Interdependency of stress relaxation and afferent nerve discharge in rat small intestine

Background and aimsTo be able to characterize intestinal mechano-electrical transduction, i.e. the mechanoreceptor behaviour, quantitative nerve studies with controlled and quantified stimulus are needed. This study aimed to determine the relationship between mechanical stress relaxation and afferent discharge adaptation evoked by fast isovolumetric bag distensions in the rat jejunum.MethodsMultiunit afferent activity was recorded in vivo from jejunum afferents from five male Wistar rats. The jejunum was distended via a bag at a distension speed of 0.5 ml/s to volumes of 0.2, 0.25, 0.3 and 0.4 ml, respectively. The distension was stopped and the volume was kept constant for 2 min to induce stress relaxation. The pressure in the bag, the afferent discharge (spike rate) and the diameter of the segment during the relaxation time were recorded simultaneously.ResultsThe afferent discharge responses to distension showed a pattern with a peak during the sudden loading followed by decreasing activity with time. At distension volumes of 0.2, 0.25, 0.3 and 0.4 ml, the afferent discharge declined faster and to a greater extent (94%, 91%,96% and 87%) than the stress decreased (55%, 45%, 59% and 56%) during stress relaxation (p<0.001). Both the stress and the afferent discharge during the constant volume distension were independent of the distension volumes (p>0.5). The stress and the afferent discharge during the distension can be described mathematically on the basis of the quasi-linear theory of viscoelasticity. The association between the stress and the afferent discharge during the constant volume distension is linear with the same slope under various distension volumes.ConclusionsIntestinal mechanoreceptors were sensitive to the stress stimulus and a linear association between the stress relaxation and afferent discharge adaptation was found. The quasi-linear theory of visco-elasticity can be transferred to analysis of mechanical stimulus evoked afferent discharge.     

躯体和内脏传入冲动在大鼠束旁核内的会聚

在麻痹的大鼠上,分别刺激迷走神经、内脏大神经、坐骨神经、腓肠神经、睾丸和副睾,在对侧丘脑束旁核记录到了45个细胞的单位放电。根据诱发反应的潜伏期、时程和放电频谱分布的不同,可将他们分为五种类型,并且认为这些类型和刺激引起的感觉性质有关。在观察到的45单位中,29个的反应具有痛放电的特性,而且对躯体及内脏的传入冲动呈聚合性反应。其中2个单位只对内脏传入冲动产生反应。这项研究的结果表明,束旁核不仅是接受内脏传入的丘脑结构,而且也是一个整合内脏与躯体传入信息的中枢。     

Modulation of the masseteric reflex by gastric vagal afferents

Several investigations have shown that the vagal nerve can affect the reflex responses of the masticatory muscles acting at level either of trigeminal motoneurons or of the mesencephalic trigeminal nucleus (MTN). The present experiments have been devoted to establish the origin of the vagal afferent fibres involved in modulating the masseteric reflex. In particular, the gastric vagal afferents were taken into consideration and selective stimulations of such fibres were performed in rabbit. Conditioning electrical stimulation of truncus vagalis ventralis (TVV) reduced the excitability of the MTN cells as shown by a decrease of the antidromic response recorded from the semilunar ganglion and elicited by MTN single-shock electrical stimulation. Sympathetic and cardiovascular influences were not involved in these responses. Mechanical stimulation of gastric receptors, by means of gastric distension, clearly diminished the amplitude of twitch tension of masseteric reflex and inhibited the discharge frequency of proprioceptive MTN units. The effect was phasic and depended upon the velocity of distension. Thus the sensory volleys originating from rapid adapting receptors reach the brain stem through vagal afferents and by means of a polysynaptic connection inhibits the masseteric reflex at level of MTN cells.     

Relations of single semicircular canals to the pontine reticular formation.

The effects of afferent vestibular impulses on single pontine reticular formation units and on a small filament of the IIIrd cranial nerve were recorded with tungsten microelectrodes in 40 curarized guinea pigs. Single-shock and repetitive electrical stimulations were applied by means of stimulating electrodes inserted bilaterally into the perilymphatic space of single ampullae of the anterior and lateral semicircular canals. The reticular unitary response consisted mainly in excitation of the resting discharge rate: most units showed vestibular convergence being affected by separate stimulation of the single four ampullae. the reticular evoked field and unitary potentials accounted for latency values ranging from 0.3 to 2.5 msec. As for the early latencies they can be interpreted as responses mediated by direct vestibulo-reticular fibres. A delimited vestibular projection field in the parameidan pontine reticular formation was not identified.     

Response of motor units of the abdominal musculus rectus to afferent input in the course of the long-lasting activity in rats

In rats, in the course of a long-lasting m. rectus abdominis motor units activity combined with repetitive afferent stimulation of the ischiadic nerve, the units' firing rate decreased. The dependence of the motor units response to afferent stimulation on the background firing rate was preserved during the long-lasting activity in spite of changes in the background firing rate.     

The tensile and stress relaxation responses of human patellar tendon varies with specimen cross-sectional area.

In order to provide insight into the mechanical response of the collagen fascicle structures in tendon, a series of constant strain rate and constant displacement, stress relaxation mechanical tests were performed on sequentially sectioned human patellar tendon specimens (protocol 1) and specimens with both small (approximately 1 mm2) and large (approximately 20 mm2) cross-sectional areas (protocol 2). These data described the stress relaxation and constant strain rate tensile responses as a function of cross-sectional area and water content. The experimental data suggested that small portions of tendon exhibit a higher tensile modulus, a slower rate of relaxation and a lower amount of relaxation in comparison to larger specimens from the same location in the same tendon. The decrease in relaxation response and the increase in tensile modulus with decreasing cross-sectional area was nonlinear. These data suggest that there may be structures other than the subfascicle, such as the epitenon and other connective tissue components, which influence the tensile and stress relaxation responses in tendon.     

糖尿病痛过敏大鼠尾神经中传入单位对交感传出的反应

实验观察了刺激交感神经（sympathetic stimulation,SS）、静脉注射去甲肾上素（noradrenaline,NA）和酚妥拉明对糖尿病痛过敏大鼠尾神经中各种传入单位的影响。结果发现,糖尿病痛过敏大鼠的具有自发放电的C单位和Aδ单位在SS后放电频率增加,α－受体阻断剂能消除这些自发放电活动;在无自发放电的C单位和Aδ单位中,SS能使部分C单位和Aδ单位由静息状态转入活动状态;它虽不能诱发C－机械感受单位（C mechanical receptive unit,C-M）产生传入放电,但可诱发部分C－机械热单位（C mechano-heat unit,C-MH）和C－多型单位（C polymodal unit,C-Pol）的活动;SS还能使部分Aδ－机械单位（Aδ mechanical receptive unit,Aδ-M）和Aδ－机械热单位（Aδ mechano-heat unit,Aδ-MH）产生传入放电;它所诱发的C单位和Aδ单位反应的潜伏期不等,但不短于5s;SS不能引起糖尿病痛过敏大鼠Aβ机械感受性单位和对照组大鼠各类感受性单位产生新的传入活动。静脉注射NA可诱发糖尿病痛过敏大鼠的部分C单位和Aδ单位产生新的传入活动。结果提示,交感神经末梢释放的NA对糖尿病痛过敏大鼠C单位和Aδ单位的兴奋作用是糖尿病大鼠产生痛过敏和感觉异常的外周因素。     

Slowly Adapting Cutaneous Mechanoreceptor Afferent Units Associated with Merkel Cells in Frogs and Effects of Direct Currents

In the bullfrog, two types of slowly adapting (SA) cutaneous mechanoreceptor afferent units have been identified physiologically: irregularly discharging frog type I (Ft I) units in both warty and nonwarty skin, and regularly discharging frog type II (Ft II) units in the nonwarty skin. In the present study, mechanosensitive spots of Ft I units were located around the skin warts in the warty skin. The quinacrine technique (Crowe and Whitear, 1978) revealed that quinacrine-accu-mulating Merkel cells were present around the skin warts and near the orifice of skin glands that also surrounded the skin warts. Thus, a significant correlation was found between the location of Merkel cells and the receptive fields (RFs) of Ft I units in the warty skin.

Direct current (DC) stimulation was applied for 1 sec to the skin inside and outside the mechanical RFs of the two types of SA units. RFs for DC stimulation were located on those for mechanical stimulation in both types of SA units. The current threshold required to produce a single spike was lower in cathodal than in anodal pulses in both types of SA units. Greater current intensity elicited an increased number of spikes, but the effective polarity of currents was anodal for Ft I units and cathodal for Ft II units. The optimal current intensity for producing prolonged discharges ranged from +60 to +100 μA in Ft I units and from -50 to -80 μA in Ft II units. The sequence of impulses evoked was irregular in Ft I units and regular in Ft II units, as seen in mechanical responses. When current of the effective polarity for each type of unit was superimposed on the mechanical indentations, it facilitated the mechanical response. Currents of opposite polarity were not effective without mechanical indentation, but when used together, they depressed the mechanical response in both the Ft I units and the Ft II units. Thus, different polarities of DC could selectively activate two different types of SA units in bullfrogs. We consider these findings in connection with a presumed receptor structure for each type of unit; it is likely that the prolonged discharges in the Ft I unit are produced by active involvement of Merkel cells, whereas those in Ft II units are the result of a direct activation of afferent nerve terminals.     

Mechanical properties of spinal nerve roots subjected to tension at different strain rates

An understanding of the biomechanical and physiological properties of spinal nerve roots, particularly in response to tension, is critical in understanding the pathomechanisms of pain and nerve root injury and subsequent management of related injuries. Biomechanical properties of dorsal nerve roots at the lumbar and sacral levels were evaluated at various strain rates. Nerve roots were stretched at two different rates, 0.01 mm/s (Group A, quasistatic) and 15 mm/s (Group B, dynamic). Load, displacement and digital video data were obtained as the nerve roots were stretched until failure. Maximum stress, strain at maximum stress and modulus of elasticity (E) were calculated from the load-displacement measurements. Comparison of mechanical properties and failure patterns of nerve roots at two different rates revealed significant differences. Maximum load, maximum stress and E values of 5.7+/-2.7 gm, 257.9+/-111.3 kPa and 1.3+/-0.8 MPa were observed for Group A and 13.9+/-7.5 gm, 624.9+/-306.8 kPa and 2.9+/-1.5 MPa were observed for Group B, respectively. Higher maximum load, maximum stress and E values occurred at the dynamic stretch rate as compared to the quasistatic stretch rate, illustrating the strain-rate dependency of spinal nerve roots. No differences were observed in the strain values. Differences in mechanical behavior of nerve roots were also observed among the four root levels (L4-S1). A significant interaction effect was observed between nerve root diameter and stretch rates. Overall, results from the present study demonstrate viscoelastic material properties of spinal nerve roots and provide better insight on the tensile properties of nerve roots at different strain rates.     

Biphasic poroviscoelastic simulation of the unconfined compression of articular cartilage: II--Effect of variable strain rates

This study investigated the abilities of the linear biphasic poroviscoelastic (BPVE) model and the linear biphasic poroelastic (BPE) model to simulate the effect of variable ramp strain rates on the unconfined compression stress relaxation response of articular cartilage. Curve fitting of experimental data showed that the BPVE model was able to successfully account for the ramp strain rate-dependent viscoelastic behavior of articular cartilage under unconfined compression, while the BPE model was able to account for the complete viscoelastic response at a slow strain rate, but only the long-term viscoelastic response at faster strain rates. We concluded that the short-term viscoelastic behavior of articular cartilage, when subjected to a fast ramp strain rate, is primarily governed by a fluid flow-independent (intrinsic) viscoelastic mechanism, whereas the long-term viscoelastic behavior is governed by a fluid flow-dependent (biphasic) viscoelastic mechanism. Furthermore, a linear viscoelastic representation of the solid stress was found to be a valid model assumption for the simulation of ramp strain rate-dependent relaxation behaviors of articular cartilage within the range of ramp strain rates investigated.     

Information processing at a central synapse suggests a noise filter in the auditory pathway of the noctuid moth

1. The central projections of the A1 afferent were confirmed via intracellular recording and staining with Lucifer Yellow in the pterothoracic ganglion of the noctuid moths, Agrotis infusa and Apamea amputatrix (Fig. 1). Simultaneous recordings of the A1 afferent in the tympanal nerve (extracellularly) and in the pterothoracic ganglion (intracellularly) confirm the identity of the stained receptor as being the A1 cell. 2. The major postsynaptic arborizations of interneurone 501 in the pterothoracic ganglion were also demonstrated via intracellular recording and staining (Fig. 2). Simultaneous recordings of the A1 afferent (extracellularly) and neurone 501 (intracellularly) revealed that each A1 spike evokes a constant short latency EPSP in the interneurone (Fig. 2Bi). Neurone 501 receives only monaural input from the A1 afferent on its soma side as demonstrated by electrical stimulation of each afferent nerve (Fig. 2Bii). EPSPs evoked in neurone 501 by high frequency (100 Hz) electrical stimulation of the afferent nerve did not decrement (Fig. 2Biii). These data are consistent with a monosynaptic input to neurone 501 from the A1 afferent. 3. The response of neurone 501 to a sound stimulus presented at an intensity near the upper limit of its linear response range (30 ms, 16 kHz, 80 dB SPL) was a plateau-like depolarization, with tonic spiking activity which continued beyond the end of the tone. The instantaneous spike frequency of the response was as high as 800 Hz, and was maintained at above 600 Hz for the duration of the tone (Fig. 3). 4. The relationship between the instantaneous spike frequency in the A1 afferent and that recorded simultaneously in neurone 501 is linear over the entire range of A1 spike frequencies evoked by white noise sound stimuli (Fig. 4). Similarly, the relationship between instantaneous spike frequency in the A1 afferent and the mean depolarization evoked in neurone 501 is also linear for all A1 spike frequencies tested (Fig. 5). No summation of EPSPs occurred for A1 spike frequencies below 100 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cosensitivity of vagal mucosal afferents to histamine and 5-HT in the rat jejunum

A complex sensitivity of afferent nerves in the mesentery of the rat jejunum to systemic administration of histamine has recently been demonstrated. In the present study, we aimed to characterize subpopulations of mesenteric afferents that mediate this afferent nerve response. Multiunit afferent discharge was recorded from mesenteric nerves supplying the proximal jejunum in anesthetized rats. The majority of mesenteric bundles (84%) exhibited biphasic responses to histamine (8 micromol/kg), and these bundles also responded to 2-methyl-5-HT (2m5HT). In contrast, monophasic responses lacked a short-latency component, and these bundles failed to respond to 2m5HT. Single-unit analysis revealed a population of afferents that possessed cosensitivity for 2m5HT and histamine. This population of afferents was absent in chronically vagotomized animals, whereas mucosal anesthesia with luminal lidocaine reversibly converted the biphasic profile to a monophasic one. Ondansetron (500 microg/kg) blocked the response to 2m5HT with no effect on the profile of the histamine response, whereas pyrilamine (5 mg/kg) blocked the histamine response without affecting the response to 2m5HT. We conclude that histamine-sensitive afferents exist in the rat proximal jejunum that also respond to 5-HT via the 5-HT3 receptor. These fibers appear to be vagal afferents originating in the intestinal mucosa and may be involved in the organization of mast cell-mediated responses.     

Gastric distension-induced release of 5-HT stimulates c-fos expression in specific brain nuclei via 5-HT3 receptors in conscious rats

We examined c-fos expression in specific brain nuclei in response to gastric distension and investigated whether 5-HT released from enterochromaffin (EC) cells was involved in this response. The role of 5-HT3 receptors in this mechanism was also addressed. Release of 5-HT was examined in an ex vivo-perfused stomach model, whereas c-fos expression in brain nuclei induced by gastric distension was examined in a freely moving conscious rat model. Physiological levels of gastric distension stimulated the vascular release of 5-HT more than luminal release of 5-HT, and induced c-fos expression in the nucleus of the solitary tract (NTS), area postrema (AP), paraventricular nucleus (PVN), and supraoptic nucleus (SON). The c-fos expression in all these brain nuclei was blocked by truncal vagotomy as well as by perivagal capsaicin treatment, suggesting that vagal afferent pathways may mediate this response. Intravenous injection of 5-HT3 receptor antagonist granisetron blocked c-fos expression in all brain nuclei examined, although intracerebroventricular injection of granisetron had no effect, suggesting that 5-HT released from the stomach may activate 5-HT3 receptors located in the peripheral vagal afferent nerve terminals and then induce brain c-fos expression. c-fos Positive cells in the NTS were labeled with retrograde tracer fluorogold injected in the PVN, suggesting that neurons in the NTS activated by gastric distension project axons to the PVN. The present results suggest that gastric distension stimulates 5-HT release from the EC cells and the released 5-HT may activate 5-HT3 receptors located on the vagal afferent nerve terminals in the gastric wall leading to neuron activation in the NTS and AP and subsequent activation of neurons in the PVN and SON.     

Peptide-containing neurons intrinsic to the gut wall

Summary Nerve fibers containing substance P, VIP, enkephalin or somatostatin are numerous in the porcine gut wall. They are particularly numerous in the submucosal and myenteric plexuses where peptide-containing cell bodies are also observed. Peptide-containing nerve fibers occur also in the vagus nerves, suggesting that the gut receives an extrinsic supply of peptidergic nerves. The extrinsic contribution to the peptide-containing nerve supply of the gut wall has not yet been quantitatively assessed. In an attempt to clarify this question pigs were subjected to bilateral subdiaphragmatic vagotomy. Another group of animals was subjected to complete extrinsic denervation by autotransplantation of a jejunal segment. The pigs were killed at various time intervals after the operations; the longest time interval studied was four months. Following vagotomy the innervation pattern of the jejunum appeared completely unaffected. Following complete extrinsic denervation the adrenergic nerve fibers disappeared, while peptide-containing and acetylcholinesterase-positive nerve fibers remained apparently unaltered. This was confirmed chemically in the case of substance P.The motor activity of smooth muscle from the jejunum was studied in vitro. At low stimulation frequencies the smooth muscle from control jejunum responded by relaxation; upon cessation of stimulation a contraction occurred. With increasing stimulation frequencies the duration of the relaxation decreased; at high frequency stimulation only a contraction was recorded. In the autotransplant low frequency stimulation induced no or only a weak relaxation; high frequency stimulation induced contraction. After cholinergic and adrenergic blockade, the muscle responded with relaxation at all frequencies; the response was similar in innervated and denervated specimens. On the whole, the effects of extrinsic denervation on the motor activity of smooth muscle from porcine jejunum were minor, possibly reflecting the high degree of autonomy of the gut.     

Diaphragmatic relaxation rate after voluntary contractions and uni- and bilateral phrenic stimulation

We compared the rate of relaxation of the diaphragm (RRdi) after unilateral phrenic nerve stimulation, bilateral phrenic nerve stimulations, and short sharp voluntary contractions (sniffs). RRdi was measured as the maximum rate of decline in transdiaphragmatic pressure (Pdi) corrected for the change in Pdi [maximum relaxation rate (MRR)/delta Pdi], the time constant (tau) of the later exponential decline in Pdi, and the time to half relaxation (1/2 RT). In five subjects there was no difference in mean RRdi apart from a smaller MRR/delta Pdi (P less than 0.05) for left unilateral compared with either right unilateral or bilateral needle stimulation. However, RRdi varied unpredictably between unilateral and bilateral stimulation of the phrenic nerve in individual subjects. In the same five subjects, sniffs were found to have a slower RRdi than bilateral stimulations (MRR/delta Pdi 0.0064 +/- 0.0007 vs. 0.0074 +/- 0.0018/ms, tau 57.2 +/- 8.7 vs. 48.2 +/- 7.4 ms, 1/2 RT 108.9 +/- 10.9 vs. 73.9 +/- 6.0 ms; all P less than 0.05). The application and inflation of an abdominal binder to an external pressure of 60 mmHg resulted in a decrease in functional residual capacity (-710 +/- 70 ml), but there was no effect on relaxation parameters. Our findings suggest that in the evaluation of RRdi 1) unilateral hemidiaphragmatic stimulations may not accurately reflect the in vivo contractile properties of the diaphragm, 2) sniff maneuvers are not voluntary equivalents of phrenic nerve stimulations, and 3) RRdi is not affected by abdominal binder inflation up to 60 mmHg.     

Interactions between inhibitions of sympathetic nerve discharge to successive stimulations of baroreceptor and cutaneous afferents

Interactions between inhibitions of sympathetic nerve discharge (SND) to consecutive stimulations of A- and C-fibres in the aortic nerve and superficial peroneal (SP) nerve have been studied in rabbits anaesthetized with urethane and chloralose. SND was recorded from the renal nerve. At interval of 10 s following conditioning stimulation of SP nerve the test response of SND to activation of C-fibres (interaction SP-C) amounts to 83.7 +/- 7.5% (means +/- SE) of control. After applying both stimulations to the same SP nerve (interaction SP-SP) the test response of SND is significantly lower amounting to 56.3 +/- 3.9%. Comparable decrease of the effect of conditioning stimulation of one type of afferents on the test response to activation of other type of fibres was observed at interval of 10 s in two other interactions (SP-A and C-A). In seven interactions the conditioning stimulation of one type of fibres had no effect on the test response to stimulation of other type of fibres. This was found in one interaction at interval of 10 s (C-SP), in four interactions at interval of 20 s (SP-A, C-SP, SP-C and C-A) and in two interactions at interval of 40 s (SP-A and SP-C). Conditioning stimulations used in above interactions depressed the test responses to activation of the same type of fibres. It is suggested that the decrease or suppression of the effect of conditioning stimulation of one type of afferents on the test response of SND to activation of other type of fibres is related to disinhibition of this response.     

Static and dynamic properties of synaptic transmission at the cyto- neural junction of frog labyrinth posterior canal

The properties of synaptic transmission have been studied at the cyto-neural junction of the frog labyrinth posterior canal by examining excitatory postsynaptic potential (EPSP) activity recorded intraaxonally from the afferent nerve after abolishing spike firing by tetrodotoxin. The waveform, amplitude, and rate of occurrence of the EPSPs have been evaluated by means of a procedure of fluctuation analysis devised to continuously monitor these parameters, at rest as well as during stimulation of the semicircular canal by sinusoidal rotation at 0.1 Hz, with peak accelerations ranging from 8 to 87 deg.s-2. Responses to excitatory and inhibitory accelerations were quantified in terms of maximum and minimum EPSP rates, respectively, as well as total numbers of EPSPs occurring during the excitatory and inhibitory half cycles. Excitatory responses were systematically larger than inhibitory ones (asymmetry). Excitatory responses were linearly related either to peak acceleration or to its logarithm, and the same occurred for inhibitory responses. In all units examined, the asymmetry of the response yielded nonlinear two-sided input-output intensity functions. Silencing of EPSPs during inhibition (rectification) was never observed. Comparison of activity during the first cycle of rotation with the average response over several cycles indicated that variable degrees of adaptation (up to 48%) characterize the excitatory response, whereas no consistent adaptation was observed in the inhibitory response. All fibers appeared to give responses nearly in phase with angular velocity, at 0.1 Hz, although the peak rates generally anticipated by a few degrees the peak angular velocity. From the data presented it appears that asymmetry, adaptation, and at least part of the phase lead in afferent nerve response are of presynaptic origin, whereas rectification and possible further phase lead arise at the encoder. To confirm these conclusions a simultaneous though limited study of spike firing and EPSP activity has been attempted in a few fibers.     

Interaction of apical and basal membrane ion channels underlies electroreception in ampullary epithelia of skates.

The exquisite sensitivity of elasmobranch fishes to electric fields is thought to reside in electroreceptive organs called ampullae of Lorenzini. We measured the stimulus-response behavior of ampullary organs excised from skates. Under open-circuit conditions, the ampullary organ showed three distinct response states: spontaneous repetitive spikes, evoked spikes, and small, damped oscillatory responses. Under short-circuit conditions, the amplitude range for a linear current response to a sinusoidal (0.5 Hz) voltage clamp of an organ (assessed by spectral analysis of the harmonics generated) was 7-200 microV rms. Changes in the spike firing rate of the afferent nerve innervating the organ were evident for voltage clamps of the ampullary epithelium of 3 microV and the spike rate saturated for clamp steps exceeding 100 microV. Thus, the linear response range of the ampullary epithelium exceeded the range in spike firing rate of the afferent nerve. The steady-state transorgan electrical properties under voltage clamp conditions were obtained by analysis of complex admittance determinations in the frequency range 0.05-20 Hz for perturbations (< 100 microV rms) in the linear range. Admittance functions were distinctly related to the preparation states observed under open-circuit conditions. A negative real part in the organ admittance (i.e., a steady-state negative conductance generated by the preparation) was a common characteristic of the two (open-circuit) excitable states. The negative conductance was also confirmed by the direction of current flow through the ampullary epithelium in response to step voltage clamps.(ABSTRACT TRUNCATED AT 250 WORDS)     

Functional properties and projection areas of cutaneous receptors in catfish

Summary Neurophysiological studies were conducted in a species of catfish,Ictalurus nebulosus where specialized living habits are coupled with enlargement of certain nervous system structures. These fish, mainly lake bottom dwellers, recognize food only by touch. The entire body surface contains structures resembling mammalian taste buds; the ganglia of the fifth and seventh nerves (receiving input from barbel and head skin) are much larger than in other fish of similar size; the facial lobes, macroscopically invisible in most fish, are prominent. Early anatomical studies reported that the cutaneous buds, considered chemosensitive, project to the brainstem in parallel with tactile paths, but the two modalities remain separate. The facial lobes, the projection area of the cutaneous buds via the seventh nerve, were considered a pure chemosensory structure. In feeding behavior, the chemosensory system was proposed to distinguish sapid from nonsapid substances, the tactile system to localize the food substances.Response properties of single neurons to mechanical and chemical skin stimulations were determined in three successively activated sites: (1) sensory ganglion, (2) facial lobe, and (3) cerebellum. In each location, the majority of neurons were mechanosensitive; only a tiny fraction responded to chemical stimulation. Contrary to early anatomical studies, the two modalities were intermingled, and, particularly the enlarged facial lobes, cannot be considered pure chemosensory structures.Some degree of topographic organization occurred in the ganglion and facial lobe on comparing input from two peripheral barbel nerves. Among chemosensory neurons, some responded only to one, others to several of the applied test solutions. The latter generated distinctive spike trains in response to different test solutions (Fig. 2).Certain response properties, absent in the ganglion, occurred in the facial lobe after synaptic transmission: convergence of afferent fibers on single neurons, indicated by larger receptive fields and by spatial summation of input from two barbel nerves. Stimulation of a single peripheral nerve gave evidence of temporal summation at short conditioning-testing intervals and of inhibition at longer intervals (Fig. 5), suggesting that in many facial lobe units a single stimulus evokes a sequence of excitation followed by inhibition. The latencies of electrically driven units suggest that input from the barbels and head region project to the brainstem, and from there via a multisynaptic path to the cerebellum.This study was supported by U.S. Public Health Service Grants DE 02152 and DE 00248. I am indebted to Mr. R. Hansen, College of Fisheries, University of Washington, for obtaining the fish from Lake Washington.     

CCK enhances response to gastric distension by acting on capsaicin-insensitive vagal afferents

Capsaicin treatment destroys vagal afferent C fibers and markedly attenuates reduction of food intake and induction of hindbrain Fos expression by CCK. However, both anatomical and electrophysiological data indicate that some gastric vagal afferents are not destroyed by capsaicin. Because CCK enhances behavioral and electrophysiological responses to gastric distension in rats and people, we hypothesized that CCK might enhance the vagal afferent response to gastric distension via an action on capsaicin-insensitive vagal afferents. To test this hypothesis, we quantified expression of Fos-like immunoreactivity (Fos) in the dorsal vagal complex (DVC) of capsaicin-treated (Cap) and control rats (Veh), following gastric balloon distension alone and in combination with CCK injection. In Veh rats, intraperitoneal CCK significantly increased DVC Fos, especially in nucleus of the solitary tract (NTS), whereas in Cap rats, CCK did not significantly increase DVC Fos. In contrast to CCK, gastric distension did significantly increase Fos expression in the NTS of both Veh and Cap rats, although distension-induced Fos was attenuated in Cap rats. When CCK was administered during gastric distension, it significantly enhanced NTS Fos expression in response to distension in Cap rats. Furthermore, CCK's enhancement of distension-induced Fos in Cap rats was reversed by the selective CCK-A receptor antagonist lorglumide. We conclude that CCK directly activates capsaicin-sensitive C-type vagal afferents. However, in capsaicin-resistant A-type afferents, CCK's principal action may be facilitation of responses to gastric distension.