Analysis of entrainment of respiratory rhythm by somatic afferent stimulation in cats using phase response curves

To elucidate how peripheral somatic afferents synchronize the respiratory rhythm to the exercise rhythm, the phrenic nerve activity in the vagotomized, paralyzed, and artificially ventilated cats anesthetized with chloralose-urethane was recorded during electrical stimulation of the superficial radial nerve afferents. At first, a single pulse train was given at various times of the respiratory cycle to obtain a phase-response curve (PRC). The stimulation given at mid to late expiration produced a phase advance, but the stimulation during inspiration produced no measurable phase shifts in most animals (8/10). The maximum phase advance changed depending on the stimulus intensity. The stronger the stimulus intensity, the greater became the maximum phase advance. Repetitive somatic afferent stimulation produced 1:1 entrainment of the respiratory frequency to the repetitive stimulation. Theoretical predictions on the stable entrainment phase and on the entrainment frequency range from the obtained PRC were close to the experimental results. The present study demonstrated the presence of a neuronal circuit synchronizing the respiratory rhythm to the periodic somatic afferents and the manner of how such entrainment occurs.     

Efferent activity in the phrenic nerve during startle reflex in chloralose anesthetized cats

Efferent activity was investigated in the phrenic nerve during startle reflex manifesting as somatic nerve discharges (lower intercostal nerves and the nerve endings) in chloralose anesthetized cats. Inhibition (usually of short duration, lasting 23–36 msec) of inspiration activity was found to be the main component of response in the phrenic nerve in the shaping of "low threshold" startle reflex produced by acoustic and tactile stimuli and stimulation of low threshold peripheral afferents. Reflex discharge prevailed amongst the response patterns produced in the phrenic nerve by stimulating high threshold afferents, i.e., early (propriospinal) and late (suprasegmental, arising from stimulating intercostal nerve) or late only (when stimulating the hindlimb nerves). Two patterns of late response could be distinguished, one on inspiration (found in roughly 3 out of 4 experiments) and other on exhalation — the respiratory homologs of somatic startle reflex. Response pattern is described throughout the respiratory cycle. Structure and respiratory modulation of reflex responses produced in the phrenic nerve by stimulating bulbar respiratory structure are also examined. Possible neurophysiological mechanisms underlying phrenic response during the shaping of startle reflex are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 4, pp. 473–482, July–August, 1987.     

Attenuation of phrenic motor discharge by phrenic nerve afferents

Short latency phrenic motor responses to phrenic nerve stimulation were studied in anesthetized, paralyzed cats. Electrical stimulation (0.2 ms, 0.01-10 mA, 2 Hz) of the right C5 phrenic rootlet during inspiration consistently elicited a transient reduction in the phrenic motor discharge. This attenuation occurred bilaterally with an onset latency of 8-12 ms and a duration of 8-30 ms. Section of the ipsilateral C4-C6 dorsal roots abolished the response to stimulation, thereby confirming the involvement of phrenic nerve afferent activity. Stimulation of the left C5 phrenic rootlet or the right thoracic phrenic nerve usually elicited similar inhibitory responses. The difference in onset latency of responses to cervical vs. thoracic phrenic nerve stimulation indicates activation of group III afferents with a peripheral conduction velocity of approximately 10 m/s. A much shorter latency response (5 ms) was evoked ipsilaterally by thoracic phrenic nerve stimulation. Section of either the C5 or C6 dorsal root altered the ipsilateral response so that it resembled the longer latency contralateral response. The low-stimulus threshold and short latency for the ipsilateral response to thoracic phrenic nerve stimulation suggest that it involves larger diameter fibers. Decerebration, decerebellation, and transection of the dorsal columns at C2 do not abolish the inhibitory phrenic-to-phrenic reflex.     

Central inspiratory influence on abdominal expiratory nerve activity

Our purpose was to determine whether the intensity of abdominal expiratory nerve discharge is conditioned by the intensity of the preceding inspiratory phrenic discharge, independent of mechanical and chemical afferent influences. In decerebrate, paralyzed, vagotomized cats with bilateral pneumothoraxes, we recorded phrenic and abdominal (cranial iliohypogastric nerve, L1) nerve activities at hyperoxic normocapnia. We reduced the duration and intensity (i.e., integrated peak height) of phrenic nerve discharge for single cycles by stimulating the cut central end of the superior laryngeal nerve (SLN) during the central inspiratory phase (75 microA, 20-50 Hz, 0.2-ms pulse). Premature termination of inspiration consistently reduced expiratory duration (TE) and abdominal expiratory nerve activity (area of integrated neurogram), but the average reduction in TE was much less than the reduction in abdominal nerve activity (14 vs. 51%). Stimulation of the cut central end of the vagus nerve yielded similar results, as did spontaneous premature terminations of inspiration, which we observed in one cat. SLN stimulation during hyperoxic hypercapnia resulted in more variable responses, and higher stimulation frequencies were usually required to abort inspiration. SLN (or vagal) stimulation during expiration consistently increased abdominal expiratory nerve activity. We speculate that this facilitatory response is gated during inspiration, thereby allowing the inspiratory conditioning effect on the subsequent expiration to be expressed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Excitation of dorsal and ventral respiratory group neurons by phrenic nerve afferents

The projections of phrenic nerve afferents to neurons in the dorsal (DRG) and ventral (VRG) respiratory group were studied in anesthetized, paralyzed, and vagotomized cats. Extracellular recordings of neuronal responses to vagal nerve and cervical phrenic nerve stimulation (CPNS) indicated that about one-fourth of the DRG respiratory-modulated neurons were excited by phrenic nerve afferents with an onset latency of approximately 20 ms. In addition, non-respiratory-modulated neurons within the DRG were recruited by CPNS. Although some convergence of vagal and phrenic afferent input was observed, most neurons were affected by only one type of afferent. In contrast to the DRG, only 3 out of 28 VRG respiratory-modulated neurons responded to CPNS. A second study determined that most of these neuronal responses were due to activation of diaphragmatic afferents since 90% of the DRG units activated by CPNS were also excited at a longer latency by thoracic phrenic nerve stimulation. The difference in onset latency of neuronal excitation indicates an afferent peripheral conduction velocity of about 10 m/s, which suggests that they are predominately small myelinated fibers (group III) making paucisynaptic connections with DRG neurons. Decerebration, decerebellation, and bilateral transection of the dorsal columns at C2 do not abolish the neuronal responses to cervical PNS.     

Activation of thalamic ventroposteriolateral neurons by phrenic nerve afferents in cats and rats.

It has been demonstrated that phrenic nerve afferents project to somatosensory cortex, yet the sensory pathways are still poorly understood. This study investigated the neural responses in the thalamic ventroposteriolateral (VPL) nucleus after phrenic afferent stimulation in cats and rats. Activation of VPL neurons was observed after electrical stimulation of the contralateral phrenic nerve. Direct mechanical stimulation of the diaphragm also elicited increased activity in the same VPL neurons that were activated by electrical stimulation of the phrenic nerve. Some VPL neurons responded to both phrenic afferent stimulation and shoulder probing. In rats, VPL neurons activated by inspiratory occlusion also responded to stimulation on phrenic afferents. These results demonstrate that phrenic afferents can reach the VPL thalamus under physiological conditions and support the hypothesis that the thalamic VPL nucleus functions as a relay for the conduction of proprioceptive information from the diaphragm to the contralateral somatosensory cortex.     

Fictive cough in the cat.

Experiments were performed to determine whether cough could be elicited in paralyzed cats ventilated on a respiratory cycle-triggered pump. Midcollicular decerebrate cats were paralyzed and artificially ventilated on a phrenic-triggered pump. Phrenic and cranial iliohypogastric nerve efferent activities were recorded. Cough was elicited by electrical stimulation of the superior laryngeal nerve (SLN) or probing the intrathoracic trachea. Fictive coughs induced by electrical stimulation of the SLN or mechanical stimulation of the intrathoracic trachea consisted of large-amplitude bursts in phrenic discharge immediately followed by large bursts in cranial iliohypogastric discharge. During fictive cough, phrenic postinspiratory discharge was reduced relative to control cycles. Codeine (0.03-1 mg/kg iv) decreased both SLN- and probe-induced fictive cough. I conclude that fictive cough can be produced in paralyzed cats ventilated on a phrenic-triggered pump. Furthermore, fictive cough can be produced in the absence of afferent feedback associated with active expiration.     

Altered breathing pattern elicited by stimulation of abdominal visceral afferents

The effect of stimulation of afferent mesenteric nerves on tidal volume (VT), phrenic nerve, and external intercostal muscle activities was studied in anesthetized spontaneously breathing cats. Both mechanical distension of the small intestine and electrical stimulation of the mesenteric nerves resulted in an initial inspiratory inhibition of VT followed by a gradual recovery above the prestimulus controls. Changes in VT were accompanied by a depression of phrenic nerve activity and an excitation of external intercostal muscle activity. During the recovery phase of VT, the amplitude of phrenic nerve activity returned only partially, whereas the activity of the external intercostal muscle was greater than the prestimulus controls. In a second group of experiments, brief tetanic stimulation at the beginning of inspiration led to a complete and maintained inhibition of phrenic nerve activity but with a simultaneous excitation of external intercostal muscle activity and without any change in VT; whereas expiratory stimulation caused a decrease in expiratory abdominal muscle activity, without changing the peak amplitude of phrenic nerve activity. The respiratory changes observed with distension of the small intestine were abolished after denervation of the mesenteric plexus. It is concluded that activation of the visceral afferents of the mesenteric region reflexly changes diaphragmatic breathing to intercostal breathing. It is assumed that such a type of breathing pattern may occur in pregnancy and in pathophysiological situations involving splanchnic viscera.     

Respiratory cycle timing and fast inspiratory discharge rhythms in the adult decerebrate rat

In supracollicular decerebrate paralyzed adult rats, neural respiration was monitored by bilateral phrenic recordings. In the study of respiratory cycle timing, the effects of vagal afferent input (lung inflation) on respiratory phase durations resembled those seen in decerebrate cats. 1) Withholding lung inflation during neural inspiration (I) produced lengthening of I phase duration by 46% (mean, n = 11). 2) Maintaining lung inflation during neural expiration (E) produced lengthening of E phase duration by 112% (mean, n = 4). In the study of fast rhythms in inspiratory discharges, phrenic nerve autospectra and bilateral (left-right) phrenic coherences in 16 rats revealed two types of fast rhythm: 1) high-frequency oscillation (HFO), which had significant coherence peaks (n = 9, range 106-160 Hz, mean 132 Hz); and 2) medium-frequency oscillation (MFO), which had autospectral peaks but no distinct coherence peaks (n = 11, range 46-96 Hz, mean 66 Hz). These rhythms resembled MFOs and HFOs in the decerebrate cat, but the modal frequency range was about twice as large. In addition, these frequency values differed markedly from the 20-40 Hz of the rhythms found in earlier studies in neonatal in vitro preparations; the difference may be due to developmental immaturity.     

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.     

Comparison of phrenic motoneuron activity during eupnea and gasping

Single-fiber phrenic nerve action potentials were recorded together with activity of contralateral whole phrenic nerve rootlets during eupnea and gasping in decerebrate, cerebellectomized, vagotomized, paralyzed, and ventilated cats. Gasping was reversibly produced by cooling a fork thermode positioned through the pontomedullary junction. In eupnea, phrenic motoneurons were distributed into "early" and "late" populations relative to their onset of activity during inspiration. During gasping, however, both fiber types typically commenced activity at the beginning of the phrenic nerve burst. Moreover, late fibers, but not early units, exhibited an augmentation of discharge frequency with the onset of gasping. The concentration of activity of all phrenic motoneurons at the beginning of inspiration and the increase in late-unit discharge frequency account for the faster rise of the gasp as compared with the eupneic breath. It is concluded that the pattern of phrenic nerve activation during gasping differs fundamentally from that during eupnea. These results support the concept that mechanisms underlying the neurogenesis of gasping and eupnea may not be identical.     

Phase locking of the respiratory rhythm in cats to a mechanical ventilator

Mechanical ventilation of paralyzed, pentobarbital-anesthetized adult cats was performed while recording phrenic nerve activity. The periodic changes in lung volume owing to mechanical ventilation affected the rhythm of central respiratory activity, resulting in a variety of regular and irregular patterns of coupling between respiratory system output, monitored by phrenic activity, and the mechanical ventilator. Phase-locked patterns, in which phrenic burst onset occurred at specific and repetitive phase(s) of the mechanical ventilator, with ratios of ventilator frequency: phrenic burst frequency of 1:2, 1:1, 3:2, 2:1, and 3:1 were observed. Regular and irregular patterns occurred over specific ranges of frequency and volume of the mechanical ventilator. A careful study was made of the 1:1 phase locking as the frequency and inflation volume of the mechanical ventilator were changed. The inspiratory time (TI) was defined as the interval between the time when phrenic activity began to rise and the onset of its rapid decline, and the expiratory time (TE) as the time between inspirations. In the 1:1 phase-locking region, as the frequency of the ventilator was increased both TI and TE decreased, and the phase of phrenic onset in the ventilator cycle changed. During ventilation with frequencies higher than the intrinsic phrenic frequency (initial burst frequency of phrenic activity with the ventilator turned off) inspiratory activity was prematurely terminated by lung inflation (Hering-Breuer inspiratory inhibitory reflex). During ventilation with frequencies lower than the intrinsic phrenic frequency, the onset of phrenic activity was delayed (TE was prolonged) by lung inflation (Hering-Breuer expiratory promoting reflex).     

Trigeminal reflex regulation of the glottis depends on central glycinergic inhibition in the rat

In an unanesthetized decerebrate in situ arterially perfused brain stem preparation of mature rat, strychnine (0.05-0.2 microM) blockade of glycine receptors caused postinspiratory glottal constriction to occur earlier, shifting from early expiration to inspiration. This resulted in a paradoxical inspiratory-related narrowing of the upper airway. Stimulation of the trigeminal ethmoidal nerve (EN5; 20 Hz, 100 micros, 0.5-2 V) evoked a diving response, which included a reflex apnea, glottal constriction, and bradycardia. After strychnine administration, this pattern was converted to a maintained phrenic nerve discharge and a reduced glottal constriction that was interrupted intermittently by transient abductions. The onset of firing of postinspiratory neurons shifted from early expiration into neural inspiration in the presence of strychnine, but neurons maintained their tonic activation during EN5 stimulation, as observed during control. Inspiratory neurons that were hyperpolarized by EN5 stimulation in control conditions were powerfully excited after loss of glycinergic inhibition. Thus the integrity of glycinergic inhibition within the pontomedullary respiratory network is critical for the coordination of cranial and spinal motor outflows during eupnea but also for protective reflex regulation of the upper airway.     

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.     

Effects of peripheral inputs from hindlimb on the monosynaptic reflex of motoneurons innervating tail muscles.

The effects of group II muscle (PBSt, GS) and cutaneous afferent (Sur, SPc, Tib) inputs from the hindlimb on the monosynaptic reflexes of motoneurons innervating tail muscles were studied in lower spinalized cats. Stimulation of the cutaneous nerves at the conditioning-test stimulus interval of about 10-20 ms facilitated and inhibited the monosynaptic reflexes of ipsilateral and contralateral tail muscles, respectively. The effects of the muscle nerve stimulation were not so prominent as those elicited by cutaneous nerve stimulation. The monosynaptic reflex was also inhibited by muscle nerve stimulation at 10-50 ms intervals. The effects of conditioning stimulation of the hindlimb peripheral nerves at short intervals were depressed or blocked by section of the ipsilateral lateral funiculus at S1 spinal segment. These findings show that the neuronal pathway from hindlimb afferents to tail muscle motoneurons passed the lateral funiculus of the spinal cord and modulates the motoneuronal activity of tail muscles.     

Internal intercostal nerve discharges in the cat: influence of chemical stimuli

We studied the influence of central and peripheral chemoreceptor stimulation on the activities of the phrenic and internal intercostal (iic) nerves in decerebrate, vagotomized, and paralyzed cats with bilateral pneumothoraces. Whole iic nerves of the rostral thorax (T2-T5) usually discharged during neural inspiration, whereas those of the caudal thorax (T7-T11) were primarily active during neural expiration. Filaments of rostral iic nerves that terminated in iic muscles generally discharged during expiration, suggesting that inspiratory activity recorded in whole iic nerves may have innervated other structures, possibly parasternal muscles. All nerves were phasically active at hyperoxic normocapnia and increased their activities systematically with hypercapnia. Isocapnic hypoxia or intra-arterial NaCN injection consistently increased phrenic and inspiratory iic nerve activities. In contrast, expiratory iic nerve discharges were either decreased (10 cats) or increased (7 cats) by hypoxia. Furthermore, expiratory responses to NaCN were highly variable and could not be predicted from the corresponding response to hypoxia. The results show that central and peripheral chemoreceptor stimulation can affect inspiratory and expiratory motoneuron activities differentially. The variable effects of hypoxia on expiratory iic nerve activity may reflect a relatively weak influence of carotid body afferents on expiratory bulbospinal neurons. However, the possibility that the magnitude of expiratory motoneuron activity is influenced by the intensity of the preceding centrally generated inspiratory discharge is also discussed.     

Integration-differentiation and gating of carotid afferent traffic that shapes the respiratory pattern.

The phase-dependent plasticity of carotid chemoafferent signaling was studied with electrical stimulation of a carotid sinus nerve during either inspiration or expiration in anesthetized, glomectomized, vagotomized, paralyzed, and ventilated rats. Stroboscopic and interferometric analyses of the resulting phase-contrast disturbances of the respiratory rhythm revealed that carotid chemoafferent traffic was dynamically filtered centrally by a parallel bank of leaky integrators and differentiators, each being logically gated to the inspiratory or expiratory phase in a stop-and-go manner as follows: 1) carotid short-term potentiation of inspiratory drive was mediated by dual integrators that both shortened inspiration and augmented phrenic motor output cooperatively in long and short timescales; 2) carotid short-term depression of respiratory frequency was mediated by a (possibly pontine) integrator that lengthened expiration with a relatively long memory; and 3) carotid "chemoreflex" shortening of expiration was mediated by an occult fast integrator, which, together with carotid short-term depression, formed a differentiator. These effects were modulated anteriorly by integrators in the nucleus tractus solitarius that were "auto-gated" to, or recruited by, the carotid sinus nerve input. Such phase-selective and activity-dependent time-frequency filtering of carotid chemoafferent feedback in parallel neurological-neurodynamic central pathways may profoundly affect respiratory stability during hypoxia and sleep and could contribute to the dynamic optimization of the respiratory pattern and maintenance of homeostasis in health and in disease states.     

Respiratory effects of stimulation of intercostal muscles and saphenous nerve in kittens

Effects of intercostal muscle stimulation were studied in 2- to 7-day-old kittens under ketamine-acepromazine anesthesia. Animals were vagotomized, paralyzed, and artificially ventilated. Stimuli applied during inspiration (TI) inhibited this phase. Stimulus strength necessary for TI inhibition decreased with time. However, an all-or-nothing effect was not always observed. Stimulation during expiration (TE) prolonged this phase. The responsiveness increased with increasing stimulus delay. The effects of intercostal muscle stimulation were compared with those recorded during saphenous nerve stimulation. Stimulation during TI prolonged this phase. Phrenic activity increased after a short-lasting decrease in the on-going activity. Stimulation during the first 50% of TE had variable effects, whereas stimulation with longer delay shortened this phase. Our results indicated that the pattern of breathing in newborns can be affected by both intercostal muscle and other somatic efferents. However, the mechanisms controlling respiratory timing may differ in newborns and in adults. Different effects of respiratory muscle and saphenous nerve stimulation suggest different transmitters involved or different sites of interaction of these inputs with the medullary respiratory rhythm generator.     

正常猫膈神经单纤维电活动特征

在麻醉猫和麻痹的切断迷走神经的清醒猫,观察了膈神经单纤维电活动特征。1.电活动类型:按膈神经单纤维放电与其总干放电的相位关系分为三种类型。(1)完全同步型,即单纤维放电与总干放电同时开始并同时停止,占76.9％。(2)部分同步型占15.4％,其中早期同步,即单纤维放电与总干放电同时开始,但提前终止,占1.9％,中期同步,即单纤维放电较总干放电开始晚,又提前终止,占5.8％,晚期同步,即单纤维放电较总干放电开始晚,但两者同时终止,占7.7％。(8)非同步型,即吸气相和呼气相都有放电,但呼气相时冲动频率较低,占7.7％。前两型为单纯的吸气性放电,共占92.3％。2.单纤维放电平均参数值:麻醉猫每次吸气发放11个冲动,其频率为21次/秒,清醒猫每次吸气发放18个冲动,其频率为34次/秒。结果表明:猫膈神经单纤维放电类型和文献上报导的直接记录膈神经运动神经元放电一致,即以单纯的吸气性放电为最多。     

Function of different medullary neurons in respiration]

1. We have studied the activity of 162 medullary respiratory neurones in the "encephale isole bas" cat. These neurones were classed into three groups : bulbospinal inspiratory (NBSI : 39) or expiratory (NBSE : 15) neurones whose axons enter the spinal cord ; inspiratory or expiratory laryngeal motoneurones (MLI : 17; MLE : 10) antidromically activated by vagus nerve stimulation ; propriobulbar inspiratory (NPBI : 59) or expiratory (NPBE : 22) neurones whose axons lie perhaps entirely within the medulla. 2. Correlation coefficients between number of spikes delivered in each burst and the duration of the corresponding respiratory phase (inspiration for NBSI, MLI, NPBI ; expiration for NBSE, MLE, NPBE) have been calculated for each neurone. 3. The activity of most of the NBSI and MLI is significantly correlated with the duration of the inspiration. These two groups of neurones are probably homogenous. 4. On the basis of this correlation test, NPBI do not constitute an homogeneous population ; 50% of NPBI are not significantly correlated. The same results are obtained if correlations are calculated between the number of spikes delivered and the amplitude of integrated phrenic nerve acitivty. According to the discharge pattern and correlation test, we can consider three groups of NPBI : early recruited neurones with decreasing frequency and non significantly correlated activity (23,7%); early and late neurones with increasing frequency and significantly correlated activity (32,2%); early and late neurones with increasing frequency and non significantly correlated acitivty (44,1%). 5. The activity of most of the NBSE and NPBE with increasing frequency is significantly correlated with the duration of the expiration. Among the MLE and NPBE with a decreasing frequency, a great number of neurones are not significantly correlated. 6. The functional significantion of the different neuronal types is discussed from these correlation tests and from the pattern of activity and axonal pathways.