Analysis of Combined Action of Antagonists of Excitatory and Inhibitory Amino Acids on Motoneuron Postsynaptic Potentials of the Frog Rana ridibunda

The effect of blockers of excitatory and inhibitory amino acid receptors on postsynaptic potentials (PSP) evoked by activation of three synaptic inputs of the lumbar motoneuron (stimulation of the dorsal root, reticular formation, ventral and lateral columns) was studied on preparation of the isolated spinal cord of the frog Rana ridibunda. It has been shown that sensitivity of PSP to antagonists differs in different motoneurons, in the same motoneuron at activation of different inputs, and in the same input in different PSP components. It has been found that many descendent (DC) PSPs resistant to kynurenate or CNQX [1] were inhibited by blockers of inhibitory receptors. In this case the early component of DC-PSP varied considerably by amplitude and changed its polarity from positive to negative on the background of a low transmembrane depolarizing current. These changes were absent under conditions of replacement of chlorine ion by sulfate in the perfusion solution or treatment of the spinal cord with a blocker of inhibitory amino acids. All this allows suggesting that these DC-PSPs or their components were inhibitory. A part of PSPs resistant to kynurenate and CNQX were also resistant to the blockers of inhibitory amino acids (strychnine, picrotoxin, and bicuculline). In some cases, as a result of treatment with convulsants, the same blockers of excitatory receptors inhibited the initially resistant PSPs.     

Superior laryngeal nerve section alters responses to upper airway distortion in sleeping dogs

Curran, Aidan K., Peter R. Eastwood, Craig A. Harms, CurtisA. Smith, and Jerome A. Dempsey. Superior laryngeal nerve sectionalters responses to upper airway distortion in sleeping dogs.J. Appl. Physiol. 83(3): 768-775, 1997.We investigated the effect of superior laryngeal nerve (SLN)section on expiratory time(TE) and genioglossuselectromyogram (EMGgg) responses to upper airway (UA) negative pressure(UANP) in sleeping dogs. The same dogs used in a similar intact study(C. A. Harms, C. A., Y.-J. Zeng, C. A. Smith, E. H. Vidruk, and J. A. Dempsey. J. Appl. Physiol. 80:1528-1539, 1996) were bilaterally SLN sectioned. After recovery,the UA was isolated while the animal breathed through a tracheostomy.Square waves of negative pressure were applied to the UA from below thelarynx or from the mask (nares) at end expiration and held until thenext inspiratory effort. Section of the SLN increased eupneicrespiratory frequency and minute ventilation. Relative to the same dogsbefore SLN section, sublaryngeal UANP caused lessTE prolongation while activation of the genioglossus required less negative pressures. Mask UANP had noeffect on TE or EMGgg activity.We conclude that the SLN 1) is notobligatory for the reflex prolongation ofTE and activation of EMGggactivity produced by UANP and 2)plays an important role in the maintenance of UA stability and thepattern of breathing in sleeping dogs.

     

Corticofugal influences on the motoneurons of the trigeminal nucleus

We studied the postsynaptic potentials evoked from 76 trigeminal motoneurons by stimulation of the motor (MI) and somatosensory (SI) cortex in the ipsilateral and contralateral hemispheres of the cat. Stimulation of these cortical regions evoked primarily inhibitory postsynaptic potentials (PSP) in the motoneuron of the masseter muscle, but we also observed excitatory PSP and mixed reactions of the EPSP/IPSP type. The average IPSP latent period for the motoneurons of the masseter on stimulation of the ipsilateral cortex was 6.1±0.3 msec, while that on stimulation of the contralateral cortex was 5.2±0.4 msec; the corresponding figures for the EPSP were 7.6±0.5 and 4.5±0.3 msec respectively. Corticofugal impulses evoked only EPSP and action potentials in the motoneurons of the digastric muscle (m. digastricus). The latent period of the EPSP was 7.6 msec when evoked by afferent impulses from the ipsilateral cortex and 5.4 msec when evoked by pulses from the contralateral cortex. The duration of the PSP ranged from 25 to 30 msec. Postsynaptic potentials developed in the motoneurons studied when the cortex was stimulated with a single stimulus. An increase in the number of stimuli in the series led to a rise in the PSP amplitude and a reduction in the latent periods. When the cortex was stimulated with a series of pulses (lasting 1.0 msec), the IPSP were prolonged by appearance of a late slow component. We have hypothesized that activation of the trigeminal motoneurons by corticofugal impulsation is effected through a polysynaptic pathway; each functional group of motoneurons is activated in the same manner by the ipsilateral and contralateral cortex. The excitation of the digastric motoneurons and inhibition of the masseter motoneurons indicates reciprocal cortical control of their activity.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 3, No. 5, pp. 512–519, September–October, 1971.     

Postsynaptic potentials of facial motoneurons evoked by afferent and corticofugal impulses

Postsynaptic potentials of motoneurons in the facial nerve nucleus, evoked by stimulation of the cranial nerves (trigeminal, hypoglossal, facial) and of the sensomotor cortex were investigated in cats anesthetized with chloralose and pentobarbital. Two functionally opposite groups of motoneurons were found to exist in the facial nucleus. Stimulation of the afferent nerves and cortex evoked the appearance of EPSPs in the first of these groups and IPSPs in the second. The latency and duration of the PSPs indicate that afferent and corticofugal impulses reach the facial motoneurons along polysynaptic pathways. Interneurons on which wide convergence of influences travelling along afferent fibers and of the cortex, were found in the region of the facial nucleus. The possible neuronal pathways concerned with the transmission of afferent and corticofugal impulses to the facial motoneurons are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol.4, No.4, pp. 391–400, July–August, 1972.     

Vagal stimulation induces expiratory lengthening in the in vitro neonate rat

Mellen, Nicholas M., and Jack L. Feldman.Vagal stimulation induces expiratory lengthening in the in vitroneonate rat. J. Appl. Physiol. 83(5):1607-1611, 1997.Respiration is modulated by lung mechanoreceptorfeedback in vivo on a cycle-to-cycle basis. We replicated thismodulation in vitro and tested four stimulus protocols to identifywhich of these most closely replicated in vivo responses to lungmechanoreceptor activation in mammals. We activated pulmonary vagalafferent pathways by electrical stimulation or by lung inflation,applied during expiration, which produces expiratory lengthening invivo. In each modality, transient and tonic stimuli were applied.Stimuli were applied over a range of delays following inspiratorytermination. Tonic stimuli were maintained until subsequent inspiratoryonset. All stimulus modalities prolonged expiration(P < 0.05). These results indicatethat the neural circuitry mediating pulmonary afferent modulation ofexpiratory duration is retained in vitro.

     

Short-term plasticity of descending synaptic input to phrenic motoneurons in rats.

Respiratory afferent stimulation can elicit increases in respiratory motor output that outlast the period of stimulation by seconds to minutes [short-term potentiation (STP)]. This study examined the potential contribution of spinal mechanisms to STP in anesthetized, vagotomized, paralyzed rats. After C(1) spinal cord transection, stimulus trains (100 Hz, 5-60 s) of the C(1)-C(2) lateral funiculus elicited STP of phrenic nerve activity that peaked several seconds poststimulation. Intracellular recording revealed that individual phrenic motoneurons exhibited one of three different responses to stimulation: 1) depolarization that peaked several seconds poststimulation, 2) depolarization during stimulation and then exponential repolarization after stimulation, and 3) bistable behavior in which motoneurons depolarized to a new, relatively stable level that was maintained after stimulus termination. During the STP, excitatory postsynaptic potentials elicited by single-stimulus pulses were larger and longer. In conclusion, repetitive activation of the descending inputs to phrenic motoneurons causes a short-lasting depolarization of phrenic motoneurons, and augmentation of excitatory postsynaptic potentials, consistent with a contribution to STP.     

Human respiratory input impedance between 32and 800Hz, measured by interrupter technique and forced oscillations

Frey, Urs, Bela Suki, Richard Kraemer, and Andrew C. Jackson. Human respiratory input impedance between 32 and 800 Hz,measured by interrupter technique and forced oscillations. J. Appl. Physiol. 82(3):1018-1023, 1997.Respiratory input impedance (Zin) over a widerange of frequencies (f) has beenshown to be useful in determining airway resistance (Raw) and tissueresistance in dogs or airway wall properties in human adults. Zinmeasurements are noninvasive and, therefore, potentially useful ininvestigation of airway mechanics in infants. However, accuratemeasurements of Zin at these f valueswith the use of forced oscillatory techniques (FOT) in infants aredifficult because of their relatively high Raw and large compliance ofthe face mask. If pseudorandom noise pressure oscillations generated bya loudspeaker are applied at the airway opening (FOT), the power of theresulting flow decreases inversely withf because of capacitive shunting intothe volume of the gas in the speaker chamber and in the face mask. Westudied whether high-frequency respiratory Zin can be measured by using rapid flow interruption [high-speed interrupter technique(HIT)], in which we expect the flow amplitude in the respiratorysystem to be higher than in the FOT. We compared Zin measured by HIT with Zin measured by FOT in a dried dog lung and in five healthy adultsubjects. The impedance was calculated from two pressure signalsmeasured between the mouth and the HIT valve. The impedance could beassessed from 32 to 800 Hz. Its real part at lowf as well as thef and amplitude of the first andsecond acoustic resonance, measured by FOT and by HIT, were notsignificantly different. The power spectrum of oscillatory flow whenthe HIT was used showed amplitudes that were at least 100 times greaterthan those when FOT was used, increasing atf > 400 Hz. In conclusion,the HIT enables the measurement of high-frequency Zin data ranging from 32 to 800 Hz with particularly high flow amplitudes and, therefore, possibly better signal-to-noise ratio. This is particularly important in systems with high Raw, e.g., in infants, when measurements have tobe performed through a face mask.

     

Reflex apneic response evoked by laryngeal exposure to wood smoke in rats: neural and chemical mechanisms

Lin, Y. S., and Y. R. Kou. Reflex apneic responseevoked by laryngeal exposure to wood smoke in rats: neural and chemical mechanisms. J. Appl. Physiol. 83(3):723-730, 1997.We investigated the neural and chemical mechanismscontributing to the immediate ventilatory responses to laryngealexposure to wood smoke in anesthetized Sprague-Dawley rats. Fivemilliliters of wood smoke were delivered into a functionally isolatedlarynx at a constant flow rate of 1.4 ml/s while the animals breathedspontaneously. Within 1 s after exposure, laryngeal wood smokeconsistently triggered an apnea in each of the 42 rats tested. Theapneic duration reached 1,636.4 ± 105.4 (SE) % (n = 42) of the baseline expiratoryduration. This apneic response was not affected by denervation ofrecurrent laryngeal nerves (n = 6) orby removal of smoke particulates (n = 14), but it was totally eliminated by topical application of ananesthetic (n = 8; lidocainehydrochloride, 8%) to the laryngeal mucosa or by sectioning of thesuperior laryngeal nerves (n = 42).Furthermore, laryngeal application of a hydroxyl radical scavenger(dimethylthiourea; 500 mg/ml; n = 8)greatly diminished or abolished the smoke-induced apneic response, butit did not affect the apneic response evoked by laryngeal exposure toair saturated with 6% ammonia. These results suggest that theimmediate apneic response to laryngeal wood smoke is a reflex resulting from the stimulation of the superior laryngeal afferents by the gasphase of wood smoke and that the stimulation is mediated through ahydroxyl radical mechanism.

     

Changes in potentiation and timing of sensorimotor transmission after adaptation of a postsynaptic transient outward current in a simulated cortical neuron

How adaptation of a postsynaptic transient outward current might affect the efficacy of sensorimotor transmission was investigated. The transmission signals that were studied were a 5 ms conditioned stimulus (CS) and a 60 ms US drawn from intracellularly recorded, depolarizing postsynaptic potentials (PSPs) elicited in pyramidal neurons of the cat motor cortex by a click CS and a glabella tap US, respectively. SPICE, a program used to analyze electrical circuits, was used to simulate the cortical neuron containing the adaptive outward current. Changes in the magnitude and latency of rise to firing threshold of the PSPs were compared i) after presynaptic augmentation of a CS input in the absence of an adaptive postsynaptic current and ii) after decreasing the magnitude of an adaptive postsynaptic current that was rapidly activated by depolarization. Effects of short (6 ms) and long (24 ms) inactivation time constants of the postsynaptic current were also studied. In both presynaptic adaptation and postsynaptic adaptation, the potentiation of the magnitude of the CS-induced PSP was similar, with the latency to threshold being reduced by " 1 ms in both cases. The effects on the US PSP differed. Presynaptic adaptation affecting the CS had no effect on the US. Adaptation of the CS by a postsynaptic outward current with a 6 ms inactivation time constant, reduced the latency to threshold of an EPSP from a nearby US synapse by up to 6 ms by augmenting the initial portion of the slowly rising US-induced PSP. Adaptation of a postsynaptic current with a 24 ms inactivation time constant reduced the latency of response to the US PSP by up to 16 ms. When the US synapse was relocated to the soma, the reduction in US latency caused by adaptation of the outward current at the CS synapse was reduced by up to one half. The latency of slowly rising components of integrated synaptic responses to compound CSs of > 5 ms duration from multiple synaptic inputs would be expected to show reductions corresponding to those of the US. We conclude that potentiation of synaptic transmission by adaptation of a postsynaptic outward current can result in reductions of latency of sensorimotor transmission that can significantly affect the timing and accuracy of controlled motor tasks. These effects depend significantly on the locations of the synaptic inputs within the cell.     

Changes in potentiation and timing of sensorimotor transmission after adaptation of a postsynaptic transient outward current in a simulated cortical neuron

How adaptation of a postsynaptic transient outward current might affect the efficacy of sensorimotor transmission was investigated. The transmission signals that were studied were a 5 ms conditioned stimulus (CS) and a 60 ms US drawn from intracellularly recorded, depolarizing postsynaptic potentials (PSPs) elicited in pyramidal neurons of the cat motor cortex by a click CS and a glabella tap US, respectively. SPICE, a program used to analyze electrical circuits, was used to simulate the cortical neuron containing the adaptive outward current. Changes in the magnitude and latency of rise to firing threshold of the PSPs were compared i) after presynaptic augmentation of a CS input in the absence of an adaptive postsynaptic current and ii) after decreasing the magnitude of an adaptive postsynaptic current that was rapidly activated by depolarization. Effects of short (6 ms) and long (24 ms) inactivation time constants of the postsynaptic current were also studied. In both presynaptic adaptation and postsynaptic adaptation, the potentiation of the magnitude of the CS-induced PSP was similar, with the latency to threshold being reduced by < or = 1 ms in both cases. The effects on the US PSP differed. Presynaptic adaptation affecting the CS had no effect on the US. Adaptation of the CS by a postsynaptic outward current with a 6 ms inactivation time constant, reduced the latency to threshold of an EPSP from a nearby US synapse by up to 6 ms by augmenting the initial portion of the slowly rising US-induced PSP. Adaptation of a postsynaptic current with a 24 ms inactivation time constant reduced the latency of response to the US PSP by up to 16 ms. When the US synapse was relocated to the soma, the reduction in US latency caused by adaptation of the outward current at the CS synapse was reduced by up to one half. The latency of slowly rising components of integrated synaptic responses to compound CSs of > 5 ms duration from multiple synaptic inputs would be expected to show reductions corresponding to those of the US. We conclude that potentiation of synaptic transmission by adaptation of a postsynaptic outward current can result in reductions of latency of sensorimotor transmission that can significantly affect the timing and accuracy of controlled motor tasks. These effects depend significantly on the locations of the synaptic inputs within the cell.     

Partitioning airway and lung tissue resistances in humans: effects of bronchoconstriction

Kaczka, David W., Edward P. Ingenito, Bela Suki, and KennethR. Lutchen. Partitioning airway and lung tissue resistances inhumans: effects of bronchoconstriction. J. Appl.Physiol. 82(5): 1531-1541, 1997.The contributionof airway resistance(Raw) and tissue resistance(Rti) to totallung resistance(RL)during breathing in humans is poorly understood. We have recentlydeveloped a method for separating Rawand Rti from measurements ofRLand lung elastance (EL)alone. In nine healthy, awake subjects, we applied a broad-band optimalventilator waveform (OVW) with energy between 0.156 and 8.1 Hz thatsimultaneously provides tidal ventilation. In four of the subjects,data were acquired before and during a methacholine (MCh)-bronchoconstricted challenge. TheRLandELdata were first analyzed by using a model with a homogeneous airwaycompartment leading to a viscoelastic tissue compartment consisting oftissue damping and elastance parameters. Our OVW-based estimates ofRaw correlated well with estimatesobtained by using standard plethysmography and were responsive toMCh-induced bronchoconstriction. Our data suggest thatRti comprises ~40% of totalRLat typical breathing frequencies, which corresponds to ~60% ofintrathoracic RL. During mildMCh-induced bronchoconstriction, Rawaccounts for most of the increase inRL. At high doses of MCh, therewas a substantial increase in RLat all frequencies and inEL athigher frequencies. Our analysis showed that bothRaw andRti increase, but most of the increaseis due to Raw. The data also suggestthat widespread peripheral constriction causes airway wall shunting toproduce additional frequency dependence inEL.

     

Phrenic motoneuron discharge during sustained inspiratory resistive loading

Iscoe, Steve. Phrenic motoneuron discharge duringsustained inspiratory resistive loading. J. Appl.Physiol. 81(5): 2260-2266, 1996.I determinedwhether prolonged inspiratory resistive loading (IRL) affects phrenicmotoneuron discharge, independent of changes in chemical drive. Inseven decerebrate spontaneously breathing cats, the discharge patternsof eight phrenic motoneurons from filaments of one phrenic nerve weremonitored, along with the global activity of the contralateral phrenicnerve, transdiaphragmatic pressure, and fractional end-tidalCO₂ levels. Discharge patterns during hyperoxic CO₂ rebreathingand breathing against an IRL (2,500-4,000cmH₂O ^· l^{1 ·} s)were compared. During IRL, transdiaphragmatic pressure increased andthen either plateaued or decreased. At the highest fractional end-tidalCO₂ common to both runs,instantaneous discharge frequencies in six motoneurons were greaterduring sustained IRL than during rebreathing, when compared at the sametime after the onset of inspiration. These increased dischargefrequencies suggest the presence of a load-induced nonchemical drive tophrenic motoneurons from unidentified source(s).

     

Potentiation of in vitro concentric work in mouse fast muscle

Grange, R. W., R. Vandenboom, J. Xeni, and M. E. Houston.Potentiation of in vitro concentric work in mouse fast muscle. J. Appl. Physiol. 84(1): 236-243, 1998.Phosphorylation of myosin regulatory light chain (R-LC) isassociated with potentiated work and power during twitch afterloadedcontractions in mouse extensor digitorum longus muscle [R. W. Grange, C. R. Cory, R. Vandenboom, and M. E. Houston.Am. J. Physiol. 269 (Cell Physiol. 38): C713-C724, 1995]. We now describe the association between R-LCphosphorylation and potentiated concentric work when the extensordigitorum longus muscle is rhythmically shortened and lengthened tosimulate contractions in vivo. Work output (at 25°C) wascharacterized at sine frequencies of 3, 5, 7, 10, and 15 Hz atexcursions of 0.6, 1.2, and 1.6 mm (~5, 9, and 13% optimal musclelength) at a low level of R-LC phosphorylation. Muscles stimulatedduring the sine function with a single twitch at specific times beforeor after the longest muscle length yielded maximal concentric work nearthe longest muscle length at a sine frequency of 7 Hz (e.g., excursion~9% optimal muscle length = 1.6 J/kg). Power increased linearlybetween sine frequencies of 3 and 15 Hz at all excursions (maximum~29 W). After a 5-Hz 20-s conditioning stimulus and coincident with a3.7-fold increase in R-LC phosphate content (e.g., from 0.19 to 0.70 mol phosphate/mol R-LC), work at the three excursions and a sinefrequency of 7 Hz was potentiated a mean of 25, 44, and 50%(P < 0.05), respectively. Thepotentiated work during rhythmic contractions is consistent withenhanced interaction between actin and myosin in the force-generatingstates. On the basis of observations in skinned skeletal muscle fibers(H. L. Sweeney and J. T. Stull. Proc. Natl. Acad. Sci.USA 87: 414-418, 1990), this enhancement couldresult from increased phosphate incorporation by the myosin R-LC. Underthe assumption that the predominant effect of the conditioning stimuluswas to increase R-LC phosphate content, our data suggest that a similarmechanism may be evident in intact muscle.

     

Inputs from upper airway affect firing of respiratory-associated midbrain neurons

Chen, Zibin, and Frederic L. Eldridge. Inputs fromupper airway affect firing of respiratory-associated midbrain neurons. J. Appl. Physiol. 83(1): 196-203, 1997.In 16 decerebrated unanesthetized cats, we studied effects ofneural inputs from upper airway on firing of 62 mesencephalic neuronsthat also developed respiratory-associated (RA) rhythmic firing whenrespiratory drive was high [Z. Chen, F. L. Eldridge, and P.G.Wagner. J. Physiol. (Lond.) 437:305-325, 1991] and on firing of 16 neurons that did notdevelop the rhythmic firing (non-RA neurons). Activity in RA neuronsincreased after mechanical expansion of pharynx (45% of those tested)or larynx (68%) and after stimulation of glossopharyngeal (50%) orsuperior laryngeal nerves (77%). The increased neuronal firingoccurred despite decreases or abolition of respiratory activity(expressed in phrenic nerve). Neuronal firing also increased aftermechanical stimulation of nasal mucosa (66%) or by jetsof air directed into the nares (48%) and after lightbrushing of nasal skin (~40%). Most stimuli led to decreased firingin a smaller number of neurons, and some neurons showed no response.None of the non-RA neurons developed an increase of firing after any ofthe stimuli, although one had decreased firing after stimulation of thesuperior laryngeal nerve. We conclude that inputs from the upper airwayand nasal skin have independent modulatory effects on the samemesencephalic neurons that are stimulated by ascending rhythmic RAinput from the medulla. These findings may have relevance to generationof the sensation of dyspnea.

     

Physiology of vibration-sensitive afferents in the femoral chordotonal organ of the stick insect

The femoral chordotonal organ in orthopterans signals proprioceptive sensory information concerning the femur-tibia joint to the central nervous system. In the stick insect, 80 out of 500 afferents sense tibial position, velocity, or acceleration. It has been assumed that the other sensory cells in the chordotonal organ would serve as vibration detectors. Extracellular recordings from the femoral chordotonal organ nerve in fact revealed a sensitivity of the sense organ for vibrations with frequencies ranging from 10 Hz to 4 kHz, with a maximum sensitivity between 200 and 800 Hz. Single vibration-sensitive afferents responded to the same range of frequencies. Their spike activity depended on acceleration amplitude and displacement amplitude of the vibration stimulus. Additionally, 80% of the vibration-sensitive afferents received indirect presynaptic inputs from themselves or from other afferents of the femoral chordotonal organ, the amplitude of which depended on stimulus frequency and displacement amplitude. They were associated with a decrease of input resistance in the afferent terminal. From the present investigation we conclude that the femoral chordotonal organ of the stick insect is a bifunctional sensory organ that, on the one hand, measures position and movement of the tibia and, on the other hand, detects vibration of the tibia. Accepted: 6 November 1998     

Hypoxia, temperature, and pH/CO2 effects on respiratory discharge from a turtle brain stem preparation

Johnson, Stephen M., Rebecca A. Johnson, and Gordon S. Mitchell. Hypoxia, temperature, andpH/CO₂ effects on respiratory discharge from a turtle brain stem preparation. J. Appl. Physiol. 84(2): 649-660, 1998.An in vitrobrain stem preparation from adult turtles (Chrysemyspicta) was used to examine the effects of anoxia andincreased temperature and pH/CO₂on respiration-related motor output. At pH ~7.45, hypoglossal (XII)nerve roots produced patterns of rhythmic bursts (peaks) of discharge(0.74 ± 0.07 peaks/min, 10.0 ± 0.6 s duration) that werequantitatively similar to literature reports of respiratory activity inconscious, vagotomized turtles. Respiratory discharge was stable for 6 h at 22°C; at 32°C, peak amplitude and frequency progressivelyand reversibly decreased with time. Two hours of hypoxia had no effecton respiratory discharge. Acutely increasing bath temperature from 22 to 32°C decreased episode and peak duration and increased peakfrequency. Changes in pH/CO₂increased peak frequency from zero at pH 8.00-8.10 to maxima of0.81 ± 0.01 and 1.44 ± 0.02 peaks/min at 22°C (pH 7.32) and32°C (pH 7.46), respectively;pH/CO₂ sensitivity was similar atboth temperatures. We conclude that1) insensitivity to hypoxiaindicates that rhythmic discharge does not reflect gasping behavior,2) increased temperature altersrespiratory discharge, and 3)central pH/CO₂ sensitivity isunaffected by temperature in this preparation (i.e.,Q₁₀ ~1.0).

     

Doublet potentiation during eccentric and concentric contractions of cat soleus muscle

Sandercock, Thomas G., and C. J. Heckman. Doubletpotentiation during eccentric and concentric contractions of cat soleusmuscle. J. Appl. Physiol. 82(4):1219-1228, 1997.The addition of an extra stimulus pulse, ordoublet, at the beginning of a low-frequency train has been shown tosubstantially increase isometric force. This study examined the effectsof muscle movement on this doublet potentiation. The soleus muscles ofanesthetized cats were stimulated at 10 Hz for 1 s, with and without anadded doublet (0.01-s interval). Isovelocity releases reduced but didnot eliminate peak and early doublet potentiation (average 0.0-0.5s after the doublet). Large releases, >0.4 s after the doublet,completely abolished sustained doublet potentiation (average0.5-1.0 s after the doublet). In contrast, early isovelocitystretches boosted peak doublet potentiation. Yet, large stretches laterin the stimulus almost completely eliminated sustained doubletpotentiation. This suggests that a different mechanism is responsiblefor early and sustained doublet potentiations. Because peak and averageinitial doublet potentiation were not strongly affected by movement,doublets still offer a viable control strategy to increase force during movement while minimizing the number of stimulus pulses.

     

Some effects of superior laryngeal nerve stimulation on sympathetic preganglionic neuron firing

Repetitive electrical stimulation of afferent fibers in the superior laryngeal nerve (SLN) evoked depressant or excitatory effects on sympathetic preganglionic neurons of the cervical trunk in Nembutal-anesthetized, paralyzed, artifically ventilated cats. The depressant effect, which consisted of suppression of the inspiration-synchronous discharge of units with such firing pattern, was obtained at low strength and frequency of stimulation (e.g. 600 mV, 30 Hz) and was absent at end-tidal CO2 values below threshold for phrenic nerve activity. The excitatory effect required higher intensity and frequency of stimulation and was CO2 independent. The depressant effect on sympathetic preganglionic neurons with inspiratory firing pattern seemed a replica of the inspiration-inhibitory effect observed on phrenic motoneurons. Hence, it could be attributed to the known inhibition by the SLN of central inspiratory activity, if it is assumed that this is a common driver for phrenic motoneurons and some sympathetic preganglionic neurons. The excitatory effect, on the other hand, appears to be due to connections of SLN afferents with sympathetic preganglionic neurons, independent of the respiratory center.     

Activity-dependent induction of facilitation,depression, and post-tetanic potentiation at an insect central synapse

Summary In Manduca sexta larvae, sensory neurons innervating planta hairs on the tips of the prolegs make monosynaptic excitatory connections with motoneurons innervating proleg retractor muscles. Tactile stimulation of the hairs evokes reflex retraction of the proleg. In this study we examined activity-dependent changes in the amplitude of the excitatory postsynaptic potentials (EPSPs) evoked in a proleg motoneuron by stimulation of individual planta hair sensory neurons. Deflection of a planta hair caused a phasic-tonic response in the sensory neuron, with a mean peak instantaneous firing frequency of >300 Hz, and a tonic firing rate of 10–20 Hz. Direct electrical stimulation was used to activate individual sensory neurons to fire at a range of frequencies including those observed during natural stimulation of the hair. At relatively low firing rates (e.g., 1 Hz), EPSP amplitude was stable indefinitely. At higher instantaneous firing frequencies (>10 Hz), EPSPs were initially facilitated, but continuous stimulation led rapidly to synaptic depression. High-frequency activation of a sensory neuron could also produce post-tetanic potentiation, in which EPSP amplitude remained elevated for several min following a stimulus train. Facilitation, depression, and post-tetanic potentiation all appeared to be presynaptic phenomena. These activity-dependent changes in sensory transmission may contribute to the behavioral plasticity of the proleg withdrawal reflex observed in intact insects.Abbreviations ACh acetylcholine - AChE acetylcholine esterase - CNS central nervous system - EPSP excitatory postsynaptic potential - I _h injected hyperpolarizing current - LTP long-term potentiation - PPR principal planta retractor motoneuron - PTP post-tetanic potentiation - R _in input resistance - V _h hyperpolarized potential - V _m membrane potential - VN ventral nerve - VNA anterior branch of the ventral nerve - V _r resting potential.     

Upper airway muscle paralysis reduces reflex upper airway motor response to negative transmural pressure in rat.

The reflex upper airway (UA) motor response to UA negative pressure (UANP) is attenuated by neuromuscular blockade. We hypothesized that this is due to a reduction in the sensitivity of laryngeal mechanoreceptors to changes in UA pressure. We examined the effect of neuromuscular blockade on hypoglossal motor responses to UANP and to asphyxia in 15 anesthetized, thoracotomized, artificially ventilated rats. The activity of laryngeal mechanoreceptors is influenced by contractions of laryngeal and tongue muscles, so we studied the effect of selective denervation of these muscle groups on the UA motor response to UANP and to asphyxia, recording from the pharyngeal branch of the glossopharyngeal nerve (n = 11). We also examined the effect of tongue and laryngeal muscle denervation on superior laryngeal nerve (SLN) afferent activity at different airway transmural pressures (n = 6). Neuromuscular blockade and denervation of laryngeal and tongue muscles significantly reduced baseline UA motor nerve activity (P < 0.05), caused a small but significant attenuation of the motor response to asphyxia, and markedly attenuated the response to UANP. Motor denervation of tongue and laryngeal muscles significantly decreased SLN afferent activity and altered the response to UANP. We conclude that skeletal muscle relaxation reduces the reflex UA motor response to UANP, and this may be due to a reduction in the excitability of UA motor systems as well as a decrease of the response of SLN afferents to UANP.