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PubMed1981

Ultrastructure of the crayfish stretch receptor in relation to its function.

The crayfish slow-adapting abdominal stretch receptor was fixed under the relaxed or stretched condition. During this procedure action potentials of the sensory neuron were recorded by a suction electrode. The receptor organ consists of a receptor muscle and a sensory neurons with its dendrites embedded in the connective tissue zone in the receptor muscle. From the cell body of the neuron, several "primary dendrites" arise, branch successively into "dendritic branches", and finally terminate as "dendritic tips," which are cylindrical processes of fairly uniform diameter. In contrast to the primary dendrites and the dendritic branches, the dendritic tips have neither mitochondria nor sheaths and are embedded in the connective tissue zone or apposed to the receptor muscle with a gap of about 15 nm. Microtubules and smooth ER are seen in all parts of the dendrites. When the receptor is stretched and then fixed with 1.6% glutaraldehyde in 0.12 M phosphate buffer (total osmolarity of this solution is isosmotic with the physiological solution), dendritic tips became more parallel to the long axis of the receptor muscle and showed marked deformation consisting of alternate regions of swelling and shrinkage, resulting in a bead-like appearance. When fixed with 1.6% glutaraldehyde in 0.2 M phosphate buffer (total osmolarity of this solution is hyperosmotic), the dendritic tips showed less tendency toward such deformation. These results suggest that the dendritic tip membrane is susceptible to stretch and might be the region where the generator potential is produced.

Animals↗
PubMed1981

Freeze-fracture study of the crayfish stretch receptor.

The crayfish slow-adapting stretch receptor was fixed under relaxed or stretched conditions (twice the relaxed length) and then processed for freeze-fracture study. The sensory neuron membrane had evenly distributed intramembrane particles mostly on its P face. The density of these particles was higher in the cell body than in the dendritic tips, which are the terminal portions of the dendrites. The dendritic tips were cylindrical under the relaxed condition and showed deformations with stretch stimuli. When they were fixed under the stretched condition with 1.6% glutaraldehyde in 0.12 M phosphate buffer (the total osmolarity of this fixative is isosmotic with the physiological solution), the dendritic tips showed regional swelling and shrinkage. The intramembrane particle density of the swollen parts decreased and there were particle-free patches of membrane, whereas the particle density of the shrunken parts increased. On the other hand when the receptor was fixed with 1.6% glutaraldehyde in 0.2 M phosphate buffer (the total osmolarity is hyperosmotic but buffer osmolarity is isosmotic), the diameter of the dendritic tips became smaller, and their membrane particle densities were almost the same as that under the relaxed condition. The sheath cells covering the sensory neuron were characterized by their sheet-like profiles, gap junctions, and crater-like protrusions. The receptor muscle membrane had longitudinal foldings, occasional invaginations, peripheral couplings, string-shaped particle aggregates, and band-shaped particle aggregates.

Animals↗
PubMed1981

Thin-section and freeze-fracture studies of crayfish stretch receptor synapses including the reciprocal inhibitory synapse.

The crayfish slow-adapting abdominal stretch receptor organ is innervated by three inhibitory and several excitatory axons. A previous study by Tisdale and Nakajima ('76) showed that under certain fixation conditions inhibitory and excitatory synapses can be distinguished on the basis of synaptic vesicle structure. Using this morphological criterion we describe six types of synapses in the receptor: (1) the inhibitory axo-dendritic synapse, (2) the excitatory neuromuscular synapse, (3) the inhibitory neuromuscular synapse, (4) the axo-axonic synapse which suggests presynaptic inhibition of the excitatory synapse, (5) the axo-axonic synapse which suggests presynaptic inhibition on the inhibitory synapse, (6) the reciprocal inhibitory axo-axonic synapse, which is a new type of synapse. The presence of these six types of synapse suggest that inhibitory and excitatory axons interact synaptically in a complicated manner, resulting in a delicate control of receptor function. In freeze fracture we have observed the presynaptic membrane structures of inhibitory and excitatory synapses. The active zone of the inhibitory synapse has ridges with loosely aggregated particles on the tops of the ridges and indentations (vesicle attachment sites) along their sides. The active zone of the excitatory neuromuscular synapse consists of bands of particle aggregates which are situated on slightly elevated membrane regions and surrounded by wide, relatively particle-free, flat membrane areas.

Animals↗
PubMed1983

Inhibitory and excitatory synapses in crayfish stretch receptor organs studied with direct rapid-freezing and freeze-substitution.

Crayfish abdominal stretch receptor organs are innervated by inhibitory (GABA) and excitatory (glutamate) synapses. Previous studies with aldehyde fixation showed that synaptic vesicles in the inhibitory synapse are flat and small, whereas those in the excitatory synapse are rounder and larger. We have reexamined these inhibitory and excitatory synapses by using direct rapid-freezing and freeze-substitution in order to preserve synaptic structure closer to its living state. Fine details of synaptic structure appear to be better preserved by this method. Synaptic vesicles in inhibitory as well as excitatory synapses are round, so the conventional flattened shape of vesicles in the inhibitory synapse must depend on some aspect of aldehyde processing. However, the average size of vesicles in the inhibitory synapse is significantly smaller than that of vesicles in the excitatory synapse, so synaptic vesicle size is regarded as having functional significance.

Animals↗
PubMed1983

Antennal neuropile in the brain of the crayfish: morphology of neurons.

The cellular composition of the antennal neuropile of the crayfish is described. As a context for this work the distribution of neuronal cell bodies throughout the supraoesophageal ganglion (brain) is also described. The neuronal cell bodies in the brain are concentrated in 19 distinct clusters. Three paired clusters are located on the dorsal side of the brain, four paired and one midline cluster bend around the brain laterally and frontally respectively. Fewer than ten somata lie outside of these clusters. The antennal neuropile is composed of primary afferent terminals, efferents, and projecting and local interneurons. The structures of individual neurons of all four types were determined by filling them with Lucifer yellow, and an overview of the neuropile structure was obtained with cobalt backfills of selected nerves. The antennal afferents are concentrated in four main tracts that run medially in the outer layer of the antennal neuropile. Up to 11 orthogonal side branches occur at equal distances (25-35 microns) along the main branches and penetrate the neuropile. The efferents contribute very thin dendrites to the antennal neuropile. The majority of the neuronal mass of the antennal lobe consists of projecting and local interneurons. The branching pattern of the interneurons within the antennal neuropile also shows an orthogonal arrangement of main branches and higher-order branches. Thus the antennal neuropile displays a strong geometrical regularity: Main processes of all four types of neurons run in bundles the length of the long axis of the neuropile (lateral to medial inside the brain) giving rise to orthogonal side branches at regular intervals. This branching pattern leads to a striped appearance of the antennal lobe.

Animals↗
PubMed1985

The structure of the fourth abdominal ganglion of the crayfish, Procambarus clarki (Girard). I. Tracts in the ganglionic core.

The organization of the fourth abdominal ganglion of the crayfish, Procambarus clarki, was studied with the light microscope in serial sections stained with osmium ethyl gallate. This ganglion is composed of a ventral rind of somata and a core of alternating layers of through-tracts and commissures. The longitudinal tracts of the ganglion are named according to the system in use for the orthopteran insects, because the basic plans of the crustacean and insect ventral ganglia exhibit striking anatomical parallels. The dorsal tracts are the largest and the most regular in their path through the ganglion. In the ventral posterior quadrant of the ganglion the tracts diverge from the basic plan to pass around the major synaptic neuropil and the bases of the peripheral nerves. This paper reports the three-dimensional anatomy of the major longitudinal through-tracts, internal tracts and commissures, and bases of peripheral nerves. Landmark features of the ganglion--including the tracts, the major artery of the vascular system, the shape of the ganglionic core in section, and prominent single cells, all of which make it possible to recognize specific regions of the ganglion--are described.

Animals↗
PubMed1985

The structure of the fourth abdominal ganglion of the crayfish, Procambarus clarki (Girard). II. Synaptic neuropils.

Four discrete regions of synaptic neuropil in the crayfish fourth abdominal ganglion are described by light and electron microscopy. The largest is the horseshoe neuropil, a horseshoe-shaped mass of synaptic glomeruli that lies horizontally in the ventral ganglionic core. This neuropil has a substructure of three rings of fused glomeruli associated with the entry of small axons from the first and second nerve roots. The lateral neuropils are large, paired bulges of neuropil that define the sides of the ganglionic core. They contain neuronal profiles of various sizes, filled with clear or dense-cored vesicles. The neurons are randomly oriented except for occasional dendritic bundles. The tract neuropil is ultrastructurally similar to the lateral neuropils but it is distributed among the largest axons of the through-tracts and commissures. The midline neuropils are small, U-shaped clumps of uniformly sized neuronal profiles that contain large numbers of dense-cored vesicles and distinctive lamellar inclusions.

Animals↗
PubMed1986

Mapping of proctolinlike immunoreactivity in the nervous systems of lobster and crayfish.

Whole-mount immunocytochemical techniques have been used to map candidate proctolin-containing cells in the central nervous systems of the lobster, Homarus americanus, and the crayfish, Procambarus clarkii. Proctolinlike immunoreactivity was detected in cell bodies and neuropil regions in all central ganglia, and immunoreactive axons were detected in most interganglionic connectives and nerve roots. Cell body staining was confined to fewer than 2% of all cells. Immunoreactive neurons include motoneurons, sensory neurons, neurosecretory cells, and interneurons. Colocalization of the proctolinlike antigen with other neurotransmitters was indicated in a number of cases. Many aspects of the distribution of immunoreactivity were similar in lobster and crayfish; however, staining differences were detected in a number of identified neurons and neural groups, including neurons that innervate the pericardial organs and hindgut motoneurons. Further studies of such neurons might provide interesting clues about the physiological functions of proctolin and the evolution of peptide transmission.

Animals↗
PubMed1986

Regional specialization in synaptic input and output in an identified local nonspiking interneuron of the crayfish revealed by light and electron microscopy.

Lateral inhibition of several mechanosensory interneurons in the crayfish terminal ganglion is mediated by a pair of identified local nonspiking interneurons called local directionally selective (LDS) interneurons. The ultrastructure and synaptic distribution of these interneurons were investigated by using intracellular labeling with horseradish peroxidase for electron microscopy. The LDS interneuron has bilateral, asymmetric arborizations. They are connected by a thick transverse neurite on which no synapses are made. The neurites on the side ipsilateral to the cell body are smooth and receive almost entirely input synapses. On the other hand, the contralateral neurites have prominent numerous swellings or varicosities from which many short fine spines arise, with intermingled input and output synapses. Both synapses, however, are not always associated with varicosities and spines. Presynaptic neurites of the LDS interneuron contain only round agranular synaptic vesicles in the vicinity of synaptic specializations. These morphological findings correlate well with the physiology of this interneuron: it is functionally polarized to receive input on one side (ipsilateral to the cell body) of the ganglion and convey it to the other side.

Afferent Pathways↗
PubMed1986

Functional organization of crayfish abdominal ganglia: I. The flexor systems.

For insect ganglia, Altman (Advances in Physiological Science, Vol. 23. Neurobiology of Invertebrates. New York: Pergamon Press, pp. 537-555, '81) proposed that individual neuropils control different motor activities. A corollary of this hypothesis is that motor neurons involved in many behavioral functions should branch in more neuropils than those active in fewer behaviors. In crayfish, the abdominal fast-flexor muscles are active only during the generation of the powerstroke for tailflips, whereas the slow-flexor muscles are involved in the maintenance of body posture. The slow flexors are thus active in many of the crayfish's behavioral activities. To test the generality of Altman's idea, we filled groups of crayfish fast-flexor and slow-flexor motor neurons with cobalt chloride and described their shapes with respect to the ganglionic structures through which they pass. Individual fast flexors were also filled intracellularly with HRP. Ganglia containing well-filled neurons were osmicated, embedded in plastic, and sectioned. Unstained sections were examined by light microscopy and pertinent sections were photographed. We found that the paths of the larger neurites were invariant, that the dendritic domains of fast and slow motor neurons occupied distinctive sets of neuropils, and that dendrites of slow motor neurons branched in more ganglionic structures than did those of fast motor neurons. These results are consistent with Altman's hypothesis.

Animals↗
PubMed1986

Distribution and ultrastructure of synapses on a premotor local nonspiking interneuron of the crayfish.

Many premotor local nonspiking interneurons are involved in the control of the uropod movements of a crayfish. One of these interneurons was impaled intracellularly, characterized physiologically, and then labeled by intracellular horseradish peroxidase (HRP) injection to examine the distribution and ultrastructure of synapses. Depolarization of this interneuron by a current injection excited the closer motoneurons of the uropod and inhibited its opener motoneurons, but hyperpolarization had no effect. Input and output synapses are distributed all over the major branches and the finer neurites except for the main neurite (7-10 micron in diameter), which runs near the dorsal surface of the neuropil. Both types of synapse are located on the same neurite and are often intermingled in close proximity, often less than 1 micron apart. Presynaptic terminals of the interneuron contain round, clear vesicles that are densely packed in fine branches and spines. The number of synaptic vesicles associated with a particular output synapse was estimated to be about 2,000. No dense-cored granules are observed in the labeled neurites. Our results support the proposal that synaptic transmission in this class of interneurons of the crayfish can sometimes be restricted to a very small region of the branches and that, therefore, different regions of the interneuron can function independently.

Animals↗
PubMed1987

Opposing parallel connections through crayfish local nonspiking interneurons.

Unilateral local nonspiking interneurons in the terminal (sixth) abdominal ganglion of crayfish (Procambarus clarkii Girard) can be classified into two major groups of PL and AL types by their gross morphology and somatic position. These premotor interneurons are the neural components of uropod motor pattern formation. They receive sensory input from the exopodite of the contralateral side as well as that of the ipsilateral side. Small fluctuations in their membrane potentials cause sustained change in activity of the motoneurons innervating the uropod muscles. PL interneurons, which make noninverting connections to an identified closer, the reductor motoneuron No. 1, mainly receive excitatory input from the afferents of the contralateral exopodite, whereas inverting PL interneurons receive inhibitory input. AL interneurons receive distinctly different input from the afferents. Noninverting AL interneurons mainly receive inhibitory input, whereas inverting AL interneurons receive excitatory input. The rate of discharge of the reductor motoneurons is increased by sensory stimulation. The PL interneurons form either excitatory or disinhibitory pathways, which are relevant in function to the observed increase of the motoneuron. Conversely, the AL interneurons form either inhibitory or disfacilitatory pathways. Thus, the PL and the AL interneurons are fractionated in function and distinguishable in terms of their physiology by their input and output correlations. Functional meaning of the presence of these two types of unilateral local nonspiking interneurons of opposing connections in the uropod motor control system is discussed.

Afferent Pathways↗
PubMed1987

Developmental and other factors affecting regeneration of crayfish CNS axons.

According to histological and ultrastructural criteria, nongiant CNS axons in newly hatched crayfish regenerate more rapidly and with greater frequency than do similar axons in adult crayfish. Regenerative ability is greater in one species (Procambarus clarkii) than in another species (Procambarus simulans), is greater at 20-25 degrees C than at 15-16 degrees C, and is greater in nongiant axons than in giant axons. In contrast to axonal regeneration, nerve cell bodies do not regenerate in newly hatched or adult crayfish of either species. While the ability to regenerate CNS axons differs between newly hatched and adult crayfish, the ultrastructural appearance of the CNS is very similar at any age it is examined.

Animals↗
PubMed1987

Functional organization of crayfish abdominal ganglia: II. Sensory afferents and extensor motor neurons.

Abdominal ganglia of crayfish contain identifiable neuropils, commissures, longitudinal tracts, and vertical tracts. To determine the functional significance of this ganglionic framework, we backfilled the following types of neurons with cobalt chloride: sensory hair afferents, slow and fast extensor motor neurons, the segmental stretch receptor neurons, and their inhibitory accessory cells. After the cobalt ions were precipitated and intensified, we studied the central projections of the filled neurons within the ganglionic structures. All of the axons of these neurons exit or enter each of the first five abdominal ganglia through the second pair of nerves. Our description of the central projections of the hair afferents is the first in the literature. These afferents innervate the large ventral horseshoe neuropil (HN) in the core of each ganglion. This neuropil is homologous to the insect ventral association centers, which also process sensory information. Furthermore, we discovered that some of the crayfish afferents innervate glomeruli within the HN. The slow and fast extensor motor neurons, the stretch receptor neurons, and the accessory cells branch mostly in the dorsal part of the ganglion. We reinterpret previous identifications of the extensor neurons that were based largely on soma position. Together with our previous descriptions of the flexor motor neurons, these results allow us to relate both rapid tail-flips and slower postural movements to the structure of the segmental ganglia.

Afferent Pathways↗
PubMed1988

Atlas of serotonin-containing neurons in the optic lobes and brain of the crayfish, Cherax destructor.

An atlas of neurons in the brain of the crayfish Cherax destructor that are immunoreactive to antibodies raised against serotonin has been compiled from whole mount preparations. Neuronal networks of serotonin-containing cells are identified in the optic lobes and protocerebrum, in the deutocerebrum, and in the tritocerebrum. The consistency of the whole-mount technique allows 50 out of a total of about 100 immunoreactive cells to be individually identified according to their neuronal architecture or the location of their cell somata or axons. Apart from six neurons with axons in the oesophageal connectives, all the immunoreactive cells are intrinsic to the optic lobes and brain.

Animals↗
PubMed1988

Immunocytochemical studies of the distribution of acetylcholine in the crayfish brain.

A number of studies indicate that acetylcholine is an important transmitter in most crustacean primary afferents and in at least several central pathways. Little is known, however, regarding the structure or distribution of cholinergic pathways in the central nervous system. The recent introduction of antibodies to choline-protein conjugates provides a potentially powerful means for localizing putative cholinergic neurons and pathways in the nervous system. Acetylcholine was localized with immunocytochemical procedures in the axons and terminals of cephalic primary afferents and in interneurons of the crayfish brain. The most intensely reactive loci were the primary sensory neuropiles, which contain the terminals of the statocyst afferents (parolfactory lobes) and antennal afferents (antennal lobe). These results are generally in accord with previous findings based upon choline uptake and enzyme assay in lobster cephalic nerves. We also found evidence consistent with the presence of acetylcholine in the globular interneurons of the accessory lobe and in descending interneurons which originate in the dorsal medial and anterior clusters of the protocerebrum. The axons of several neurons in the circumesophageal connective (descending interneurons and primary afferents) are also reactive to the choline antibody.

Acetylcholine↗
PubMed1989

Distribution of radioactive leucine following uptake by olfactory sensory neurons in normal and heteromorphic crayfish antennules.

Surgical excision of one of the compound eyes from juvenile crayfish leads to the regeneration of a heteromorphic antennule in 30% of the cases. Most of the heteromorphic antennules generated this way are bifurcate appendages possessing morphologically distinct medial and lateral branches. These structures are identical to the internal and external flagella of the normal antennules, and the homolog of the external flagellum bears aesthetascs supplied by olfactory sensory neurons. Autoradiographic analysis of the brain following exposure of heteromorphic antennules to tritiated leucine indicates that the supernumerary sensory axons transport the labeled amino acid into their central terminals at appropriate target locations within the ipsilateral olfactory lobe. The data suggest that olfactory input from heteromorphic antennules is incorporated into the organized central projection of olfactory afferents from the normal antennule.

Animals↗
PubMed1989

Comparative study of neuronal and glial gap-junctions in crayfish nerve cords.

Electrical synapses (neuronal gap-junctions) and glial gap-junctions were compared by using thin sectioning, freeze fracturing, and negative staining techniques. Neuronal and glial gap-junctions differed in the length of the extracellular domains of the channels, in the presence of a cytoskeleton associated to neuronal gap-junctions, and in their unit cell dimensions. The difference in length of the channels and the fact that both glial and neuronal gap-junctions had the same particle diameter suggest that the proteins forming glial and neuronal gap-junctions might have different molecular weights. The cytoskeleton associated to neuronal gap-junctions consisted of a beaded layer of densities located parallel to the membrane in the synaptic regions. Synaptic vesicles associated to neuronal gap-junctions were attached to this cytoskeleton, which was in turn anchored to the synaptic membrane through densities about 20 nm apart, a spacing similar to the neuronal unit cell dimension. These results suggest that the cytoskeleton might be responsible for the association of vesicles to neuronal gap-junctions and for maintaining the crystalline appearance of neuronal gap-junctions in situ.

Abdomen↗