by cluster t3. The possible role of the lozenge gene in sensillar determination is discussed. The homeotic mutant spineless-aristapedia (ssa) transforms the arista into a leg-like tarsus. Unlike leg discs, neurons are missing in the larval antennal disc of ssa. However, the first neurons differentiate earlier than in normal antennal discs. Despite these changes, the pattern of afferents in the ectopic tarsus appears leg specific, whereas in the non-transformed antennal segments a normal antennal pattern is formed. This suggests that neither larval leg neurons nor early aristal neurons are essential for the outgrowth of subsequent afferents.     

Abnormal impulse discharge in primary afferent axons injured in the peripheral versus the central nervous system

Chronic injury to sensory axons in the rat peripheral nerve induces pathophysiologic changes in the axolemma at the cut nerve end, which are reflected in spontaneous ectopic impulse discharge and hyperexcitability to a range of depolarizing stimuli. We asked whether sensory axons injured in the central nervous system (CNS) also respond in this way. Primary afferent axons were severed in the sciatic nerve and, alternatively, in the midcervical or upper lumbar dorsal column (DC). Measurements of abnormal discharge from myelinated afferents showed high levels of spontaneous activity generated at the nerve injury site, especially during the period 3-16 days postoperatively, but comparatively little activity generated at the DC lesion site at any postoperative time. There was a corresponding difference in ectopic hyperexcitability to mechanical and adrenergic stimulation, and to depolarization with topical K+. DC lesion sites were not made more excitable by concurrent transection of the sciatic nerve, or by placing an autologous graft of excised sciatic nerve tissue into the DC defect at the time of initial surgery. Transection sites on dorsal roots L4 and L5 yielded abnormal discharge similar to that of sciatic nerve neuromas, indicating that the relative silence of DC transection sites was related to the CNS environment and not to position with respect to the sensory cell body.     

The role of Wg signaling in the patterning of embryonic leg primordium in Drosophila

Cellular interaction between the proximal and distal domains of the limb plays key roles in proximal-distal patterning. In Drosophila, these domains are established in the embryonic leg imaginal disc as a proximal domain expressing escargot, surrounding the Distal-less expressing distal domain in a circular pattern. The leg imaginal disc is derived from the limb primordium that also gives rise to the wing imaginal disc. We describe here essential roles of Wingless in patterning the leg imaginal disc. Firstly, Wingless signaling is essential for the recruitment of dorsal-proximal, distal, and ventral-proximal leg cells. Wingless requirement in the proximal leg domain appears to be unique to the embryo, since it was previously shown that Wingless signal transduction is not active in the proximal leg domain in larvae. Secondly, downregulation of Wingless signaling in wing disc is essential for its development, suggesting that Wg activity must be downregulated to separate wing and leg discs. In addition, we provide evidence that Dll restricts expression of a proximal leg-specific gene expression. We propose that those embryo-specific functions of Wingless signaling reflect its multiple roles in restricting competence of ectodermal cells to adopt the fate of thoracic appendages.     

Anatomy and physiology of neurons composing the commissural ring nerve of the cricket, Acheta domesticus

The commissural ring nerve (RN) of the cricket Acheta domesticus links together the two cercal motor nerves of the terminal abdominal ganglion. It contains the axons of almost 100 neurons including two bilateral clusters of eight to 13 ventrolateral neurons and approximately 75 neurons with midline somata within the terminal abdominal ganglion. The ventrolateral neurons have an ipsilateral dendritic arborization within the dorsal neuropil of the ganglion and their axons use the RN as a commissure in order to enter the contralateral nerves of the tenth ganglionic neuromere. In contrast, most midline neurons have bifurcating axons projecting bilaterally into the neuropil of the ganglion as well as into the RN where they often branch extensively before entering the contralateral tenth nerves. Most RN neurons have small, non-spiking somata with spike initiation zones distant from the soma. Many midline neurons also produce double-peaked spikes in their somata, indicative of multiple spike initiation zones. Spontaneous neuronal activity recorded extracellularly from the RN reveals several units, some with variable firing patterns, but none responding to sensory stimuli. The RN is primarily composed of small (50 nm diameter) axon profiles with a few large (0.5-1 microm diameter) profiles. Occasionally, profiles of nerve terminals containing primarily small clear vesicles and a few large dense vesicles are observed. These vesicles can sometimes be clustered about an active zone. We conclude that the primary function of the RN is to serve as a peripheral nerve commissure and that its role as a neurohemal organ is negligible. J. Exp. Zool. 286:350-366, 2000.Copyright 2000 Wiley-Liss, Inc.     

Homologies of positional information in thoracic imaginal discs ofDrosophila melanogaster

Summary The regulative behavior of fragments of the imaginal discs of the wing and first leg was studied when these fragments were combined with fragments of other thoracic imaginal discs. A fragment of the wing disc which does not normally regenerate when cultured could be stimulated to regenerate by combination with certain fragments of the haltere disc. When combined with a haltere disc fragment thought to be homologous by the criteria of morphology and the pattern of homoeotic transformation, such stimulated intercalary regeneration was not observed. Combinations of first and second leg disc fragments showed that a lateral first leg fragment could be stimulated to regenerate medial structures when combined with a medial second leg disc fragment but not when combined with a lateral second leg disc fragment. Combinations of wing and second leg disc fragments showed that one fragment of the second leg disc is capable of stimulating regeneration from a wing disc fragment while another second leg disc fragment fails to stimulate such regeneration. It is suggested that absence of intercalary regeneration in combinations of fragments of different thoracic imaginal discs is a result of homology or identity of the positional information residing in the cells of the fragments. The pattern of correspondence of positional information revealed by this analysis is consistant with the pattern of homology determined by morphological observation and by analysis of the positional specificity of homoeotic transformation among serially homologous appendages. The implications of the existence of homologous positional information in wing and second leg discs which share a common cell lineage early in development are discussed.     

Synaptic connections between sensory afferents and the common inhibitory motoneuron in crayfish

The sensory inputs to the common inhibitory motoneuron that innervates every leg muscle of the crayfish Procambarus clarkii (Girard) were analyzed by performing intracellular recordings from its neurite within the neuropil of the 5th thoracic ganglion. Two types of sensory inputs involved in locomotion were studied, those from a movement coding proprioceptor (the coxobasal chordotonal organ) and those from sensory neu rons coding contact forces exerted at the tip of the leg on the substrate (the dactyl sensory afferents). Sinusoidal movements applied to the chordotonal organ strand induced a stable biphasic response in the common inhibitory motoneuron that consisted of bursts of spikes during release and stretch of the strand, corresponding to raising and lowering of the leg, respectively. Using ramp movements imposed on the chordotonal strand, we demonstrated that only movement-coding chordotonal afferents produce excitatory post-synaptic potentials in the common inhibitory motoneuron; these connections are monosynaptic. Mechanical or electrical stimulation of the dactyl sensory afferents resulted in an increase in the tonic discharge of the common inhibitory motoneuron through polysynaptic excitatory pathways. These two types of sensory cues reinforce the central command of the common inhibitory motoneuron and contribute to enhancing its activity during leg movements, and thus facilitate the relaxation of tonic muscle fibres during locomotion.Abbreviations ADR anterior distal root - A Lev anterior levator nerve - CB coxo-basipodite joint - CBCO coxo-basal chordotonal organ - CI common inhibitory motoneuron - Dep depressor nerve - DSA dactyl sensory afferents - EPSP excitatory post-synaptic potential - IN interneuron - MN motoneuron - PDR posterior distal root - P Lev posterior levator nerve - Pro promotor nerve - Rem remotor nerve     

Structure of allotransplanted ganglia and regenerated neuromuscular connections in crayfish

In adult crayfish, Procambarus clarkii, motoneurons to a denervated abdominal superficial flexor muscle regenerate long-lasting and highly specific synaptic connections as seen from recordings of excitatory postsynaptic potentials, even when they arise from the ganglion of another crayfish. To confirm the morphological origins of these physiological connections we examined the fine structure of the allotransplanted tissue that consisted of the third abdominal ganglion and the nerve to the superficial flexor muscle (the fourth ganglion and the connecting ventral nerve cord were also included). Although there is considerable degeneration, the allotransplanted ganglia display intact areas of axon tracts, neuropil, and somata. Thus in both short (6–8 weeks) and long (24–30 weeks) term transplants approximately 20 healthy somata are present and this is more than the five axons regenerated to the host muscle. The principal neurite and dendrites of these somata receive both excitatory and inhibitory synaptic inputs, and these types of synaptic contacts also occur among the dendritic profiles of the neuropil. Axon tracts in the allotransplanted ganglia and ventral nerve cord consist largely of small diameter axons; most of the large axons including the medial and lateral giant axons are lost. The transplanted ganglia have many blood vessels and blood lacunae ensuring long-term survival. The transplanted superficial flexor nerve regenerates from the ventral to the dorsal surface of the muscle where it has five axons, each consisting of many profiles rather than a single profile. This indicates sprouting of the individual axons and accounts for the enlarged size of the regenerated nerve. The regenerated axons give rise to normal-looking synaptic terminals with well-defined synaptic contacts and presynaptic dense bars or active zones. Some of these synaptic terminals lie in close proximity to degenerating terminals, suggesting that they may inhabit old sites and in this way ensure target specificity. The presence of intact somata, neuropil, and axon tracts are factors that would contribute to the spontaneous firing of the transplanted motoneurons. © 1996 John Wiley & Sons, Inc.     

Ectopic CNS projections guide peripheral neuron axons along novel pathways in leech embryos

Previous studies have indicated that the formation of stereotyped segmental nerves in leech embryos depends on the interactions between CNS projections and ingrowing afferents from peripheral neurons. Especially, CNS-ablation experiments have suggested that CNS-derived guidance cues are required for the correct navigation of several groups of peripheral sensory neurons. In order to directly test this hypothesis we have performed transplantations of CNS ganglia into ectopic sites in segments from which the resident ganglia have been removed. We find that the transplanted ganglia extend numerous axons distributed roughly equally in all directions. When these CNS projections reach and make contact with peripheral sensory axons they are used as guides for peripheral neurons to grow toward and into the ectopic ganglia even when this means following novel pathways that cross the midline and/or segmental boundaries. The peripheral sensory axons turn and grow toward the ectopic ganglia only when in physical contact with CNS axons, suggesting that diffusible chemoattractants are not a factor. These results demonstrate that the guidance cues provided by ectopic CNS projections are both necessary and sufficient to steer peripheral sensory neuron axons into the CNS.     

Integument and sensory nerve differentiation ofDrosophila leg and wing imaginal discs in vitro

Summary An ultrastructural analysis is presented of the cuticular and neural structures formed by the prothoracic leg and wing imaginal discs of maleDrosophila melanogaster larvae during culture in vitro with 0.2 g/ml of -ecdysone. A pupal cuticle, and subsequently an imaginal cuticle with a well-defined epicuticle and a laminated endocuticle is formed. The ultrastructure of the epidermis and of cuticular structures such as bristles, trichomes, apodemes, and tracheoles is very similar to that found in situ. Dendrites and nerve cell bodies are formed in vitro, and sensory axons form nerve bundles similar to those of normal appendages in situ, despite their isolation from the central nervous system. It is concluded that at the ultrastructural level, differentiation in vitro closely parallels the normal course of development.     

Metamorphosis of wing motor system in the silk moth,Bombyx mori L. (Lepidoptera: Bombycidae): Anatomy of the sensory and motor neurons that innervate larval mesothoracic dorsal musculature,stretch receptors,and epidermis

Anatomy of dorsal mesothoracic structures, such as muscles, sensory organs, and innervation, was studied in the silkworm, Bombyx mori L. (Lepidoptera : Bombycidae), and compared with the adult wing motor system. Musculature and nerve innervation were investigated by dissection and electron micrograph; and central projection of sensory fibers and morphology of somata and dendrites of motor neurons by cobalt back-filling, followed by silver intensification. There are 23 muscle bundles (DLM) and 2 stretch receptors (SR). The DLMs, SRs, and epidermis are innervated by a branch of the dorsal nerve trunk emerging from the mesothoracic ganglion (MSG). The branch bifurcates into a dorsal sensory branch of about 300 sensory fibers and a dorsal motor branch of 14 fibers. The sensory fibers project mainly to a longitudinal portion near the mid line in the ventral neuropil of MSG and the metathoracic ganglion. Several fibers extend into the prothoracic ganglion (PG) and a few into the subesophageal and 1st abdominal ganglia. At least 13 (probably 14) motor neurons send axons to DLMs: 9 (probably 10) in PG, and 4 in MSG. Their dendrites are located mostly on the dorsoipsilateral side of the neuropil, but several branches cross the mid line and give rise to many fine branches on the contralateral side. Comparison between the larval (present study) and adult motor system shows a significant similarity in the musculature, peripheral nerve pattern, and motor neurons with some peculiarities.     

Functional organization of the arctiid moth tymbal (insecta,lepidoptera)

The exoskeletal morphology, muscular organization, and innervation patterns of the tymbals of seven sound-producing species of tiger moths (Arctiidae) were compared with the undifferentiated episterna of two silent species. At least three muscles are involved in sound production: the tymbal muscle, pv2, and the accessory muscles, pvl and/or pv6. All of the tymbal muscles are innervated by the IIIN2a branch of the metathoracic leg nerve, which contains two axons larger than the others. Backfills of the tymbal branch of the IIIN2a reveal a medial sensory neuropil and a population of five ipsilateral motor neurons whose somata are clustered into three groups along the anterior edge of the metathoracic ganglion. The dendritic arborizations of the motor neurons extend to the ganglionic midline but are separate from one part of the auditory neuropil observed in other noctuoids. The study concludes that the arctiid tymbal reveals only minor modifications (e.g., cuticle thinning) of the episterna of silent moths and represents a primitive form of the tymbal compared to those of the Cicadidae.     

Central projections of a homoeotic regenerate, antennapedia, in a stick insect, Carausius morosus (Phasmida)

Homoeotic appendages provide a system for the analysis of neural path-finding in which the appendage is mismatched with its segmented ganglion. Central projections of sensory neurons from homoeotic antennapedia regenerates induced by antennal amputation in the stick insect, Carausius morosus, are described. The majority of afferent axons project to the olfactory lobe as in the normal antennal nerve, but they do not give rise to compact glomeruli. Nor does the form of the projection resemble that of leg sensory nerves in thoracic ganglia. The projection of antennapedia regenerate neurons in Carausius resembles the antennapedia mutant of Drosophila except that some primary afferents bypass the olfactory lobe and take several courses through the brain, sometimes reaching distant contralateral areas. It appears that these wandering fibers, having bypassed the olfactory lobe, tend to follow established tracts and to arborize or to deviate at circumscribed synaptic areas. The behavioral evidence for sensory input from antennapedia regenerates is equivocal.     

Laser microbeam axotomy and conduction block show that electrical transmission at a central synapse is distributed at multiple contacts

Touch (T) sensory neurons in the leech innervate defined regions of skin and synapse on other neurons, including other T cells, within the ganglionic neuropil. The cells' receptive fields in the periphery are comprised of a central region, innervated by thick axons, and adjoining regions (minor fields) innervated by thinner axons. Secondary branches, known to be sites of synapses, emerge from the thinner and thicker axons. Pairs of T cells appear to make up to 200 separate contacts distributed within the neuropil. When the T cell is hyperpolarized, as occurs during natural stimulation of the cell, action potentials generated in the minor field and travelling into the ganglion along the thin axons may fail to conduct at central branch points. Evidence is presented, using axon conduction block and laser axotomy of cells filled with 6-carboxy-fluorescein, that synapses between separate groups of branches can function independently. Thus, selective activation of branches of the thin anterior axon produced a synaptic potential 36 +/- 6% of control amplitude, which was consistent with counts of 39 +/- 6% of contacts made by these branches. Laser axotomy of postsynaptic neurons showed that the anterior contacts indeed made the principal or only contacts activated during anterior conduction block. The results show that conduction block can modulate transmission within the ganglion, and it operates by silencing particular contacts between cells.     

Developmental changes of the antenna and its neurons in the silkworm,Bombyx mori,with special regard to larval-pupal transformation

The larval antenna of Bombyx mori has 13 sensilla and about 52 sensory neurons in its distal portion. The axons form two nerve cords which unite in the cranial hemocoel to supply the brain as the olfactory nerve. The antennal imaginal disc, which is a thick pseudostratified epithelium continuous with the antennal epidermis, thickens markedly during the 5th instar by rapid cell proliferation. At the prepupal stage cell proliferation ceases and the disc everts to form a large pupal antenna. Simultaneously, an extensive cell rearrangement occurs in the antennal epidermis and the disc tissue becomes much thinner because of the abrupt expansion of antennal surface area. The two larval nerve cords thin down markedly by degeneration of axons, but they do not disintegrate totally even after the onset of pupation. The epidermis of the larval antenna forms the distal portion of the pupal antenna, while the imaginal disc forms the more basal portion. Development to the adult antenna occurs almost immediately after the onset of pupation; many adult neurons appear in the simple epidermis facing toward the thick outer side of the newly formed pupal cuticle. By 12 hours after the onset of pupation, these neurons align themselves in many transverse rows which are the first sign of the adult antennal configuration. Addition of these neuronal axons to the once-thinned nerve cords causes resumed thickening of the cords during the first 24 hours and thereafter. Differentiation of adult sensilla begins in the next 24 hours and is almost completed at the third day of pupation, which requires a total of 10 days.     

The cerebral ganglia of Milnesium tardigradum Doyère (Apochela, Tardigrada): Three dimensional reconstruction and notes on their ultrastructure

Differential interference contrast micrographs from stretched animals, serially sectioned semi-thin and ultrathin sections revealed that the cerebral ganglia (supraoesophageal mass) of the eulardigrade Milnesium tardigradum lie above the buccal tube and adjacent tissue like a saddle. It has an anterior indentation which is penetrated by two muscles that arise from the cuticle of the forehead. The cerebral ganglia consist of lateral outer lobes bearing an eye on each side, and two inner lobes which extend caudally. Between the inner lobes a cone-like projection tapers into a nerve bundle. Each outer lobe is joined with the first ventral ganglion. From the outer lobe near the eye the ganglion for a posterolateral sensory field extends to the epidermis. Anterior to the supraoesophageal mass are three dorsal ganglia for the upper three peribuccal papillae. Two additional ganglia attached to the cerebral mass supply the lateral cephalic papillae. The cerebral ganglia are covered by a thin neural lamella. The pericarya which surround the neuropil have large nuclei. Near the axons in the centre of the supraoesophageal mass the cytoplasm is crowded with vesicles of different size and appearance. Some of them resemble synaptic vesicles while others resemble dense core bodies. Structurally different types of synapses and axons can be distinguished within the neuropil.     

Effects of Latrodectus spider venoms on sensory and motor nerve terminals of muscle spindles

The effects of the venoms of the spiders Latrodectus mactans tredecimguttatus (black widow) and Latrodectus mactans hasselti (red back) on sensory nerve terminals in muscle spindles were studied in the mouse. A sublethal dose of venom was injected into tibialis anterior and extensor digitorum longus muscles of one leg. After survival from 30 minutes to 6 weeks muscles were examined in serial paraffin sections impregnated with silver or by electron microscopy. Sensory endings became swollen, some within 30 minutes, while over the next few hours there was progressive degeneration of annulospiral endings. By 24 hours every spindle identified by light or electron microscopy was devoid of sensory terminals. Degenerated nerve endings were taken up into the sarcoplasm of intrafusal muscle fibres. Regeneration of sensory axons began within 24 hours, new incomplete spirals were formed by 5 days and by 1 week annulospiral endings were almost all normal in appearance. Intrafusal motor terminals underwent similar acute degenerative and regenerative changes. These experiments show that intrafusal sensory and motor terminals are equally affected by Latrodectus venoms. Sensory nerve fibres possess a capacity for regeneration equal to that of motor fibres and reinnervate intrafusal muscle fibres close to their original sites of innervation.