Peak Firing Rate Research Articles

Regeneration and recovery of cat muscle spindles after devascularization.

1. We have assessed the sensory reinnervation and recovery of regenerated muscle spindles in extensor digitorum longus (EDL) 6, 8 and 13 weeks after the muscle, with its nerve left intact, had been devascularized. Recordings were made from the dorsal roots of the responses of single afferent fibres to ramp-and-hold stretch of the regenerated spindles whose sensory reinnervation was subsequently examined in teased, silver preparations. 2. The spindle population in four normal EDL muscles ranged from 53 to 83 (mean 69); analysis of the afferent innervation of 166 normal b1b2c spindles showed that 23% had primary endings supplied by two Ia afferents. Regenerated spindles were identified as belonging to one of four groups in which afferents establish sensory endings on intrafusal muscle fibres in groups 1-3, but not in group 4. Sensory reinnervation was complete after 6 weeks recovery and similar proportions of group 1-3 spindles occurred after each recovery period, i.e. 58% after 6 weeks, 65% after 8 weeks and 62% after 13 weeks. We estimate that about half the original spindle population was lost owing to persistent ischaemic necrosis; that 30% regenerated and acquired functional afferent connections (group 1-3 spindles); and that the total loss of spindle afferents was over 60%. 3. The conduction velocities of the regenerated spindle afferents were very similar to those of normal EDL spindle afferents. The proportions that responded normally to ramp-and-hold stretch at the end of each recovery period increased from 58% after 6 weeks to 61% after 8 and 88% after 13. Other responsive spindle afferents were either predominantly phasic or only responded to supramaximal stretch. The proportions of these decreased as recovery progressed reducing from 19% after 6 weeks to 9% after 13 weeks in the case of those giving predominantly phasic responses and from 23% after 6 weeks to 3% after 13 weeks in the case of those unresponsive to physiological stretch. 4. The mean peak and held firing rates of regenerated spindle afferents responsive to 10 mms-1 ramp-and-hold stretches were all significantly lower than normal. There was no marked trend towards higher firing rates after longer periods of recovery through, considered separately, the mean peak firing rates of the normally responding afferents did show a distinct improvement. The mean dynamic and velocity indexes were not significantly different from normal. 5. The ratio of Ia:spindle II afferents, as identified by their responses to stretch, was 1:1.22 in the control animals and 1:1.26 in the experimental series.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of passive eye position changes on retinogeniculate transmission in the cat

1. Extracellular recordings were made from single neurons in layer A of the left dorsal lateral geniculate nucleus (LGNd) of anesthetized and paralyzed adult cats. Responses to retinotopically identical visual stimuli (presented through the right eye) were recorded at several positions of the left eye in its orbit. Visual stimuli consisted of drifting sinusoidal gratings of optimal temporal and spatial frequencies at twice threshold contrast. Visual stimulation of the left eye was blocked by a variety of methods, including intravitreal injection of tetrodotoxin (TTX). The change in position of the left eye was achieved by passive movements in a randomized and interleaved fashion. Of 237 neurons studied, responses were obtained from 143 neurons on 20-100 trials of identical visual stimulation at each of six eye positions. Neurons were classified as X- or Y- on the basis of a standard battery of physiological tests (primarily linearity of spatial summation and response latency to electrical stimulation of the optic chiasm). 2. The effect of eye position on the visual response of the 143 neurons was analyzed with respect to the number of action potentials elicited and the peak firing rate. Fifty-seven (40%) neurons had a significant effect [by one-factor repeated-measure analysis of variance (ANOVA), P less than 0.05] of eye position on the visual response by either criterion (number of action potentials or peak firing rate). Of these 57 neurons, 47 had a significant effect (P less than 0.05) with respect to the number of action potentials and 23 had a significant effect (P less than 0.05) by both criteria. Thus the permissive measure by either criterion and the conservative measure by both criteria resulted in 40% and 16%, respectively, of all neurons' visual responses being significantly affected by eye position. 3. For the 47 neurons with a significant effect of eye position (number of action potentials criterion), a trend analysis of eye position versus visual response showed a linear trend (P less than 0.05) for 9 neurons, a quadratic trend (P less than 0.05) for 32 neurons, and no significant trend for the 6 remaining neurons. The trends were approximated with linear and nonlinear gain fields (range of eye position change over which the visual response was modulated). The gain fields of individual neurons were compared by measuring the normalized gain (change in neuronal response per degree change of eye position). The mean normalized gain for the 47 neurons was 4.3. 4. The nonlinear gain fields were generally symmetric with respect to nasal versus temporal changes in eye position.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of passive eye movement on retinogeniculate transmission in the cat

1. The nature and time window of interaction between passive phasic eye movement signals and visual stimuli were studied for dorsal lateral geniculate nucleus (LGNd) neurons in the cat. Extracellular recordings were made from single neurons in layer A of the left LGNd of anesthetized paralyzed cats in response to a normalized visual stimulus presented to the right eye at each of several times of movement of the left eye. The left eye was moved passively at a fixed amplitude and velocity while varying the movement onset time with respect to the visual stimulus onset in a randomized and interleaved fashion. Visual stimuli consisted of square-wave modulated circular spots of appropriate contrast, sign, and size to elicit an optimal excitatory response when placed in the neurons' receptive-field (RF) center. 2. Interactions were analyzed for 78 neurons (33 X-neurons, 43 Y-neurons, and 2 physiologically unclassified neurons) on 25-65 trials of identical visual stimuli for each of eight times of eye movement. 3. Sixty percent (47/78) of the neurons tested had a significant eye movement effect (ANOVA, P less than 0.05) on some aspect of their visual response. Of these 47 neurons, 42 (89%) had a significant (P less than 0.05) effect of an appropriately timed eye movement on the number of action potentials, 36 (77%) had a significant effect on the mean peak firing rate, and 31 (66%) were significantly affected as evaluated by both criteria. 4. The eye movement effect on the neurons' visual responses was primarily facilitatory. Facilitation was observed for 37 (79%) of the affected neurons. For 25 of these 37 neurons (68%), the facilitation was significant (P less than 0.05) as evaluated by both criteria (number of action potentials and mean peak firing rate). Ten (21%) of the affected neurons had their visual response significantly inhibited (P less than 0.05). 5. Sixty percent (46/78) of the neurons were tested for the effect of eye movement on both visually elicited activity (visual stimulus contrast = 2 times threshold) and spontaneous activity (contrast = 0). Eye movement significantly affected the visual response of 23 (50%) of these neurons. However, spontaneous activity was significantly affected for only nine (20%) of these neurons. The interaction of the eye movement and visual signals was nonlinear. 6. Nine of 12 neurons (75%) tested had a directionally selective effect of eye movement on the visual response, with most (8/9) preferring the temporal ward direction.(ABSTRACT TRUNCATED AT 400 WORDS)

Head-direction cells recorded from the postsubiculum in freely moving rats. II. Effects of environmental manipulations

The discharge characteristics of postsubicular head-direction cells in a fixed environment were described in the previous paper (Taube et al., 1990). This paper reports changes in the firing properties of head-direction cells following changes in the animal's environment. Head-direction cells were recorded from rats as they moved freely in a 76-cm-diameter gray cylinder. A white card, occupying 100 degrees of arc, was taped to the inside wall of the cylinder and served as the major orienting spatial cue in the animal's environment. Rotation of the cue card produced near-equal rotation in the preferred firing direction of head-direction cells, with minimal changes in peak firing rate, directional firing range, or asymmetry of the firing-rate/head-direction function. Card removal had no effect on peak firing rate or range of firing, but in 8/13 cells the preferred direction rotated by at least 24 degrees. Similarly, changing the shape of the environment to a rectangular or square enclosure caused the preferred firing direction to rotate by at least 48 degrees for 8/10 cells in the rectangle and 3/8 cells in the square, with minimal changes in the peak firing rate or directional firing range. Hand holding the animals and moving them around the cylinder had no effect on the preferred direction or firing range of the cell, but decreased the maximal firing rate in 7/9 cells. On 2 occasions, 2 head-direction cells were recorded simultaneously. The rotation of the preferred firing direction for one cell was the same as the rotation of the preferred direction for the second cell after each environmental manipulation. These results demonstrate that specific visual cues in the environment can exert control over the preferred firing direction and indicate that head-direction cell firing is not a simple sensory response to visual cues, but rather represents more abstract information concerning the animal's spatial relationship with its environment. The constancy of the angle between the preferred firing directions of pairs of simultaneously recorded head-direction cells suggests that there is a fixed mapping of the population onto direction within the environment. Thus, environmental manipulations appear to cause only a change in the reference direction, but leave all other discharge characteristics of directional cells unchanged. In the discussion, comparisons are drawn between the responses of head-direction cells and hippocampal place cells to similar environmental manipulations (Muller and Kubie, 1987), and ways in which these 2 spatial systems interact in navigation are discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Head-direction cells recorded from the postsubiculum in freely moving rats. I. Description and quantitative analysis

This paper is a study of the behavioral and spatial firing correlates of neurons in the rat postsubiculum. Recordings were made from postsubicular neurons as rats moved freely throughout a cylindrical chamber, where the major cue for orientation was a white card taped to the inside wall. An automatic video/computer system monitored cell discharge while simultaneously tracking the position of 2 colored light emitting diodes (LEDs) secured to the animal's head. The animal's location was calculated from the position of one of the LEDs and head direction in the horizontal plane calculated from the relative positions of the 2 LEDs. Approximately 26% of the cells were classified as head-direction cells because they discharged as a function of the animal's head direction in the horizontal plane, independent of the animal's behavior, location, or trunk position. For each head-direction cell, vectors drawn in the direction of maximal firing were parallel throughout the recording chamber and did not converge toward a single point. Plots of firing rate versus head direction showed that each firing-rate/head-direction function was adequately described by a triangular function. Each cell's maximum firing rate occurred at only one (the preferred) head direction; firing rates at head directions on either side of the preferred direction decreased linearly with angular deviation from the preferred direction. Results from 24 head-direction cells in 7 animals showed an equal distribution of preferred firing directions over a 360 degrees angle. The peak firing rate of head-direction cells varied from 5 to 115 spikes/sec (mean: 35). The range of head-direction angles over which discharge was elevated (directional firing range) was usually about 90 degrees, with little, if any, discharge at head directions outside this range. Quantitative analysis showed the location of the animal within the cylinder had minimal effect on directional cell firing. For each head-direction cell, the preferred direction, peak firing rate, and directional firing range remained stable for days. These results identify a new cell type that signals the animal's head direction in its environment.

Dynamics of neuronal activity in the cingulate gyrus during the sleep-wake cycle

Dynamics of neuronal activity in the cingulate gyrus (CG) was investigated during the sleep-wake cycle (SWC) of free-ranging cats. The highest activity rate was found to occur in 65.4% of neurons during active emotional consciousness (EC) and the so-called emotional phase of paradoxical sleep (PS), while firing rate decreased during passive consciousness (PC) and slow-wave sleep (SS). Peak firing rate was observed during SS in 15% of neurons, while divisions of activity between stages of the SWC remained the same in 19.6%. In addition, discharge patterns in 75.2% of neurons changed consistently in step with phase shifts. More particularly, neurons fired during EC and PS with single action potentials with a more or less even time distribution, whereas a burst-phase activity pattern occurred in the course of SS. Firing rate declined (in 42.6% of units) or remained unchanged (in 50.4%) in the majority of CG neurons (even amongst those manifesting highest activity during EC and PS) as episodes of isolated EEG arousal developed (whether in the context of SS or as PS wore off). This would indicate that the CG actually contributes to shaping the behavioral state of consciousness. The CG, therefore, as one of the higher divisions of the limbic system, must play a major part in controlling the basic mechanisms of the SWC, as well as emotionally motivated processes evolving in the cycle.

Quantitative Comparisons Between the Electrical Activity of Supraoptic Neurons and Vasopressin Release in vitro

Abstract The firing rate of antidromically identified neurons in the tuberal portion of the rat supraoptic nucleus and vasopressin release were compared in an in vitro preparation of the hypothalamo-neurohypophysical system. Extracellular potentials were isolated and held while the perifusing medium was collected for radioimmunoassay of vasopressin. Neurons in the tuberal portion of the supraoptic nucleus were induced to first bursts of action potentials by injecting supra-threshold concentrations (0.01, 0.1 and 1 mM) of the alpha(1)-agonist phenylephrine into the perifusing line. Phenylephrine caused a dose-dependent release of vasopressin into the perifusate and a dose-dependent increase in the peak firing rate, initial burst duration and the total number of action potentials (compared to a 10 min control period) of the recorded neurons. Peak firing rate and peak vasopressin release both increased linearly with the log of the phenylephrine dose, but the efficiency ratio of the increase in vasopressin release to the increase in firing rate was greater from the middle to the highest dose than it was for the lowest to the middle dose, indicating a possible facilitation of hormone release with higher firing frequencies. The total amount of vasopressin released in 10 min post-injection and the total number of evoked action potentials in the same period also increased linearly, but in this case there was no change in the efficiency ratio between the low to middle and the middle to high doses of phenylephrine. Release of vasopressin to phenylephrine was dependent on an intact neural stalk, and in a separate group of isolated neural lobes, 1 mM phenylephrine did not significantly alter the peak or total amount of vasopressin released to an electrical stimulus given in the pattern of an evoked burst. In another group of explants the temporal relationship between firing rate and vasopressin release was examined after a single dose of 1 mM phenylephrine. In these explants the true time-course of release was estimated using a deconvolution procedure which corrected for diffusion of the hormone. Mean vasopressin release and spike activity were still elevated above baseline 4 to 5 min after the first elicited burst, suggesting that the latter parts of the initial burst and/or the after-discharges contribute to the prolonged vasopressin release. However, there was a dramatic drop in the efficiency ratio (vasopressin release/peak firing rate) from the first to the second minute following stimulation. Thus, changes in the frequency of action potentials generated by supraoptic neurons can be directly related to simultaneous changes in the rate of vasopressin release in the same preparation. The results suggest that elevations in firing rate are accompanied by an increased rate of vasopressin release, but as has been demonstrated in isolated neural lobes, this relationship is probably not constant across different stimulation strengths or within a single burst.

Response properties of single units in the lateral terminal nucleus of the accessory optic system in the behaving primate.

1. To determine the potential role of the primate accessory optic system (AOS) in optokinetic and smooth-pursuit eye movements, we recorded the activity of 110 single units in a subdivision of the AOS, the lateral terminal nucleus (LTN), in five alert rhesus macaques. All monkeys were trained to fixate a stationary target spot during visual testing and to track a small spot moving in a variety of visual environments. 2. LTN units formed a continuum of types ranging from purely visual to purely oculomotor. Visual units (50%) responded best for large-field (70 x 50 degrees), moving visual stimuli and had no response associated with smooth-pursuit eye movement; some responded during smooth pursuit in the dark, but the response disappeared if the target was briefly extinguished, indicating that their smooth-pursuit-related response reflected activation of a parafoveal receptive field. Eye movement and visual units (36%) responded both for large, moving visual stimuli and during smooth-pursuit eye movements made in the dark. Eye movement units (14%) discharged during smooth-pursuit or other eye movements but showed no evidence of visual sensitivity. 3. Essentially all (98%) LTN units were direction selective, responding preferentially during vertical background and/or smooth-pursuit movement. The vast majority (88%) preferred upward background and/or eye movement. During periodic movement of the large-field visual background while the animal fixated, their firing rates were modulated above and below rather high resting rates. Although LTN units typically responded best to movement of large-field stimuli, some also responded well to small moving stimuli (0.25 degrees diam). 4. LTN units could be separated into two populations according to their dependence on visual stimulus velocity. For periodic triangle wave stimuli, both types had velocity thresholds less than 3 degrees/s. As stimulus velocity increased above threshold, the activity of one type reached peak firing rates over a very narrow velocity range and remained nearly at peak firing for velocities from approximately 4-80 degrees/s. The firing rates of the other type exhibited velocity tuning in which the firing rate peaked at an average preferred velocity of 13 degrees/s and decreased for higher velocities. 5. A close examination of firing rates to sinusoidal background stimuli revealed that both unit types exhibited unusual behaviors at the extremes of stimulus velocity.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphology of augmenting inspiratory neurons of the ventral respiratory group in the cat.

The present study examined, in Nembutal-anesthetized and artificially ventilated cats, the morphologic properties of the inspiratory neurons of the ventral respiratory group (VRG). Horseradish peroxidase (HRP) was injected into 21 augmenting inspiratory or late inspiratory neurons with peak firing rates in the late inspiratory phase. The majority of the stained neurons were antidromically activated by stimulation of the cervical cord. Thirteen somata, located within or around the nucleus ambiguus (AMB), between 100 microns caudally and 2,000 microns rostrally to the obex, were stained. In ten cases, the stem axons issuing from the cells of origin coursed medially to cross the midline without giving off any axonal collaterals. Three neurons gave rise to axonal collaterals on the ipsilateral side, distributing boutons in the medullary reticular formation, in the vicinity of the AMB, hypoglossal nucleus, solitary tract, and dorsal motor nucleus of the vagus. In eight neurons, only the axons were labeled; in four of these, which were antidromically activated from the spinal cord, the stem axons crossed the midline 2,000-3,000 microns rostral to the obex and descended in the reticular formation around the AMB down to the cervical cord. They issued several axonal collaterals, distributing terminal boutons at the level of the caudal end of the retrofacial nucleus and about 1,000 microns rostral and caudal from the obex. Terminals were found mainly in and around the AMB, and a few were found in the vicinity of the dorsal motor nucleus of the vagus. The remaining four nonactivated axons distributed their terminal boutons widely in the reticular formation around the AMB. Thus, the augmenting inspiratory neurons of the VRG were shown to project not only to the spinal cord, but also to the VRG, hypoglossal nucleus, and dorsal motor nucleus of the vagus.

Responses of electroreceptors in the platypus bill to steady and alternating potentials.

1. This is a report of further observations on the response characteristics of electroreceptors in the bill of the platypus, Ornithorhynchus anatinus, first described by Gregory, Iggo, McIntyre & Proske (1987). 2. The main finding is that, with the bill immersed in water, applying a potential difference between large plate electrodes on either side of the bill, produced detectable responses in a population of electroreceptors to field strengths as low as 4 mV cm-1. Threshold for individual receptors lay between 4 and 25 mV cm-1. 3. An electric dipole placed in the water close to the receptive field could also elicit responses, threshold being lowest when the cathode was near the centre of the field. On several occasions the most sensitive spot was seen, under the microscope, to correspond to the mouth of a mucous sensory gland (Andres & Von Düring, 1984). Response intensity fell when the dipole was moved further away, the drop being less steep in a direction over the top of the bill towards the mid-line. 4. For individual receptors the latency of the first impulse initiated by supramaximal voltage pulses was 1.1-1.8 ms. Latencies tended to be shorter when the site of the receptor lay closer to the recording electrodes. Plotting each latency against conduction path length for eleven receptors gave an approximately linear relation from which was calculated an average axonal conduction velocity of 56 m s-1. The plot yielded an estimate of impulse initiation time of 0.8 ms. It is argued that this is too short to include a synaptic delay. A peripheral synapse is found in all non-mammalian electroreceptors. 5. Electroreceptors responded to both steady and rapidly changing potential gradients. For ramp-shaped gradients of 1-50 V s-1 peak firing rate was approximately proportional to log stimulus velocity. In response to sinusoidal potential changes a 1:1 relation between each afferent impulse and the peak of the stimulating waveform could be obtained over the range 12-300 Hz. Threshold was at its lowest at 50-100 Hz. Tuning curves measured with the bill immersed in water were little different from those obtained by focal stimulation with the bill in air. 6. It is concluded that platypus electroreceptors, supplied by the trigeminal nerve, and which are therefore not part of the acoustico-lateralis system as in non-mammalian electroreceptors, are also unique in not having a peripheral synapse. Furthermore, they are able to respond to both steady and rapidly changing voltage gradients.(ABSTRACT TRUNCATED AT 400 WORDS)

GABAergic inhibition shapes temporal and spatial response properties of pyramidal cells in the electrosensory lateral line lobe of gymnotiform fish.

1. The amplitude-coding pyramidal neurons of the first-order nucleus in weakly electric gymnotiform fish (Eigenmannia), the electrosensory lateral line lobe (ELL), exhibit 2 major physiological transformations of primary afferent input. Pyramidal cells rapidly adapt to a step change in amplitude, and they have a center/surround receptive-field organization. This study examined the physiological role of GABAergic inhibition on pyramidal cells. GABAergic synapses onto the somata of pyramidal cells primarily originate from granule-cell interneurons along with descending input. 2. Pyramidal cells fall into two physiologically distinct categories: E units, which are excited by a rise in stimulus amplitude, and I units, which are inhibited by a rise in stimulus amplitude. Microiontophoretic application of bicuculline methiodide onto both types of pyramidal cells increased the time constant of adaptation, defined as the time required for the neuron's response to decay to 37% of its maximum value, by 70-90%. The peak firing rate of E units to a step increase in stimulus amplitude increased by 49%, while the firing rate of I units did not change significantly. 3. Bicuculline application demonstrated that GABAergic inhibition may contribute to the strict segregation of E and I response properties. In the presence of bicuculline, many E units (normally excited only by stimulus amplitude increases) became excited by both increases and decreases; many I units (normally excited only by amplitude decreases) also became excited to increases. 4. The size of the excitatory receptive-field of E units was not affected by bicuculline, although response magnitude increased. The inhibitory surround increased in spatial extent by 175% with bicuculline administration. Neither the size of the I unit receptive-field center nor the response magnitude changed in the presence of bicuculline. The antagonistic surround of I units, however, increased by 49%. 5. The anatomy of the ELL is well understood (see Carr and Maler 1986). The physiological results obtained in this study, along with the results of Bastian (1986a, b), further our understanding of the functional role of the ELL circuitry. Our results suggest that spatial and temporal response properties of pyramidal cells are regulated by different but interacting inhibitory interneurons, some of which use GABA as a neurotransmitter. The activity of these interneurons is in turn controlled by descending feedback systems.

Discharges of interpositus and Purkinje cells of the cat cerebellum during locomotion under different conditions.

1. Extracellular microelectrodes were used in free-to-move cats to study the locomotor-related discharges of Purkinje cells in the intermediate part of lobule V of the cerebellar anterior lobe and of neurones in the underlying nucleus interpositus anterior. All cells studied discharged rhythmically during locomotion. 2. The discharges during walking at a speed of 0.5 m/s on a horizontal exercise belt were compared with those during (a) walking at 0.9 m/s (when the duration of the step cycle is shortened considerably and the amplitudes of the locomotor electromyograms (EMGs) recorded from flexor and extensor muscles of the limbs are markedly increased) and (b) during walking at 0.5 m/s with the belt tilted uphill by 30 deg (when step duration is little changed but locomotor EMGs are increased by 70-100%). 3. In each of thirty Purkinje cells the timing of the discharges relative to the forelimb step cycle showed no major difference between the two speeds of walking. Most cells discharged at slightly higher overall rates at the faster walking speed but the increase was usually modest, the average being only 5.6 impulses/s (i.e. an increase of 8%). Peak rates sometimes underwent larger increases but the average was only 11.9 impulses/s (+11%). Changes in minimum rate were generally small (an average increase of 0.3 impulses/s). 4. Among twenty-one interpositus neurones there was only one in which discharge timing relative to the step cycle was different between the two speeds. Like the Purkinje cells, most neurones discharged slightly faster at the higher speed but the average increase was only 5.5 impulses/s (+8.5%). Peak firing rates also usually showed a modest increase (averaging 6.2 impulses/s; +6.5%) while minimum rates were little changed. 5. Among nineteen Purkinje cells compared between walking uphill and on the flat only one showed any major difference in discharge phasing; overall firing rates were on average only 1.3 impulses/s (2%) higher for uphill locomotion. 6. Among twenty-one interpositus neurones discharge phasing differed markedly between walking uphill and on the flat in only two cells. Overall discharge rates were on average slightly higher uphill (by 3.5 impulses/s; 6.7%) and peak rates also usually increased slightly (on average, by 6 impulses/s; 7.7%). Minimum rates were higher, on average, by 1.6 impulses/s (+5%). 7. The findings are discussed in relation to current notions of how the intermediate part of the cerebellum may contribute to movement control and it is concluded that the neurones studies probably make little contribution to determining the vigour of the movements of steady walking.

Laryngeal afferents activated by phenyldiguanide and their response to cold air or helium-oxygen

In anesthetized cts, sensory in the superior laryngeal nerves (SLN) were identified with respect to their response to (1) phenyldiguanide (PDG) i.v., (2) mechanical stimulation and (3) lowering temperature in an isolated tracheolaryngeal segment. The activity originating from 107 SLN afferent units activated by PDG was recorded using glass microelectrodes in the nodose ganglion. All tested afferent units increased their discharge rate during direct touching of the airway mucosa. None showed flow or pressure related activity during abrupt changes in constant laryngeal flow or transmural pressure in the isolated segment. Fifteen units were inhibited by cold air. Sixty-two units significantly increased their firing rate when the temparature approached 18°C, reached a peak discharge near 15°C, then their activity decreased or stopped. The response to cold air was compared to cold heliox (79% He-21 O 2), which enhanced the respiratory heat loss by conduction. The peak firing rate was significantly higher with heliox (+ 356% compared to + 246% with air), the temperature threshold higher (25° C ± 1.0° C) amd the temperature range broader (25-11.5°C). Present results show that a large proportion (58%) of afferent SLN fibres activated by PDG are likely non-proprioceptive units, which are stimulated by cooling the inspired gas. Thermosensitive units in the upper airways may act as sensors of the thermal flux through the airway wall more than as detectors of the absolute value of temperature in the airway lumen.

Cat hindlimb motoneurons during locomotion. II. Normal activity patterns.

Activity patterns were recorded from 51 motoneurons in the fifth lumbar ventral root of cats walking on a motorized treadmill at a range of speeds between 0.1 and 1.3 m/s. The muscle of destination of recorded motoneurons was identified by spike-triggered averaging of EMG recordings from each of the anterior thigh muscles. Forty-three motoneurons projected to one of the quadriceps (vastus medialis, vastus lateralis, vastus intermedius, or rectus femoris) or sartorius (anterior or medial) muscles of the anterior thigh. Anterior thigh motoneurons always discharged a single burst of action potentials per step cycle, even in multifunctional muscles (e.g., sartorius anterior) that exhibited more than one burst of EMG activity per step cycle. The instantaneous firing rates of most motoneurons were lowest upon recruitment and increased progressively during a burst, as long as the EMG was still increasing. Firing rates peaked midway through each burst and tended to decline toward the end of the burst. The initial, mean, and peak firing rates of single motoneurons typically increased for faster walking speeds. At any given walking speed, early recruited motoneurons typically reached higher firing rates than late recruited motoneurons. In contrast to decerebrated cats, initial doublets at the beginning of bursts were seen only rarely. In the 4/51 motoneurons that showed initial doublets, both the instantaneous frequency of the doublet and the probability of starting a burst with a doublet decreased for faster walking speeds. The modulations in firing rate of every motoneuron were found to be closely correlated to the smoothed electromyogram of its target muscle. For 32 identified motoneurons, the unit's instantaneous frequencygram was scaled linearly by computer to the rectified smoothed EMG recorded from each of the anterior thigh muscles. The covariance between unitary frequencygram and muscle EMG was computed for each muscle. Typically, the EMG profile of the target muscle accounted for 0.88-0.96 of the variance in unitary firing rate. The EMG profiles of the other anterior thigh muscles, when tested in the same way, usually accounted only for a significantly smaller fraction of the variance. Brief amplitude fluctuations observed in the EMG envelopes were usually also reflected in the individual motoneuron frequencygrams. To further demonstrate the relationship between unitary frequencygrams and EMG, EMG envelopes recorded during walking were used as templates to generate depolarizing currents that were applied intracellularly to lumbar motoneurons in an acute spinal preparation.(ABSTRACT TRUNCATED AT 400 WORDS)

Paired recordings from supraoptic and paraventricular oxytocin cells in suckled rats: recruitment and synchronization.

Oxytocin cells in the paraventricular (p.v.) and contralateral supraoptic (s.o.) nuclei were pair-recorded (with two micro-electrodes) in suckled rats after being anaesthetized with urethane (1.2 g/kg), to study the synchronization of their neurosecretory bursts, the importance of cell recruitment and their firing characteristics. The synchronization of paired bursts was determined by measuring the onset time-lag (time in milliseconds between the onset of two corresponding bursts) and the maximum firing time-lag (time in milliseconds between the two shortest interspike intervals for the corresponding bursts). For each cell, the characteristics studied were: the background activity and the frequency and amplitude (total number of spikes) of the neurosecretory bursts. All paired p.v.-s.o. cells recorded were activated simultaneously 12-18 s before each milk ejection. The onset of a burst could vary either way, up to 680 ms, in relation to the other (mean onset time-lag was 206 +/- 18 ms; n = 85) but the maximum activation periods fitted more closely, the mean maximum firing time-lag being 122 +/- 14 ms (n = 64). Both parameters varied randomly, in duration and order from one pair of cells to another, from one pair of bursts to another for successive bursts of a given pair of cells and independently, whether the cells were in the p.v. or the s.o. nucleus. However, in most cases, the neurosecretory burst with the highest amplitude began and reached its peak firing rate before the corresponding burst from the other cell. Cell recruitment was observed when the milk ejection reflex began, for both the p.v. and the s.o. cells. The bursts of the non-responsive cells developed progressively with the reflex, but, as soon as a cell was recruited, all its successive bursts were simultaneous with those of the first-recruited oxytocin cells. During a regular pattern of milk ejections, the mean background activity of sixty p.v. cells (3.1 +/- 0.2 spikes/s) was significantly higher than that of their s.o. counterparts (1.9 +/- 0.2 spikes/s). Nevertheless, the mean amplitude of the neurosecretory bursts of the sixty p.v. cells (49 +/- 3 spikes) did not differ significantly from that of their s.o. counterparts (55 +/- 4 spikes).(ABSTRACT TRUNCATED AT 400 WORDS)

W-like response properties of interlaminar zone cells in the lateral geniculate nucleus of a primate ( Galago crassicaudatus)

Recent anatomical studies have suggested that the cells located in the interlaminar zones (ILZs) of the primate dorsal lateral geniculate nucleus (LGN) relay visual information from the retina to the striate cortex in a manner similar to that of W-cells in the LGN of cat. In the present study, we examined this idea directly by recording the response properties of single cells localized to the ILZs in the prosimian primate, Galago crassicaudatus. The properties of the cells in the ILZs were found to be physiologically distinct from the X-like and Y-like properties of the parvocellular and magnocellular LGN layers. Moreover, the small cells located in the interlaminar zones were physiologically similar to the W-like cells found in the specialized small-celled koniocellular layers in these primates. As is the case with the koniocellular layer cells, the ILZ cells exhibited a broad range of properties which, as a group, were distinguished by the following characteristics: the ILZ cells had long latencies to stimulation of the optic chiasm (mean, 3.95 ms) and to antidromic stimulation from striate cortex (mean, 3.31 ms) and had relatively large receptive-field centers (mean, 1.79°). They also had low maintained discharge rates (5.5 spikes/s), relatively long response latencies to light (mean onset, 82 ms; peak, 112) and low peak firing rates (59 spikes/s). Few (25%) had standard receptive-field organization (ON-center, OFF-surround, or vice versa). Only 29% responded well to sine-wave gratings and all were influenced by non-visual (auditory and tactile) stimuli. Our results support the conclusion that cells located in the ILZs of primates, and perhaps all mammals, are part of a W-like system.

A neural pathway for the shift response in the cat

The shift response (McIlwain effect) was elicited by moving a large grating situated greater than 15° from the conventional receptive field center (RFC). We examined the change in the amplitude of the shift response induced by placing a steady target on the RFC or the RF surround. We found that appropriate stimulation of the RFC or the RF surround will increase and inappropriate stimulation will decrease the amplitude of the shift response in a graded manner. The amplitude of the shift response was not correlated with the maintained activity but was correlated with the transient peak firing rate which is evoked by flashing the enhancing stimulus. A shift stimulus which elicits a strong shift response can be blocked by inappropriate stimulation. The results suggest that the shift signal is modulating a tonic signal which is present in the RF. Because the shift response is a transient excitation, we suggest that the shift response results from a disinhibitory process. A possible neural pathway for the shift response is presented.

Periaqueductal gray neuronal activity associated with laryngeal EMG and vocalization in the awake monkey

Extracellular recordings were made from 73 periaqueductal gray (PAG) neurons in an awake, vocalizing monkey ( Macaca fascicularis). About 50% of the neurons were inactive and only began discharging prior to laryngeal EMG activity and vocalization; they reached their peak firing rate near the onset of vocalization and then became inactive again. Spike-triggered averaging of laryngeal EMG from the PAG unit discharge revealed in most cases an increase in laryngeal EMG immediately following the unit spike. Other cells appeared to be related to other oral-facial behaviors. The role of the PAG in vocalization is discussed.

Effects of skin cooling and warming on stretch responses of the muscle spindle primary and secondary afferent fibers from the cat's tibialis anterior

Fusimotor activation induced by thermal stimuli to the skin was investigated by recording the stretch responses of primary as well as secondary spindle afferent fibers from the tibialis anterior of the barbiturate-anesthetized, spinal cats. A standard rampand-hold stretch was repeatedly given during warming or cooling (20 to 40°C) the skin of the ipsilateral foot pads. In each response of the spindle afferent fiber to stretch, the peak (P) and static firing rates (S) were measured and the dynamic index was calculated to assess the type of fusimotor fiber influence as dynamic or static. Of 47 primary spindle afferent fibers, 22 (46%) were affected by changes in skin temperature. Cooling of the skin below a certain temperature produced a marked increase in the dynamic sensitivity of their response to stretch, and this also occurred on warming above it. Such a “critical temperature” at which a directional relationship between skin temperature and muscle spindle activity was completely altered was determined (mean ± SD: 35.10 ± 1.66°C, N = 7). However, a marked increase in static firing accompanied by a significant decrease in the dynamic index was observed at the dynamic phase of heating the skin beyond the critical temperature. None of the seven secondary afferent fibers studied showed a detectable temperature response. We conclude that the activation of primary spindle afferent fibers by thermal stimulation of the skin is to be attributed predominantly to activation of dynamic fusimotor neurons and is significant for the initiation of cold shivering.

Response of pulmonary stretch receptors to shifts of functional residual capacity

The response of slowly adapting pulmonary stretch receptors (PSRs) to sustained elevations of functional residual capacity (FRC) was investigated in spontaneous breathing anesthetized cats. A subatmospheric pressure was produced around the thorax and abdomen to increase FRC by approximately one tidal volume (V T) for up to 60 min. During eupneic breathing the PSR frequency (f PSR) was closely related to changes in transpulmonary pressure (P TP), bu occasionally hysteresis was observed in the f PSR - P TP relationship. Elevation of FRC caused most phasic PSRs to discharge continously for a few breaths before returning to a phasic discharge pattern. During the shift in FRC there were increases in mean f PSR, peak f PSR and PSR firing threshold which were sustained throughout the period of elevated FRC. PSRs that normally showed discharges at FRC similarly increased their mean and peak firing rates. For all PSRs the y-intercept ( f PSR at P TP = 0) of the f PSR - P TP relationship was decreased but the sensitivity of the PSRs as defined by Δf PSR/ΔP TP was not changed until the period of elevated FRC exceeded 30 min. Thereafter, PSR sensitivity tended to decline. There results suggest that PSRs undergo some modification of their discharge parameters during prolonged elevation of FRC.