Membrane properties of the stretch receptor neurones of crayfish with particular reference to mechanisms of sensory adaptation

Quantifying the Effects of Two Local Anesthetics on the Crayfish Stretch Receptor Organ: An Integrated Neurophysiology Lab

The crayfish stretch receptor organ (SRO) preparation represents a robust experimental model for undergraduate laboratory experiences. For example, this preparation may be included as part of a course-based undergraduate research experience (CURE), where students work independently to plan and carry out their own experiments. In the current paper, we provide an example of how local anesthetics may be used to manipulate the SRO preparation and to perform quantitative analyses of SRO action potential firing rates. Local anesthetics provide interesting tools for manipulating physiological responses within the nervous system. A variety of inexpensive anesthetics are available for student use and each of these is expected to inhibit neurophysiological responses. While specific anesthetics exhibit subtle differences in chemical organization, they are generally understood to block voltage gated sodium channels. In the current study, we investigated the effects of two local anesthetics, MS-222 and procaine, on the action potential firing rate from the crayfish SRO. Using quantitative analyses of SRO action potential generation, we determined that each anesthetic has unique inhibitory effects on action potential firing rate that may be explained by their neuropharmacological properties. This manipulation may thus be utilized as an interesting experimental tool in undergraduate teaching laboratories. Local anesthetics applied to crayfish SRO preparations can thus be used to deepen student understanding of local anesthetics, exercise quantitative analyses, and provide experimental tools for independent experimental design.

Realignment of signal processing within a sensory brainstem nucleus as brain temperature declines in the Syrian hamster, a hibernating species

Crucial for survival, the central nervous system must reliably process sensory information over all stages of a hibernation bout to ensure homeostatic regulation is maintained and well-matched to dramatically altered behavioral states. Comparing neural responses in the nucleus tractus solitarius of rats and euthermic Syrian hamsters, we tested the hypothesis that hamster neurons have adaptations sustaining signal processing while conserving energy. Using patch-clamp techniques, we classified second-order neurons in the nucleus as rapid-onset or delayed-onset spiking phenotypes based on their spiking onset to a depolarizing pulse (following a -80 mV prepulse). As temperature decreased from 33 to 15°C, the excitability of all neurons decreased. However, hamster rapid-onset spiking neurons had the highest spiking response and shortest action potential width at every temperature, while hamster delayed-onset spiking neurons had the most negative resting membrane potential. The frequency of spontaneous excitatory postsynaptic currents in both phenotypes decreased as temperature decreased, yet the amplitudes of tractus solitarius stimulation-evoked currents were greater in hamsters than in rats regardless of phenotype and temperature. Changes were significant (P < 0.05), supporting our hypothesis by showing that, as temperature falls, rapid-onset neurons contribute more to signal processing but less to energy conservation than do delayed-onset neurons.

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.

Distinct tachykinin NK(1) receptor function in primate nucleus tractus solitarius neurons is dysregulated after second-hand tobacco smoke exposure

BACKGROUND AND PURPOSE

Second-hand tobacco smoke (SHS) exposure in children increases the risk of asthma and sudden infant death syndrome. Epidemiological and experimental data have suggested SHS can alter neuroplasticity in the CNS, associated with substance P. We hypothesized that exposure to SHS in young primates changed the effect of substance P on the plasticity of neurons in the nucleus tractus solitarius (NTS), where airway sensory information is first processed in the CNS.

EXPERIMENTAL APPROACH

Thirteen-month-old rhesus monkeys were exposed to filtered air (FA, n= 5) or SHS (n= 5) for >6 months from 50 days of their fetal age. Whole-cell patch-clamp recordings were performed on NTS neurons in brainstem slices from these animals to record the intrinsic cell excitability in the absence or presence of the NK(1) receptor antagonist, SR140333 (3 µM).

KEY RESULTS

Neurons were electrophysiologically classified based on their spiking onset from a hyperpolarized membrane potential into two phenotypes: rapid-onset spiking (RS) and delayed-onset spiking (DS) types. In RS neurons, SR140333 reduced the spiking response, similarly in both FA- and SHS-exposed animals. In DS neurons, SR140333 almost abolished the spiking response in FA-exposed animals, but had no effect in SHS-exposed animals.

CONCLUSIONS AND IMPLICATIONS

The contribution of NK(1) receptors to cell excitability depended on firing phenotype of primate NTS neurons and was disrupted by SHS exposure, specifically in DS neurons. Our findings reveal a novel NK(1) receptor function in the primate brainstem and support the hypothesis that chronic exposure to SHS in children causes tachykinin-related neuroplastic changes in the CNS.

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.

Secondhand smoke exposure alters K+ channel function and intrinsic cell excitability in a subset of second-order airway neurons in the nucleus tractus solitarius of young guinea pigs

Extended exposure to secondhand smoke (SHS) in infants and young children increases the incidence of cough, wheeze, airway hyper-reactivity and the prevalence and earlier onset of asthma. The adverse effects may result from environmentally-induced plasticity in the neural network regulating cough and airway function. Using whole-cell patch-clamp recordings in brainstem slices containing anatomically identified second-order lung afferent neurons in the nucleus tractus solitarius (NTS), we determined the effects of extended SHS exposure in young guinea pigs for a duration equivalent to human childhood on the intrinsic excitability of NTS neurons. SHS exposure resulted in marked decreases in the intrinsic excitability of a subset of lung afferent second-order NTS neurons. The neurons exhibited a decreased spiking capacity, prolonged action potential duration, reduced afterhyperpolarization, decrease in peak and steady-state outward currents, and membrane depolarization. SHS exposure effects were mimicked by low concentrations of the K+ channel blockers 4-aminopyridine and/or tetraethyl ammonium. The data suggest that SHS exposure downregulates K+ channel function in a subset of NTS neurons, resulting in reduced cell excitability. The changes may help to explain the exaggerated neural reflex responses in children exposed to SHS.

A novel postsynaptic group II metabotropic glutamate receptor role in modulating baroreceptor signal transmission

The nucleus tractus solitarius (NTS) is essential for orchestrating baroreflex control of blood pressure. When a change in blood pressure occurs, the information is transmitted by baroreceptor afferent fibers to the central network by glutamate binding to ionotropic glutamate receptors on second-order baroreceptor neurons. Glutamate also activates presynaptic group II and III metabotropic glutamate receptors (mGluRs), depressing both glutamate and GABA release to modulate baroreceptor signal transmission. Here we present a novel role for postsynaptic group II mGluRs to further fine-tune baroreceptor signal transmission at the first central synapses. In a brainstem slice with ionotropic glutamate and GABA receptors blocked, whole-cell patch-clamp recordings of second-order baroreceptor neurons revealed that two group II mGluR agonists evoked concentration-dependent membrane hyperpolarizations. The hyperpolarization remained when a presynaptic contribution was prevented with Cd(2+), was blocked by a postsynaptic intervention of intracellular dialysis of the G-protein signaling inhibitor, was mimicked by endogenous release of glutamate by tractus solitarius stimulation, and was prevented by a group II mGluR antagonist. Postsynaptic localization of group II mGluRs was confirmed by fluorescent confocal immunohistochemistry and light microscopy. Group II mGluR induced-currents consisted of voltage-dependent outward and inward components, prevented by tetraethylammonium chloride and tetrodotoxin, respectively. In contrast to group II mGluR-induced hyperpolarization, there was no effect on intrinsic excitability as determined by action potential shape or firing in response to depolarizing current injections. The data suggest a novel mechanism for postsynaptic group II mGluRs to fine-tune baroreceptor signal transmission in the NTS.

Behavior of solutions of the Hodgkin-Huxley equations and its relation to properties of mechanoreceptors

The membrane current in the Hodgkin-Huxley equations is considered to be a stimulus to the membrane and the responses to the simulus are numerically calculated. Responses of the Hodgkin-Huxley model to an alternating current superimposed upon a constant bias current show qualitative analogy to responses of biological mechanoreceptors. The intensity of the bias current seems to correspond to the degree of adaptation of actual receptors.

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.

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.

Tension sensitivity of the heart pacemaker neurons in the isopod crustacean Ligia pallasii

In the crustacean neurogenic heart, the cardiac ganglion (CG) acts as a peripherally located central pattern generator (CPG) by producing rhythmic motor output that initiates the heartbeat. In the isopod Ligia, the CG consists of six electrically coupled neurons that all function both as endogenous oscillators and as glutamatergic motoneurons innervating heart muscle. In the present study, we present several lines of evidence to suggest that the CG neurons are sensitive to passive stretch and active tension of the heart muscle. Stretching the heart wall caused a sustained decrease in the burst frequency of the CG neuron. Releasing from the stretch caused a rebound increase in burst frequency above the control rate. A brief stretch (200-300 ms duration) caused either phase advance or phase delay of the following CG bursts, depending on the timing at which the stretch was applied. Repeated brief stretches could entrain the CG bursts to either higher or lower frequencies than the free-run burst frequency. Intracellular recording from one of the CG neurons revealed that it exhibited hyperpolarization during the stretch. The stretch-induced hyperpolarization was followed by a burst discharge upon release from the stretch. With increased stretch amplitude, the amplitude of hyperpolarizing response increased and the timing of the following burst was advanced. When the myogenic activity of the heart muscle was pharmacologically isolated from the ganglionic drive by applying a glutamatergic antagonist, Joro spider toxin (JSTX), the spontaneous muscle contraction caused a hyperpolarizing deflection in the CG neuron. Under specific conditions made by JSTX and tetrodotoxin, the CG burst became entrained to the myogenic rhythm. These results suggest that the Ligia CG neurons have tension sensitivity in addition to their pacemaker and motoneuronal functions. Such multifunctional neurons may form a single neuron reflex arc inside the heart.

Activity-dependent sensitivity of proprioceptive sensory neurons in the stick insect femoral chordotonal organ

Mechanosensory neurons exhibit a wide range of dynamic changes in response, including rapid and slow adaptation. In addition to mechanical factors, electrical processes may also contribute to sensory adaptation. We have investigated adaptation of afferent neurons in the stick insect femoral chordotonal organ (fCO). The fCO contains sensory neurons that respond to position, velocity, and acceleration of the tibia. We describe the influence of random mechanical stimulation of the fCO on the response of fCO afferent neurons. The activity of individual sensory neurons was recorded intracellularly from their axons in the main leg nerve. Most fCO afferents (93%) exhibited a marked decrease in response to trapezoidal stimuli following sustained white noise stimulation (bandwidth = 60 Hz, amplitudes from +/-5 to +/-30 degrees ). Concurrent decreases in the synaptic drive to leg motoneurons and interneurons were also observed. Electrical stimulation of spike activity in individual fCO afferents in the absence of mechanical stimulation also led to a dramatic decrease in response in 15 of 19 afferents tested. This indicated that electrical processes are involved in the regulation of the generator potential or encoding of action potentials and partially responsible for the decreased response of the afferents. Replacing Ca(2+) with Ba(2+) in the saline surrounding the fCO greatly reduced or blocked the decrease in response elicited by electrically induced activity or mechanical stimulation when compared with control responses. Our results indicate that activity of fCO sensory neurons strongly affects their sensitivity, most likely via Ca(2+)-dependent processes.

Different spatial distributions of sodium channels in the slowly and rapidly adapting stretch receptor neuron of the crayfish

Inward Na+ currents were studied, using a two-microelectrode intracellular voltage-clamp technique, in the slowly adapting (SA) and rapidly adapting (RA) stretch receptor neurons of the crayfish after the axons were cut at different distances from the soma. In the SA neuron, inward Na+ currents were recorded in the soma even when the axon was cut as close as 100 microm from the center of the soma, indicating the presence of Na+ channels in these parts. Also, two populations of Na+ channels seem to exist in the SA neuron. In the RA neuron, only minute Na+ currents were observed if the axon was shorter than 250 microm. The results strongly indicate that the voltage-gated Na+ channels in the SA and RA neurons have different distributions and that the difference in the spatial distribution of Na+ channel types may be important for the difference in firing properties in the two types of neurons.

Voltage-activated potassium outward currents in two types of spider mechanoreceptor neurons

We studied the properties of voltage-activated outward currents in two types of spider cuticular mechanoreceptor neurons to learn if these currents contribute to the differences in their adaptation properties. Both types of neurons adapt rapidly to sustained stimuli, but type A neurons usually only fire one or two action potentials, whereas type B neurons can fire bursts lasting several hundred milliseconds. We found that both neurons had two outward current components, 1) a transient current that activated rapidly when stimulated from resting potential and inactivated with maintained stimuli and 2) a noninactivating outward current. The transient outward current could be blocked by 5 mM tetraethylammonium chloride, 5 mM 4-aminopyridine, or 100 microM quinidine, but these blockers also reduced the amplitude of the noninactivating outward current. Charybdotoxin or apamin did not have any effect on the outward currents, indicating that Ca2+-activated K+ currents were not present or not inhibited by these toxins. The only significant differences between type A and type B neurons were found in the half-maximal activation (V50) values of both currents. The transient current had a V50 value of 9. 6 mV in type A neurons and -13.1 mV in type B neurons, whereas the V50 values of noninactivating outward currents were -48.9 mV for type A neurons and -56.7 mV for type B neurons. We conclude that, although differences in the activation kinetics of the voltage-activated K+ currents could contribute to the difference in the adaptation behavior of type A and type B neurons, they are not major factors.

Action potential and sodium current in the slowly and rapidly adapting stretch receptor neurons of the crayfish (Astacus astacus)

Action potentials (APs) and sodium current from the slowly and the rapidly adapting stretch receptor neurons in the crayfish (Astacus astacus) were recorded with a two microelectrode voltage- and current-clamp technique. In the rapidly adapting neuron the APs had a duration of 3.2 +/- 0.2 ms (means +/- SE) and an amplitude of 55.2 +/- 1.5 mV. In the slowly adapting receptor neuron APs had a duration of 4.1 +/- 0.2 ms and an amplitude 79.9 +/- 2.0 mV. APs in the rapidly adapting neuron had a larger amplitude if they were recorded from the axon. In the rapidly adapting neuron adaptation of the impulse response was prolonged by hyperpolarization or by exposure to scorpion venom. Also, sinusoidal current stimulation added to the current steps prevented impulse adaptation. Block of the potassium currents in the slowly adapting neuron resulted in a rapid adaptation of the impulse response. The maximum sodium current amplitude was 313 +/- 15 nA in slowly adapting neuron and 267 +/- 11 nA in the rapidly adapting neuron. The current-voltage relationship showed a hump most marked in the slowly adapting neuron and abolished when a depolarizing prepulse was given. In the rapidly adapting neuron the inactivation starts at a more negative potential (Eh = -45 mV) and is faster compared with the slowly adapting neuron (Eh = -41 mV). The crude scorpion venom of Leiurus quinquestriatus (ScVLq) shifted hinfinity curve toward more positive potentials and slowed down the rate of inactivation. The results indicate the possible presence of more than one Na+ channel population and that the relative density and the spatial distribution is different in the slowly and rapidly adapting neuron. The difference contributes to the adaptive properties of the two receptor neurons.

The structure and function of a connective chordotonal organ in the cockroach leg

The micromorphology of the tibio-tarsal joint of Periplaneta is described, based on the mesothoracic limb. There are two muscles acting on the joint, numbered 144 and 145. There is a connective chordotonal organ, which branches off from the trunk of N5 proximally and inserts on the intersegmental membrane distally. In addition, a previously undescribed group of campaniform sensilla is reported from the tibiotarsal joint. The tibio-tarsal chordotonal organ subdivides into a main branch and two side branches. It contains 26 bipolar sensory neurons, whose dendrites are associated distally with 14 scolopales. These scolopidia are arranged in three groups which differ in position, fine structure and number of cells per scolopale. There is one group 1 scolopidium in each of the side branches and two proximally in the main branch. Group 2 scolopidia are spread out along the main branch and group 3 scolopidia occur distally in the main branch. Group 1 scolopidia consist of two bipolar neurons, 15 to 20 µm in diameter, whose dendrites, heavily sheathed, insert in a single scolopale. Group 2 scolopidia consist of two bipolar neurons, 8 to 15 µm in diameter, whose dendrites are less well sheathed and insert in a single scolopale. Group 3 scolopidia consist of a single bipolar neuron, about 10 µm in diameter, with a short, poorly sheathed dendrite inserting in a single scolopale. The scolopales of groups 1 and 2 are identical, consisting of the usual pattern of a ring of scolopale rods inserting into a distal cap and enclosing the cilia on the dendrite terminations. In this case, the cap is particularly long and pointed and both the cilia and the scolopale rods penetrate a long way into it. The cilia of the two members of a pair of dendrites are identical but the ciliary roots differ between the two members of a pair. Electrophysiological recordings show that the organ responds to downward and backward deflexion of the tarsus. The response comprises at least two classes of sensory fibre distinguishable both by their size and by their behaviour. The larger fibres show a unidirectional phasic and tonic response to extreme deflexion of the tarsus and are identified with the group 1 scolopidia. The smaller fibres show a unidirectional tonic response to the full range of deflexion of the tarsus and are identified with the group 2 scolopidia. On structural evidence, it is suggested that the differences in adaptation between these two groups of scolopidia is not likely to be caused by differences in mechanical attachment. The combination of electrophysiological and fine structural evidence indicates that in this chordotonal organ the adequate stimulus of the scolopidia is an increase in their longitudinal tension but it is not possible to say which fine structural component responds to strain.

Dynamics of analog-to-frequency transduction in sensory receptors

We analyze the analog-to-frequency transduction by sensory neurons in vivo. Spatially extended neural models are made to fire by applying the stimulus to the membrane potential at the boundary of the trigger zone. The membrane property that provides a broad dynamical range of frequency is a prolonged hyperpolarizing afterpotential. In support of this PDE model, we study the stimulus-dependent location of pulse-initiation in crayfish stretch receptors Ringham (1971).

Periodically modulated inhibition and its postsynaptic consequences--I. General features. Influence of modulation frequency

Our aim was to examine the relation, or "synaptic coding", between spike trains across a synapse with inhibitory postsynaptic potentials when the presynaptic rate is modulated periodically and the postsynaptic cell is a pacemaker. Experiments were on the synapse in crayfish stretch receptor organs. Spike trains were considered point processes along time; the time series of corresponding pre- and postsynaptic intervals were extracted. Analyses used displays of intervals along time and order ("basic graphs", and "rasters", respectively), displays of differences between intervals along order ("recurrence plots"), cycle histograms (as such and as Lissajous diagrams with presynaptic and postsynaptic on the abscissae and ordinate, respectively), and correlation histograms. Cycle histograms and correlation histograms demonstrated that all presynaptic modulation frequencies (1/60-10 Hz) are reflected postsynaptically; novel frequencies may arise, not always relating simply to the pre- or postsynaptic ones. The transferred frequency domain is broad and physiologically meaningful. Indeed, vitally important functions have strong periodicities in all portions of the explored domain, and so do the discharges of participating neurons. Overall, pre- and postsynaptic discharges change oppositely, one accelerating while the other slows. Locally, however, pre- and postsynaptic discharges contrast clearly in other ways. The presynaptic evolution is everywhere smooth and orderly, half-cycles usually are symmetric, and there is a single kind of discharge, as expected because the presynaptic axon follows well the controlling stimuli. The postsynaptic cycle shows marked local distortions. These involve presynaptic domains called "congruent portions" where changes are in the same sense (e.g., joint accelerations), "saturated" domains where postsynaptic discharges are arrested, and asymmetric sensitivities to presynaptic change with hysteretic loops in the Lissajous diagrams; the postsynaptic discharge is heterogeneous showing dissimilar forms in succession. Congruent portions are either "positive segments" with pre- to postsynaptic rate ratios practically 1:1, 2:1, 1:1, or parts of Lissajous loops. Different modulation frequencies have different postsynaptic consequences. Differences involve the width and number of positive segments, the proportion of the cycle with saturation, the sense, magnitude and lead-lag characteristics of the hysteretic loops, etc. Because their consequences are separable, frequencies are classified within categories labelled "low" (under 0.5 Hz), "high" (between 0.5 and 5.0 Hz) and "very high" (over 5.0 Hz). Categories arise widely but each prevails in different biological functions (e.g., low or high in, respectively, respiration or vibratory sensitivity). The refactoriness of the inhibitory fibre affects how it can be modulated: consequently, the very high category resembles pacemaker discharges and was not analysed.(ABSTRACT TRUNCATED AT 250 WORDS)

Transients in the inhibitory driving of neurons and their postsynaptic consequences

The presynaptic fiber at an inhibitory synapse on a pacemaker neuron was forced to generate transients, defined here as spike trains with a trend, unceasingly accelerating or slowing. Experiments were on isolated crayfish stretch receptor organs. Spike train analyses used tools and notions from conventional point processes and from non-linear dynamics. Pre- and postsynaptic discharges contrasted clearly in terms of rates and interspike intervals. The inhibitory train evolved monotonically and smoothly, following tightly the simple prescribed curves; it was uniform, exhibiting throughout a single and simple discharge form (i.e. interval patterning). The inhibited postsynaptic train alternately accelerated and slowed, not following tightly any simple curve; it was heterogeneous, exhibiting in succession several different and often complex discharge forms, and switching abruptly from one to another. The inhibited trains depended on the inhibitory transient's span, range and average slope. Accordingly, transients separated (not cuttingly) into categories with prolonged spans (over 1 s) and slow slopes (around 1/s2) and those with short spans (under 1 s) and fast slopes (around 30/s2). Special transients elicited postsynaptic discharges that reproduced it faithfully, e.g. accelerated with the transient and proportionately; no transient elicited postsynaptic discharges faithful to its mirror image. Crayfish synapses are prototypes, so these findings should be expected in any other junction, as working hypotheses at least. Implications involve the operation of neural networks, including the role of distortions and their compensation, and the underlying mechanisms. Transients have received little attention, most work on synaptic coding concentrating on stationary discharges. Transients are inherent to the changing situations that pervade everyday life, however, and their biological importance is self-evident. The different discharges encountered during a transient had strong similarities to the stationary forms reported for different pacemaker drivings that are called locking, intermittency, erratic and stammering; they were, in fact, trendy versions of these. Such forms appear with several synaptic drivings in the same order along the presynaptic rate scale; they may constitute basic building blocks for synaptic operation. In terms of non-linear science, it is as if the attractors postulated for stationary drivings remained strongly influential during the transients, though affected by the rate of change.

Transducer properties of the rapidly adapting stretch receptor neurone in the crayfish (Pacifastacus leniusculus)

1. The transducer properties of the rapidly adapting stretch receptor neurone of the crayfish (Pacifastacus leniusculus) were studied using a two-microelectrode voltage clamp technique. 2. The impulse response to ramp-and-hold extensions of the receptor muscle typically consisted of a high frequency burst followed by cessation of impulses within a relatively short time depending on the amplitude of extension. The type of adaptation was consistent with earlier studies. The stimulus-response relationship for the impulse frequency was non-linear and had a slope in a log-log plot of 2.9. 3. When impulse generation was blocked by tetrodotoxin (TTX), (block of Na+ channels) the receptor potential was extension dependent and similar to that found in the slowly adapting receptor. For small extensions there was an initial peak followed by a fall to a steady potential level. For large extensions the potential response during the ramp phase consisted of a peak followed by a constant potential level lasting to the end of the ramp. When the extension changed to the hold phase the potential fell towards a steady state. The relation between extension and amplitude of receptor potential was non-linear and saturated at -40 to -30 mV (extensions > 15% of zero length, lo). 4. When potassium channels were blocked by TEA (50 mM) and 4-aminopyridine (4-AP, 5 mM) (and Na+ channels blocked by TTX) the shape of the generator potential become less complex with an increased amplitude for large extensions. 5. When the receptor neurone was voltage clamped at the resting potential, extension of the receptor muscle produced an inwardly directed receptor current, the stretch-induced current (SIC). The response consisted of a fast transient phase which decayed towards a steady state. The SIC peak amplitude was dependent on extension in a sigmoidal fashion and saturated at 190 nA (extensions > 25% of lo). The slope of the steepest part of the stimulus-response relation (between 10 and 20% extension) was 4.7 +/- 0.25 (mean +/- S.E.M.) in a log-log plot. 6. The peak amplitude of the SIC increased with increasing extension speed (ramp steepness), the relation between the slope of the ramp and current amplitude being a first order (hyperbolic) function. The amplitude of the receptor current was voltage dependent and had a reversal potential of +16.2 +/- 1.8 mV (mean +/- S.E.M., 32 cells). From the reversal potential the permeability ratio, PNa/PK, of the transducer permeability system was calculated to be 1.5. The I-V curve of SIC was non-linear.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of baroreceptor adaptation in dogs: attenuation of adaptation by the K+ channel blocker 4-aminopyridine

1. Increased arterial pressure increases baroreceptor activity but activity declines (i.e. baroreceptors adapt) as the pressure is maintained at the higher level. The purpose of this study was to investigate the role of a 4-aminopyridine (4-AP)-sensitive K+ current in causing baroreceptor adaptation. 2. Multi- and single fibre recordings of baroreceptor activity were obtained from the vascularly isolated carotid sinus in anaesthetized dogs during step increases in carotid sinus pressure sustained for periods up to 5 min. 3. Baroreceptor activity increased with the rise in pressure, declined markedly over the first minute, and continued to decline at a slower rate during the remainder of the 5 min period of elevated pressure. Exposure of the isolated carotid sinus to 4-AP (10(-5) and 10(-4) M) attenuated adaptation in a dose-dependent and reversible manner (P < 0.05). 4-AP attenuated the gradual decline in single fibre activity and also prevented derecruitment or dropout of fibres that occurred over time. 4-AP did not alter peak nerve activity measured within the first 2 s of the pressure step. 4. Ouabain (5 x 10(-7)-10(-6) M), an inhibitor of Na+,K(+)-ATPase, increased baroreceptor activity but did not attenuate baroreceptor adaptation. 5. Neither 4-AP nor ouabain altered the distensibility of the carotid sinus as measured with sonomicrometer crystals suggesting that the agents act directly on the nerve endings. 6. The results suggest that activation of a 4-AP-sensitive K+ current contributes significantly to baroreceptor adaptation with little or no contribution of Na+,K(+)-ATPase.

Signal transduction and nonlinearities revealed by white noise inputs in the fast adapting crayfish stretch receptor

Input-output relations were investigated in the fast adapting stretch receptor organ (RM2) of the crayfish by matching gaussian white noise (GWN) length inputs, with the resulting spike output. The analysis revealed the expected sensitivity to lengthening velocity, a behavior termed phasic. It also disclosed a sensitivity to sustained elongation, a performance termed tonic and previously not recognized in the RM2. Spectral analysis indicated the properties of a low-pass filter, confirming the tonic sensitivity. A variety of individual length trajectories could lead to a spike. The average trajectory consisted in a biphasic shortening-lengthening wave. The range of possible trajectories and their averages changed with stimulus prestretch and GWN amplitude, indicating that system properties depended on the input characteristics; i.e., a nonlinear operation. Length waveforms in the GWN were isolated by computing methods and the corresponding responses were calculated. Symmetric stimuli led to responses that reflected magnitudes and velocities asymmetrically. Nonlinear interactions between responses in the past and present were negligible. In conclusion, depending on the input, the RM2 modifies its operation to enhance the detectability of the functionally relevant signal in each natural situation.

Potential-dependent potassium currents in the rapidly adapting stretch receptor neuron of the crayfish

The outward current was analysed in the rapidly adapting stretch receptor neuron of the crayfish Pacifastacus leniusculus with a two-micropipette potential clamp technique and K(+)-selective microelectrodes in an attempt to establish if the properties of this current could explain the difference in adaptive behaviour compared to the slowly adapting receptor. A fast activating outward current carried by K+ was revealed. The time constant of activation(tau n) was dependent on potential and had a mean value of 0.5 ms at potential steps to 0 mV. Activation followed a second-order process according to the Hodgkin-Huxley model. The potential dependence of activation (n infinity) followed by a sigmoid curve n infinity = 1/(1 + exp/[(E - En)/a]) with a half maximal activation potential En = -44 mV and a = -13 mV. When long pulses were applied the outward potassium current decreased with two time constants, one that was potential independent (0.2 s) and one that was potential dependent (2-8 s). The latter could be explained by accumulation of K+ in the extracellular space of the neuron. The potential dependence of inactivation followed a sigmoid function infinity = 1/(1 + exp[(E - Ek)/+a]) with Ek = -36 mV and a = 13 mV. The inactivation properties are different from those of the classical fast transient (IA) current. The transport system for the outward potassium current during depolarizing potential steps in the rapidly adapting stretch receptor is similar to the current found in the slowly adapting receptor neuron. However, the activation is faster and seems to occur at potentials more negative than in the slowly adapting receptor. These differences can contribute to but not entirely explain the difference in adaptive behaviour between the slowly and rapidly adapting receptor.

Potential-dependent potassium currents in the slowly adapting stretch receptor neuron of the crayfish

The outward current in the stretch receptor neuron of the crayfish Pacifastacus leniusculus was analysed using a two-micropipette potential-clamp technique. The outward current was shown to be carried by K+. When the sodium-dependent inward current was blocked by tetrodotoxin a fast-activating potassium current was revealed, resembling the delayed rectifier. The time-course of activation (Tau n) was dependent on potential and had a mean value of I ms at potential steps of to mV. The activation followed a second-order process according to the Hodgkin-Huxley model. The potential dependence of activation (n infinity) followed a sigmoid curve, n infinity = I/(I + exp [(E-En)/a]) with half-maximal activation potential En = -31 mV and a = -13 mV. When long pulses were applied, the potassium current showed marked inactivation with a fast time constant of 0.5 s that was potential independent and a slow component that was slightly potential dependent. The minimum value for the slow time constant was 4 s for steps to about 0 mV. The potential dependence of inactivation followed a sigmoid function k infinity = I/(I + exp [(E-Ek)/a]) with Ek = -39 mV and a = II mV. No transient potassium outward current (IA) was found in the crayfish stretch receptor neuron. In experiments on tail currents after depolarizing potential steps of different duration, it was found that the reversal potential changed in the positive direction when the duration of the pre-pulse increased. This could be due to K- accumulation in a space close to the neuronal membrane. The potassium current during depolarizing potential steps in the crayfish stretch receptor is similar to the delayed current found in other cells, for example the frog myelinated nerve, but different from many other invertebrate neurons.

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.

Immunocytochemical evidence for the GABAergic innervation of the stretch receptor neurons in crayfish

The GABAergic innervation of the stretch receptor neurons of the crayfish Orconectes limosus has been investigated by means of light- and electron microscope immunocytochemistry using an antibody to GABA. Both whole-mount preparations and post-embedding semithin sections revealed a massive GABAergic innervation of both the slowly and the fast adapting receptor neurons. The stretch receptor organ is supplied by one principle GABA-immunoreactive axon, which gives off several branches that innervate the receptor neurons. Cell body, initial axon segment and dendritic region of the sensory neurons are covered by numerous GABA-immunoreactive varicose fibers. Electron microscopy revealed that the GABA-immunoreactive varicosities establish specialized synaptic contacts with the sensory neurons. The functional significance of the occurrence of GABA-immunoreactive varicosities on the different parts of the sensory neurons is discussed. The results support the physiological and pharmacological evidence that GABA is a transmitter substance of the efferent inhibitory neurons which innervate the crayfish stretch receptor neurons.

Removal of rapid sensory adaptation from an insect mechanoreceptor neuron by oxidizing agents which affect sodium channel inactivation

1. The femoral tactile spine of the cockroach is a mechanoreceptor with a single sensory neuron. The response to a step movement is a burst of action potentials which decays to zero in about 1 s. This rapid adaptation is a property of the action potential initiating region of the neuron. 2. The oxidizing agents chloramine-T and N-chlorosuccinimide selectively and irreversibly remove sodium channel inactivation from neurons in several preparations and are believed to act by oxidation of methionine or cysteine residues in the proteins of the sodium channel. 3. Chloramine-T and N-chlorosuccinimide, applied for a controlled time period, eliminated the rapid adaptation of the tactile spine neuron to an electrical depolarization. After treatment it fired tonically in response to a steady current stimulus. Longer applications of the agents eventually raised the threshold for action potential initiation. 4. Threshold behavior in the tactile spine neuron was characterized by measuring strength-duration relationships for stimulation with extracellular current pulses at the action potential initiating region. The two oxidizing agents caused a voltage-dependent modification of the dynamic threshold properties which led to the change from rapidly adapting to tonic behavior. 5. Two stronger oxidizing agents, N-bromoacetamide and N-bromosuccinimide, raised the threshold of the neuron without removing rapid adaptation. These two agents act similarly to chloramine-T and N-chlorosuccinimide on sodium inactivation in other neurons but are believed to oxidize the tryptophan, tyrosine and histidine residues of proteins in addition to cysteine and methionine.(ABSTRACT TRUNCATED AT 250 WORDS)

Sustained sensitivity modifications induced by brief length perturbations in the crayfish slowly adapting stretch receptor

These experiments in the slowly adapting stretch receptor of crayfish test the effects of brief length perturbations (i.e., pulses) when presented in isolation at different constant elongations or superimposed on trapezoidal stretches of different amplitudes. Within "in vivo" lengths, during static responses, perturbations reduced firing rates to below control values and, in extreme cases, could silence the receptor. This effect, or "down-step," was sustained, occurred above a threshold pulse amplitude and background stretch, and increased with both stimulus characteristics, but was not present during dynamic responses. Beyond "in vivo" lengths, and in a few cases within those limits but close to the extremes, the receptor was silent but perturbations could restore activity. Lengthening pulses were more effective than shortening ones in generating after-effects. Perturbations change, during indefinitively long periods, the receptor's length or static sensitivity acting as a negative feedback which tends to maintain the discharge rate within fixed values. Perturbations disclose marked nonlinearities, which suggest that the classical view of a proportional control in the reflex loop in which the receptor participates may not operate in natural conditions.

Dynamic properties of the action potential encoder in an insect mechanosensory neuron

A variety of sensory receptors show adaptation to dynamic stimuli that can be well characterized as fractional differentiation of the input signal. The cause of this behavior is unknown, but because it can be represented by linear systems theory, it has been assumed to arise during early linear processes of transduction or adaptation, rather than during the nonlinear process of action potential encoding. I measured the action potential encoding properties of an insect mechanoreceptor by direct electrical stimulation of the sensory cell axon and found a dynamic response that is identical to the response given by mechanical stimulation. This indicates that the fractional differentiation is a property of the encoder rather than the transducer.

Intracellular ion control in lobster stretch receptor neurone

The control of intracellular ion concentrations by means of passive and active transmembrane ion transports was investigated in the lobster stretch neurone using electrophysiological and pharmacological techniques in combination with recording with ion-sensitive microelectrodes. In resting conditions [Na+]i, [K+]i, and [Cl-]i were, in both slowly and rapidly adapting cells, found to be in the order of 20, 155, and 50 mM, respectively. In the slowly adapting cell impulse firing at stationary frequencies of 7-10 Hz caused an increase in [Na+]i and a decrease in [K+]i of 20-30 mM; [Cl-]i was only little affected, the rise in [Na+]i led to an enhanced Na-K pump activity noticeable as an increase in pump current production. In stationary conditions the quotient between pump current and Na+ influx increments was about 0.3, which is compatible with 3:2 Na-K pumping ratio in the present preparation. From measurements of the pump current activation during stationary firing at maximum tolerable frequencies an estimate was made of the cell's maximum pump current production. The measurements were used in the formulation of a mathematical model of the intracellular ion control in which expressions of active and passive transmembrane ion transports are incorporated into the continuity equation for the ion fluxes involved.

Kinetics of the TEA and 4-AP sensitive K+ current in the slowly adapting lobster stretch receptor neurone

The kinetics of the TEA and 4-AP sensitive K+ current (IK) in the slowly adapting lobster stretch receptor neurone were investigated in sub- and near-threshold voltage regions using electrophysiological and pharmacological techniques. In dynamic conditions IK was found to display both fast and slow reactions. These were attributed to a Hodgkin-Huxley type of K activation, and a slow type of K inactivation, respectively. The slow K inactivation could be shown to be unrelated to K+ flux dependent changes in intra- and pericellular K+ concentrations. Its stationary voltage dependence was however shifted in a depolarizing direction by increasing, and in hyperpolarizing direction by decreasing the extracellular Ca++ concentration. In view of these findings, and of its kinetic properties, the slow K inactivation was classified as a genuine channel gating process. The process of K activation was too fast for a dynamic analysis with the recording technique available. An estimate of its stationary voltage dependence could however be obtained in a voltage range from about -100 to about -40 mV. The experimental observations were utilized in the formulation of a mathematical model describing the kinetic behaviour of IK in the present preparation based on constant field and state transition theories.

Impulse-coded and analog signaling in single mechanoreceptor neurons

Although most sensory neurons convey temporally coded impulses to the central nervous system, certain nonspiking receptors use only graded afferent signals. Each of three large nerve fibers from the lobster oval organ, a mechanoreceptor subserving ventilation, carry both impulses and graded potentials. Thus, both impulse frequency and receptor potential amplitude are available for information transfer.

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.

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.

Fast inward and outward current channels in a non-spiking neurone

Although the crustacean coxal receptors and no-spiking, indirect pharmacological and electrophysiological evidence suggests that fast sodium channels may be present in their membrane. The properties of these channels are not known, but it has been suggested that they might be "incompletely differentiated", perhaps lacking "appropriate gating mechanisms", and/or "too sparsely distributed". The former hypothesis is not supported by the results of voltage-clamping experiments done on dendritic segments isolated from these mechanoreceptors. Instead, the results reported here provide direct evidence for a voltage-dependent fast inward current, sensitive to tetrodotoxin (TTX) and requiring external sodium (but not calcium). This current is shunted by a transient fast outward current, also voltage dependent, and it is suggested that this shunting may account, at least in part, for the non-spiking behaviour of the coxal receptors.

Dynamic properties of cockroach cercal "bristlelike" hair sensilla

Responses of cercal "bristlelike" hair sensilla (BHS) on Periplaneta americana L. to movement were investigated by recording generator (GP) and spike potentials with an extracellular pipette electrode which held the bristle by its tip. BHSs had no resting discharge, were purely phasic, with sensitivity only to stimulus transitions. They were directionally sensitive. Sinusoidal analysis suggested, to a first approximation because of the important nonlinearities, the behavior of a first-order lead system with corner frequencies distributed between 8 and 20 Hz. Responses elicited by step- and ramplike displacements were roughly in accord with the above behavior. Nonlinearities occurred both at GP level and at the level of spike generation. The phasic and the nonlinear behaviors at GP level may have a mechanical origin. The lack of spontaneous activity and the threshold nature of the spike generator account for other linearities. The operation of BHS could be separated conceptually into a linear element followed by nonlinear elements. A computer simulation based on these concepts showed a close fit to the biological responses.

Dynamic properties of cockroach cercal "threadlike" hair sensilla

Input--output (I--O) relationships were studied in cercal "thread-hair" sensilla (THS) on Periplaneta americana L. by recording from individual axons of THS while displacing the corresponding hair with a galvanometric device. Sinusoidal analysis was attempted and pulse- and ramplike displacements were then tested. The effects of stimulus orientation were also investigated. THS were spontaneously active and purely phasic and did not respond to sustained displacements. With small sinusoidal displacements (less than 30 degrees) they behaved as a linear, second-order lead system sensitive to velocity. With larger amplitudes, however, they exhibited prominent nonlinear features with minimal consequences of displacements at the extremes. Responses to other waveforms indicated second-order response components. THS were directionally sensitive. Phasic behavior and the nonlinearities may be due to mechanical properties at the base of the hair. The spike-firing threshold may also contribute. Resting activity appears to be due to neuronal factors since it was not abolished by preventing hair movements. Transducer operations were simulated in a digital computer.

Crotaline pit organs analyzed as warm receptors

Afferent impulses from single-fiber preparations of the trigeminal nerve in Agkistrodon blomhoffi brevicaudus were recorded during steady and dynamic temperature stimulation of the sensory membrane in the facial pit. The thermoreceptors of the pit showed high sensitivity to the rate of change in receptor temperature. Changing the heat capacity of the pit membrane (a drop of water in the pit in the case of the laser and halogen lamp, and a drop of water covered by a plastic film in the case of flowing water) changed the pattern of response. When the heat capacity of the pit membrane is increased, responses approach those obtained in other warm receptors. The spatial gradient theory of Williams, whereby a reversal of heat energy flow is supposed to produce a reverse of response, was shown to be inapplicable to the pit receptors. Reversal of heat energy flow in the pits produced neither off-silence nor depression of response, and therefore direction of heat flow is not an important component of the stimulus for these receptors.

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

Receptor muscles of the abdominal muscle receptor organs of the crayfish, Procambarus clarkii, were examined by electron microscopy. Both the fast and the slow receptor strand comprises a single muscle fibre which is divided by invagination of the cell membrane into numerous cytoplasmic processes in its intermediate region (the so-called intercalated tendon). Most of these myofibrillar processes insert in this region, but some of them pass through the intermediate region without interruption and join the other portion of the fibre. Thus the receptor muscles, whilst maintaining cytoplasmic continuity throughout their whole length, are modified in their intermediate regions, becoming fasciculated and providing spaces which are occupied by the connective tissue and the dendrites of the sensory neurone. Clear-cut differences in fine structure are shown between the muscle of the two types of receptor unit. The fast receptor muscle shows the typical features of arthropod fast muscles, including short sarcomere length (on average 3.3 micron), cylindrical myofibrils, well-developed sarcoplasmic reticulum, and regular hexagonal array of the myofilaments. By contrast, the slow receptor muscle fibre is characterized by long sarcomeres (average 6.5 micron) and unique organization of the myofilaments, with very thick 'thick' filaments having diameters in the range of 25-36 nm surrounded by about 12 thin filaments.