Ultrastructure of the crayfish stretch receptor in relation to its function

Protection of the Crayfish Mechanoreceptor Neuron and Glial Cells from Photooxidative Injury by Modulators of Diverse Signal Transduction Pathways

Oxidative stress is the reason of diverse neuropathological processes. Photodynamic therapy (PDT), an effective inducer of oxidative stress, is used for cancer treatment, including brain tumors. We studied the role of various signaling pathways in photodynamic injury and protection of single neurons and satellite glial cells in the isolated crayfish mechanoreceptor. It was photosensitized with alumophthalocyanine Photosens in the presence of inhibitors or activators of various signaling proteins. PDT eliminated neuronal activity and killed neurons and glial cells. Inhibitory analysis showed the involvement of protein kinases Akt, glycogen synthase kinase-3β (GSK-3β), mammalian target of rapamycin (mTOR), mitogen-activated protein kinase kinases 1 and 2 (MEK1/2), calmodulin, calmodulin-dependent kinase II (CaMKII), adenylate cyclase, and nuclear factor NF-κB in PDT-induced necrosis of neurons. Nitric oxide (NO) and glial cell-derived neurotrophic factor (GDNF) reduced neuronal necrosis. In glial cells, protein kinases Akt, calmodulin, and CaMKII; protein kinases C and G, adenylate cyclase, and p38; and nuclear transcription factor NF-κB also mediated PDT-induced necrosis. In contrast, NO and neurotrophic factors nerve growth factor (NGF) and GDNF demonstrated anti-necrotic activity. Phospholipase Cγ, protein kinase C, GSK-3β, mTOR, NF-κB, mitochondrial permeability transition pores, and NO synthase mediated PDT-induced apoptosis of glial cells, whereas protein kinase A, tyrosine phosphatases, and neurotrophic factors NGF, GDNF, and neurturin were involved in protecting glial cells from photoinduced apoptosis. Signaling pathways that control cell survival and death differed in neurons and glia. Inhibitors or activators of some signaling pathways may be used as potential protectors of neurons and glia from photooxidative stress and following death.

Ultrastructure of neuroglial contacts in crayfish stretch receptor

In order to explore neuroglial relationships in a simple nervous system, we have studied the ultrastructure of the crayfish stretch receptor, which consists of only two mechanoreceptor neurons enwrapped by glial cells. The glial envelope comprises 10-30 glial layers separated by collagen sheets. The intercellular space between the neuronal and glial membranes is generally less than 10-15 nm in width. This facilitates diffusion between neurons and glia but restricts neuron communication with the environment. Microtubule bundles passing from the dendrites to the axon through the neuron body limit vesicular transport between the perikaryon and the neuronal membrane. Numerous invaginations into the neuron cytoplasm strengthen glia binding to the neuron and shorten the diffusion pathway between them. Double-membrane vesicles containing fragments of glial, but not neuronal cytoplasm, represent the captured tips of invaginations. Specific triads, viz., "flat submembrane cisterns - vesicles - mitochondria", are presumably involved in the formation of the invaginations and double-membrane vesicles and in neuroglial exchange. The tubular lattice in the glial cytoplasm might transfer ions and metabolites between the glial layers. The integrity of the neuronal and glial membranes is impaired in some places. However, free neuroglial passage might be prevented or limited by the dense diffuse material accumulated in these regions. Thus, neuroglial exchange with cellular components might be mediated by transmembrane diffusion, especially in the invaginations and submembrane cisterns, by the formation of double-walled vesicles in which large glial masses are captured and by transfer through tubular lattices.

Mechanotransduction and the crayfish stretch receptor

Mechanotransduction or mechanosensitivity is found in almost every cell in all organisms from bacteria to vertebrates. Mechanosensitivity covers a wide spectrum of functions from osmosensing, cell attachment, classical sensory mechanisms like tactile senses in the skin, detection of sound in hair cells of the hearing apparatus, proprioceptive functions like recording of muscle length and tension in the muscle spindle and tendon organ, respectively, and pressure detection in the circulation etc. Since most development regarding the molecular aspects of the mechanosensitive channel has been made in nonsensory systems it is important to focus on mechanosensitivity of sensory organs where the functional importance is undisputed. The stretch receptor organ of the crustaceans is a suitable preparation for such studies. The receptor organ is experimentally accessible to mechanical manipulation and electrophysiological recordings from the sensory neuron using intracellular microelectrode or patch clamp techniques. It is also relatively easy to inject substances into the neuron, which also makes the neuron accessible to measurements with fluorescent techniques. The aim of the present paper is to give an up to date summary of observations made on the transducer properties of the crayfish stretch receptor (Astacus astacus and Pacifastacus leniusculus) including some recent unpublished findings. Finally some aspects on future line of research will be presented.

Ion channels for mechanotransduction in the crayfish stretch receptor

Mechanosensitivity is found in almost every cell in all organisms from bacteria to vertebrates and covers a wide spectrum of function from osmosensing to mechanical sensing in the specialized receptors, such as the hair cells of the cochlea. The molecular substrate for such mechanosensitivity is thought to be mechanosensitive ion channels (MSCs). Because most development regarding the molecular aspects of the MSC has been made in nonsensory or sensory systems, which have not been accessible to recordings from ion channels, it is important to focus on the mechanosensitivity of sensory organs where their functional importance is undisputed. The stretch receptor organ (SRO) of the crustaceans is a suitable preparation for such studies. Each organ contains two receptors: one slowly and one rapidly adapting receptor neurons. The primary mechanosensitivity is generated by two types of MSC of hitherto unknown molecular type located in the neuronal dendrites, which are inserted into a receptor muscle fiber. In addition to the MSCs, the neurons contain voltage-gated Na(+) channels, which seem to be differently located in the slowly and rapidly adapting neurons. At least three types of voltage-gated K(+) channels are present in the sensory neurons, the location of which is not known. The spatial distribution of ion channels and the kinetics of the channels, together with the viscoelastic properties of the receptor muscles, determine the overall transducer properties and impulse firing of the two receptor neurons, including their typical adaptive characteristics.

Structure and function relationship in the abdominal stretch receptor organs of the crayfish

The structure/function relationship in the rapidly and slowly adapting stretch receptor organs of the crayfish (Astacus leptodactylus) was investigated using confocal microscopy and neuronal modeling methods. Both receptor muscles were single muscle fibers with structural properties closely related to the function of the receptors. Dendrites of the rapidly adapting neuron terminated in a common pile of nerve endings going in all directions. Dendrites of the slowly adapting neuron terminated in a characteristic T shape in multiple regions of the receptor muscle. The slowly adapting main dendrite, which was on average 2.1 times longer and 21% thinner than the rapidly adapting main dendrite, induced larger voltage attenuation. The somal surface area of the slowly adapting neuron was on average 51% larger than that of the rapidly adapting neuron. Variation in the neuronal geometry was greatest among the slowly adapting neurons. A computational model of a neuron pair demonstrated that the rapidly and the slowly adapting neurons attenuated the dendritic receptor potential like low-pass filters with cut-off frequencies at 100 and 20 Hz, respectively. Recurrent dendrites were observed mostly in the slowly adapting neurons. Voltage signals were calculated to be propagated 23% faster in the rapidly adapting axon, which is 51% thicker than the slowly adapting axon. The present findings support the idea that the morphology of the rapidly and the slowly adapting neurons evolved to optimally sense the dynamic and the static features of the mechanical stimulus, respectively.

Peripheral synaptic contacts at mechanoreceptors in arachnids and crustaceans: morphological and immunocytochemical characteristics

Two types of sensory organs in crustaceans and arachnids, the various mechanoreceptors of spiders and the crustacean muscle receptor organs (MRO), receive extensive efferent synaptic innervation in the periphery. Although the two sensory systems are quite different-the MRO is a muscle stretch receptor while most spider mechanoreceptors are cuticular sensilla-this innervation exhibits marked similarities. Detailed ultrastructural investigations of the synaptic contacts along the mechanosensitive neurons of a spider slit sense organ reveal four important features, all having remarkable resemblances to the synaptic innervation at the MRO: (1) The mechanosensory neurons are accompanied by several fine fibers of central origin, which are presynaptic upon the mechanoreceptors. Efferent control of sensory function has only recently been confirmed electrophysiologically for the peripheral innervation of spider slit sensilla. (2) Different microcircuit configuration types, identified on the basis of the structural organization of their synapses. (3) Synaptic contacts, not only upon the sensory neurons but also between the efferent fibers themselves. (4) Two identified neurotransmitter candidates, GABA and glutamate. Physiological evidence for GABAergic and glutamatergic transmission is incomplete at spider sensilla. Given that the sensory neurons are quite different in their location and origin, these parallels are most likely convergent. Although their significance is only partially understood, mostly from work on the MRO, the close similarities seem to reflect functional constraints on the organization of efferent pathways in the brain and in the periphery.

Ultrastructural characterization of the accessory lobes of Lachi in the lumbosacral spinal cord of the pigeon with special reference to intrinsic mechanoreceptors

The lumbosacral spinal cord of birds is unique among vertebrates in that segmentally organized accessory lobes protrude from the ventrolateral spinal cord into the vertebral canal. Recently, it has been suggested that these lobes may be part of an extralabyrinthine sense organ of equilibrium. For a better understanding of such a function, a complete analysis of the structural basis was performed by means of electron microscopy. The lobes consist of multipolar neurons, myelinated and unmyelinated axons, glia-derived glycogen cells, glial cells, and capillaries. The dorsal part of the lobe is covered by a loose mesh of pia mater. Ventrolaterally, an arachnoidal trabecle is in close contact with the lobe. Extracellular lacunae extend from the periphery deep into each lobe. The lacunae are separated from the subarachnoidal space by a loose mesh of processes of the glycogen cells with its basal lamina. The lacunae are filled by a network of processes of glycogen cells, glial cell, dendrites, and small axons. Both neuronal somata and dendrites are contacted by numerous axon terminals that form rather uniform synapses. Finger-like processes emerge from both the somata and the dendrites. The dendrites branch deeply into the extracellular lacunae and form lateral ramifications, which consist of narrow stalks with serially arranged bulbous portions, from which finger-like processes emerge. Finger-like processes are well-known elements in mechanotransduction. Glycogen cells and lacunae may contribute to transmission of hydrostatic pressure changes during movements of the body.

Adaptive motor control in crayfish

This article reviews the principles that rule the organization of motor commands that have been described over the past five decades in crayfish. The adaptation of motor behaviors requires the integration of sensory cues into the motor command. The respective roles of central neural networks and sensory feedback are presented in the order of increasing complexity. The simplest circuits described are those involved in the control of a single joint during posture (negative feedback-resistance reflex) and movement (modulation of sensory feedback and reversal of the reflex into an assistance reflex). More complex integration is required to solve problems of coordination of joint movements in a pluri-segmental appendage, and coordination of different limbs and different motor systems. In addition, beyond the question of mechanical fitting, the motor command must be appropriate to the behavioral context. Therefore, sensory information is used also to select adequate motor programs. A last aspect of adaptability concerns the possibility of neural networks to change their properties either temporarily (such on-line modulation exerted, for example, by presynaptic mechanisms) or more permanently (such as plastic changes that modify the synaptic efficacy). Finally, the question of how "automatic" local component networks are controlled by descending pathways, in order to achieve behaviors, is discussed.

Multiterminal proprioceptors in Squilla mantis (Stomatopoda, Crustacea)

A series of morphologically simple multiterminal proprioceptors is described in the thoracopods of the hoplocaridian crustacean, Squilla mantis, which is a primitive offshoot of the main malacostracan evolutionary line. The description given here is primarily for the largest and most complex receptor that occurs in the second thoracopod, the raptorial limb. These stretch receptors, which lie distally in the coxopodite, consist of single multiterminal sensory neurons with widely branching dendrites innervating broad connective tissue strands. The single primary dendrite divides dichotomously or, occasionally, trichotomously to produce a large number of terminal dendrites. The terminal dendrites, which exhibit a beaded structure, run parallel to the connective tissue strands' longitudinal axis. Dendrites possess an elaborate connective tissue sheath consisting of a single inner glial layer and a multiple outer connective tissue layer. The presence of the beaded structure has been confirmed by transmission electron microscopy (EM), and, because microtubule density does not vary with dendrite diameter, the beads are not fixation artifacts. Beads have high numbers of mitochondria and glycogen deposits, which suggests that they have a high metabolic activity. Dendritic tips are not associated with beads, arise from the terminals of smaller dendrites, and are embedded in the extracellular ground matrix of the connective tissue strand, where they lie parallel to and surround bundles of elastic fibrils. The different degree of development within the receptor series allows comparison between functionally different limbs, and comparison is made with other segmental receptor series. The evolution of multiterminal proprioceptors is discussed.

Visco-elastic properties of the rapidly adapting stretch receptor muscle of the crayfish

The visco-elastic properties of the receptor muscle associated with the rapidly adapting stretch receptor organ of crayfish (Pacifastacus Leniusculus) were studied by recording the tension responses to various length changes. Steady-state length changes resulted in a non-linear tension development in the receptor muscle. The tension increased slowly for small extensions and more rapidly when extension increased. Muscle tension responses to ramp-and-hold extension were characterized by a transient peak followed by a gradual non-exponentional decline in tension. At the onset of the ramp the tension increased rapidly, similar to what has been observed in the muscle of the slowly adapting receptor (SM). The steeper rise in tension during the first part of the ramp indicating higher initial stiffness, resulted in a 'hump' when large extensions (> 15%) were applied. The results show that the rapidly adapting receptor muscle has a more pronounced dynamic component; the ratio between the amplitude of the peak and the steady state response was larger in the rapidly than in the slowly adapting receptor muscle. Accordingly, different values for the elements of a visco-elastic model of the muscle had to be set for the two types of receptors. The different properties of the rapidly and slowly adapting receptor muscles are in line with the differences in the overall adaptive behaviour of the organ and give further support to the idea that mechanical factors contribute to the adaptive properties.

Stimulus-response properties of the slowly adapting stretch receptor neuron of the crayfish

The receptor potential and receptor current in response to ramp-and-hold extensions were measured in the slowly adapting stretch receptor of the crayfish, using potential clamp technique. The stimulus-response relationship for the peak amplitude of the receptor current showed a linear behaviour for extensions less than 2% and a nonlinear behaviour for extensions larger than 5%. Using the Stevens power law, R = k(S--S0)n, where R is response, S is stimulus, S0 is threshold stimulus and n the power coefficient, n was found to be 3 for extensions between 5 and 15%. The receptor current saturated at extensions above 20-25% of the zero length of the muscle, resulting in a lower n value. However, the n value is difficult to define in this region due to the saturation. The stimulus-response relation for the receptor current can be explained by the properties of the stretch-activated channels for which the open probability is exponentially dependent on the square of the membrane tension, as suggested by recent findings. The receptor potential, using tetrodotoxin, in response to identical ramp-and-hold extensions as those used to record current responses showed a more complex time-course, indicating involvement of potential-dependent channels, potassium channels being the most probable candidate. This was supported by a mathematical model which takes into account the viscoelastic properties of the receptor muscle, the properties of the stretch-activated channels and a potential dependent K+ current.

Potassium channels in crustacean glial cells

Unitary currents through single ion channels in the glial cells, which ensheath the abdominal stretch receptor neurons of the crayfish, were characterized with respect to their basic kinetic properties. In cell-attached and excised patches two types of Ca(++)-independent K+ channels were observed with slope conductances of 57 pS and 96 pS in symmetrical K+ solution. The 57 pS K+ channel was weakly voltage-dependent with a slope of the Po vs. membrane potential relationship of +95 mV for an e-fold change in Po. In addition to the main conductance level, the channel displayed conductance levels of 80 and 109 pS. In excised patches, channel activity of this "subconductance" K+ channel showed "rundown" that could be prevented with 2 mM ATP-Mg on the cytoplasmic side of the membrane. The 96 pS K+ channel was strongly voltage-dependent with a slope of +12 mV for an e-fold change in Po. Averaged single-channel currents elicited by voltage jumps proved the channel to be of the delayed rectifying type. Channel activity persisted in excised patches with minimal salt solution and in virtually Ca(++)-free saline. Because of its dependence on intracellular ATP-Mg, the subconductance K+ channel is discussed as a target of modulation by transmitters or peptides via phosphorylation of the channel.

Viscoelastic properties of the slowly adapting stretch receptor muscle of the crayfish

The viscoelastic properties of the muscle associated with the slowly adapting stretch receptor organ of the crayfish (Astacus astacus) were studied by recording the tension response to various length changes. When steady-state length changes were applied to the muscle, the tension developed in a non-linear way, increasing slowly for small extensions and rapidly when extension increased. Muscle tension responses to ramp-and-hold extensions were characterized by a transient peak followed by a gradual decline in tension. At the onset of the ramp the tension increased rapidly, similar to the response seen in resting skeletal muscle. The relation between peak dynamic tension and extension was non-linear. In a log-log plot the relation was linear with a mean slope of 1.4. At small extensions (less than 5%) the slope seemed to be lower. The experimental results have been analysed in relation to a viscoelastic model consisting of a Voigt element in series with a non-linear spring. The model could describe both the static length-tension relation and the dynamic response, but different parameters for the springs had to be used for the two cases. When the measured tension response was transformed by an exponential function of the squared tension, in accord with recent findings on stretch-activated channels, a good agreement was obtained with the time course of the receptor currents. Adaptation is thus likely to be caused by both the mechanical properties of the receptor muscle and the characteristics of stretch-activated channels of the neuron.

Functional morphology of the telson-uropod stretch receptor in the sand crab Emerita analoga

The telson-uropod stretch receptor in Emerita analoga belongs to a segmental array of axial-coxal receptors having sensory neurons with central somata. It consists of an elastic strand innervated by four giant, nonspiking, mechanoreceptive neurons. We examined the elastic (sensory) strand by light and electron microscopy to discover whether 1) functional differences between the four sensory neurons could be surmised from their peripheral terminations and 2) the process of mechano-electric transduction in this type of receptor might be revealed by the ultrastructure of the dendritic terminals. The elastic strand is divided into nonoverlapping domains by the mechanoreceptive neurons. We discerned no differences other than their serial order along the strand. Each neuron has three morphological zones in the periphery. In the zone of dendrite entry, the dendrite bifurcates into 40-60 microns primary branches that enter the strand at acute to right angles and turn rostrally and caudally along the strand's axis. The zone of branching consists of 4-35 microns diameter dendritic branches within the strand. Approximately 21,000 dendritic tips (diameters, 0.1-0.7 micron; lengths, 3-20 microns) per sensory neuron arise via short stubs from all levels of the zone of branching and constitute the zone of dendrite termination. The repeated bifurcations of dendrites within the zone of branching take place by radial intrusion of longitudinally oriented, partitions of extracellular matrix each of which extends the length of the pair of dendritic branches it divides. Tips project into these partitions. Since they change shape when the elastic strand is stretched, which compresses the extracellular matrix of the partitions, they are probably the site of mechanosensory transduction. Cross-sectional profiles of dendritic tips are significantly larger but less numerous in stretched than in relaxed receptors. We propose a model in which stretch compresses distal portions of tips until they are too small to be recognized. The large profiles are the proximal portions of tips that have been expanded by hydrostatic pressure. The very large number of dendritic tips/nonspiking mechanoreceptor may be what endows each neuron with 1) high sensitivity to stretch of the elastic strand and 2) ability to respond to a broader range of tensions than is usual for single sensory cells (the entire range experienced by the stretch receptor).

Structure and localization of synaptic complexes in the cardiac ganglion of a portunid crab

The cardiac ganglion of Portunus sanguinolentus exhibits spontaneous rhythmic activity when isolated. The ganglion contains five large and four small intrinsic neurons and is innervated by three pairs of fibres originating in the thoracic ganglia. We have identified the processes of the large neurons in electron micrographs by injecting these cells with two electron-dense markers, horseradish peroxidase (HRP) and Procion Rubine (PR). In addition we have studied the processes of the four smaller neurons by light microscopy serial reconstructions and by electron microscopy of selected regions. Both markers were found only in neuronal processes and not in glial cells nor in the extracellular space, except close to the soma of the injected cell. We found contacts between the small secondary (collateral) processes of the large cells but not between their somata or their primary processes (axons and dendrites). Two specialized structures present at the contacts between the collateral processes were small membrane close appositions, possibly the site of electrotonic junctions, and chemical synapses. Contacts between processes marked by HRP and those marked by PR were common, as were contacts between processes marked by either HRP or PR and those of the other intrinsic neurons. Adjacent processes stained by PR could contain PR deposits of different densities, but it is unclear whether this finding was due to intercellular diffusion of the dye or to its diffusing at different rates into branches of the same process. Identified processes of all the intrinsic neurons contained the same type of vesicles, which were different from those found in processes of the extrinsic fibres. Chemical synapses were present at contacts between processes of the extrinsic and intrinsic neurons, as well as at contacts between processes of the intrinsic neurons. The axons of three small cells made a series of contacts at which extensive arrays of membrane close appositions, but not chemical synapses, were found. These three axons also formed contacts, either directly or through their collateral branches, with processes of the large cells, at which both membrane close appositions and chemical synapses were present. The axon of the fourth small cell could not be followed in our series.

Ultrastructure of the phasic stretch receptor in the crayfish abdominal nerve cord

The ultrastructure of the abdominal ganglionic cord stretch receptor of the crayfish Pacifastacus leniusculus is described. This bilaterally-paired, segmentally-repeating phasic receptor monitors stretch applied to the central nervous system itself. It consists of a connective tissue mass closely applied to the medial margin of each medial giant fibre, into which ramifies a collection of specialized terminal dendrites originating from branches (primary dendrites) of a single axon. The connective tissue consists of an electron-opaque matrix in which are embedded many short, electron-lucent, tubular structures whose lumens are continuous with the matrix. Some filamentous material penetrates the connective tissue from its boundaries, and glial cells are present. The primary dendrites are irregular in size and orientation, and contain many microtubules and much filamentous material. The terminal dendrites are of consistent diameter and longitudinal orientation, containing very regularly-spaced microtubules with no microfilaments. The terminal dendrites contain a well-defined cytoskeletal 'tube' or lamina 6 nm thick, evenly spaced about 25 nm below the plasma membrane and connected to it by filamentous material 5 nm in diameter, which is deposited in rings or helices. This lamina arises just at the point where the primary dendrites gave rise to the terminal dendrites. Its function is not known, but it shows some similarities to the subaxolemmal lamina found in some regions of spike initiation.

Is the intimate relationship between ligaments and marginal specialized cells in the snake's spinal cord indicative of a CNS mechanoreceptor?

Large neurons at the ventrolateral edge of the snake's spinal cord are intimately associated with ligaments that closely adhere to the whole length of the spinal cord. Ultrastructural studies show close similarities of these cells and their processes with those of other mechanoreceptors. Furthermore, the ligaments undergo changes within the intervertebral areas that would enhance focusing the stimulus to the receptor area.

Ultrastructural changes of the sensory nerve terminals in frog muscle spindle during dynamic stretch

The sensory ending of the frog muscle spindle consists of bulbous swellings interconnected by thin, tube-like axonal branches. This study was made to determine if the bulb or thin tube regions are deformed to the same degree during dynamic stretch, by comparing spindles prepared in the relaxed and stretched states. Isolated muscle spindles were rapidly frozen, either in a completely relaxed state or at the end of dynamic stretch to 130% of the in situ length. Longitudinal sections for ultra-high-voltage electron microscopy revealed that dynamic stretch caused a decrease of 61.7% in bulb diameter, an increase of 88% in bulb length and an increase of 34.6% in bulb volume. The cross-sectional areas of the nonmyelinated terminals in the reticular and compact zones measured in electron micrographs decreased by 79.2% in the bulb region and 66.7% in the tube region during dynamic stretch.

The structure of cytoplasm in directly frozen cultured cells. I. Filamentous meshworks and the cytoplasmic ground substance

Cultured fibroblasts or epithelial cells derived from Xenopus laevis embryos were directly frozen, freeze-substituted by an improved method, and then either critical-point-dried and viewed as whole mounts, or embedded and thin sectioned. In thin regions of these cells, where ice crystal artifacts are absent, the cytoplasm consisted of a dense, highly interconnected meshwork of filaments, embedded in a finely granular ground substance. The meshwork in directly frozen, intact cells was compared with that in cells that were lysed (physically, with detergents, or with filipin), or fixed with glutaraldehyde before freezing. Although filaments tended to be less numerous in lysed cells, their overall organization was the same as that in intact cells. However, fixation with glutaraldehyde before freezing distorted the meshwork to variable degrees depending on the osmolarity of the fixation buffer, and also obscured the granular ground substance which is obvious in directly frozen cells. With optimal preparative methods, the cytoplasm of these directly frozen cells is shown to consist of a cytoskeleton composed of discrete interwoven filaments interconnected by numerous finer filaments and a readily extractable granular matrix which presumably represents aggregations of cytoplasmic proteins.

The edge cell, a possible intraspinal mechanoreceptor

In the lateral edge of the "white matter" in the lamprey spinal cord, there is a group of nerve cells referred to as edge cells. The results of a combined physiological, light microscopical, and electron microscopical study suggest that these cells serve as intraspinal mechanoreceptors. Edge cells are depolarized on stretch of the lateral margin of the spinal cord, and they have nestlike ramifications in this region oriented in a rostrocaudal plane. These cells exhibit a close structural similarity with the crayfish stretch receptor.

Electrical and mechanical properties of the crustacean stretch receptor during sinusoidal length changes

Isolated slowly adapting stretch receptors of the crayfish (Astacus fluviatilis) were exposed to sinusoidal length changes. The mechanical force, the receptor current and the receptor potential were analysed in terms of frequency response (Bode plots) and input/output functions. Within the frequency range investigated (0.3 to 80 Hz) the mechanical force was found to be only slightly dependent on the frequency of the sinusoidal stimuli, the slope of the gain function in the Bode plot being close to zero. Dynamic length-force curves exhibited a characteristic hysteresis. In potential clamp experiments, the receptor current induced by sinusoidal length changes was dependent on the clamp potential attaining maximal amplitude at a membrane potential of about -20 to -30 mV for 80 Hz mechanical stimulation. The membrane potential at which the receptor current changed sign (the reversal potential) increased with increasing frequency. In Bode plots of the receptor current the gain was represented by a straight line, having a slope of about 1.2 dB/octave. The phase shift was positive at low frequencies. Bode plots of the receptor potential gain exhibited a characteristic peak in the region of 15 Hz. Below this peak the gain increased with about 1.2 dB/oct, above the peak the gain decreased with about 4.8 dB/oct. There was a small positive phase shift at low frequencies; at high frequencies the phase shift became negative. The impedance of the cell, as calculated from the receptor potentials and the current responses at resting membrane potential, gave Bode plots comparable to a simple resistive-capacitive filter compatible with the electrical properties of the receptor cell membrane. The cut-off frequency of this low pass filter decreased with decreasing stretch amplitude. One physiological significance of this finding, is that the performance of the receptor is improved at low levels of stimulation by reducing the bandwidth of the system.

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.

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.

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.