Fine structural study of the abdominal muscle receptor organs of the crayfish (Procambarus clarkii). Fast and slow receptor muscles

Antidromic potential spread modulates the receptor responses in the stretch receptor neurons of the crayfish

The effects of antidromic potential spread were investigated in the stretch receptor neurons of the crayfish. Current and potential responses to conductance changes were recorded in the dynamic clamp condition and compared to those obtained by using some conventional clamp methods and a compartmental neuron model. An analogue circuit was used for dynamic calculation of the injected receptor current as a function of the membrane potential and the given conductance change. Alternatively, receptor current responses to a mechanical stimulus were recorded and compared when the cell was voltage clamped to a previously recorded impulse wave form and the resting potential, respectively. Under dynamic clamp, the receptor current had an oscillating waveform which contrasts with the conventional recordings. Frequency, amplitude and sign of the oscillations were dependent on the applied conductance level, reversal potential and electrotonic attenuation. Mean current amplitude and frequency of the evoked impulse responses were smaller under dynamic clamp, especially for large conductance increases. However, firing frequency was larger if plotted against the mean current response. Recorded responses were similar to those calculated in the model. It was not possible to evoke any adaptation in the slowly adapting neuron by using the dynamic clamp. Evoked potential change served as a self limiting response, preventing the depolarization block. However, impulse duration was significantly shorter in the rapidly adapting neuron when the dynamic clamp was used. It was concluded that, in the stretch receptor neurons during a conductance increase, antidromic potential spread modulates the receptor responses and contributes to adaptation.

Muscle anatomy is a primary determinant of muscle relaxation dynamics in the lobster (Panulirus interruptus) stomatogastric system

We stained sarcomere thin filaments with fluorescently labeled phalloidin, measured sarcomere and muscle length, and calculated sarcomere number in pyloric and gastric mill muscles. A wide range of sarcomere lengths (3.25-12.29 microm), muscle lengths (5.9-21.1 mm), and sarcomere numbers (648-3,036) were observed. Sarcomere number differences occurred both because of changes in sarcomere length and muscle length, and sarcomere and muscle length varied independently. This independence, the wide range of sarcomere numbers present, and the muscles being all 'slow', graded muscles allowed us to use these data to test Huxley and Neidergerke's (1954) hypothesis that muscle dynamics depend on sarcomere number. The time constants of exponential fits to contraction relaxations were used to measure muscle dynamics, and comparison of theoretical predictions and experimental results quantitatively confirm the predicted dependence. The differing dynamics of the various pyloric muscles are likely functionally important, and the dependence of muscle dynamics on sarcomere number implies that sarcomere number is likely closely regulated in these muscles. The stomatogastric system may thus be an excellent model system for studying the mechanisms regulating muscle sarcomere number.

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.

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.

Differences in maximal activation properties of skinned short- and long-sarcomere muscle fibres from the claw of the freshwater crustacean Cherax destructor

Single fibres of different sarcomere length at rest have been isolated from the claw muscle of the yabby (Cherax destructor), a decapod crustacean. Fibres of either long (SL > 6 microns) or short (SL < 4 microns) sarcomere length have been mechanically skinned and were maximally activated by Ca2+ and Sr2+ under various experimental conditions (ionic strength, in the presence of 2,3 butanedione monoxime (BDM)) to determine differences in their contractile properties. Isometric force was measured simultaneously with either myofibrillar MgATPase or fibre stiffness in both fibre types. The ultrastructure of individual long- and short-sarcomere fibres was also determined by electron microscopy. The long-sarcomere fibres developed greater tension (30.48 +/- 1.72 N cm-2) when maximally activated by Ca2+ compared with the short-sarcomere fibres (18.60 +/- 0.80 N cm-2). The difference in the maximum Ca(2+)-activated force can be explained by the difference in the amount of filament overlap between the two fibre types. The maximum Ca(2+)-activated myofibrillar MgATPase rate in the short-sarcomere fibres (1.60 +/- 0.27 mmol ATP l-1s-1) was higher, but not significantly different from the ATPase rate in fibres with long-sarcomeres (1.09 +/- 0.14 mmol ATP l-1s-1). As the concentration of myosin is estimated to be higher only by a factor of 1.22 in the short-sarcomere preparations there is no evidence to suggest that the myofibrillar MgATPase activity is different in the long- and short-sarcomere preparations. The maximum Ca(2+)-activated force (P0) of both short- and long-sarcomere fibres was quite insensitive to BDM compared with vertebrate muscle. Force decreased to 60.2 +/- 5.3% and 76.1 +/- 2.7% in the short- and long-sarcomere fibres respectively in the presence of 100 mmol l-1 BDM. The difference in the force depression between the long- and short-sarcomere fibres is statistically significant (p < 0.05). Fibre stiffness during maximum Ca(2+)-activation expressed as percentage maximum force per nm per half sarcomere was higher by a factor of 3.5 in short-sarcomere fibres than in long-sarcomere fibres suggesting that the compliance of the filaments in the long-sarcomere fibres is considerably higher than in the short-sarcomere fibres. Sr2+ could not activate the contractile apparatus to the same level as that seen by Ca2+ in either fibre type: the maximum Sr(2+)-activated force was (20 +/- 3%) and (63 +/- 3%) of the maximum Ca(2+)-activated force response in short- and long-sarcomere fibres, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Higher mitochondrial density in slow versus fast lobster sensory neurons

The stretch-sensitive muscle receptor organ (MRO) in the abdomen of the lobster Homarus americanus contains an identifiable fast and a slow sensory neuron. Morphometric analysis of electron micrographs of areas through the somata of these neurons revealed a higher density of mitochondria in the slow versus the fast cell (19 vs 15%). Such differences in oxidative capacity are closely matched with differences in their physiological performances.

Fine structural study of the abdominal muscle receptor organs of the crayfish (Procambarus clarkii). Sensory endings and synaptic structures

The sensory endings, neuromuscular junctions and interneuronal synapses in the crayfish muscle receptor organ have been studied by electron microscopy. The dendrites of the receptor neuron terminate as endings which are either free in the connective tissue matrix of the central region of the receptor strands, or abut on the muscle membrane forming a specialized junction with a narrow cleft of about 18 nm. Efferent nerve endings are classified into three types on the basis of their fine structural features. Type 1 endings contain mainly spherical vesicles with a diameter of about 55 nm and a few large granular vesicles with a diameter of about 100 nm, and synapse exclusively on muscle fibres. Type 2 endings have a high proportion of elongated vesicles measuring about 30 X 80 nm and a few large granular vesicles, and synapse on both sensory neurons and muscle. Type 3 endings are characterized by the high electron density of the axoplasm and numerous large granular vesicles with a diameter of about 100 nm; they synapse only on the sensory neuron of the slow receptor unit. It is suggested that Type 1 endings are excitatory, and Type 2 and 3 endings are inhibitory. Several differences in postsynaptic structure were observed between the putative excitatory and inhibitory neuromuscular junctions. Axo-axonal synapses between endings of Type 1 and Type 2, the latter being presynaptic to the former, are also found. Functional implications and possible roles of these structures are discussed.