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.     

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 neck muscle motoneurons evoked by horizontal semicircular canal stimulation in cats

Postsynaptic potentials evoked by stimulation of ipsilateral and contralateral horizontal semicircular canals in motoneurons of muscles tilting and turning the head were investigated in acute experiments on cats anesthetized with chloralose and pentobarbital. Stimulation of the ipsilateral canal evoked EPSPs with latent periods varying from 1.8 to 10.0 msec in 25 of these motoneurons and IPSPs with latent periods varying from 1.9 to 3.9 msec in 10 of them. Calculation of the impulse conduction time from the ipsilateral semicircular canal through Deiters' nucleus to the cervical motoneurons indicates that EPSPs with latent periods of under 3.8 msec may be regarded as disynaptic, and those with latent periods of over 3.8 msec as polysynaptic. Stimulation of the contralateral canal evoked EPSPs with latent periods varying from 1.8 to 6.0 msec in 19 motoneurons and IPSPs with latent periods varying from 3.2 to 3.9 msec in two cells. The possible pathways of transmission of these influences and their functional role are discussed.     

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.     

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.     

Corticofugal influences on the motoneurons of the trigeminal nucleus

We studied the postsynaptic potentials evoked from 76 trigeminal motoneurons by stimulation of the motor (MI) and somatosensory (SI) cortex in the ipsilateral and contralateral hemispheres of the cat. Stimulation of these cortical regions evoked primarily inhibitory postsynaptic potentials (PSP) in the motoneuron of the masseter muscle, but we also observed excitatory PSP and mixed reactions of the EPSP/IPSP type. The average IPSP latent period for the motoneurons of the masseter on stimulation of the ipsilateral cortex was 6.1±0.3 msec, while that on stimulation of the contralateral cortex was 5.2±0.4 msec; the corresponding figures for the EPSP were 7.6±0.5 and 4.5±0.3 msec respectively. Corticofugal impulses evoked only EPSP and action potentials in the motoneurons of the digastric muscle (m. digastricus). The latent period of the EPSP was 7.6 msec when evoked by afferent impulses from the ipsilateral cortex and 5.4 msec when evoked by pulses from the contralateral cortex. The duration of the PSP ranged from 25 to 30 msec. Postsynaptic potentials developed in the motoneurons studied when the cortex was stimulated with a single stimulus. An increase in the number of stimuli in the series led to a rise in the PSP amplitude and a reduction in the latent periods. When the cortex was stimulated with a series of pulses (lasting 1.0 msec), the IPSP were prolonged by appearance of a late slow component. We have hypothesized that activation of the trigeminal motoneurons by corticofugal impulsation is effected through a polysynaptic pathway; each functional group of motoneurons is activated in the same manner by the ipsilateral and contralateral cortex. The excitation of the digastric motoneurons and inhibition of the masseter motoneurons indicates reciprocal cortical control of their activity.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 3, No. 5, pp. 512–519, September–October, 1971.     

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.     

EPSPs of masseter motoneurons evoked by stimulation of low-threshold infraorbital nerve afferents in cat

Stimulation of the infraorbital nerve at strengths 1.4–2.5 times higer than the threshold of excitation of A fibers in cats anesthetized with chloralose and pentobarbital evoked EPSPs with an amplitude up to 3.0 mV and a duration of 9–15 msec in 69% of masseter motoneurons after 1.5–3.0 msec. These EPSPs were complex and formed by summation of simpler short-latency and long-latency EPSPs. The short-latency EPSPs appeared in response to infraorbital nerve stimulation at 1.1–1.5 thresholds and had a slow rate of rise (2.5–4.5 msec, mean 3.7±0.4 msec), low amplitude (under 2.0 mV), and short duration (5–6 msec). Their latent period varied from 1.5 to 3.0 msec (mean 2.1±0.2 msec). The shortness of the latent period and its constancy during stimulation of the nerve at increasing strength, and also the character of development of facilitation and inhibition of the EPSP during high-frequency stimulation suggests that these EPSPs are monosynaptic. The slow rate of rise suggested that these EPSPs arise on distal dendrites of the motoneurons. Long-latency EPSPs appeared 7–9 msec after stimulation of the infraorbital nerve at 1.1–1.5 thresholds. Their amplitude reached 1.5–2.0 mV and their duration 7–9 msec. The long duration of the latent period combined with low ability to reproduce high-frequency stimulation (up to 30/sec) points to the polysynaptic origin of these EPSPs.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 9, No. 6, pp. 583–591, November–December, 1977.     

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 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.     

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.     

Postsynaptic potentials of facial motoneurons evoked by afferent and corticofugal impulses

Postsynaptic potentials of motoneurons in the facial nerve nucleus, evoked by stimulation of the cranial nerves (trigeminal, hypoglossal, facial) and of the sensomotor cortex were investigated in cats anesthetized with chloralose and pentobarbital. Two functionally opposite groups of motoneurons were found to exist in the facial nucleus. Stimulation of the afferent nerves and cortex evoked the appearance of EPSPs in the first of these groups and IPSPs in the second. The latency and duration of the PSPs indicate that afferent and corticofugal impulses reach the facial motoneurons along polysynaptic pathways. Interneurons on which wide convergence of influences travelling along afferent fibers and of the cortex, were found in the region of the facial nucleus. The possible neuronal pathways concerned with the transmission of afferent and corticofugal impulses to the facial motoneurons are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol.4, No.4, pp. 391–400, July–August, 1972.     

Effects of stimulating the superior colliculus on motoneurons of the neck muscle in the cat

Postsynaptic potentials produced by stimulating three sites of the midbrain superior colliculus were examined in motoneurons innervating the sternocleidomastoid, the trapezius, and the platysma cervical muscles in anesthetized cats. Stimulating the ipsilateral colliculus produced EPSP in the motoneurons as well as action potentials with a latency of 1.5–3.5 msec, averaging 2.6 ± 0.1 msec. Stimulation of the contralateral colliculus evoked EPSP with a latency of 1.5–3.2 msec and averaging 2.1 ± 0.1 msec together with IPSP with latency ranging from 2.6 to 5.0 msec. It is postulated that these postsynaptic responses are both monosynpatic and bisynaptic in nature. This type of synaptic action is assumed to be one of the mechanisms responsible for coordinated head movements produced by tectofugal impulses.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 2, pp. 197–202, March–April, 1986.     

Connections of neurons of the cat somatosensory cortex with the thalamic relay nuclei

Of 103 neurons in the rostral part of the posterior sigmoid gyrus of the cat cortex 30 responded to stimulation of the ventro-posterolateral and ventrolateral nuclei of the thalamus (VPL and VL), 42 responded to stimulation of VL only, and 31 to stimulation of VPL only. It was shown by intracellular recording that stimulation of VPL induces a spike response with or without subsequent IPSPs in some neurons and an initial IPSP in others. The spike frequency of single neurons reached 60/sec, but the IPSP frequency never exceeded 10–20/sec. Stimulation of VL was accompanied by: a) antidromic spike responses; b) short-latency monosynaptic EPSPs and spikes capable of following a stimulation frequency of 100/sec; c) long-latency polysynaptic EPSPs and spikes appearing in response to stimulation at 4–8/sec; d) short-latency IPSPs; e) long-latency IPSPs increasing in intensity on repetition of infrequent stimuli. It is concluded that the afferent inputs from the relay nuclei to neurons of the somatosensory cortex are heterogeneous. An important role is postulated for recurrent inhibition in the genesis of the long-latency IPSPs arising in response to stimulation of VL, and for direct afferent inhibition during IPSPs evoked by stimulation of VPL. It is shown that the rostral part of the posterior sigmoid gyrus performs the role of somatic projection and motor cortex simultaneously.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 3, pp. 245–255, May–June, 1972.     

Unit responses of the ventral posterior and ventral lateral thalamic nuclei to electrical stimulation of the thalamic reticular nucleus

A microelectrode investigation was made of responses of 72 physiologically identified neurons of the ventral posterior (VP) and 116 neurons of the ventral lateral (VL) thalamic nuclei to electrical stimulation of the reticular (R) thalamic nucleus. Mainly those neurons of VP and VL (73.7 and 86.2% respectively) which responded to stimulation of the first motor area and nucleus interpositus of the cerebellum responded to stimulation of R; 19.8% of VL neurons tested responded to stimulation of R by an antidromic action potential with latent period of 0.5–2.0 msec and 46.6% of neurons responded by orthodromic excitation; 23% of orthodromic responses had a latent period of 0.9–3.5 msec and 77% a latent period of 4.0–21.0 msec; 19.8% of VL neurons tested were inhibited. Among IPSPs recorded only one was monosynaptic (1.0 msec) and the rest polysynaptic. It is postulated that both R neurons are excitatory and that the inhibition which develops in VL neurons during stimulation of R are connected mainly with activation of inhibitory interneurons outside the reticular nucleus.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 9, No. 5, pp. 477–485, September–October, 1977.     

Unit responses in the cat auditory cortex to medial geniculate body stimulation

Responses of 98 auditory cortical neurons to electrical stimulation of the medial geniculate body (MGB) were recorded (45 extracellulary, 53 intracellularly) in experiments on cats immobilized with tubocurarine. Responses of the same neurons to clicks were recorded for comparison. Of the total number of neurons, 75 (76%) responded both to MGB stimulation and to clicks, and 23 (24%) to MGB stimulation only. The latent period of extracellularly recorded action potentials of auditory cortical neurons in response to clicks varied from 7 to 28 msec (late responses were disregarded), and that to MGB stimulation varied from 1.5 to 12.5 msec. For EPSPs these values were 8–13 and 1–4 msec respectively. The latent period of IPSPs arising in response to MGB stimulation varied from 2.2 to 6.5 msec; for 34% of neurons it did not exceed 3 msec. The difference between the latent periods of responses to clicks and to MGB stimulation varied for different neurons from 6 to 21 msec. Responses of 11% of neurons to MGB stimulation, recorded intracellularly, consisted of sub-threshold EPSPs, while responses of 23% of neurons began with an EPSP which was either followed by an action potential and subsequent IPSP or was at once cut off by an IPSP; 66% of neurons responded with primary IPSPs. Neurons responding to MGB stimulation by primary IPSPs are distributed irregularly in the depth of the cortex: there are very few in layers III and IV and many more at a depth of 1.6–2 mm. Conversely, excited neurons are predominant in layer III and IV, and they are few in number at a depth of 1.6–2 mm. It is concluded that the afferent volley reaching the auditory cortex induces excitation of some neurons therein and, at the same time, by the principle of reciprocity, induces inhibition of others. This afferent inhibition takes place with the participation of inhibitory interneurons, and in some cells the inhibition is recurrent. The existence of reciprocal relationships between neurons in different layers of the auditory cortex is postulated.A. A. Bogomolets' Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 1, pp. 23–31, January–February, 1972.     

Inhibitory effects of reticulospinal fibers of the lateral funiculus on neurons of thoracic spinal reflex pathways

Experiments on anesthetized cats with partial transection of the spinal cord showed that reticulo-spinal fibers in the ventral part of the lateral funiculus participate in the inhibition of polysynaptic reflexes evoked by stimulation of the ipsi- and contralateral reticular formation. The reticulo-fugal wave in the ventrolateral funiculus evoked comparatively short (up to 70 msec) IPSPs in some motoneurons of the internal intercostal nerve investigated and at the same time evoked prolonged (up to 500 msec) inhibition of IPSPs caused by activation of high-threshold segmental afferents. This wave also led to the appearance of IPSPs in 14 of 91 (15.5 %) thoracic spinal interneurons studied. The duration of these IPSPs did not exceed 100 msec; meanwhile, segment excitatory responses of 21 of 43 interneurons remained partly suppressed for 120–500 msec. It is concluded that the inhibitory action of the lateral reticulo-spinal system on segmental reflexes is due to several synaptic mechanisms, some of them unconnected with hyperpolarization of spinal neurons. The possible types of mechanisms of this inhibition are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 162–172, March–April, 1978.     

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.     

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.