TTX sensitive plateau potentials in the crayfish slowly adapting stretch receptor neuron

Summary Electrophysiological experiments showed that a tetrodotoxin (TTX) sensitive slowly inactivating Na⁺ current contributed to the excitability of the sensory neuron (SN1) that innervates the slow receptor muscle in the abdominal muscle receptor (MR1) of crayfish, Procambarus clarkii. Following either tetraethylammonium (TEA) blockage of the K⁺ delayed rectifier currents or exposure to high temperature, a depolarizing plateau potential was evoked by the slow Na⁺ current. Ca⁺⁺ substitution by other divalent cations had no effect on the plateau potential, demonstrating that Ca⁺⁺ is not involved in plateau potential genesis. Simultaneous intrasomatic and extraaxonic recordings coupled with 4-aminopyridine (4-AP) exposure indicated that the slowly inactivating Na⁺ current is primarily somatic, and does not contribute significantly to spiking.Abbreviations 4-AP 4-aminopyridine - HAP hyperpolarizing after-potential - MR1 slowly adapting muscle receptor organ - SR1 sensory neuron of MR1 - TEA tetraethylammonium - TTX tetrodotoxin     

Quantitative studies on the slowly adapting stretch receptor of the crayfish

Summary In isolated receptors the impulse frequency following step stretches had a highly significant correlation with both muscle length and tension; any deviations from linearity were in opposite directions, impulse frequency rising more quickly than linearly with length and more slowly than linearly with tension. The impulse frequency decayed according to a power function of time from application of a step increase in length. A transfer function was derived and used to predict responses to sinusoidal and constant velocity stretches. The experimental data generally agreed with predictions. The deviations that were found could be accounted for by considering quantitatively any non-linearity between frequency and length, the adaptation of the impulse frequency to constant currents, the all-or-none nature of the action potential, and the viscous forces present during dynamic stretch. The approximately linear relationship between impulse frequency and muscle length and muscle tension is discussed. Muscle tension appears to be the more direct causal agent of impulse generation. Possible physical bases for the transfer function are also considered.     

Chemosensitivity of crayfish slowly adapting stretch receptors to nicotine

It has recently been demonstrated that slowly adapting stretch receptors (SASRs) in the airways of the dog respond directly to nicotine (Federation Proc. 43: 318, 1984). The purpose of the present experiment was to investigate this chemical effect on an isolated stretch receptor. The crayfish muscle receptor organ was chosen, since crayfish muscle is reported to be insensitive to nicotine or acetylcholine and therefore permits the testing of any direct chemical effect of nicotine on the muscle stretch receptors. The tail was removed and pinned out in a tissue bath, and a stretch receptor organ was surgically isolated. Single-unit SASR extracellular nerve recordings were made while simultaneously measuring tension in the tail. Drugs were prepared in Van Harreveld's solution and administered into the bath kept at 18 degrees C. When resting muscle tension was essentially reduced to zero by cutting both ends of the receptor organ muscle, nicotine (0.07 microM) added to the bath increased receptor activity fourfold. This response was abolished by treatment with hexamethonium (690 microM). In a second group of animals in which the muscle was left intact, nicotine was shown to significantly increase receptor sensitivity to step changes in muscle tension. Once again hexamethonium blocked the response to nicotine. These results demonstrate that the sensitivity of mechanoreceptor can be altered by chemical interaction with nicotinic receptors, which dramatically alter sensory receptor activity.     

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.     

Measures of complexity in neural spike-trains of the slowly adapting stretch receptor organs

Discrete sequence analysis methods were applied to study spike-trains generated by the isolated neuron of the slowly adapting stretch receptor organ. Calculation of the algorithmic complexity and block entropies of digitized individual spike-train forms allowed us to distinguish different classes of neural behavior. While some spike-trains exhibited significant structure, others displayed diverse degrees of randomness. The sequences recorded during the stimulated portions of the intermittent and walk-through forms, differed considerably from their randomly shuffled surrogates. Informational and grammar complexity measures (in two, four and eight-letter alphabets), tell us things about the structure of spike-trains that are not obtained with conventional spike analysis. Comparison of the conditional entropies for the digitized signals showed that the method distinguishes between different stimulated conditions. Additionally, comparison of the different stimulated conditions with their corresponding surrogates showed that, both, conditional entropies and complexities were significantly different for the two groups. Although the original and the randomly shuffled sequences had the same distribution and average firing rate, their complexity values were different. The results obtained with both measures of sequence structure were quite consistent.     

Repetitive firing: quantitative analysis of encoder behavior of slowly adapting stretch receptor of crayfish and eccentric cell of Limulus

Techniques developed for determining summed encoder feedback in conjunction with the leaky integrator and variable-gamma models for repetitive firing are applied to spike train data obtained from the slowly adapting crustacean stretch receptor and the eccentric cell of Limulus. Input stimuli were intracellularly applied currents. Analysis of data from cells stringently selected by reproducibility criteria gave a consistent picture for the dynamics of repetitive firing. The variable-gamma model with appropriate summed feedback was most accurate for describing encoding behavior of both cell types. The leaky integrator model, while useful for determining summed feedback parameters, was inadequate to account for underlying mechanisms of encoder activity. For the stretch receptor, two summed feedback processes were detected: one had a short time constant; the other, a long one. Appropriate tests indicated that the short time constant effect was from an electrogenic sodium pump, and the same is presumed for the long time constant summed feedback. Both feedbacks show seasonal and/or species variations. Short hyperpolarizing pulses inhibited the feedback from the long time constant process. The eccentric cell also showed two summed feedback processes: one is due to self inhibition, the other is postulated to be a short time constant electrogenic sodium pump similar to that described in the stretch receptor.     

Effects of acetazolamide and 4-aminopyridine on CO2-induced slowly adapting pulmonary stretch receptor inhibition in rats

Inhibitory responses of slowly adapting pulmonary stretch receptor (SAR) activity to CO(2) inhalation (maximal tracheal CO(2) concentration ranging from 9.5 to 12.5%) for approximately 60 s were examined before and after administration of acetazolamide (a carbonic anhydrase inhibitor) or 4-aminopyridine (4-AP, a K(+) channel blocker). The experiments were performed in 35 anesthetized, artificially ventilated rats after unilateral vagotomy. Sixty-eight of eighty-four SARs were inhibited by CO(2) inhalation. The SAR inhibition was attenuated by pretreatment with either acetazolamide (20 mg/kg, n = 10) or 4-AP (0.7 and 2.0 mg/kg, n = 10). In other series of experiments, stainings to show the existence of carbonic anhydrase (CA) enzymatic reaction were not found in the smooth muscle of either extrapulmonary or intrapulmonary bronchi. Protein gene product 9.5 (PGP 9.5)-immunoreactive SAR terminals to form leaflike extensions were found in the bronchioles at different diameters and were smooth-muscle-related receptors. But in the same sections, CA isozyme II-like (erythrocyte CA) immunoreactive SAR terminals were not identified. These results suggest that CO(2)-induced inhibition of SARs may be involved in the CA-dependent CO(2) hydration in addition to the activation of 4-AP sensitive K(+) currents.     

Effects of ouabain and flecainide on CO(2)-induced slowly adapting pulmonary stretch receptor inhibition in the rabbit

The inhibitory effect of CO2 on slowly adapting pulmonary stretch receptors (SARs) was examined before and after administration of ouabain, a Na+-K+ ATPase inhibitor, and flecainide, a Na+ channel blocker. The experiments were performed in anesthetized, artificially ventilated rabbits after vagus nerve section. CO2 inhalation (maximal tracheal CO2 concentration ranging from 9.2 % to 10.4%) for about 60 sec decreased the receptor activity during both inflation and deflation. The magnitude of decreased SAR activity during deflation was greater than that seen during inflation. Administration of ouabain (25 microg/kg) initially stimulated SAR activities during inflation and deflation, and after 20 min, the SAR response was still kept excitatory in both inflation and deflation phases. Under these conditions, CO2 inhalation inhibited SAR activities during inflation and deflation. Flecainide treatment (3 mg/kg) that abolished veratridine (30 microg/kg)-induced SAR excitation had no significant effect on the inhibitory responses of SAR activity to CO2. These results suggest that the inhibitory effect of CO2 occurs when ouabain results in intracellular Na+ concentration ([Na+]i) increases in the SAR endings, and that CO2-induced SAR inhibition may not be related to the reduction of influx of Na+ through voltage-gated Na+ channels.     

哇巴因刺激肺慢适应感受器

钠钾泵抑制剂——哇巴因能引起气道内慢适应感受器异相发放,表现为冲动在正常时的吸气相发放,呼气相终止转变为在呼气相发放,吸气相终止。我们推测异相发放由过度兴奋所致,如果假设正确,那么降低气道压力从而减少对感受器刺激,将能防止异相发放。本工作在麻醉、开胸、机械通气（在呼气末附加3cm水柱的正压）的家兔中记录颈迷走神经中慢适应感受器的单位放电,向感受野注射微量哇巴因（1μmol／L,20μ1）,可观察到感受器活动发生变化。感受器放电经历紧张性发放、异相发放、以及不规则发放三个时期,随后放电终止,进入静息状态。在紧张期,感受器呈持续发放,冲动频率随肺部通气变化的波动幅度明显减小。在异相发放期,感受器活动出现突然发放（呼气相）与终止（吸气相）,其冲动快速转换于高频发放和静止之间。此时,若撤除呼气末正压而减少气道内压力,感受器活动恢复正常,即冲动频率于气管压峰值时为最高,在呼气相减少或终止。在不规则期,感受器通常处于静止状态,时而出现突发高频冲动,且与呼吸周期无关。可以设想：在吸气相,感受器受到牵拉,引起钠、钙等阳离子内流,产生感受器电位。正常时,由于激活钠泵,将钠离子泵出细胞,使感受器电位回复。当钠泵受到抑制后,钠外流受阻,感受器电位加大。在异相发放期,肺充气时牵拉感受器,进一步增加感受器电位,当它超越了产生动作电位的活动范围后,则感受器因过度去极化而失去兴奋性。     

Intracellular regulation of oxygen consumption in the isolated crayfish stretch receptor neuron

Morphological correlations of the functional regulation of oxygen consumption have been studied on single isolated crayfish mechanoreceptor neurons. An enhancement of oxygen consumption is promoted by the following: (1) redistribution of mitochondria and an increase in cytochrome oxidase (CO) activity in mitochondria near the plasmatic membrane, (2) coordination of mitochondria aggregation rhythms with pO₂ rhythms in the medium external for a cell, (3) a decrease in the area of high CO activity and mitochondria and a shortening of the oxygen diffusion pathway, (4) an increase of the CO activity gradient from the neuron body periphery to its center, (5) a transfer of oxygen with the water flow during neuron body hydration and cytoplasm dilution during the transfer of a portion of the gel into sol, (6) cyclic changes in the ratio of the neuron body and hillock sizes at which there is a transfer of oxygen with the water flow into the neuron body, its mitochondrial uptake in the neuron body, and transfer of the oxygen-free water from the neuron body into the axonal hillock and further into the external medium.     

Tracheal slowly adapting stretch receptors: Theoretical models

Explanations and conditions given for the occurrence of diffusive structure in two-species ecosystem models do not generalize to systems with three or more species.     

Structure of slowly adapting pulmonary stretch receptors in the lung periphery.

Pulmonary sensory receptors are the initiating sites for lung reflexes; however, little is known about their structure, especially the relationship between the structure and function of these receptors. Using a novel approach (combining electrophysiological and morphological techniques), we examined the structures of the typical slowly adapting pulmonary stretch receptors (SARs) located in the lung periphery. We recorded SAR activities in the cervical vagus nerve, identified the receptive field, dissected the SARs in blocks, fixed and processed these blocks for immunohistochemical staining using anti-Na+/K+-ATPase, and examined the blocks under a confocal microscope. These SAR structures have multiple endings that have terminal knobs. Some structures that are located in the airway walls have terminal knobs buried in smooth muscle. Others are in the most peripheral part of the lung, and their terminal knobs have no obvious relation to smooth muscle, suggesting that muscle contraction may not be a direct factor for SAR activation.     

Effects of sodium and potassium channel blockers on hyperinflation-induced slowly adapting pulmonary stretch receptor stimulation in the rat

The excitatory responses of slowly adapting pulmonary stretch receptor (SAR) activity to hyperinflation (inflation volume = 3 tidal volumes) for approximately 10 respiratory cycles were examined before and after administration of flecainide, a Na+ channel blocker, and 4-aminoprydine (4-AP), a K+ channel blocker. The experiments were performed in anesthetized, artificially ventilated rats after unilateral vagotomy. During hyperinflation the SARs increased their activity during inflation and decreased their discharge during deflation. The magnitude of increased SAR activity during inflation became more prominent as compared to that of decreased receptor activity during deflation. Flecainide treatment (6 mg/kg) that was sufficient to block veratridine (50 microg/kg)-induced SAR stimulation did not significantly alter the excitatory response of SAR activity to hyperinflation. Subsequent administration of 3 mg/kg flecainide (a total dose, 9 mg/kg) resulted in a greater inhibition of hyperinflation-induced SAR stimulation. Although administration of 4-AP (2 mg/kg) usually stimulated SAR activity, particularly in the deflation phase, in the control ventilation, 4-AP treatment had no significant effect on hyperinflation-induced SAR stimulation. These results suggest that the excitatory effect of hyperinflation on SAR activity may not be involved in the activation of either flecainide-sensitive Na+ channels or 4-AP-sensitive K+ channels.     

Light and electron microscopic studies of slowly adapting abdominal stretch receptors of the American river crayfish Orconectes limosus (RAF)]

The slowly adapting abdominal stretch receptors of Orconectes limosus (RAF) have been investigated morphologically; 1. Despite their variety of size and shape all slowly adapting receptor neurons show common characteristic features which in addition distinguish them clearly from the fast adapting receptor neuron type SN2. The slightly globular cells have always several dendrites (often 4-6). They originate apical or lateral to the neuron, are oriented mainly longitudinal to the muscle fibres and are brodly ramified. The fine dendrites form a 3-dimensional fibrilar network. 2. The structure and distribution of the connective tissue in the "intertendon" of the muscle receptor organ correspond to the dendrite ramification; In this area, some muscle fibres end direktly at tendon-like connective tissue structures, but a number of different fibres run uninerruptedly through the whole muscular fascicle. 3. The perikaryon of every sensory neuron shows 2 "cytoplasm types" which are clearly distinguishable one against the other. A characteristic feature of the granular-lamellar neuroplasm that closely surrounds the nucleus are many flat vesicles of the granular endoplasmatic reticulum, accumulations of free ribosomes, numerous mitochondria and Golgi fields. The fibril-rich neuroplasm on the contrary contains only few mitochondria, but very many neurofilaments, here and there also neurotubuli. It projects directly into the dendrites and neuritek. Cell bodies, axon and dendrites are surrounded alternatingly by sheath cells and connective tissue of collagenous nature. The innermost layer of the coat cells borders directly on the neuron membrane. Finer dendrites are enclosed by nothing more but a thin layer of sheath cell plasm and intercellular substance. The dendrite terminals are either stored directly in connective tissue ground substance or border immediately on the sarcoplasm. 5. The axo-dendritic or axo-somatic synapses, respectively, contain numerous ellipsoidal (250-350 X 400-500 A), but also many spherical, vesicles. Some vesicles have a slightly larger diameter (700-900 A) and contain an electron-dense core. The synaptic gap measures 150 to 200 A. The neuromuscular (supposedly excitatory) synapses are filled much lighter with vesicles as compared with those just mentioned, which show a relatively unique form and size (nearly all spherical, phi 400-500 A). There are less vesicles with an electron-dense centre. On the average, the synaptic gap is broader (200-250 A) and the contact zone is larger. Apart from these, terminals could be observed in the dendritic ramification area, too, resembling the axo-dendritic and axo-somatic ones, respectively. 6. Finer dendrite branches contain vesicles differing slightly from those mentioned above as far as shape and size are concerned. Their diameters vary between 500 and 1 000 A. "Dense bodies" could be observed sporadically in these vesicles.     

Summation of excitation and inhibition in a second order sensory neuron investigated by computer simulation

Dorsal spinocerebellar tract (DSCT) neurones adequately activated by primary afferents from the muscle spindles in one muscle in the hindleg of the cat, is regularly inhibited by primary afferents from other muscles. The inhibitory input causes a constant reduction in firing frequency independent of the excitatory drive of the cell. In a simple model the effects of presynaptic inhibition and of postsynaptic inhibition with different time course of the inhibitory action, have been explored. Within the scope of this model, only postsynaptic inhibition with a very long time-constant could explain the experimental results. It is suggested that the inhibitory action is transmitted to the DSCT-cell through a number of synapses distributed over the dendrites.     

Response of slowly adapting pulmonary stretch receptors to reduced lung compliance

We examined the steady-state response of slowly adapting pulmonary stretch receptors (SAPSRs) to reduced lung compliance in open-chest cats with lungs ventilated at eupneic rate and tidal volume (VT) and with a positive end-expiratory pressure (PEEP) of 3-4 cmH2O. Transient removal of PEEP decreased compliance by approximately 30% and increased transpulmonary pressure (Ptp) by 1-2.5 cmH2O. Reduction of compliance significantly decreased SAPSR discharge in deflation and caused a small increase in discharge at the peak of inflation; it had little effect on discharge averaged over the ventilatory cycle. Increasing VT to produce a comparable increase in Ptp significantly increased peak discharge. Thus unlike rapidly adapting receptors, whose discharge is increased more effectively by reduced compliance than by increased VT, SAPSRs are stimulated by increased VT but not by reduced compliance. We speculate that the most consistent effect of reduced compliance on SAPSRs (the decrease in deflation discharge) was due to the decreased time constant for deflation in the stiffer lung. This alteration in firing may contribute to the tachypnea evoked as the lungs become stiffer.     

Effect of pulmonary arterial PCO2 on slowly adapting pulmonary stretch receptors

We recorded pulmonary stretch receptor (PSR) activity in anesthetized dogs and examined the effect of varying pulmonary arterial PCO2 (PpCO2) in both the naturally perfused and vascularly isolated pulmonary circulations while ventilating the lungs with room air. Steady-state increases in PpCO2 from approximately 25 to 50 Torr and from 50 to 70 Torr decreased PSR activity (impulses/ventilatory cycle) by 15 and 9%, respectively (P less than 0.001). Rapid increases in PpCO2 from approximately 50 to 80 Torr in a right-heart bypass preparation (with pulmonary blood flow constant) decreased PSR activity by 27%. Depression of firing, which was proportionately greater in deflation, was not dependent on changes in lung mechanics. Results show that loading CO2 intravascularly depresses PSR activity, the effects extending above as well as below resting PpCO2. Rapidly increasing PpCO2 above the resting level markedly depresses PSR activity during the transient. We conclude that PSRs may contribute to altered breathing resulting from changes in mixed venous PCO2 over the physiological range.