Muscle Receptor Organ Research Articles

Pharmacological profiling of stretch activated channels in proprioceptive neurons

Proprioception in mammals and invertebrates occurs through stretch activated ion channels (SACs) localized in sensory endings. In mammals, the primary organs for proprioception are the intrafusal muscle spindles embedded within extrafusal muscle. In invertebrates there are varied types of sensory organs, from chordotonal organs spanning joints to muscle receptor organs (MRO) which are analogous to the mammalian muscle spindles that monitor stretch of muscle fibers. A subset of SACs are the PIEZO channels. They are comprised of a distinct type of protein sequence and are similar among species, from mammals to invertebrates. We screened several new agents (YODA 1, JEDI 2, OB 1 and DOOKU) which have been identified to act on SACs of the PIEZO 1 subtype. JEDI 2 increased activity in the crayfish MRO but not the crab chordotonal organs. The SACs of the crustacean proprioceptors have not been satisfactorily pharmacologically classified, nor has their molecular makeup been identified. We screened these pharmacological agents on model sensory organs in crustaceans to learn more about their subtype classification and compare genomic profiles of related species.

The Effect of CO2, Intracellular pH and Extracellular pH on Mechanosensory Proprioceptor Responses in Crayfish and Crab

Proprioceptive neurons monitor the movements of limbs and joints to transduce the movements into electrical signals. These neurons function similarly in species from arthropods to humans. These neurons can be compromised in disease states and in adverse environmental conditions such as with changes in external and internal pH. We used two model preparations (the crayfish muscle receptor organ and a chordotonal organ in the limb of a crab) to characterize the responses of these proprioceptors to external and internal pH changes as well as raised CO2. The results demonstrate the proprioceptive organs are not highly sensitive to changes in extracellular pH, when reduced to 5.0 from 7.4. However, if intracellular pH is decreased by exposure to propionic acid or saline containing CO2, there is a rapid decrease in firing rate in response to joint movements. The responses recover quickly upon reintroduction of normal pH (7.4) or saline not tainted with CO2. These basic understandings may help to address the mechanistic properties of mechanosensitive receptors in other organisms, such as muscle spindles in skeletal muscles of mammals and tactile as well as pressure (i.e., blood pressure) sensory receptors. KEYWORDS: Proprioception; Sensory; Invertebrate; Carbon Dioxide; Protons; Mechanosensory; Intracellular pH; Extracellular pH

The effects of potassium and muscle homogenate on proprioceptive responses in crayfish and crab.

Proprioception of limbs and joints is a basic sensory function throughout most of the animal kingdom. It is important to understand how proprioceptive organs and the associated sensory neurons function with altered environments such as increased potassium ion concentrations ([K+]) from diseased states, ionic imbalances, and damaged tissues. These factors can drastically alter neuronal activity. To assess this matter, we used the chordotonal organ in a walking leg of a blue crab (Callinectes sapidus) and the muscle receptor organ of the crayfish (Procambarus clarkii). These organs serve as tractable models for the analysis of proprioception. The preparations can help serve as translational models for these effects, which may be observed in other invertebrate species as well as mammalian species (including humans). When extracellular potassium concentration ([K+]o) is increased to 20mM in both preparations, mixed results are observed with activity increasing in some preparations and decreasing in others after mechanical displacement. However, when [K+]o is increased to 40mM, activity drastically decreases in all preparations. Additionally, proprioceptor sensory activity declines upon exposure to a diluted muscle homogenate, which contains a host of intracellular constituents. The robust effects of altered [K+] on proprioception in these models illuminate the potential detriments on neuronal function in cases of severe tissue damage as well as altered [K+]o.

Muscle Receptor Organs in the Crayfish Abdomen: A Student Laboratory Exercise in Proprioception

The primary purpose of this experiment is to demonstrate primary sensory neurons conveying information of joint movements and positions as proprioceptive information for an animal. An additional objective of this experiment is to learn anatomy of the preparation by staining, dissection and viewing of neurons and sensory structures under a dissecting microscope. This is performed by using basic neurophysiological equipment to record the electrical activity from a joint receptor organ and staining techniques. The muscle receptor organ (MRO) system in the crayfish is analogous to the intrafusal muscle spindle in mammals, which aids in serving as a comparative model that is more readily accessible for electrophysiological recordings. In addition, these are identifiable sensory neurons among preparations. The preparation is viable in a minimal saline for hours which is amenable for student laboratory exercises. The MRO is also susceptible to neuromodulation which encourages intriguing questions in the sites of modulatory action and integration of dynamic signals of movements and static position along with a gain that can be changed in the system.

Muscle Receptor Organs in the Crayfish Abdomen: A Student Laboratory Exercise in Proprioception

The primary purpose of this experiment is to demonstrate primary sensory neurons conveying information of joint movements and positions as proprioceptive information for an animal. An additional objective of this experiment is to learn anatomy of the preparation by staining, dissection and viewing of neurons and sensory structures under a dissecting microscope. This is performed by using basic neurophysiological equipment to record the electrical activity from a joint receptor organ and staining techniques. The muscle receptor organ (MRO) system in the crayfish is analogous to the intrafusal muscle spindle in mammals, which aids in serving as a comparative model that is more readily accessible for electrophysiological recordings. In addition, these are identifiable sensory neurons among preparations. The preparation is viable in a minimal saline for hours which is amenable for student laboratory exercises. The MRO is also susceptible to neuromodulation which encourages intriguing questions in the sites of modulatory action and integration of dynamic signals of movements and static position along with a gain that can be changed in the system.

Morphological and physiological development of anterior thoracic stretch receptors in two isopods, Armadillidium vulgare and Ligia exotica

Abdominal muscle receptor organs (MROs) monitor the position and movement of abdomen in crustaceans. Thoracic segments of decapods are fused and immovable. It is speculated that MROs had retrograded simple shape, N-cells that lost receptor muscles, a receptor cell and accessory nerves. We focused on the effect of segmental movement in respect to thoracic N-cells and MROs in isopods that have movable thoracic segments. Armadillidium vulgare rolled up its body segments. Ligia exotica swam by quick movement of the posterior thoracic segments. Both isopods possessed N-cells and MROs in the thorax. N-cells were a simple structure, but N-cells from the second and third thoracic segments of A. vulgare had a muscle strand. MROs(T3-T4) (from the third and fourth thoracic segments) of A. vulgare had two receptor muscles. MROs(T3-T4) of L. exotica had one long receptor muscle. N-cells of both species and MROs of A. vulgare showed slowly adapting stretch-activated discharges. MROs of L. exotica showed both slowly and rapidly adapting discharges. The stretch-activated responses of N-cells and MROs inhibited each other. N-cells or MROs in the thorax of isopods are not related to the segmental structure. The morphology and physiology of N-cells and MROs are specialized to species-specific behaviors.

Information transfer rate of nonspiking afferent neurons in the crab.

The thoracic-coxal muscle receptor organ (TCMRO) is the only proprioceptor at the thoracic-coxal joint in the crab leg. The S and T afferent neurons of the TCMRO convey signals to the CNS solely by means of graded changes in membrane potential. The rate of information transfer of these afferents was determined by measuring the signal-to-noise ratio (SNuR) of these cells after repeated stimulation of the receptor with identical sequences of random movement and applying the Shannon formula for the information capacity of a Gaussian channel. Intracellular recordings were made from the S and T afferents adjacent to the transduction site at the origin of the receptor and along the axon 5-7 mm distal to this site. These nonspiking afferents transduce receptor movement and transmit this information with extremely high fidelity. The SNR of both neurons near the transduction site was >1000 over most of the 200 Hz stimulation bandwidth, and the mean information transfer rate was approximately 2,500 bits/s. When calculated over a wider bandwidth of 500 Hz, the information rate was >4,600 bits/s. The effect of axonal cable properties on the information rate was evaluated by determining the SNR from membrane potential recordings made 5-7 mm distal to the transduction region. The major effect of graded transmission along the axon was attenuation and low-pass filtering of the sensory signal. The consequent reduction in signal power and bandwidth decreased the information transfer by approximately 10-15% over 200 Hz and approximately 30% over a 500 Hz bandwidth.

Peripheral targets of centrally located putative accessory neurons of MRO in the isopod Ligia exotica.

The three centrally located putative accessory neurons of the muscle receptor organ (MRO) of the isopod Ligia exotica were identified to the third segmental nerve (N3) of the thoracic ganglion by backfilling with Lucifer Yellow. These neurons were then studied intracellularly and extracellularly to determine whether they suppressed the stretch-activated responses of thoracic stretch receptors. Intracellular injection of depolarizing currents into these three putative accessory neurons revealed that only neuron #3 had an inhibitory effect, suggesting that it is an inhibitory accessory neuron related to thoracic stretch receptors. We searched for the peripheral targets of neurons #1 and #2 by intracellular filling with Lucifer Yellow or by recording of junctional potentials in extensor muscles, and show that they are motor neurons that innervate the deep extensor and superficial extensor muscles, respectively.

Nonspiking and spiking proprioceptors in the crab: nonlinear analysis of nonspiking TCMRO afferents.

The proprioceptor that signals the position and movement of the first joint of crustacean legs provides an excellent system for investigating information processing and transmission in neurons that function in a graded (nonspiking) manner in the context of a simple motor system. The thoracic-coxal muscle receptor organ (TCMRO) spans the thoracic-coxal joint and transmits graded signals to the CNS via two large nonspiking axons. The response characteristics and nonlinear models of the input-output relationship for the two nonspiking TCMRO afferents (S and T fibers) were determined using white noise analysis (Wiener kernel) methods. The best-fitting linear responses of these neurons was similar, as were their second-order kernels. The gains of the afferents slowly increased with increasing frequency and reached a maximum at approximately 40-60 Hz for the S fiber and 60-80 Hz for the T fiber. Above this corner frequency, the gains of both afferents decreased at approximately 20 dB/decade for the remainder of the 220-Hz stimulus bandwidth. The shape of the first-order kernels, and hence the corresponding (linear) gain functions, of both afferents were similar when driven with different amplitudes of noise, covering a 40-fold amplitude range. Predictions of the S fiber response based on the first two Wiener kernels were accurate, with the second-order model producing a mean square error of 6-8%. Second-order Wiener models for the T fiber were less accurate with a mean square error of approximately 22-26%, but this accuracy improved to 10-16% with the incorporation of the third-order term in the Wiener expansion. The effect of cable properties on the transmission of the sensory potentials to the CNS was evaluated by determining the system characteristics using membrane potentials 5-7 mm distal to the transduction site. The major change after transmission along the axon was a low-pass filtering of the sensory signals and consequent reduction in signal bandwidth.

Nonspiking and spiking proprioceptors in the crab: white noise analysis of spiking CB-chordotonal organ afferents.

The proprioceptors that signal the position and movement of the first two joints of crustacean legs provide an excellent system for comparison of spiking and nonspiking (graded) information transfer and processing in a simple motor system. The position, velocity, and acceleration of the first two joints of the crab leg are monitored by both nonspiking and spiking proprioceptors. The nonspiking thoracic-coxal muscle receptor organ (TCMRO) spans the TC joint, while the coxo-basal (CB) joint is monitored by the spiking CB chordotonal organ (CBCTO) and by nonspiking afferents arising from levator and depressor elastic strands. The response characteristics and nonlinear models of the input-output relationship for CB chordotonal afferents were determined using white noise analysis (Wiener kernel) methods. The first- and second-order Wiener kernels for each of the four response classes of CB chordotonal afferents (position, position-velocity, velocity, and acceleration) were calculated and the gain function for each receptor determined by taking the Fourier transform of the first-order kernel. In all cases, there was a good correspondence between the response of an afferent to deterministic stimulation (trapezoidal movement) and the best-fitting linear transfer function calculated from the first-order kernel. All afferents also had a nonlinear response component and second-order Wiener kernels were calculated for afferents of each response type. Models of afferent responses based on the first- and second-order kernels were able to predict the response of the afferents with an average accuracy of 86%.

The effects of serotonin and ecdysone on primary sensory neurons in crayfish.

The overall behaviors and motivational states observed during social interactions and throughout the molting cycle of crayfish have been linked to the effects of humoral neuromodulators. Both serotonin (5-HT) and a molt-related hormone, 20-hydroxyecdysone (20-HE), are known to be present in the hemolymph of crustaceans. To determine if they alter the activity of a primary sensory neuron that monitors proprioceptive information, we examined their effects on the activity of the slow-adapting muscle receptor organ (MRO) of the crayfish abdomen, a model sensory system that has been extensively studied. 5-HT within the range of 100 nM to 1 microM, increases the firing frequency of the neuron during sustained stimulation. In experiments in which 20-HE was added alone, an increase in the firing frequency also occurred, although to a lesser degree than that for 5-HT at the same concentrations. When the MRO is first exposed to 20-HE, followed sequentially by 5-HT, the activity increases to about the same degree as in the reverse order of exposure. This outcome indicates that mixtures of these endogenous neuromodulators, at various levels, are more important in alternating behavior than the absolute level of any one of them introduced alone.

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.

Effects of Removal of Muscle Receptor Organ Input on the Temporal Structure of Non-Giant Swimming Cycles in the Crayfish, Cherax Destructor

Most studies of the muscle receptor organs (MROs) of decapod crustaceans have focused on their role in local reflex loops. This may not be their only function. We examine their involvement in the regulation of non-giant swimming cycles by removing stretch receptor (SR) input from the MROs in abdominal segments 2-5 of the crayfish Cherax destructor. SR input was left intact in two control groups, one of which had sham surgery and the other no surgery at all. We recorded electromyograms (EMGs) from selected uropod muscles during tailflipping in sequences of non-giant swimming in tethered animals. The removal of SR input had a significant effect. The opener muscle period was shorter in the experimental group than in either of the control groups. This suggests that by using SR afference, crayfish sacrifice speed for increased control of the swimming movement.

Muscle receptor organs do not mediate load compensation during body roll and defense response extensions in the crayfish Cherax destructor

Muscle receptor organs do not mediate load compensation during body roll and defense response extensions in the crayfish Cherax destructor

Morphology and organization of muscle fibres in the thoracic coxal muscle receptor organ and the associated promotor muscle, in a crayfish, Cherax destructor, and mud crab, Scylla serrata

AbstractUsing confocal laser scanning and conventional light microscopy, the morphology and organization of the muscle fibres in a proprioceptor, the thoracic coxal muscle receptor organ (TCMRO), and the associated ‘extrafusal’ promotor muscle were investigated in two species of decapod crustacea, the crayfish Cherax destructor and the mud crab Scylla serrata. The diameter of the TCMROs was shown to increase distally, with an increase up to 350% recorded for the crayfish. The tapered shape of the crayfish TCMRO was demonstrated to amplify movements mechanically at the transducer region where the afferent nerves attach. Serial sectioning of the TCMROs, showed that the fibre number increased in the proximal to distal direction from 14 to 30 fibres in the crayfish and from 7 to 20 in the crab. Optical sectioning with the laser scanning confocal microscope revealed that the increase in fibre numbers was the result of muscle fibres branching in the distal third section of the TCMRO. The percentage of muscle tissue in the cross‐sectional area in the TCMRO was found to be only 35.2% and 64.6% in the crayfish and crab, respectively. Longitudinal sectioning using laser scanning confocal microscopy revealed the average sarcomere length of the TCMRO muscle fibres of both species to be in the intermediate range for crustacean muscle fibres (4.1 ± 0.1 µm and 4.55 ± 0.34 µm for the crayfish and crab) compared with the long sarcomere muscle fibres in the associated promotor muscles (7.87 ± 0.2 and 10.6 ± 0.6 µm). The distinct morphology of the TCMRO muscle fibres – smaller diameter, intermediate sarcomere length and branching of fibres compared to the larger, long sarcomere promotor fibre muscle fibres – suggest that the TCMRO muscle fibres are specialized in their role of proprioception.

Muscle receptor organs do not mediate load compensation during body roll and defense response extensions in the crayfish Cherax destructor

It has been proposed that the abdominal muscle receptor organ (MRO) of decapod crustaceans acts in a sensory feedback loop to compensate for external load. There is not yet unequivocal evidence of MRO activity during slow abdominal extension in intact animals, however. This raises the possibility that MRO involvement in load compensation is context-dependent. We recorded from MRO tonic stretch receptors (SRs) in freely behaving crayfish (Cherax destructor) during abdominal extension occurring during two different behaviors: body roll and the defense response. Abdominal extensions are similar in many respects in both behaviors, although defense response extensions are more rapid. In both situations, SR activity typically ceased when the abdominal extension commenced, even if the joint of the SR being monitored was mechanically prevented from extending by a block. Since extensor motor neuron activity increased when the abdomen was prevented from extending, we concluded that the load compensation occurring in these behaviors was not mediated by the MROs.

Functional organisation of anterior thoracic stretch receptors in the deep-sea isopod Bathynomus doederleini: behavioural, morphological and physiological studies.

The relationship between segmental mobility and the organisation of thoracic stretch receptors was examined in the deep-sea isopod Bathynomus doederleini, which shows a developed adaptive behaviour during digging. The movements of segments during digging were analysed from video recordings, which showed that a large excursion occurred in the anterior thoracic segments. Dye-fills of axons revealed four types of thoracic stretch receptor (TSR): an N-cell type (TSR-1), a differentiated N-cell type (TSR-2), a muscle receptor organ (MRO)-type with a long, single receptor muscle (TSR-3) and an MRO-type with a short, single receptor muscle (TSR-4 to TSR-7). Physiologically, TSR-1 and TSR-2 are tonic-type stretch receptors. TSR-3 to TSR-7 show two kinds of stretch-activated responses, a tonic response and a phasico-tonic response in which responses are maintained as long as the stretch stimulus is delivered. Both TSR-2, with a long muscle strand, and TSR-3, with a single, long receptor muscle, have a wide dynamic range in their stretch-activated response. In addition, TSR-2 is controlled by an intersegmental inhibitory reflex from TSR-3. These results suggest that, although TSR-1 has no receptor muscle and TSR-2 has a less-differentiated receptor-like muscle, they are fully functional position detectors of segmental movements, as are the MRO-type receptors TSR-3 to TSR-7.

Morphology and organization of muscle fibres in the thoracic coxal muscle receptor organ and the associated promotor muscle, in a crayfish, Cherax destructor, and mud crab, Scylla serrata

Morphology and organization of muscle fibres in the thoracic coxal muscle receptor organ and the associated promotor muscle, in a crayfish, Cherax destructor, and mud crab, Scylla serrata

Organization of efferent peripheral synapses at mechanosensory neurons in spiders.

The mechanosensory neurons of arachnids receive diverse synaptic inputs in the periphery. The function of most of these synapses, however, is unknown. We have carried out detailed electron microscopic investigations of the peripheral synapses at sensory neurons in the compound slit sense organ VS-3 of the spider Cupiennius salei. Based on the localization of discrete presynaptic vesicle populations, it is possible to discriminate at least four different synapse types, containing either: (1) small round, electron-lucent vesicles 32 nm in diameter; (2) large round, clear 42-nm vesicles; (3) a mixture of small and large clear, round vesicles, similar in size to those in Type 1 and Type 2 synapses, respectively, and granular and dense-core vesicles; or (4) clear, round 37- to 65-nm vesicles. Combined immunocytochemical labeling at the light and the electron microscopic level suggests that gamma-aminobutyric acid (GABA) is the transmitter in many of the 32-nm vesicle synapses, and glutamate in many of the 42-nm ones. Based on vesicle type and particular synaptic configuration, various forms of presumed efferent synaptic contacts are distinguishable with the sensory neurons, the surrounding glia, and between the putative efferent fibers themselves. These include simple unidirectional synapses, reciprocal synapses, serial synapses, and convergent as well as divergent dyads. These various synaptic microcircuits are suited to serve a variety of functions. Among these are direct postsynaptic inhibition or excitation of the mechanosensory neurons, and disinhibition or sensitization via presynaptic inhibition or excitation. The observed synaptic configurations are compared with those at the crustacean muscle receptor organ. They reveal a remarkable complexity of synaptic microcircuits at spider sensilla and suggest manifold possibilities for subtle, efferent control of sensory activity.

Ca2+- and Sr2+-activation properties of muscle fibres from a muscle receptor organ and the associated extrafusal muscle of the crab and crayfish.

In this study on decapod crustaceans, we examined the Ca2+- and Sr2+-activation properties of skeletal muscle fibres from an identified proprioceptor, the thoracic coxal muscle receptor organ (TCMRO) and its extrafusal promotor muscle fibres. Proprioceptors and extrafusal muscles were isolated from a walking leg from the crayfish (Cherax destructor) and the rear swimming leg of the mud crab (Scylla serrata). The crayfish and mud crab TCMROs had very low Hill coefficient (nCa) values (1.86 +/- 0.08 and 1.64 +/- 0.03, respectively). In comparison to other skeletal muscle fibre types these low Hill coefficients would enable the length of the receptor muscles to be finely controlled over a wide range of [Ca2+]. Maximum force was found to be significantly lower in the TCMROs (crayfish: 5.76 +/- 0.98; crab: 4.80 +/- 0.56 Ncm(-2)), compared to their associated extrafusal promotor muscle fibres (crayfish: 10.69 +/- 1.63; crab: 20.07 +/- 1.98 Ncm(-2)), which is consistent with their sensory role. The muscle fibres of the crayfish TCMRO had faster contractile properties than the mud crab TCMRO, we discuss how these contractile properties relate to the type of locomotion undergone by each leg. The mud crab 'red' promotor and all crayfish promotor fibres were characterised as slow with low Hill coefficients (nCa: crayfish: 3.22 +/- 0.29; crab: 3.34 +/- 0.29) and a contractile apparatus with a high sensitivity to Ca2+ (pCa50: crayfish: 6.42 +/- 0.03; crab: 6.18 +/- 0.03). In contrast the 'white' mud crab promotor fibres from the swimming leg had contractile properties that were characteristic of fast fibres with a high mean Hill coefficient (nCa: 5.27 +/- 0.76) and a lower Ca2+ sensitivity (pCa50: 6.03 +/- 0.03). The sensitivity of the contractile apparatus to Sr2+ was very low (range of mean pSr50: 4.23 +/- 0.03-3.48 +/- 0.06) and low force levels were produced in comparison to that produced with Ca2+. The results of this study show that the muscle fibres of the sensory receptor, produce less force and have been adapted to enable the length of the receptor to be finely set in relation to the length of the extrafusal muscle. We discuss how the striated fibres of the receptor have been adapted to perform a sensory role and how this is related to the type of locomotion undergone by the legs. We also discuss how the fibre types of the extrafusal muscle have adapted to the mode of locomotion.