Mean Firing Frequency Research Articles

Across-fiber patterns may contain a sensory code for stimulus intensity

Various models for sensory coding have used statistical approaches based on the assumption that the stimulus intensity parameter is represented in the afferent neurons as mean firing frequency. In this paper we question the assumption that this is the only code for intensity. We show that in lobster olfactory receptors narrowly tuned to hydroxyproline, an across-fiber pattern (AFP) code distinguished more concentration levels over a 5 log step range than a response magnitude code and, unlike the latter, was unaffected by response summation time. AFP discrimination of stimulus intensity appears to be based on high inter-cell response variability and low intra-cell response variability.

Quantitative stimulus-response studies on sustained ganglion cells in the frog retina

"Sustained" visual units, i.e. nonerasable (R1) and erasable (R2) units as commonly described, were recorded in the superficial layers of the rostral optic tectum of Rana esculenta and their neuronal activity (expressed as the mean firing frequency R) was quantitatively analyzed. The mean value of the exponent alpha of the velocity function of the erasable R1 units was found to be 0.46 and the constant k to be about 24.7. The optimal stimulus diameter was equal to 2.6 deg. For the whole population of the erasable units, the exponent alpha was found to be between 0.36-0.75 (mean value approximately 0.56, i.e. largely different from the values currently reported for such units) and was independent of the stimulus diameter (D = 0.6-6 deg). However, the constant k of the velocity function (range: 8.2-37.7) varied with D. Most of the erasable units (called "mixed R1-R2" units) therefore showed an R1-type velocity function with an exponent alpha approximately 0.5 and a constant k approximately 25 but reacted to other parameters as did "typical" R2 units: (i) their neuronal response was related to the stimulus diameter by a logarithmic function with a maximum obtained for D = 2.4 deg; and (ii) a variation of about 4-17% on either side of the average value of R was observed, independent either of the stimulus velocity or of the diameter of the stimulus. Results are compared to known quantitative data, and the use of stimulus-response relationships as a tool for classification is discussed.

Quantitative studies on ipsilateral type 2 retinotectotectal (IRTT) units in frogs: homologies with R3 ganglion cells.

Contralateral and ipsilateral responses from deep layers of the rostral neuropil of the optic tectum of Rana esculenta were recorded extracellularly and quantitatively analyzed. Effects of the velocity and diameter of the stimulus on the neuronal response (measured as the mean firing frequency, R) were mainly tested in this work. 1. Among the population of changing contrast or event ganglion cells, R3-like units (with a weak response to background off-on stimulation) were defined in addition to typical R3 ganglion cells. 2. A power function relating R and the stimulus velocity (v) was established in all units (R = k v alpha), with alpha = 0.80-1.07 and k = 2.1-9.5 for R3 units, alpha = 0.55-0.77 and k = 5.8-15.2 for R3-like units, and alpha = 0.80-1.16 and k = 1.3-5.1 for ipsilateral I2 units. 3. The area function was expressed by a logarithmic function. In all classes the maximal response was obtained with 4.4 degrees-7.5 degrees targets, independent of the test velocity. 4. Both the velocity and the diameter of the stimulus influenced the value of the dynamic receptive field diameter. 5. Finally, results show that qualitative and quantitative properties of I2 units are similar to those of R3 ganglion cells.

Electrophysiological evidence for the dendritic localization of a calcium conductance in guinea-pig substantia nigra neurones in vitro.

Within the substantia nigra, anatomical, neurochemical and pharmacological findings strongly suggest that transmitter and protein are secreted from the dendrites of nigrostriatal neurones. This phenomenon may underlie a non classical modulatory cellular mechanism. Two conductances are generated in nigrostriatal neurones independent of somatic action potentials, that might mediate this modulation. However, these conductances have never been directly nor precisely located specifically within the dendrites. The aim of this study was to record the membrane properties of substantia nigra zona compacta neurones in response to selective sectioning of the population of long 'apical' dendrites i.e. the removal of the zona reticulata. Intracellular recordings from substantia nigra zona compacta neurones were made from mesencephalic slices of the guinea-pig brain maintained in vitro. In cells without the apical dendrites, the membrane potential, input resistance and mean firing frequency was not significantly different from the control neurones. However, removal of the substantia nigra zona reticulata virtually abolished one conductance in particular. This conductance, seen in control neurones, is a long lasting slow depolarization which is resistant to tetrodotoxin blockade of sodium channels: rather, it is mediated by the entry of calcium ions and is optimally deinactivated at a hyperpolarised membrane potential. Hence, this study strongly suggests that this conductance is generated exclusively in the 'apical' dendrites. It has been postulated that this long lasting calcium conductance is central to the modulation of nigrostriatal neuronal excitability. Thus, the 'apical' dendrites could play a specific and active role in the functioning of nigrostriatal neurones.

Inhibition of expiratory muscle EMG and motor unit activity during augmented breaths in cats

To test the hypothesis that expiratory muscle activity is reduced during augmented breaths, electromyographic activity (EMG) of the triangularis sterni (TS) was recovered from eight pentobarbital anesthetized cats. Augemented breaths significantly increased tidal volume and peak diaphragm EMG, and prolonged inspiratory time and the first phase of expiration. However, the duration of the second phase of expiration was unchanged. Peak TS EMG was reduced during sighs in all animals, from 25 ± 5 to 12 ± 2 arbitrary units ( P < 0.005). Furthermore, the onset of TS activity during expiration was significantly delayed during augmented breaths ( P < 0.002), whereas the duration of expiratory firing tended to decrease but not significantly. Electrical activity was recorded from eight motor units of the TS in five cats. During testing breathing the motor units had a mean relative expiratory onset time of 46 ± 4% of expiration, and a mean firing frequency of 19 ± 2 impulses/sec. Two motor units became quiescent during augmented breaths. Of the remaining six motor units, three minimally shortened their duration of activity (by less than 15%) while three substantially abbreviated their period of firing (by 50% or more). In addition, all TS motor units reduced their mean firing frequency ( P < 0.05) and number of impulses per breath ( P < 0.002) during sighs. We conclude that expiratory activity of the triangularis sterni muscle is reduced augmented breaths, due to a combination of motor unit derecruitment and a slowing of motor unit firing frequency.

Persistent reduction of Purkinje cell inhibition on neurones of the cerebellar nuclei after climbing fibre deafferentation.

The long lasting effects of inferior olive (IO) destruction were studied in rats treated with 3-acetylpyridine two years before. The activities of the Purkinje cells (PCs) and of their target neurones in the cerebellar nuclei (ECNs), were investigated and compared to those of a group of non-treated rats. Our results show that long-term deafferented PCs recovered a mean firing frequency similar to that of the controls while the ECNs discharge is enhanced. Furthermore, the cryodestruction of the cerebellar cortex produces a significant release of the ECNs firing in the control rats but not in the poisoned animals. Thus, it appears that IO destruction induces a permanent impairment of the inhibitory control exerted by the PCs on the activity of their target neurones.

Influences from laryngeal afferents on expiratory bulbospinal neurons and motoneurons.

The purpose of this study is to analyze the reflex effects of laryngeal afferent activation on respiratory patterns in anesthetized, vagotomized, paralyzed, ventilated cats. We recorded simultaneously from the phrenic nerve, T10 internal intercostal nerve, and single bulbospinal expiratory neurons of the caudal ventral respiratory group (VRG). Laryngeal afferents were activated by electrical stimulation of the superior laryngeal nerve (SLN) or by cold-water infusion into the larynx. Both types of stimuli caused inhibition of phrenic activity and facilitation of internal intercostal nerve activity, indicating expiratory effort. The activity of 46 bulbospinal expiratory cells was depressed during SLN electrical stimulation, and 13 of them were completely inhibited. In 44 of 56 neurons tested, mean firing frequency (FFmean) was decreased in response to cold-water infusion and 8 others responded with increased FFmean; in the remaining 4 neurons, FFmean was unchanged. Possible reasons for different neuronal responses to SLN electrical stimulation and water infusion are discussed. We conclude that bulbospinal expiratory neurons of VRG were not the source of the reflex motoneuronal expiratory-like activity produced by SLN stimulation. Other, not yet identified inputs to spinal expiratory motoneurons are activated during this experimental condition.

The pineal gland of the Mongolian gerbil: nocturnal increase of electrical activity.

Extracellular single unit recordings were made during day- and nighttime from spontaneously active pineal gland cells in male Mongolian gerbils. Units exhibited an irregular firing pattern. Discharge frequencies ranged from 0.1 to 8.5 Hz and showed distinct time-dependent differences. During daytime, mean firing frequencies were in the range of 1 Hz and increased to 2 Hz in the early dark period, reaching a peak (ca. 5 Hz) in the middle of the night, followed by a decrease to 2 Hz in late night. The results of this first report on pineal electrical activity in Mongolian gerbils are discussed in comparison to those obtained from rat, guinea pig and golden hamster.

Nonlinear relationship between impulse flow and dopamine released by rat midbrain dopaminergic neurons as studied by in vivo electrochemistry

Mesolimbic dopaminergic neurons discharge either in a single spike mode or in a bursting pattern. In order to investigate the influence of these patterns on dopamine release, extracellular dopamine was electrochemically monitored in vivo in the olfactory tuberculum of anaesthetized rats by means of two approaches. In the first, a pure signal, unequivocally corresponding to extracellular dopamine, was recorded every minute from pargyline treated rats by differential normal pulse voltammetry combined with electrochemically treated carbon fibre electrodes. In the second, the differential current solely due to oxidation of all the catechols was monitored every l s in drug-free rats by differential pulse amperometry. In basal conditions this current was mainly due to extracellular DOP AC. However, electrical stimulation of the dopaminergic pathway for 20 s elicited an immediate increase in this signal. This effect was due to evoked dopamine release since it was strongly enhanced by amphetamine (2 mg/kg) or pargyline (75 mg/kg) injections. As studied with both approaches, the evoked increase in extracellular dopamine concentration was immediate and lasted as long as the stimulation. The amplitude of the effect depended on the frequency of the stimulation (from 3 to 14 Hz) in an exponential manner but never exceeded 1 μ M dopamine. Bursting stimulations (frequency within the trains: 14 Hz) were twice as potent as regularly spaced ones, having the same average frequency (5 Hz). In conditions which mimicked the spontaneous activity of dopaminergic neurons when they switch from one pattern to the other (4 Hz regularly spaced stimulation versus trains at a mean frequency of 6 Hz), the bursting stimulations were found to be up to 6 times more potent. Therefore, as regards the functional efficacy of DA neurons, bursting might be much more important than mean firing frequency.

A quantitative analysis of ipsilateral tectal unit responses to moving stimuli in frogs

Ipsilateral retino-tecto-tectal (IRTT) units were recorded extracellularly in the rostral optic tectum of the frog (Rana esculenta). The activity of 79 superficial units (II type) was quantified in response to black disks of various sizes, moved vertically at various angular velocities and against a white background. The contrast ¦C¦ was constant during the experiments. Neuronal activity was analysed by two methods, yielding identical results: (1) I1 units responded transiently to moving and movement gated stationary stimuli; these units did not seem to be directionally sensitive nor responsive to changes in background illumination. Fifty-three % of units had a low spontaneous activity. (2) A power function relating mean firing frequency (¯R) and angular velocity (v) was established in the majority (78%) of units. The exponentα and the constantk were 0.44–0.8 and 8.9–20, respectively. (3) The relationship between¯R and stimulus diameter (D) was best expressed by a logarithmic function. The maximum response occurred forD= 2‡–4‡. The optimal stimulus diameter was found to be independent of stimulus velocity. (4) When stimulated repetitively under steady conditions, I1 units showed about 10% fluctuations in mean response, which seemed to increase with stimulus diameter.

An EMG analysis on regulation of the diaphragm and abdominal muscles.

Neural mechanisms regulating activities of diaphragm and abdominal muscles were investigated on anesthetized, tracheostomized and spontaneously breathing dogs. EMGs of costal diaphragm and external oblique (EO)abdominal muscle were recorded by fine-wire electrodes. The electrodes for diaphragm-EMG were implanted without opening abdominal cavity using laparoscopy. During quiet breathing the mean firing frequency of diaphragm-EMG was 7.6 +/- 0.7 (S.D.) Hz. The discharge spikes of diaphragm-EMG observed in early-inspiration or in post-inspiratory phase, were low in amplitudes, consistent with previous reports that phrenic motoneurons were comprised of two populations. During quiet breathing spikes were not observed in EO-EMG in a half of the animals, and were tonically-firing in the remaining dogs. Hypercapnia increased firing frequency of diaphragm-EMG, but did not recruit spikes of high amplitude. On the other hand, hypercapnia recruited EO-spikes of higher amplitude, and increased firing frequency. When spikes fired tonically in EO-EMG, hypercapnia suppressed these spikes during inspiration. Airway occlusion gradually recruited EO units of higher amplitude. Mechanical stimulation of upper airway suppressed tonic-EO activity during inspiration. In conclusion, we demonstrated that 1) abdominal wall is not related with control mechanism of diaphragm; 2) the control mechanism of dog's EO differs from cat's internal intercostal muscles; and 3) the tonic component of EO is affected by respiratory commands from higher neuronal architecture.

Nature of the maintained discharge of Q, X, and Y retinal ganglion cells of the cat.

Cat retinal ganglion cells with center-surround receptive fields have an irregular discharge whose rate is altered by visual stimulation. In assessing the detectability of stimulus-induced changes in the discharge, a consideration of the power spectral density of the discharge is helpful. The power spectral density of Q, X, and Y cells is flat at low frequencies, rises to a peak at the mean frequency of firing, and then decays away at higher frequencies in an oscillatory manner to an asymptotic level equal to the mean rate of discharge. Measured spectra correspond closely with spectra predicted by a renewal-point process with gamma-distributed intervals. When the rate of the discharge is altered by visual stimulation, the spectral density at low frequencies remains roughly constant. Assuming that it is the noise power at these frequencies that is effective in limiting the detectability of visual stimuli, it appears that at the retinal level the irregularity of the discharge can be treated as an additive noise.

Mechanoreceptor activity in the gills of the carp. II. Gill arch proprioceptors

The presence of gill arch proprioceptors in the gills of a teleost, Cyprinus carpio L., is demonstrated and their firing characteristics are analysed. Spontaneous activity of gill arch proprioceptors was recorded from epibranchial ganglia. In paralysed fish the mean discharge rate for 16 receptors ranged from 11.3 impulses · sec −1 (SD 0.3) to 25.7 impulses · sec −1 (SD 0.4). The discharge rate could be influenced by displacement of the main elements of the respiratory pumping system. The receptors showed a tonic response. Their firing frequency was approximately linearly related to gill arch position and, hence, showed a respiratory modulation during ventilation. Gill arch adduction caused a decrease and abduction an increase in discharge rate. In actively breathing fish the mean firing frequency of 30 neurons ranged from 9.9 impulses · sec −1 (SD 0.7) to 40.1 impulses · sec − (SD 6.7). These gill arch proprioceptors are located in the cartilaginous strip between the epibranchial and the ceratobranchial of each gill arch and are innervated by the pretrematic branches of the vagal branchial nerves. The role these proprioceptors play in the regulation of gill movement during both feeding and ventilation is discussed.

Electrophysiological characterization of the pineal gland of golden hamsters.

In one of the most-widely used species in pineal gland research, the strongly photoperiodic golden hamster, Mesocricetus auratus, no electrophysiological data on pinealocytes are currently available. To fill this gap, in the present study 185 spontaneously active pinealocytes from male golden hamsters were recorded extracellularly, both during day- and night-time (light: dark cycle 12: 12, lights on at 07.00 h). As in other species, pinealocytes exhibited action potentials of 1-2 ms duration. An irregular firing pattern was observed in 95% of the pinealocytes, the remainder fired more regularly or showed a phasic discharge pattern. The firing frequencies ranged from 0.2 to 25 Hz and showed clear time-dependent differences. From 07.00 h to 22.00 h the mean firing frequencies were identical, i.e. in the range of 2 Hz; between 22.00 h and 01.00 h mean discharge rate increased to 5 Hz and exhibited a peak of 7 Hz between 01.00 h and 04.00 h, followed by a decrease to 4 Hz between 04.00 h and 07.00 h. Electrical stimulation of the superior cervical ganglion during day-time resulted in an augmentation of firing frequency in some pinealocytes and a decrease in others; during night-time, inhibitory responses only were observed. Photic stimulation, or electrical stimulation of either the optic chiasm or the habenular nuclei mostly decreased the firing rate of pinealocytes. Compared to other mammalian species, the electrophysiological properties of golden hamster pinealocytes appear to be basically similar.

Motor unit activity and surface electromyogram power spectrum during increasing force of contraction.

Twelve male subjects were tested to determine the relationship between motor unit (MU) activities and surface electromyogram (EMG) power spectral parameters with contractions increasing linearly from zero to 80% of maximal voluntary contraction (MVC). Intramuscular spike and surface EMG signals recorded simultaneously from biceps brachii were analyzed by means of a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and an EMG frequency power spectral analysis. All measurements were made in triplicate and averaged. Results indicate that there were highly significant increases in surface EMG amplitude (71 +/- 31.3 to 505 +/- 188 microV, p less than 0.01) and mean power frequency (89 +/- 13.3 to 123 +/- 23.5 Hz, p less than 0.01) with increasing force. These changes were accompanied by progressive increases in the firing frequency of MU's initially recruited, and of newly recruited MU's with relatively larger spike amplitudes. The group data in the ISAF histograms revealed significant increases in mean spike amplitude (412 +/- 79 to 972 +/- 117 microV, p less than 0.01) and mean firing frequency (17.8 +/- 5.4 to 24.7 +/- 4.1 Hz, p less than 0.01). These data suggest that surface EMG spectral analysis can provide a sensitive measure of the relative changes in MU activity during increasing force output.

The effect of potassium on carotid body chemoreceptor discharge in the anaesthetized cat.

1. In exercise the arterial plasma potassium rises; we have investigated the possibility that such rises might affect the carotid body chemoreceptor. 2. Intravenous infusions of KCl were used to produce hyperkalaemia in anaesthetized cats. 3. Intra-arterial catheter tip potassium electrodes were used to monitor changes in plasma potassium. 4. The effects of 5 min infusions of KCl on afferent carotid chemoreceptor preparations were studied. 5. Infusions of KCl, which produced increases in plasma potassium similar to those occurring in exercise in man, caused an initial large increase in mean firing frequency (237% of control). A phase of rapid adaptation of this response was followed by a phase of slower adaptation, but after 5 min of hyperkalaemia mean firing frequency was still significantly greater than control. The amplitude of the breath-by-breath oscillation in frequency appeared to increase in parallel with mean frequency so that the amplitude/mean ratio remained constant. 6. We conclude that plasma potassium changes during exercise may contribute to the chemical drive to breathe.

Firing properties and hypercapnic responses of single phrenic motor axons in the rat.

Inspiratory phase activity was recorded from 33 phrenic motoneuron (PM) axonal fibers in anesthetized, vagotomized, artificially ventilated adult rats. During control conditions (no inspired CO2 added), the population of PM fibers could be separated into early and late onset types based on the time of firing onset relative to the onset of whole phrenic nerve activity. Mean discharge frequencies of both types were not significantly different. Compared with late PM's, early PM's had more spikes per inspiration, fired for a longer period, and the last spike occurred later and during the postinspiratory period. Further, the mean minimal interspike interval was shorter for early PM's than for late PM's. Increasing inspired CO2 to 0.03 and 0.05 resulted in earlier firing onsets and a greater number of spikes per inspiration, particularly for late PM's. Increases in mean firing frequency occurred for both PM types. Mean minimal interspike intervals for both types of PM's showed progressive reductions as CO2 rose. For almost all of the firing properties examined in this study, responses of rat PM axons were similar to those previously reported for the cat.

Endogenous burst capability in a neuron of the gastric mill pattern generator of the spiny lobster Panulirus interruptus.

The gastric system of the lobster stomatogastric ganglion has previously been thought to include no neurons capable of endogenous bursting. We describe conditions under which one of the motorneurons, the CP cell, can burst endogenously in a free-running manner in the absence of other phasic network activity. Isolated preparations of the foregut nervous system were used, and the CP bursting was either spontaneous or was activated by continuous stimulation of an input nerve. Three criteria were applied to establish the endogenous nature of such burst generation in CP: absence of phasic input, reset of the bursting pattern by pulses of current in a characteristic phase-dependent manner, and modulation of burst rate by sustained injected current. (1) The firing of other cells which are known to be related synaptically to CP was monitored in nerve records. These other cells were either silent or fired only tonically. Cross-correlograms showed that CP bursting was not ascribable to phasic activity in these other network cells. (2) A depolarizing current pulse of sufficient strength injected intracellularly between bursts triggered a burst prematurely and reset the subsequent rhythm. A hyperpolarizing pulse during a burst terminated it and reset the subsequent rhythm. Reset behavior was similar to that described for other endogenous bursters. (3) Application of a positive-going ramp current initially caused an increase in burst rate, as described for other endogenous bursters. However, further depolarization caused a slower burst rate due to lengthening of the individual bursts, although mean firing frequency continued to increase throughout the range tested. Such free-running endogenous repetitive bursting appeared to result from the CP's ability to produce slow regenerative depolarizations ("plateau potentials"). When bursting was present, so was the plateau property, as determined by I-V analysis and by the ability of brief current pulses to trigger and terminate bursts. The previous inability to observe endogenous bursting in preparations with central input removed may be due to the usual absence of the plateau property in such preparations. CP bursting during normal gastric mill rhythms, while underlain by plateau potentials, is strongly controlled by network interactions. CP appears not to be well placed in the network to be considered a source of normal gastric rhythmicity. Nevertheless, endogenous bursting in CP may explain some of the partial gastric rhythms seen in behavioral studies, and illustrates one way that cellular properties might contribute to rhythmic behaviors.

Supraoptic neurons sustain high frequency firing when extracellular Ca 2+ is replaced with other divalent cations in rat brain slices

In slices of hypothalamus, maintained in vitro, the discharge of supraoptic neurons in standard artificial cerebrospinal fluid was compared with that produced when extracellular Ca 2+ was replaced with Mg 2+, Co 2+ or Mn 2+. Interspike interval histograms were constructed for periods before, during and after replacement of extracellular Ca 2+. Of the 31 cells recorded in normal medium, 16 fired slowly and irregularly, 9 were phasic, 3 fired continuously at more than 3 spikes/s and 3 produced short, high frequency bursts of activity that were separated by slow irregular discharge. Interspike interval distributions were broad showing little preference for any one interval and intervals shorter than 30–50 ms were rare. The cell firing rate could be increased by the electrophoretic application of glutamate and under these conditions, the interval distributions became narrower as shorter intervals predominated. However, when cells discharged above 20 spikes/s, the spike amplitude declined rapidly and became indistinguishable from the noise. Replacement of extracellular Ca 2+ with Mg 2+, Co 2+ or Mn 2+ produced reversible changes in interspike interval distribution, although no consistent change in mean firing frequency was observed. Supraoptic neurons were now able to maintain relatively high frequency discharge (15–25 spikes/s) for longer periods; firing either continuously or periodically and interspike intervals became grouped more closely at the shorter end of the normal distribution. However, no very short interspike intervals were recorded. Less than 2% of all recorded intervals were shorter than 30 ms, even in cells exposed to test medium for 1–3 h and excited by application of glutamate. In cells that were transynaptically activated by electrical stimulation of the slice, exposure to test medium blocked stimulus-evoked spike activity. Calcium may be a necessary factor in the mechanism that prevents sustained high frequency discharge in these neurons under normal conditions, but the mechanism that regulates the minimum interspike interval, which is a striking feature of the normal firing of these cells, appears to be less dependent on extracellular Ca 2+.

The influence of acute hypoxia and sleep states on the electrical activity of the cerebellum in the sheep fetus

The experiments were designed to examine (1) the effects of acute hypoxia and (2) the influence of sleep states on the electrical activity of the cerebellum in the fetus. Five fetal sheep were instrumented between 106 and 133 days of gestation and survived in utero until term (140–146 days). Insulated stainless electrodes (75 μm diam.) were introduced into the cerebellar cortex and cemented in place. Electrodes were also implanted into 4 fetuses to monitor the electrocorticogram, rapid eye movements and the EMG of the dorsal neck (nuchal) muscles. Recordings of cerebellar activity were made for 252 h in 4 fetuses. The recordings were derived from many units (probably Purkinje cells) with a high level of basal discharge. The mean firing frequency was related to fetal sleep state. The highest frequencies coincided with episodes of rapid eye movements with low-voltage electrocortical activity and the lowest occurred during high-voltage electrocortical activity when rapid eye movements are absent. Acute hypoxaemia (fetal p aO 2 6–10 mm Hg for up to 30 min) abolished rapid eye movements and caused a marked reduction in cerebellar activity. High-voltage electrocortical activity and nuchal EMG activity were maintained indicating that the effects of hypoxia on the cerebellum were not due simply to a general CNS depression. Cerebellar neural activity returned to normal at the termination of the hypoxic stimuli demonstrating that fetal Purkinje cells can withstand short episodes of severe hypoxia.