Role of different types of fibers of the infraorbital nerve in synaptic activation of masseter and digastric motoneurons

Postsynaptic potentials of motoneurons of the masseter and digastric muscles evoked by stimulation of the infraorbital nerve with a strength of between 1 and 10 thresholds were investigated in cats anesthetized with a mixture of chloralose and pentobarbital. Depending on their ability to be activated by low-threshold afferents of this nerve, motoneurons of the masseter were divided into two groups. Stimuli with a strength of 1.2–2.5 times above threshold for the most excitable fibers of the infraorbital nerve evoked short-latency EPSPs in the motoneurons of the first group; a further increase in stimulus strength (3–9 thresholds) led to the appearance of IPSPs with latent periods of 2.8–3.5 msec. Motoneurons of the second group responded to stimulation of the infraorbital nerve with a strength of 3–9 thresholds by IPSPs whose latent periods varied from 6 to 8 msec. Stimuli below 3 thresholds in strength evoked no responses in these motoneurons. Stimulation of the infraorbital nerve with pulses of between 1 and 2 thresholds in strength evoked EPSPs in digastric motoneurons, but an increase in the strength of stimulation led to action potential generation. The presence of many excitatory and inhibitory inputs formed by afferent fibers of different types evidently provides a basis for functional diversity of jaw-opening and jaw-closing reflexes.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 6, pp. 596–603, November–December, 1980.     

Monosynaptic excitation of motoneurons of the accessory nerve nucleus by reticulofugal impulses

Acute experiments on cats anesthetized with chloralose and pentobarbital showed that excitation of fast-conducting (130 m/sec) reticulospinal fibers, arising during stimulation of the ipsilateral medullary reticular gigantocellular nucleus evoked monosynaptic EPSPs in motoneurons of the accessory nerve nucleus. The EPSPs had latent periods of between 0.6 and 1.0 msec (mean 0.7 msec), they reached their maximal amplitude (4.0 mV) after 2.0–2.5 msec, and lasted about 10 msec. The EPSPs underwent only weak potentiation through the different types of stimulation of the gigantocellular nucleus and were not transformed into action potentials.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 1, pp. 62–66, January–February, 1980.     

Rubrofugal activation of motoneurons of the cat accessory nucleus

Postsynaptic potentials evoked in accessory nerve motoneurons by stimulation of the ipsilateral and contralateral red nuclei were investigated in acute experiments on cats anesthetized with chloralose and pentobarbital. Polysynaptic EPSPs with latent periods of 5.2 to 16 (mean 9.1 ± 0.7) msec and from 5.5 to 18 (mean 10.3 ± 0.9) msec, respectively, appeared in motoneurons of the accessory nerve in response to stimulation of the contralateral and ipsilateral red nuclei. A minimum of two or three stimuli was necessary to produce EPSPs in these motoneurons. In response to single stimulation of the contralateral and ipsilateral red nuclei EPSPs appeared in four motoneurons of the trapezius muscle with latent periods of 2.5 to 5.0 and 3.0 to 5.2 msec, respectively. An increase in the number of stimuli led to action potential generation by motoneurons. The functional role of such activation is discussed.A. A. Bogomolets Institue of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 14, No. 5, pp. 532–536, September–October, 1982.     

Excitation of accessory motoneurons by stimulation of Deiters' nucleus in the cat

Acute experiments on cats under chloralose-pentobarbital anesthesia showed that application of single stimuli to Deiters' nucleus evoked monosynaptic EPSPs in motoneurons of the accessory nucleus. Latent periods of EPSPs ranged from 1.3 to 2.3 msec (mean 1.8±0.3 msec), their rise time was 0.5–1.0 msec, and their duration 7–10 msec. During repetitive stimulation the EPSPs were weakly potentiated, but with an increase in the strength of stimulation applied to Deiters' nucleus they readily changed into action potentials. In some motoneurons polysynaptic EPSPs with latent periods of the order of 6.0 msec appeared on the descending phase of these EPSPs.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 5, pp. 515–519, September–October, 1981.     

Synaptic potentials of motoneurons of the masseter muscle

Postsynaptic potentials of 93 motoneurons of the masseter muscle evoked by stimulation of different branches of the trigeminal nerve were studied. Stimulation of the most excitable afferent fibers of the motor nerve of the masseter muscle evoked monosynaptic EPSPs with a latent period of 1.2–2.0 msec, changing into action potentials when the strength of stimulation was increased. A further increase in the strength of stimulation produced an antidromic action potential in the motoneurons with a latent period of 0.9 msec. In some motoneurons polysynaptic EPSPs and action potentials developed following stimulation of the motor nerve to the masseter muscle. The ascending phase of synaptic and antidromic action potentials was subdivided into IS and SD components, while the descending phase ended with definite depolarization and hyperpolarization after-potentials. Stimulation of cutaneous branches of the trigeminal nerve, and also of the motor nerve of the antagonist muscle (digastric) evoked IPSPs with a latent period of 2.7–3.5 msec in motoneurons of the masseter muscle. These results indicate the existence of functional connections between motoneurons of the masseter muscle and its proprioceptive afferent fibers, and also with proprioceptive afferent fibers of the antagonist muscle and cutaneous afferent fibers.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 1, No. 3, pp. 262–268, November–December, 1969.     

Monosynaptic excitation of facial motoneurons during electrical stimulation of the red nucleus

Synaptic effects of the red nucleus on motoneurons of the facial nucleus were studied in cats. Impulses from the red nucleus activate motoneurons innervating the auricular, buccal, and orbicularis oculi muscles. Monosynaptic EPSPs appeared in all motoneurons which responded to stimulation. Their mean latent period was 1.5±0.04 msec, duration 12.3 ± 0.34 msec, and rise time between 1.5 and 3.2 msec. Repetitive stimulation of the red nucleus led to marked facilitation of the testing EPSP. Facilitation was maximal when the interval between stimuli was 3.5 msec; it was reduced by either a decrease or an increase in the interval. The functional role of the monosynaptic connections of neurons of the red nucleus and of the facial motoneurons is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 3, pp. 272–279, May–June, 1972.     

Postsynaptic potentials of motoneurons evoked by rubrofugal impulsation in the hypoglossal nucleus of the cat

The effect of stimulation of the ipsilateral and contralateral red nuclei on motoneurons of the hypoglossal nucleus was studied in cats anesthetized with chloralose and pentobarbital. In 35 (69%) of the 51 motoneurons tested, PSPs were generated in response to stimulation of the red nuclei by series of 3 to 5 stimuli of threshold strength and with a frequency of 500–600/sec. Of this number, 33 motoneurons responded to stimulation by EPSPs, whose latent periods varied from 3.5 to 14.0 msec (mean value for the ipsilateral red nucleus 5.7±0.75, for the contralateral nucleus 6.8±0.8 msec), whereas two motoneurons responded (after 6.2 msec) by IPSPs. Of the 35 motoneurons responding to stimulation of the red nuclei, stimulation of the lingual nerve evoked EPSPs in 31 and IPSPs in 4 (two of them were inhibited by rubrofugal impulses). IPSPs were generated as a result of stimulation of the lingual nerve in 16 motoneurons which did not respond to rubrofugal impulses.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 1, pp. 62–66, January–February, 1978.     

Corticofugal influences on postsynaptic processes in rubrospinal neurons in the cat brain

Composite and unitary EPSPs of red nucleus neurons evoked by stimulation of the sensomotor and association parietal cortex and nucleus interpositus of the cerebellum were studied in acute experiments on cats anesthetized with pentobarbital. A monosynaptic connection was shown to exist between not only the sensomotor, but also the association cortex, and rubrospinal neurons, in which unitary EPSPs appeared during stimulation of the association cortex after a latent period of 1.5–2.7 msec, with a peak rise time of 1.1–3.1 msec and an amplitude of 0.22–0.65 mV. Analysis of the temporal characteristics of the unitary EPSP suggested that synapses formed by fibers from the association cortex occupy a position nearer the soma than synapses formed by axons of sensomotor cortical cells.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 67–74, January–February, 1984.     

Unit responses in the anterior zone of the suprasylvian gyrus to peripheral stimuli of different modalities

Unit responses in the anterior zone of the suprasylvian gyrus to visual, electrodermal, and acoustic stimulation were investigated in experiments on unanesthetized cats immobilized with tubocurarine. Electrical activity was recorded from 131 units, 121 of which were spontaneously active. In 65.5% of cells responses consisted of a short or long increase or a decrease in intensity of spike activity. Most cells (58.2%) were monosensory. Responses to visual stimulation were given by 72% of neurons, to electrodermal by 61.6%, and to acoustic by 9.3%. The corresponding latent periods were 20–40, 20–30, and 15–20 msec. Responses of the same neurons to different peripheral stimuli were uniform or they differed in their dynamics. Intracellular recording gave responses in the form of EPSPs (amplitude 4–5 mV, duration 60–80 msec) or, rarely, IPSPs (amplitude 2–3 mV, duration 160–200 msec). The functional organization of the associative cortex and mechanisms of analysis of incoming afferent information are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 4, pp. 368–374, July–August, 1972.     

Monosynaptic inhibitory postsynaptic potentials of cortical neurons

Monopolar intracortical stimulation of the auditory cortex was carried out in cats immobilized with D-tubocurarine. A macroelectrode (tip diameter 100 µ) or a microelectrode (tip diameter 10–15 µ) was used for stimulation. In both cases, besides excitatory responses, primary IPSPs with latent periods of 0.4–1.2 and 1.4–6.0 msec were recorded in cortical neurons close to the point of stimulation. The first group of IPSPs are considered to be generated in response to direct stimulation of bodies or axons of inhibitory cortical neurons, i.e., monosynaptically. The amplitude of these IPSPs varied in different neurons from 3 to 15 mV, and their duration from 4 to 150 msec. Additional later inhibitory responses were superposed on many of them. Of the IPSPs generated in auditory cortical neurons in response to stimulation of geniculocortical fibers 1.5% had a latency of 0.8–1.3 msec. They also are assumed to be monosynaptic. It is concluded that the duration of synaptic delay of IPSPs in cortical neurons and spinal motoneurons is the same, namely 0.3–0.4 msec. Axons of auditory cortical inhibitory neurons may be 1.5 mm long. The velocity of impulse conduction along these axons is 1.6–2.8 m/sec. The genesis of some special features of IPSPs of cortical neurons is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 7, No. 5, pp. 458–467, September–October, 1975.     

Action potentials of masseter motoneurons evoked by stimulation of low-threshold infraorbital nerve afferents in cats

In cats anesthetized with chloralose and pentobarbital stimulation of the infraorbital nerve by a volley of 3 or 4 stimuli 1.2 times stronger than the threshold for excitation of A-fibers caused the generation of action potentials in motoneurons of the masseter muscle if the frequency of stimuli in the volley exceeded 300/sec. Paired stimuli with a strength of 2.0 thresholds, and with an interval of 1.3–4.0 msec between stimuli, led to generation of an action potential by the motoneurons. If the interval exceeded 4 msec stimulation with a strength of 1.2–2.0 thresholds caused biphasic facilitation of the second EPSP with a facilitation factor of between 0.2 and 1.0. The small number of stimuli, combined with their high frequency in the volley, required for action potential generation by masseter motoneurons suggests that they are due to activation of A-fibers of the infraorbital nerve connected with fast-adapted receptors of the vibrissae.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 4 pp. 385–389, July–August, 1978.     

Neuronal and synaptic mechanisms of cortical inhibition

The latent periods, amplitude, and duration of IPSPs arising in neurons in different parts of the cat cortex in response to afferent stimuli, stimulation of thalamocortical fibers, and intracortical microstimulation are described. The duration of IPSPs evoked in cortical neurons in response to single afferent stimuli varied from 20 to 250 msec (most common frequency 30–60 msec). During intracortical microstimulation of the auditory cortex, IPSPs with a duration of 5–10 msec also appeared. Barbiturates and chloralose increased the duration of the IPSPs to 300–500 msec. The latent period of 73% of IPSPs arising in auditory cortical neurons in response to stimulation of thalamocortical fibers was 1.2 msec longer than the latent period of monosynaptic EPSPs evoked in the same way. It is concluded from these data that inhibition arising in most neurons of cortical projection areas as a result of the arrival of corresponding afferent impulsation is direct afferent inhibition involving the participation of cortical inhibitory interneurons. A mechanism of recurrent inhibition takes part in the development of inhibition in a certain proportion of neurons. IPSPs arise monosynaptically in 2% of cells. A study of responses of cortical neurons to intracortical microstimulation showed that synaptic delay of IPSPs in these cells is 0.3–0.4 msec. The length of axons of inhibitory neurons in layer IV of the auditory cortex reaches 1.5 mm. The velocity of spread of excitation along these axons is 1.6–2.8 msec (mean 2.2 msec).A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 394–403, May–June, 1984.     

Unit responses of the first and second somatosensory areas to stimulation of the ventroposterior thalamic nucleus

Single unit responses of the first (SI) and second (SII) somatosensory areas to stimulation of the ventroposterior thalamic nucleus (VP) were investigated in cats immobilized with D-tubocurarine. In response to VP stimulation 12.0% of reacting SI neurons and 9.5% of SII neurons generated an antidromic spike. In most antidromic responses of both SI and SII neurons the latent period did not exceed 1.0 msec. The minimal latent period of spike potentials during orthodromic excitation was 1.5 msec in SI and 1.7 msec in SII. Neurons with an orthodromic spike latency of not more than 3.0 msec were more numerous in SI than those with a latency of 3.1–4.5 msec. The ratio between the numbers of neurons of these two groups in SII was the opposite. In SII there were many more neurons with a latency of 5.6–8.0 msec than in SI. EPSPs appeared after a latent period of 1.1–9.0 msec in SI and of 1.4–6.6 msec in SII. The latent period of IPSPs was 1.5–6.8 msec in SI and 2.2–9.4 msec in SII. The relative importance of different pathways for excitatory and inhibitory influences of VP on SI and SII neurons is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 2, pp. 115–121, March–April, 1976.     

Convergence of corticospinal and rubrospinal influences on cervical motoneurons

Synaptic responses of 121 identified cervical motoneurons to stimulation of the pyramidal tract and red nucleus were investigated by intracellular recording in cats. Responses of EPSP or EPSP-IPSP type were predominant in motoneurons of distal groups of muscles and proximal flexors, while responses of IPSP type were predominant in motoneurons of the proximal extensors. The minimal effective number of stimuli for most motoneurons was 2 or 3. The mean latent period, counted from the first stimulus in the series, was 7.86 msec for EPSPs for stimulation of the pyramidal tract and 7.91 msec for stimulation of the red nucleus, while the corresponding periods for IPSPs were 8.68 and 8.75 msec. The segmental delay of 1.3–2 msec for EPSPs and IPSPs generated in certain motoneurons in response to stimulation of both structures indicates that the shortest pathway for transmission of activity from the fibers of these tracts to the motoneurons may be disynaptic. At the same time, the possible presence of an additional neuron for most inhibitory pathways cannot be ruled out. Analysis of the results also suggests the presence of a common interneuronal apparatus for both systems.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol.3, No.6, pp. 599–608, November–December, 1971.     

Changes in postsynaptic responses in spinal motoneurons during repetitive stimulation of the locus coeruleus

Experiments on cats anesthetized with chloralose showed that repetitive stimulation of the locus coeruleus is accompanied by a decrease in IPSPs evoked by stimulation of flexor reflex afferents in extensor motoneurons. The effect appeared 600 msec after the beginning of stimulation and reached its maximum after 1500–2000 msec. Repetitive stimulation of the locus coeruleus did not change the membrane potential and did not affect EPSPs or IPSPs evoked by stimulation of low-threshold muscle afferents; EPSPs due to activation of high-threshold cutaneous and muscle afferents likewise remained unchanged. Repetitive stimulation of more central regions of the brain stem was accompanied not only by a decrease in IPSPs evoked by stimulation of flexor reflex afferents in extensor motoneurons, but also by a decrease in amplitude of EPSPs arising in response to stimulation of these same afferents in flexor motoneurons. These effects were not connected with activation of monoaminergic structures, for unlike effects arising during stimulation of the locus coeruleus, they were also found in previously reserpinized animals.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 14, No. 1, pp. 51–59, January–February, 1982.     

Synaptic potentials of digastric-muscle motoneurons

We studied the antidromic and synaptic potentials evoked from 32 digastric-muscle motoneurons by stimulation of the motor nerve to this muscle, different branches of the trigeminal nerve, and the mesencephalic trigeminal nucleus. Antidromic potentials appeared after 1.1 msec and lasted about 2.0 msec. Stimulation of the infraorbital, lingual, and inferior alveolar nerves led to development of excitatory postsynaptic potentials (EPSP) and action potentials in the motoneurons. The antidromically and synaptically evoked action potentials of the digastric-nerve motoneurons were characterized by weak after-effects. We were able to record EPSP and action potentials in two of the motoneurons investigated in response to stimulation of the mesencephalic trigeminal nucleus, the latent period being 1.3 msec. This indicates the existence of a polysynaptic connection between the mesencephalic-nucleus neurons and the digastric-muscle motoneurons. Eight digastric-muscle motoneurons exhibited inhibitory postsynaptic potentials (IPSP), which were evoked by activation of the afferent fibers of the antagonistic muscle (m. masseter). The data obtained indicate the presence of reciprocal relationships between the motoneurons of the antagonistic muscles that participate in the act of mastication.A. A. Bogomol'ts Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 52–57, January–February, 1971.     

Synaptic effects evoked in motoneurons by stimulation of single propriospinal neurons

Experiments on cats with simultaneous extracellular recording, stimulation of single propriospinal neurons, and intracellular recording of unitary postsynaptic potentials from motoneurons, followed by computer averaging showed that direct stimulation of individual propriospinal cells receiving mono- and disynaptic influences from the medial reticular formation can evoke monosynaptic EPSPs and IPSPs in lower lumbar motoneurons. The amplitude of these EPSPs was 49.6±6.0 and of the IPSPs 28.9±2.9 µV and their synaptic delay was 0.34±0.05 msec. The same propriospinal neuron of the ventral horn of the upper lumbar segments may be connected with several motoneurons of the hind limb muscles.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 9, No. 3, pp. 300–306, May–June, 1977.     

Synaptic responses of medullary neurons of turtles to stimulation of the locomotor region

Responses of medullary neurons to microstimulation of the locomotor region by a current of up to 30 µA were studied by intracellular recording in turtles. The resting potential recorded in these neurons was from 22 to 42 mV. Depolarization PSPs (EPSPs) were recorded in 43 neurons, hyperpolarization PSPs (IPSPs) in 12, and mixed in 36. Synaptic discharges were observed in 29 neurons. Of these cells 11 generated action potentials without visible PSPs. The latent period of the PSPs was most frequently between 2 and 8 msec. The time from the beginning of the EPSP to the beginning of the action potential was 1–3 msec if the response index was high, but in the case of weaker stimulation, it began to fluctuate strongly and lengthened. Unitary EPSPs were recorded in 15 neurons and IPSPs in three. Their amplitude was 0.6–0.8 mV, from 2 to 12 times less than the depolarization threshold (1–7 mV). These results, together with those obtained previously by extracellular recording of medullary unit activity in turtles and cats, are used to discuss the possible mechanism of spread of locomotor activity.Institute for Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 14, No. 2, pp. 122–129, March–April, 1982.     

Postsynaptic potentials of neurons of the cat auditory cortex

The electrical reactions of 294 neurons of the auditory cortex to a click were recorded in experiments on cats immobilized with tubocurarine (174 intra- and 120 extracellularly). The value of the membrane potential varied from 30 to 70 mV with intracellular leads. The following types of reactions were obtained (the number of neurons is given in parentheses): a peak without slow oscillations in the membrane potential (4), EPSP (3), EPSP-peak (6), EPSP-peak-IPSP (17), EPSP-IPSP (9), primary IPSP (114, including 23 with an after-discharge). Twenty one neurons did not react to a click. The amplitude of the sub-threshold EPSP was 1–1.5 mV, the duration of the ascending part was about 10 and of the descending part 20–30 msec. The peak potential on the ascending part of the EPSP developed at the critical level of 3–4 mV. The amplitude of the peaks varied from several millivolts to 50–60. In 17 neurons prolonged hyperpolarization having all the properties of an IPSP, developed after the peak. The amplitude of these IPSP varied in different neurons from 1 to 10 mV and the duration varied from 20 to 80 msec. IPSP without preceding excitation of the given neuron were the predominant types of reaction. The latent period of these primary IPSP varied from 7 to 20 msec and the amplitude from 1 to 15 msec with a duration of 30–200, more frequently 80–100 msec. It is suggested that two types of inhibition develop in neurons of the auditory cortex in response to a click: recurrent and afferent. The functional significance of the first consists in limiting the duration of the discharge in the reacting neurons, the second prevents the development of excitation in adjacent neurons, thereby limiting the area of neuronal activity.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSSR, Kiev. Translated from Neirofiziologiya, Vol. 3, No. 4, pp. 339–349, July–August, 1971.     

Unit responses in the auditory cortex to stimulation of geniculocortical fibers

Extracellular and intracellular single unit responses of neurons of the auditory cortex to electrical stimulation of geniculocortical fibers (GCF) were recorded in experiments on cats immobilized with tubocurarine. The latent period of responses of 15% of neurons to GCF stimulation was 0.3–1.5 msec. It is postulated that they were excited anti-dromically. The latent period of spikes generated by neurons responding to GCF stimulation orthodromically varied from 1.6 to 12 msec. In 28.6% of neurons the latent period was 1.6–2.5 msec. It is postulated that these neurons were excited monosynaptically. Intracellular recording revealed primary IPSPs in response to GCF stimulation in 63.3% of neurons, a brief EPSP followed by a prolonged IPSP in 17.7%, an EPSP-spike-IPSP complex in 12.3%, and subthreshold EPSPs in 7% of neurons. The latent period of the primary IPSPs varied from 1.8 to 11 msec, being 1.8–3.7 in 72%, 3.8–5.7 in 20.0%, and 5.8–11 msec in 8.0% of neurons. The latent period of responses beginning with an EPSP was 1–4 msec (mean 1.8 msec). Orthodromic responses arising 3–10 msec after the antidromic response, and consisting of 3–5 spikes, were recorded in some antidromically excited neurons. Hypotheses regarding the functional organization of the auditory cortex and mechanisms of inhibition in its neurons are put forward on the basis of the results obtained.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 3, pp. 227–235, May–June, 1972.