Fine structure of an octopaminergic neuron and its terminals

The large octopaminergic dorsal unpaired median neuron of the locust that innervates the extensor tibiae muscle, DUMETi, was examined electronmicroscopically. Its soma contains many Golgi complexes apparently making dense-core vesicles similar to those found in peripheral branches and terminals. There are also larger stores of the dense material in the soma, especially near the exit of the principal neurite, that are not in vesicular form. Since the neurons can be penetrated and stimulated by microelectrodes, they form favorable subjects for direct studies of the control of neurosecretion. Preterminal fine branches of the neuron were located in proximal outer bundles of muscle fibers into which they had been traced electrophysiologically. They contain numerous large dense-core vesicles arrayed in rows near microtubules. These fine branches have a thick layer of collagenous connective tissue between the axon and the muscle fiber. Final terminals have varicosities containing many vesicles, lying inside the outer layers of the sarcolemmal complex of muscle fibers. They do not form synaptic structures. Terminals of another DUM neuron, one that innervates the dorsal longitudinal flight muscles (DUMDL), were similar in detail to those of DUMETi. DUMETi swelled about 20-fold in cross-sectional area above a ligature, in a 12-hr period, indicating that there is an extensive centrifugal flow of material in it, and sprouted a branch.

Sensory projections from the wind-sensitive head hairs of the locust Schistocerca gregaria. Distribution in the central nervous system

The neurones from the wind-sensitive hairs on the locust head have been filled with cobalt chloride and intensified with silver. All the neurones project through the brain to the suboesophageal ganglion, some continue to the prothoracic ganglion and a few as far as the mesothoracic ganglion. Three different types of projection are described and a regrouping is proposed of Weis-Fogh's five hair fields into three areas. The distribution of the neurones from these areas is described in relation to other structures in the ganglion and is discussed in relation to the function of the hair fields in stability control and grooming.

Electron microscopy of the spindle in locally heated cells

Individual living cells in metaphase were exposed to a steep temperature gradient by placing a microheater near one spindle pole. The cells were then fixed and the spindle was examined by electron microscopy. The structure of the warmer half-spindle differed from the cooler half-spindle in several ways. Kinetochore microtubules were nearly parallel in the warmer half-spindle but were divergent in the cooler. The total length of microtubules in the warmer half-spindle was 52 per cent greater and the number of kinetochore microtubules per kinetochore averaged 16 per cent higher than in the cooler half-spindle. The warmer half-spindle was longer than the cooler. These observations clearly demonstrate a locally enhanced assembly of microtubules in the warmer half-spindle. The electron microscope study makes still clearer the unusual character of chromosome movement in the differentially heated cells: the structure of the warmer half-spindle is hard to distinguish from that in normal cells, yet chromosome movement there is far slower than normal (Nicklas, 1979).

Suboesophageal neurons involved in head movements and feeding in locusts

The projections of nerves 6 and 7 of the locust suboesophageal ganglion (SOG) were stained by axonal filling with cobalt chloride. Nerve 6 contains two motoneurons which innervate neck muscles 50 and 51. Sensory neurons innervating hairs on the dorso-occipital region of the head also enter the ganglion through nerve 6 and terminate in a small bilateral plexus. The projections of the head hairs in nerve 6 do not overlap the arborizations of the motoneurons or the neurons of nerve 7, but lie in the same area as descending sensory neurons from wind-sensitive hairs of the front of the head. One branch of nerve 7 (7B) contains two fibres which innervate the salivary gland. These 'salivary' neurons (labelled SN1 and SN2) have their cell bodies in the ganglion. The second branch, 7A, contains sensory neurons from the submentum of the labium, which form four sensory plexuses, two dorsal and two ventral. The sensory plexuses from the submentum have specific regions of overlap with the salivary neurons and with the neck muscle motoneurons. We interpret these as indicating a flow of information from labial receptors signalling head and mouthpart movement to neurons involved in salivation and head movement. We further postulate that the anatomical separation of the various sensory plexuses is indicative of functional localization within the ganglion.

The fine structure of the ocelli of Schistocerca gregaria. The neural organisation of the synaptic plexus

A study of the organisation of the locust dorsal ocellus shows that the structure is designed to provide the maximum possible effective aperture. The condenser-like cuticular lens and the dispersal of the rhabdome over a large proportion of the circumferential area of the retinula cells increases the light gathering power of the eye. The synaptic plexus of the ocellus has two major features: (i) the retinula cells are repeatedly and reciprocally connected by synapses and junctions, and (ii) there is an extensive lateral and feedback network between the receptors and interneurons. A unified structure is described for a synapse that presents differing profiles dependent upon the angle of section. A distinct morphological class of junction is described between retinula cells. The synaptic arrangements of morphologically identical retinula cells vary from cell to cell and the synaptic plexus is not organised with a high degree of spatial precision. The overall synaptic configurations are discussed in terms of the varied response characteristics of units in the ocellar nerve.

The structure and function of serially homologous leg motor neurons in the locust. I. Anatomy

Twenty-one prothoracic and 17 mesothoracic motor neurons innervating leg muscles have been identified physiologically and subsequently injected with dye from a microelectrode. A tract containing the primary neurites of motor neurons innervating the retractor unquis, levator and depressor tarsus, flexor tibiae, and reductor femora is described. All motor neurons studied have regions in which their dendritic branches overlap with those of other leg motor neurons. Identified, serially homologous motor neurons in the three thoracic ganglia were found to have: (1) cell bodies at similar locations and morphologically similar primary neurites (e.g., flexor tibiae motor neurons), (2) cell bodies at different locations in each ganglion and morphologically different primary neurites in each ganglion (e.g., fast retractor unguis motor neurons), or (3) cell bodies at similar locations and morphologically similar primary neurites but with a functional switch in one ganglion relative to the function of the neurons in the other two ganglia. As an example of the latter, the morphology of the metathoracic slow extensor tibiae (SETi) motor neurons was similar to that of pro- and mesothoracic fast extensor tibiae (FETi) motor neurons. Similarly the metathoracic FETi bears a striking resemblance to the pro- and the mesothoracic SETi. It is proposed that in the metathoracic ganglion the two extensor tibiae motor neurons have switched functions while retaining similar morphologies relative to the structure and function of their pro- and mesothoracic serial homologues.

The morphology of local non-spiking interneurones in the metathoracic ganglion of the locust

The morphology is described of a number of non-spiking interneurones in the metathoracic ganglion of the locust that control motor neurones innervating muscles in the coxa and femur of a hind leg. The non-spiking interneurones are penetrated with microelectrodes, physiologically characterized, injected with cobalt, and the stain subsequently intensified with silver. The interneurones have diverse shapes but all are local, intraganglionic interneurones. Their cell bodies are 10-20 micrometer in diameter and lie in either the ventral or dorsal layers of cell bodies that form a cortex around the ganglion. The branches of the interneurones are profuse and overlap those of the motor neurones that they affect. On interneurone may have branches in both the most ventral and the most dorsal areas of the neuropile. Most interneurones have branches only in one half of the ganglion, but one interneurone has extensive and asymmetrical regions of branches in both halves of the ganglion (fig. 4). Similar physiological effects can be mediated by interneurones with distinct morphologies. For example, the single slow extensor motor neurone is excited by six distinct morphological types of interneurones (figs. 10-13). It is suggested that as many as 65% of the neurones within a ganglion may be local interneurones, many of which in turn may be non-spiking.

The unit structure of the locust compound eye

1. The ommatidium or functional unit of the locust compound eye comprises a compound corneal lens, 4 cone cells, 2 primary pigment cells, 16 secondary pigment cells and 8 retinula cells. 2. All retinula cells run the entire length from the cone to the basal lamina, although two, called the proximal cells, only contribute to the lowest third of the rhabdom, and one of either cell 6 or cell 7 on our arbitrary numbering system forms its axon one third the way up the ommatidium. 3. 84% of the 417 ommatidia examined had five cone cell processes. The position of three cone cell processes (cone threads) is almost invariable with respect to numbered retinula cells but the remaining threads can take any of three intercellular locations. 4. The position of these threads correlates with the number of the cell distally displaced from the rhabdom. We suggest that cone thread position in the developing ommatidium determines some features of retinula cells and we propose a simple model to account for this.

The ultrastructure of campaniform sensilla on the eye of the cricket, Gryllus campestris

The structure of the campaniform sensilla of the cricket eye was investigated by light and electron microscopy. Each sensillum is innervated by a single bipolar neuron. Its axon extends through the retina into a side-branch of the nervus tegumentarius. The dendrite extends through a cuticular channel to the surface of the cornea. The distal part of the dendrite, the sensory process, contains a tubular body and is attached to a cuticular cap which is obliquely inserted into the exocuticle between the corneal lenslets. Some particular structural features as well as the function of the campaniform sensillum of the cricket eye are discussed.

Postembryonic development of the visual system of the locust, Schistocerca gregaria. II. An experimental investigation of the formation of the retina-lamina projection

In the compound eye of the locust, Schistocerca gregaria, neurons from the retina project to the lamina in a precise topographical mapping. The formation of this projection was investigated in grafting experiments which altered the spatial or temporal relationship between the retina and the lamina. The results show that retina axons tend to grow along the paths of adjacent axons, with no indication of specificity for their normal termination sites. It is suggested that the orderly sequence of retina differentiation during normal development plays a major role in imposing pattern both upon the developing projection and, through some form of inductive interaction between retina and lamina neurons, upon the lamina.

Freeze-fracturing of biological material with a new specimen table

The freeze-fracturing of various biological material is described with the aid of a modified Balzers specimen table. The advantages of this table are easier and faster handling of the specimens and a better thermal contact between the specimen and the cold stage.

The dorsal, unpaired, median neurons of the locust metathoracic ganglion

Neurons having large cell bodies in the anterior dorsal median cluster in the metathoracic ganglion of the locust Schistocerca gregaria and the grasshopper Romalea microptera were studied by direct dye injection and reverse filling combined with elyctrical stimulation and recording. Eight, possibly nine, are of the unpaired type, with a T branch leading into left and right axons that leave the ganglion to terminate in muscles. Another six are probably paired, and may be interneurons. Five of the 8 or 9 unpaired neurons have one axonal branch in both N4 and N5, on both sides: the others have but a single branch. One of the nine, DUMETi, has left and right axons exclusively innervating the jumping muscles, and another, DUMDL, has left and right axons exclusively innervating the dorsal longitudinal flight muscles. Neither the locations, sizes or numbers of somata, nor their locations were as constant as is the case for ordinary ventral motoneurons.

The neuronal control of dragonfly flight. I. Anatomy

The mechanical action and innervation of the major flight muscles of dragonflies are described. All flight muscles investigated are innervated by at least 3 motor neurones and one by as many as 15. Cell bodies of motor neurones that innervate the same muscle are clustered together and have similar, widespread dendritic branching patterns. Motor neurones of leg muscles have greater variety in cell body size and position than the major flight motor neurones. Striking similarities between the positions of cell bodies of motor neurones in dragonflies and the positions of homologous motor neurones in other insects raise interesting questions about the evolution of insect nervous systems.

The ultrastructure of an olfactory sensillum on the maxillary palps of locusta migratoria (L.)

The olfactory sensilla on the maxillary palp tip of Locusta migratoria (L.) resemble the surrounding contact chemoreceptors in general morphology. The perforated peg has a thicker wall than is commonly found in olfactory sensilla. The form and position of the sensilla are considered in relation of the olfactory function. The fine structure of the dendrites is discussed in relation to that described in other olfactory sensilla.

Identification of a secretomotor centre in the brain of Locusta migratoria, controlling the secretory activity of the adipokinetic hormone producing cells of the corpus cardiacum

After retrograde filling of axons terminating in the glandular lobe of the corpus cardiacum (CC) of Locusta migratoria with cobalt chloride, a paired group of about 15 cobalt containing cells was demonstrated in the lateral area of the protocerebrum. The axons of these cells run via the NCC II into the glandular lobe of the CC. These small neurons have the characteristics of secretory cells; they contain secretory granules of about 1000 A in diameter. The axon terminals in the glandular lobe, making synaptic contacts with the glandular cells, contain secretory granules of the same size. It is therefore concluded that the cell groups in the protocerebrum control the activity of the glandular cells which produce an adipokinetic hormone. Arborizations of fibers of the lateral secretomotor cells are present in the dorsal neuropile of the protocerebrum, ventral of the mushroom bodies and along the tracts of the NCC I within the brain. It is proposed that these arborizations are sites of synaptic input. It is discussed that the axons of these cells might receive additional synaptic input in the storage lobe of the CC. The localization of cell bodies, the axons of which enter the storage part of the CC is described. The course of the axon tracts of the various cell groups in the protocerebrum and their connections with the NCC I and NCC II are demonstrated.

Innervation of the ventral diaphragm of the locust (Locusta migratoria)

1. Innervation and some electrical properties of the locust ventral diaphragm were investigated with electrophysiological and histological methods. 2. Muscle fibres are coupled electrically. Electrical stimulation evokes a graded active membrane response. 3. Each segment is innervated by four motor neurones as follows. Two motor neurones are situated in each abdominal ganglion. Branches of their axons supply the ventral diaphragm in the respective and the next posterior segment. 4. This pattern of innervation was confirmed by axonal Co and Ni staining of the motor nerve endings. 5. Neuromuscular junctions are excitatory. EPSPs show summation but no facilitation. 6. Spontaneous electrical activity of the diaphragm is to a certain degree coupled to activity of the main inspiratory muscles.

The locust wing hinge stretch receptors. I. Primary sensory neurones with enormous central arborizations

In locusts a single-celled stretch receptor (SR) neurone at the base of each wing monitors wing elevation and contributes to the control of the flight motor output. The central projections of these neurones are very complex but consistent in detail in the three species studied (Chortoicetes, Locusta and Schistocerca). The hindwing SR projects to the second and third thoracic ganglia, the forewing SR to the first, second and third thoracic ganglia. Both send fine axons into the abdominal connective. Within the ganglia each SR forms an extensive arborization, entirely ipsilateral and mainly in the dorsal neuropile, divided into medial, mediolateral, and lateral branches. The projections of the two ipsilateral SR neurones overlap almost completely in the second and third ganglia. There are recurrent loops between branches of a single neurone both within and between ganglia. Light microscope analysis shows apparent contacts between the SR aneurones and flight motor neurones and other wing sensory afferents, as well as long interneurones, other motor neurones and two types of multiaxonal neurones of unknown function. There are three groups of contacts between each SR and a flight motor neurone: laterally on the main branches, medially with the terminal twigs; and in the anterior dorso-medial glomerulus, where the inter ganglionic recurrent branch also terminates. All contacts are ipsilateral except for those with the contralateral branches of the dorsal longitudinal muscle motor neurones. We suggest that the SR neurones are multifunctional. Differential information transfer could result both from the spatial distribution of synaptic connections with the motor neurones and from filtering caused by low safety factors at branch junctions. Information in the lateral branching could be used for general excitation and control of firing frequency of the motor neurones; that in the medial branch for wing control and co-ordination.

The locust wing hinge stretch receptors. II. Variation, alternative pathways and "mistakes" in the central arborizations

The central arborizations of the stretch receptor (SR) neurones are very consistent from one individual to another. Superimposition of normalized neurones from eight individuals of Locusta show very little variation even in the detailed branching pattern. There is, however, a commonly found alternative course for the main medial branch in the metathoracic ganglion. Rare, radical departures from the normal branching pattern are termed "mistakes." Only three have been found, all in the forewing SR projection, one with an extra branch and two with missing branches. Terminals of twigs in the alternative region of the medial branch occupy consistent positions in the neuropile, although these are reached by different routes. Mistakes have terminals in areas normally containing SR endings. Both these findings suggest that there may be labelled sites in the neuropile which the growing tips of the SR neurones seek out.

Origin, destination and mapping of tritocerebral neurons of locust

The connectivities of the tritocerebrum of locust (Locusta migratoria L., Schistocerca gregaria (Forsk.)) were studied histologically and by means of cobalt chloride infusion. Its neuropil consists partly of fibers which traverse the tritocerebrum and areas consisting of neuropilar agglomerizations ("glomeruli"). The following direct connections between the tritocerebrum and other regions were observed: connections to 1) dorsal and lateral brain regions (mushroom body, optic lobe), 2) the ventral nerve cord, 3) the stomatogastric nervous system (here the protocerebrum and the subesophageal ganglion are also involved in these connections), 4) the retro-cerebral glands (corpora cardiaca, corpora allata), and 5) muscles of the foregut.

The locust jump. II. Neural circuits of the motor programme

1. Neural circuits which co-ordinate the motorneurones of the meta-thoracic tibiae of the locust in jumping and kicking have been investigated. 2. The fast extensor motorneurone is reflexly excited by the subgenual organ, by a network of cuticle strain receptors, and by Brunner's organ. The subgenual organ and the cuticle strain receptors are excited by tension in the extensor muscle and mediate a positive feedback which could help to produce the burst of fast extensor spikes which precedes a jump or kick. Brunner's organ is stimulated by pressure from the flexed tibia, and will be excited by the initial flexion and throughout the co-contraction phase of a kick. 3. A central excitatory connexion from the fast extensor to the slow extensor ensures that extensor muscle tension is as great as possible early in the co-contraction phase of a kick. 4. A central excitatory connexion from the fast extensor to flexor motorneurones is confirmed. This ensures that flexor muscle tension is great enough to keep the tibia flexed when the extensor muscle tension starts to develop before a jump or kick. 5. Reflex excitation of flexor motorneurones occurs in response to an extensor muscle twitch when the tibia is flexed. This helps to maintain the flexor connexion. 6. A receptor, the ‘lump receptor’, which is stimulated by flexor muscle tension only when the tibia is flexed, can inhibit the flexor motorneurones and may activate the trigger system which allows the extension of the tibia in a jump or kick. 7. Recptors in the suspensory ligaments of the joint inhibit the fast extensor when the tibia extends.

The organization of perpendicular fibre pathways in the insect optic lobe

High resolution serial photomicrography has been used to plot the axonal projection patterns between retina, lamina and medulla in the optic lobes of various insects with differing ommatidial receptor arrangements. Observations are reported on the cabbage white and skipper butterflies, the bee, locust, fly, backswimmer and waterbug. The patterns of these fibre pathways have previously eluded non-rigorous analyses primarily because of their physical dimensions but are revealed in this study to have striking precision and uniformity between species when examined at the level of individually identifiable cells. Axon bundles of the tracts between retina and lamina or lamina and medulla project between a single ommatidium and its corresponding lamina cartridge or between corresponding lamina and medulla cartridges. Lateral interweaving of axons between adjacent bundles is absent. The bundles preserve the retinotopic order within their total array, so transferring the pattern of retinulae directly upon the lamina and thence after horizontal inversion in the chiasma upon the medulla. Within the lamina neuropile on the other hand the trajectories of the individual terminals from a bundle have patterns which are species-specific, sometimes involving lateral divergences. In species with open-rhabdomere ommatidia the terminals distribute to a group of lamina cartidges with a pattern which resembles the receptor pattern in the overlying ommatidium. In species with fused-rhabdome ommatidia the terminals of a single retinula behave less interestingly and all enter the same cartridge, within which, again, each occupies a position related to its cell body position within the retinula. Long visual fibres in both eye types penetrate the lamina and terminate in the particular medulla cartridge that connects with the lamina cartridge underlying their ommatidium. The perpendicular fibre pathways therefore project the visual field exactly upon the medulla in all species while the lack of interweaving between adjacent fibre bundles precludes their involvement in lateral interactions between pathways with differing visual axes. Uniformity of these projection patterns between cell layers and species differences in retinular terminal locations in the lamina can be correlated with different modes of axon growth between and within neuropile layers during optic lobe neurogenesis. Further discussion surrounds the question of which particular receptors give rise to which type of axon, for which no clear generalization has yet emerged.