Total Number Of Spikes Research Articles

Evidence for peripheral serotonergic mechanisms in the early sensitization after carrageenin-induced inflammation: electrophysiological studies in the ventrobasal complex of the rat thalamus using a potent specific antagonist of peripheral 5-HT receptors

The effect of ICS 205–930 (ICS), a specific 5-HT 3 receptor antagonist, was analyzed on the sensitization of ventrobasal (VB) thalamic neuronal responses produced by an intraplantar injection of carrageenin. ICS was injected locally in the plantar paw, simultaneously, or after carrageenin (at 20 min or later than 70 min). The progressive increase of the VB neuronal responses to pinch (total number of spikes in the discharge) due to carrageenin sensitization, was prevented, blocked, or reversed, by intraplantar ICS, at a dose as low as 3.2 ng/kg, when injected, simultaneously or in the first half-hour following the carrageenin injection itself. The carrageenin sensitization then reappeared, 50–90 min after the initiation of the inflammation. By contrast to these early injections of ICS, a later administration of ICS (70 min or more, after the carrageenin injection), did not influence the sensitization. The time course of the effects of this 5-HT 3 antagonist receptor agrees well with the time course of 5-HT release into the inflammatory exudate. These data, and those previously reported on the action of aspirin and of a peripheral antihistamine on carrageenin sensitization, are compared. These results indicate the relative participation of the various inflammatory substances released in the exudate, and the importance of timing of administration for an effective antagonism of the hyperalgesia elicited by this inflammation.

Distribution of activity within the cat's peroneus longus muscle when activated in different ways via the central nervous system

We have studied whether a mechanically homogenous muscle (all units giving joint-torques in the same direction) would display different distributions of activity in response to different types of activation via the central nervous system. In adult cats anaesthetized with pentobarbitone, long-lasting contractions were evoked in m. peroneus longus (PerL), a mixed ankle dorsiflexor muscle. The contractions were elicited by continuous repetitive stimulation of: (1) the superficial peroneal nerve (NP, flexion reflex); and (2) the contralateral motor cortex (MC). During these contractions, isometric force was monitored and fine-wire electromyographic recordings (EMG) were simultaneously obtained from anterior and posterior portions of the PerL. The relative degree of activation in anterior vs posterior muscle portions was quantified by measuring, at various force levels: (1) the average spike amplitude (Spike); and (2) the total number of spikes per unit time (Count; 0.5-s measurement periods). For both types of stimulation (MC and NP), the results indicated that activation was more effective posterior than for anterior muscle portions for anterior muscle portions: the Spike-measurements as typically higher in posterior than in anterior PerL and, for MC-elicited contractions, this was true for the Count-measurements as well. With respect to both types of EMG-quantification (Spike and Count), the ‘posterior bias’ of activity was significantly more pronounced for the cortically evoked contractions than for those elicited via stimulation of the peroneal nerve. As it is known that anterior and posterior PerL portions tend to be innervated by rostral and caudal motoneurones, respectively 7, these findings indicate that different inputs to the PerL motoneurone pool may differ significantly with respect to the intraspinal spatial distribution of synaptic effects among the motoneurones. The results were discussed in relation to the organization of task-related recruitment schemes.

RELATIONSHIPS AMONG TILLERING, SPIKE NUMBER AND GRAIN YIELD IN WINTER WHEAT (Triticum aestivum L.) IN ONTARIO

Two field experiments were conducted to examine the relationships between tillering, spike number and grain yield in three winter wheat (Triticum aestivum L.) cultivars. Treatments were designed to manipulate both the production and survival of tillers, and to provide a high number of spikes per unit area. One experiment involved growth regulator treatments with cycocel and gibberellic acid while the second involved various rates of nitrogen. Grain yield was linearly related to total spike number over a range of 400–1200 spikes m−2 in a combined analysis over locations and years. When only spikes with at least nine fertile spikelets were included, a greater amount of the variability in yield could be explained, and differences among cultivars were related to the number of small, unproductive spikes. When locations and years were analyzed separately, little evidence was found for a diminishing response between grain yield and total spike number. Spike number was related to maximum tiller number in 1982, when winterkill and early spring conditions were unfavorable. Thus, although good fall tillering and winter survival contribute most to producing high spike numbers and grain yield, cultivars must also have the ability both to tiller rapidly in the spring and to sustain high-yielding tillers to provide sufficient compensation following winterkill.Key words: tillering, spikes, yield, wheat, nitrogen, regulators

Discharge patterns of locust visual interneurones

The spike discharges of the descending contralateral movement detector (DCMD) neurone, and of some smaller visual interneurones (S‐units), were recorded in the ventral nerve cord of adult Schistocerca gregaria Forsk., in response to a stationary disc (25o) or a small spot (0.2o) stimulus. The discharge rate of each neurone was plotted over a period of 5 s exposure; the total number of spikes in this period was also noted. The DCMD response to the 25o disc was a high‐frequency burst falling off quickly to a low rate; the 0.2o spot evoked a prolonged discharge with an early peak in rate. In S‐units the discharge was prolonged with both targets; the rate rose to an early peak in each case, with a much higher rate for the larger disc. For the DCMD the total number of spikes per stimulus (5 s) was greater for the 0.2o spot; for S‐units it was greater for the 25o disc. Thus an increase of about 30‐fold in the number of ommatidia stimulated resulted in a fall in the total DCMD response to about one‐quarter; a similar increase evoked a rise of about 6‐fold in the S‐unit response. When the 25o disc was presented at progressively reduced intensities the total spike response of the DCMD rose steadily to a maximum at about 2.9 μW m‐2; using the same procedure the spike output of S‐units, initially high, declined monotonically. The role of inhibition in these results is discussed.

Characteristics of early- and late-recruited oxytocin bursting cells at the beginning of suckling in rats.

1. Paired or single recordings of paraventricular and/or supraoptic oxytocin cells at the beginning of suckling in urethane-anaesthetized rats enabled us to study cell recruitment and compare the characteristics of the early- and late-recruited cells. This was done under different experimental conditions, i.e. when the reflex was triggered in less than 1 h suckling (control), and when its triggering was facilitated either by the intraventricular (i.c.v.) injection of oxytocin, of apomorphine (a dopamine agonist) or by the intravenous (i.v.) injection of propranolol (a beta-adrenoceptor antagonist) into suckled rats with no milk ejection. 2. Under control conditions, the amplitude (total number of spikes) of the successive bursts of the early-recruited cells progressively increased, generally reaching maximum by the 6th burst. This increase was more rapid and greater after oxytocin than under control conditions or after apomorphine injection, and was delayed and reduced after propranolol. The burst frequency was higher after oxytocin and apomorphine injections than under control conditions and very low after propranolol. 3. Late-recruited cells were observed under all experimental conditions, except after oxytocin injection, since all cells displayed bursts right away. Moreover, when injected during the recruitment period of a control reflex, oxytocin greatly speeded up the recruitment of the late-recruited cells. These cells generally displayed smaller amplitude bursts than the early-recruited cells. Moreover, the increase in burst amplitude was less marked for the late- than for the early-recruited cells and often was not sustained. 4. Neither the likelihood of recruitment of an oxytocin cell nor its burst amplitude could be correlated with background activity level and there was no clear relationship between the recruitment period or the bursting characteristics on one hand and the background activity on the other. 5. In conclusion, the differences between the early- and late-recruited cells in recruitment time and in burst amplitude reflected differences in cell excitability which may depend mainly on the presence of oxytocin in the magnocellular nuclei.

The Effect of Dietary Protein Levels and Haemolymph Composition on the Sensitivity of the Maxillary Palp Chemoreceptors of Locusts

ABSTRACT Previous work has shown that fifth-instar nymphs of Locusta migratoria (L.) compensate for a dilution of their dietary protein by reducing intermeal interval (Simpson & Abisgold, 1985). The effect of dietary protein on intermeal interval is regulated, either directly or indirectly, by the osmolality and free amino acid content of the haemolymph (Abisgold & Simpson, 1987). The possibility that levels of dietary protein and haemolymph composition affect the response of the maxillary palp gustatory receptors is investigated. Insects fed a low-protein diet had a significantly greater receptor response (measured as the total number of spikes in the first second of stimulation of a sensillum) to stimulation with 0·0125 moll-1 leucine in 0·05 moll-1 NaCl, 0·05 mol 1-1 NaCl alone or 0·025 mol 1-1 sucrose in 0·05 mol 1-1 NaCl than did insects fed a high-protein diet, although for both diets the response to sucrose was significantly lower than the response to the other two solutions. Increasing the free amino acid profile of the haemolymph of a low-protein-fed locust up to that of a high-protein-fed locust by injection markedly reduced the response of the receptors to subsequent stimulation with a 0·0125 mol 1-1 mix of eight of the 10 amino acids injected, but did not reduce the response to stimulation with 0·025 mol 1-1 sucrose in 0·05 mol 1-1 NaCl. This reduction was independent of the effect of injection on blood osmolality and was sustained for 50 min after the injection. The response to 0·05 mol 1-1 NaCl alone was influenced both by increases in blood amino acid levels and by osmolality, but the effect was less marked than the specific reduction in response to amino acid stimulation. The possible significance of a reduction in receptor sensitivity on feeding behaviour and the relative roles of blood osmolality and free amino acid content are discussed.

Effect of Sennosides on Colon Motility in Cats

The actions of sennosides on colonic motility are incompletely understood. We therefore studied the effects of sennosides A + B on colonic myoelectric activity and transit of a radio-opaque meal in 7 conscious cats. Intraduodenal application of sennosides (2 mg/kg body weight) accelerated the half colon transit time from 60 +/- 10 (SEM) to 43 +/- 7 min. At the same time the ratio of long-spike bursts to short-spike bursts was changed from 0.22 to 10.1. Loperamide, an antidiarrheal agent, had the opposite effect. The overall spike activity was not altered by sennosides, but increased by loperamide. It is concluded that the propulsive action of sennosides in the colon is reflected by myoelectric patterns and not by the total number of spikes.

A functional role of cholinergic innervation to neurons in the cat visual cortex

1. Effects of microionophoretic application of acetylcholine (ACh) and its antagonists on neuronal responses to visual stimuli and to electrical stimulation of the lateral geniculate nucleus were studied in the cat striate cortex. 2. Responses elicited visually and electrically were facilitated by ACh in 74% of the cells tested, whereas the responses were suppressed in 16%. These ACh effects were blocked by a muscarinic antagonist, atropine, but not by a nicotinic antagonist, hexamethonium, indicating that the ACh effects are mediated through muscarinic receptors. A single application of atropine suppressed visual responses of cells facilitated by ACh, whereas it enhanced those of cells inhibited by ACh, suggesting that endogenous ACh may tonically modulate visual responsivity of cortical neurons. 3. In most cells with the facilitatory ACh effect, responses with single spikes to the electrical stimulation became more consistent, often with double spikes, during the ACh application. The suppressive effects of ACh were noted most often in cells with a longer response latency to electrical stimulation of lateral geniculate nucleus. 4. In most of the facilitated cells the spontaneous activity remained null or very low during ACh application, in spite of marked enhancement of visual responses, suggesting that ACh may improve the signal-to-noise ratio (S/N) of cortical neuron activity. To confirm this suggestion, we calculated a S/S + N index by counting the total number of spikes in the responses (S) and that in peristimulus time histogram (S + N) and found that it was improved during the ACh application in about a half of the cells, whereas it became worse in about one-fifth. 5. In most of the facilitated cells, ACh enhanced visual responses not only to optimal but also to nonoptimal stimuli, resulting in no improvement or even worsening of the orientation selectivity. This was also the case in the selectivity of direction of stimulus movement. 6. The laminar location of the facilitated cells was biased toward layers V and VI of the cortex, although they also made up the majority in layers II + III and about half the tested cells in layers IVab and IVc. 7. In the light of recent understanding of cortical circuitry, these results suggest that the cholinergic innervation to cortical neurons may play a role in improvement of the S/N ratio of information processing in the striate cortex and in facilitation of sending processed informations to other visual centers.

Relationship between oxytocin release and amplitude of oxytocin cell neurosecretory bursts during suckling in the rat.

Plasma concentrations of oxytocin and vasopressin were measured in relationship to oxytocin cell firing during suckling in urethane-anaesthetized rats. Preliminary experiments showed that plasma concentrations of oxytocin and vasopressin, which were increased immediately after anaesthesia, reverted to basal concentrations 3 h later. Moreover, it was found that exogenous oxytocin had entirely disappeared 5 min after i.v. bolus injections of known doses of oxytocin. Suckling did not modify the basal plasma concentration of oxytocin (14.6 +/- 2.9 compared with 14.6 +/- 1.5 pmol/l before suckling) except during a brief period immediately after neurosecretory bursts on oxytocin cells (37.8 +/- 5.2 pmol/l; P less than 0.001, n = 11). The plasma concentration of oxytocin did not differ significantly from the basal concentration 1.5 min later. The plasma concentration of vasopressin never varied. After two neurosecretory bursts of similar amplitude (total number of spikes during the burst) recorded on the same oxytocin cell, the variations in plasma concentration of oxytocin were also similar. When, for a given cell, the amplitude of neurosecretory bursts increased or decreased, the amount of oxytocin released changed in the same way. These data demonstrate (1) that suckling induces pulsatile release of oxytocin without vasopressin, and (2) a direct relationship between the amounts of oxytocin released and the amplitude of oxytocin cell neurosecretory bursts which argue in favour of simultaneous increases or decreases in the neurosecretory burst amplitudes on all oxytocin cells.

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)

Electrophysiological evidence for facilitatory control of oxytocin neurones by oxytocin during suckling in the rat.

Antidromically identified paraventricular neurones were recorded simultaneously with intramammary pressure in urethane (1.2 g/kg) anaesthetized rats during suckling. The correlation of the firing pattern of these neurones with milk ejection enabled distinction between oxytocin and vasopressin neurones. Oxytocin neurones displayed a short (2-6 s) characteristic high-frequency burst of spikes. This activation probably occurred simultaneously in all oxytocin neurones 12-18 s before milk ejection and was regular in both frequency and amplitude (total number of spikes). The role of neurohypophysial peptides and analogues in the control of these characteristics was studied. Injecting 10 pg, 100 pg and 1 ng of oxytocin into the 3rd ventricle increased background activity of slow-firing oxytocin neurones (less than 3 spikes/s) and had a strong dose-dependent facilitatory effect on the milk ejection reflex, increasing both the amplitude and frequency of neurosecretory bursts. No effect was observed on non-neurosecretory neurones. Such injection also triggered the milk ejection reflex when it had not appeared an hour after suckling began. Oxytocin did not itself induce neurosecretory activation, which only appeared if the young rats were sucking. Injecting oxytocin into the lateral ventricle was less effective than into the 3rd ventricle. No effect was observed after injection into the venous blood or into the 4th ventricle, which suggested that oxytocin acts in the hypothalamus. Injecting mesotocin or isotocin into the 3rd ventricle had a facilitatory effect similar to that of oxytocin but vasopressin, vasotocin, MIF I (pro-leu-gly-NH2, terminal triplet oxytocin) or bovine neurophysins I and II did not modify neurosecretory activation or the milk ejection pattern. Injecting an oxytocin antagonist, ([1(beta-mercapto-beta, beta cyclopentamethylene propionic acid), 8-ornithine] vasotocin, d(CH2)5OVT) into the 3rd ventricle decreased milk ejection frequency and considerably delayed the reappearance of the first milk ejection. This resulted from a decrease in both frequency and amplitude of neurosecretory bursts, which were too small to induce detectable oxytocin release. Moreover, d(CH2)5OVT suppressed the facilitatory effect of exogenous oxytocin. Under normal conditions, endogenous oxytocin seemed to be involved in the control of neurosecretory activation. Injecting 1 ng oxytocin or 1 or 10 ng vasopressin into the 3rd ventricle did not modify the firing pattern of vasopressin neurones whether activated by hyperosmotic stimulation (1 ml NaCl, 9% solution (w/v) I.P.) or not.(ABSTRACT TRUNCATED AT 400 WORDS)

Construction of a simple spike counter for analysis of electrophysiological recording data

A method for constructing a simple spike counter from a conventional calculator by taking advantage of the “adding display to memory” function [(M+) key] is described. The modification involves connecting the (M+) key to a dissecting pin and to a metallic notebook binder which is taped on to a transparent plastic sheet. The plastic sheet can then be placed on top of a polygraph record. After entering “1” into the calculator, a spike can be counted by making a single contact between the pin and the metal fastener. The total number of spikes counted can be displayed on the calculator by pressing the key for memory recall. The spike counter described herein is suitable for counting extracellular action potentials recorded on chart paper as well as other biological signals such as the rate of contraction of muscle.

Low dose of morphine strongly depresses responses of specific nociceptive neurones in the ventrobasal complex of the rat

Effects of low intravenous doses of morphine (30, 100 and 1000 μg/kg) upon unitary responses of 22 ‘nociceptive’ and 5 ‘ non-nociceptive’ units recorded in the ventrobasal (VB) complex of the rat were analyzed. The responses of the ‘non-noxious’ neurones were not depressed by morphine. By contrast, for all these doses there was a decrease of the total number of spikes and of the maximal firing rate of the responses of the noxious neurones. The depressive effect was significantly dose-related (with linear semi-logarithmic dose-response curve) and naloxone-reversible. Similar effects were observed upon responses to pinches and noxious heat. The ED 50 which was close to 90 μg/kg for these thalamic responses to pinches is much more lower than that evaluated for spinal dorsal horn responses under the same anaesthetic conditions. Therefore the depressive effect of low doses observed for VB neurones seems to be mainly of supraspinal origin.

Noise impairment in the guinea pig. II. Changes of single unit responses in the inferior colliculus.

Spontaneous and evoked activity of neurons in the inferior colliculus of guinea pigs was recorded before and after exposure to noise (continuous or intermittent white noise, 115 dB SPL for 30 min). A single unit was investigated in each animal, and its activity was monitored for several hours. Exposure to noise elevated the threshold of the tip of the tuning curve, resulting in a broadening of the tuning curve. Threshold elevation at the characteristic frequency was greater after exposure to intermittent noise (200 ms noise and 200 ms pause), reaching values of 22.8 +/- 3.7 dB (means +/- S.E.) than it was after exposure to continuous noise (threshold elevation of 13.1 +/- 1.7 dB). The average threshold shift was 17.1 +/- 2 dB. Neither the shape of the poststimulus histograms nor the slope of the spike-intensity curves changed with the noise exposure. The total number of spikes during the response was, however, reduced, and the reduction was in proportion to the threshold elevation. Monaural noise exposure had no effect on the neuronal activity evoked by stimulation of the opposite, nonexposed ear. The latencies of responses recorded after exposure to noise were also longer than the latencies at the same absolute intensity recorded before the exposure. Thus the latencies during the original pre-exposure and acquired postexposure thresholds were practically identical.

Influence of Drilling Muds on the Primary Chemosensory Neurons in Walking Legs of the Lobster, Homarus americanus

The effects of whole drilling muds on the normal activity of walking leg chemosensory neurons of the lobster, Homarus americanus, were examined using extracellular neurophysiological recording techniques. Exposure of legs for 3–5 min to 10 mg/L drilling mud suspended in seawater altered responses to food odors of 29% of the chemoreceptors examined (data pooled for the two drilling muds tested); similar exposure to 100 mg/L drilling mud resulted in interference with 44% of all receptors studied. The effects of both of these concentrations are statistically significant, although they are not different from each other. Interference was usually manifested as a marked reduction in the number of action potentials in a response. In one preparation, the exposure to drilling mud caused a change in the temporal pattern of the spikes without affecting the total number of spikes. Other chemosensory neurons were excited by 10 mg/L drilling mud itself. However, not all chemoreceptors are inhibited by these drilling muds since responses to feeding stimuli were recorded from the legs of lobsters that had been exposed to drilling mud for 4–8 d before the neurophysiological experiments.Antennular and leg chemoreceptors are important in eliciting normal feeding behavior in lobsters. Although behavioral assays have demonstrated that feeding behavior is altered following exposure to drilling muds and petroleum fractions, there is no conclusive proof for a causal relationship between chemoreceptor interference and behavioral deficits. The two techniques complement each other as pollution detection assays, perhaps reflecting a common interference mechanism.Key words: chemoreception, drilling mud, feeding behavior, lobster, pollution, neurophysiology

Dynamics of visual cortical unit activity during elementary conditioning to time and association in rabbits.

1. During assessment of reproduction of the discharge pattern on the PST histogram 83% of visual cortical neurons reveal the formation of a reflex to time and 57% the formation of a defensive reflex of the light-light plus ESL type. 2. During the formation of a reflex to time the initial components of the response to reinforcement are reproduced in the omissions (0–400 msec from the beginning of the stimulus), whereas during defensive conditioning the late components of this response were repeated (300–1000 msec from the beginning of the stimulus). 3. It is suggested that there are several methods of coding of conditioned-reflex transformations of unit discharges: those based on the total number of spikes in the response, the total length of the inhibitory periods in the response, and the discharge pattern. 4. Neurons exhibiting conditioned-reflex transformations of their discharge possess polysensory convergence in most cases and sensitization of their discharge is manifested in the habituation procedure.

Pattern and correlation of fast and slow spontaneous unit activity in the visual cortex

Spontaneous unit activity in the visual cortex and its changes during stimulation by continuous light or flashes were investigated in waking rabbits. The study of distributions of adjacent intervals showed that the neurons differ in the ratio of burst (fast, with intervals of up to 15–40 msec) and nonburst (slow) activity and in the character of changes from one type of activity to the other. Of the total number of spikes 63% were outside bursts; the ratio of their number to the number of spikes within bursts consisting of two or of three or more spikes was 27:3:1. The relative stability of the burst structure of spontaneous activity and the limited number of spikes in them (on average 2.4) were demonstrated. Bursts of three or more spikes (mean 3.6) were irregular, and in 79% of them a longer interval (18.6±2.4 msec) was observed before the shortest interval (7.9±0.9 msec). Bursts of spikes of most neurons during photic stimulation contain more spikes with shorter intervals; they also began more frequently with the shortest interval, possibly signifying an increase in the steepness and amplitude of the EPSPs lying at their basis. However, in 20% of neurons spontaneous bursts included more spikes and with shorter intervals than bursts evoked by flash stimulation.

An analysis of the variability of unit activity in the cat's visual cortex.

The spontaneous firing of single units in the cat's visual cortex, and their responses to repeated presentations of a constant visual stimulus, have been analysed quantitatively. Although the responses of most cells showed some habituation to the stimulus, this was less extensive than the fluctuations in responsiveness which occurred from one set of presentations to the next, over the course of several minutes. Fluctuations occurred largely independently in both the spontaneous and the stimulus-evoked firing of the cell. This suggests that separate mechanisms determine spontaneous and evoked firing. Quantitative estimates of the reliability (the reciprocal of the variability) of responses were higher if the peak firing rate of the cell was measured, rather than the total number of spikes fired by the cell. Thus, peak firing rate is more likely than total spike count to be the signal of biological significance.

Dynamics of single unit responses of the rabbit visual cortex to repetitive photic stimulation

Single unit responses in the visual cortex of the waking rabbit to repetitive photic stimulation at a frequency of once every 2.5 sec were studied. Depending on the total number of spikes in the response, the dynamics of the responses could be divided into two types: "fast" and "slow." From 5 to 15 stimuli were required to establish a stable level of response with changes of the first type, but 50 to 100 stimuli were needed for the response with changes of the second type. About 50% of all neurons did not change the characteristics of response. In the group of neurons with changing responses, partial habituation was found in 55–59% of cells; there were 25% of neurons with sensitization of discharge and 17–20% with a humpbacked type of response dynamics. A "slow" dynamics of unit responses in most cases was accompanied by changes in the duration of inhibitory pauses in the response; negative correlation of these values was observed in 65% of neurons.

Elektrophysiologische Untersuchungen am Frontalganglion von Periplaneta americana L.

The frontal ganglion is an important part of the stomodeal division of the visceral nervous system. It is involved in the control of intake of food and water and of crop emptying as well as in the control of production and/or release of neurosecretory materials. Consequently it also has an influence upon the waterbalance, oocyte development, vitellogenesis etc. Investigations on the electrophysiological activities in the stomatogastric nervous system have not been published until now. This study deals with the spontaneous activities of the frontal ganglion of the cockroach, Periplaneta americana. Extracellular recordings were made with suction electrodes. From both frontal connectives as well as from the recurrent nerve one can detect clear irregular activities coming from the isolated ganglion. Newly moulted animals show in 70% of the cases volley like discharges, which are identical with the rhythm in the nervus connectivus (see below) and are lost in a higher ground activity at the latest four hours after ecdysis. The nervus connectivus consists of only two axons and its cell bodies are located in the protocerebrum. Each axon splits into three branches inside the neuropile of the frontal ganglion. One branch goes to the recurrent nerve, the others to the frontal connectives. From this nerve one can detect extreme regular volley-like discharges from the ganglion ( in vitro and in vivo) to the cell bodies in the protocerebrum (antidromic transmission!). Each cycle lasts 1–4 sec separated by a silent period of few seconds. The volleys consist in our records with suction electrodes of two types of spikes differing in height. Each type supplies half of the total spike number and may relate with the two axons of the nerve. At the end of each volley both the spike types show an increase in frequency and at the same time a decrease in amplitude. From the nerves N1, N2, N3 and N5, only in the cases of N3 and N5 is there spontaneous activity coming from the ganglion. The nerve N3 is more active than N5. From the tritocerebrum heavy spontaneous activity occurs between the frontal connectives and the frontal ganglion. This is similar to the activity detected in the opposite direction. It is essential that the brain-suboesophageal ganglion complex remains undamaged. If the two connectives between subesophageal ganglion and the first thoracic ganglion are dissected carefully and successively the activity may remain. These experiments are performed in situ. The recurrent nerve as well as the other nerves (N. connectivus, N1, N2, N3 and N5) never show spontaneous activity in the direction of the frontal ganglion.