Unit responses in the "locomotor strip" of the cat hindbrain to microstimulation

Synaptic responses of single units in the "locomotor strip" of the hindbrain were recorded extracellularly. Short-latency responses appeared in neurons of the rostral part of the strip to stimulation of the "locomotor region" of the mesencephalon. Neurons of the caudal part of the strip responded to microstimulation of its other regions, including rostral. If the distance between the neuron and point of stimulation was under 2–3 mm, short-latency (1.2–1.6 msec) responses could be observed. The thresholds and latent periods of the responses increased when the distance apart increased. Polysynaptic responses with a latent period of 3–4 msec could be potentiated by an increase in the frequency of stimulation up to 30–40 Hz. It is suggested that axons of the "locomotor strip" are oriented in the rostrocaudal direction for a distance of 2–3 mm and give off collaterals which run toward neighboring neurons. The strip may be an integrative center, "intercalated" between the rostral portions of the brain stem and spinal cord.Deceased.Institute for Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 10, No. 5, pp. 510–518, September–October, 1978.     

Synaptic responses of medulla oblongata neurons to stimulation of the spinal stepping atria in the cat

Synaptic responses of medulla oblongata (bulbar) neurons to microstimulation of stepping points in the spinal dorsolateral funiculi were recorded in decrebrate cats. Upon stimulation of the stepping point both in the ipsi- and contralateral funiculi, 40% of the neurons generated synaptic responses; the remaining cells responded to stimulation of only one stepping point. A part of the bulbar neurons responds to stimulation of stepping points both at the C₂ and Th₁₂ level. The latent periods of the synaptic responses of the bulbar neurons to stimulation of the stepping point at the C₂ level were in the 2–10 msec range. The data obtained indicate that bulbar neurons, along with propriospinal neurons, can participate in evoking locomotion upon stimulating the stepping point.Institute of Information Transmission Problems, Academy of Sciences of the USSR, Moscow. M. V. Lomonosov State University, Moscow. Translated from Neirofiziologiya, Vol. 23, No. 3, pp. 328–333, May–June, 1991.     

Responses of medullary and spinal cord neurons to simulataneous stimulation of two locomotor points

Responses of neurons in the medulla and cervical segments of the spinal cord to simultaneous repetitive stimulation (50 pps) of two locomotor points (LP) that exceed the threshold for evoking walking by one to two times were investigated in mesencephalic cats. In most cases a neuron responsed to stimulation of only one LP. Stimulation of a second LP usually enhanced the firing index of that response, if it was low. Or it decreased the firing index if it was high, and it had no effect if the firing index was about 0.1. Some of the neurons responded to stimulation of one LP by increasing the baseline activity, though the pulses were not locked to individual stimuli. The effect of stimulation of the second LP on this increase in activity depended on the size of the increase. Data were also obtained on the convergence of effects on single neurons from the ipsi- and contralateral LPs of the midbrain and medulla. Possible mechanisms of the summation of subthreshold excitation of a pair of LPs during the initiation of locomotion are discussed.Institute of Information Transmission Problems, Russian Academy of Sciences, Moscow. Translated from Neirofiziologiya, Vol. 24, No. 4, pp. 471–481, July–August, 1992.     

Responses of medullary neurons to microstimulation of the "locomotor strip" in cats

Synaptic responses of medullary neurons to stimulation of the bulbar locomotor strip with a current of about 20 µA were studied by an extracellular recording method in mesencephalic cerebellectomized cats. The mean latent period of response of 177 neurons was 3.2 msec. Neurons in which synaptic responses appeared were located in both the lateral and the medial parts of the reticular formation, but short-latency responses were recorded predominantly in the lateral part. In response to a single stimulus 32% of neurons generated a discharge of 2–4 spikes. "Respiratory" neurons were not excited by stimulation of the locomotor point. The results indicate that neurons of the locomotor strip may have an excitatory action not only on each other, but also on neurons located medially. The possible mechanisms of the spread of activity to the superior cervical segments of the spinal cord are discussed.Institute for Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 13, No. 3, pp. 275–282, May–June, 1981.     

The rhombencephalic "locomotor region" in turtles

Electrical stimulation (10–20 µA, 20–30 Hz) of the rhombencephalon in decerebrate turtles can induce cyclic coordinated limb movemnts. The "locomotor region" is a strip, oriented in the rostro-caudal direction, which coincides in its location with the lateral reticular formation. Both in the medial and in the lateral reticular formation extracellular ipsilateral and contralateral synaptic responses of single neurons evoked by stimulation of the "locomotor region," (10–30 µA, 2 Hz), were recorded. Usually these responses had latent periods of between 3 and 12 msec (mode 5–6 msec). Excitation of the "locomotor region" thus leads to extensive spread of activity in the rhombencephalon. The possible mechanisms of this spread are discussed.Institute of Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 12, No. 4, pp. 382–390, July–August, 1980.     

Neurons of upper cervical segments responding to stimulation of the bulbar locomotor strip

Synaptic responses of single neurons to stimulation of the bulbar "locomotor strip" were recorded extracellularly from superior cervical segments in mesencephalic cats. With a strength of stimulation of about 30 µA these responses usually had a latent period of 2–7 msec and they arose in neurons located at a depth of between 2 and 4 mm from the dorsal surface (Rexed's laminae V–VIII). These neurons could not be excited antidromically by stimulation of the lumbar or lower cervical segments. However, antidromic responses could be evoked by stimulation of a region located 3–5 mm caudally to the site of recording. It is suggested that neurons of segments C2 and C3 excited by stimulation of the locomotor strip are components of a cell column along which activity spreads polysynaptically in the direction of spinal stepping generators.Institute for Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 11, No. 3, pp. 245–253, May–June, 1979.     

Responses of bulbar neurons to stimulation of locomotor and inhibitory sites in the cat brainstem

Bulbar locomotor and inhibitory sites were located in the pons of mesencephalic decerebellate cats. Rhythmic stimulation of locomotor sites through microelectrodes at the rate of 60 Hz elicited stepping movements in the forelimbs which were halted when the inhibitory sites were rhythmically stimulated. Neuronal response was elicited by single or paired stimulation of locomotor sites at the rate of 1.5 Hz or by applying a series of 2–4 stimuli spaced 2 msec apart to the inhibitory site. Medial neurons generated synaptic responses (postsynaptic potentials or action potentials) to stimulation of the inhibitory site twice as frequently as when the locomotor site was stimulated. Responses in lateral neurons, however, occurred twice as frequently to stimulation of the locomotor site, while IPSP were only observed half as often as EPSP in neurons of both groups. In neurons excited by stimulation of the locomotor site, stimulation of the inhibitory site did not normally produce IPSP. Possible mechanisms underlying the halt of locomotion occurring in response to stimulation of the inhibitory site are discussed.Information Transmission Institute, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 18, No. 4, pp. 525–533, July–August, 1986.     

Synaptic responses of propriospinal neurons to stimulation of the stepping strip of the dorsolateral funiculus in cats

Synpatic responses were recorded extracellularly from single neurons at levels T12–T13 in response to microstimulation of the stepping strip of the dorsolateral funiculus in the thoracic and cervical portions of the spinal cord in cats decerebrated at the precollicular level [4]. The latent periods of these responses increased when the distance between the stepping point and recording point exceeded 20 mm, and when two stimuli had to be applied in order to evoke responses. Axons of neurons respondingtrans-synaptically to stimulation of the stepping strip were located on the boundary between the lateral and ventral funiculi close to the gray matter. Antidromic responses of neurons sending their axons in the caudal direction were found on average in 37% of cases, provided that the distance between the recording and stimulation points did not exceed 40 mm. Half of the neurons which generated synaptic responses to stimulation of spinal stepping points could be excited also by a series of three to six stimuli, applied to the mesencephalic locomotor point [15]. It is suggested that the neurons discovered may participate in the spread of activity toward the generator of stepping movements of the hind limb.Institute for Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 17, No. 2, pp. 270–278, March–April, 1985.     

Activity of propriospinal neurons in segments C3 and C4 during fictitious locomotion in cats

Activity of propriospinal neurons in segments C3 and C4 was recorded in immobilized decerebrate cats, whose spinal cord was divided at the lower thoracic level, during locomotor activity of neuronal mechanisms controlling the forelimbs (fictitious locomotion of the forelimbs). Neurons were identified according to antidromic responses to stimulation of the lateral column of the spinal cord at level C6. Antidromic responses also appeared in 70% of these neurons to stimulation of the medullary lateral reticular nucleus. During fictitious locomotion, i.e., in the absence of afferent signals from the limb receptors, rhythmic modulation of the discharge of most neurons was observed, correlating with activity of motoneurons. If the rostral region of the cervical enlargement of the spinal cord was cooled, causing generation of the locomotor rhythm to cease, rhythmic activity of propriospinal neurons in segments C3 and C4 also ceased. The main source of modulation of activity of propriospinal neurons in segments C3 and C4 is thus the central spinal mechanisms controlling activity of the forelimbs.Institute for Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. M. V. Lomonosov Moscow University. Translated from Neirofiziologiya, Vol. 17, No. 3, pp. 320–326, May–June, 1985.     

Efferent projections of mesencephalic locomotor neurons in the cat

Efferent neuronal projections of the mesencephalic locomotor region were investigated in cats using a horseradish peroxidase retrograde axonal transport technique. It was found that neurons located within the locomotor area form ascending and descending projections to many structures of the spinal cord and the brain but that short-axon connections running to the reticular formation of the midbrain and the medulla predominate. Small numbers of long-axon fibers may merge into the locomotor strips of the medulla and the spinal cord. The locomotor regions of the two halves of the midbrain are interlinked.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 1, pp. 117–125, January–February, 1986.     

Two patterns of bulbar neuronal response to microstimulation of locomotor and inhibitory brain stem sites

Synaptic responses (postsynaptic potentials and action potentials) were evoked in mesencephalic decerebellated cats by stimulating pontine bulbar locomotor and inhibitory sites (LS and IS, respectively) with a current of not more than 20 µA in "medial" and "lateral" neurons of the medulla. Some neurons even produced a response to presentation of single (actually low — 2–5 Hz — frequency) stimuli. The remaining cells responded to stimulation at a steady rate of 30–60 Hz only. Both groups of medial neurons were more receptive to input from LS. Lateral neurons responding to even single stimuli reacted more commonly to input from LS and those responding to steady stimulation only to input from IS. Many neurons with background activity (whether lateral or medial) produced no stimulus-bound response, but rhythmic stimulation either intensified or inhibited such activity. This response occurs most commonly with LS stimulation. Partial redistribution of target neurons in step with increasing rate of presynaptic input may play a major part in control of motor activity.Institute for Research into Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 22, No. 2, pp. 257–266, March–April, 1990.     

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.     

Propagation of activity along the “stepping strip” of the cat spinal cord

Three points located approximately 8 mm apart were identified in a dorsolateral funiculus of the lower thoracic spinal cord in mesencephalic cats, each producing stepping movements on the ipsilateral hindlimb when stimulated. An area 5–17 mm caudal to the caudal stepping point (SP) was scanned for neurons responding synaptically to stimulating the rostral or caudal SP prior and subsequent to electrolytic coagulation of the medial SP. Relative incidence of neurons excited by stimulating the caudal SP did not change following this type of lesioning, although stimulation of the rostral SP at the rate of 4 Hz induced response 5 times less frequently than before. Even stimulation of the rostral SP at the rate of 40–60 Hz, which had considerably increased firing index prior to coagulation, could only produce excitation in tiny numbers of neurons. This indicates that synaptic excitation of neurons becomes considerably more difficult once the stepping strip between stimulation and recording sites has been damaged.Institute for Research into Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 20, No. 6, pp. 763–769, November–December, 1988.     

Connections of the subthalamic and mesencephalic "locomotor regions" in rats

Responses of single neurons to stimulation of the subthalamic "locomotor region" were recorded extracellularly in the tegmentum mesencephali of rats. Latent periods of response of different neurons varied from 1 to 13 msec. The thresholds of the unit responses usually did not exceed the threshold for eliciting locomotion. The results are evidence of direct and oligosynaptic connections between the "locomotor regions" of the subthalamus and mesencephalon.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 9, No. 3, pp. 275–280, May–June, 1977.     

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.     

Frequency characteristics of skate electroreceptive central neurons responding to electrical and magnetic stimulation

Responses of single neurons in the lateral lobes of the medulla to stimulation of the electroreceptive system by homogeneous sinusoidal electrical and magnetic fields were investigated in acute experiments on the skateRaja radiata. Thresholds of neuronal responses to electrical stimulation varied from 0.03 to 10 µV/cm. The optimal frequency ranges for electrical and magnetic reception were in the regions of 0.05–5 and 2–3 Hz respectively. The possible mechanisms and functional significance of frequency characteristics of central neurons are discussed.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. All-Union Cardiologic Scientific Center, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 16, No. 4, pp. 464–470, July–August, 1984.     

Transformation of the afferent tactile signal into a motor command in the cat motor cortex

Activity of 112 neurons of the precruciate motor cortex in cats was studied during a forelimb placing reaction to tactile stimulation of its distal parts. The latent period of response of the limb to tactile stimulation was: for flexors of the elbow (biceps brachii) 30–40 msec, for the earliest reponses of cortical motor neurons about 20 msec. The biceps response was observed 5–10 msec after the end of stimulation of the cortex with a series of pulses lasting 25 msec. Two types of excitatory responses of the neurons were identified: responses of sensory type observed to each tactile stimulation of the limb and independent of the presence or absence of motion, and responses of motor type, which developed parallel with the motor response of the limb and were not observed in the absence of motion. The minimal latent period of the responses of motor type was equal to the latent period of the sensory responses to tactile stimulation (20±10 msec). Stimulation of the cortex through the recording microelectrode at the site of derivation of unit activity, which increased during active flexion of the forelimb at the elbow (11 stimuli at intervals of 2.5 msec, current not exceeding 25 µA), in 70% of cases evoked an electrical response in the flexor muscle of the elbow.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 9, No. 2, pp. 115–123, March–April, 1977.     

Classification of lumbosacral neurons by their discharge pattern during evoked locomotion

Unit activity was recorded in the lumbosacral division of the spinal cord during evoked locomotion in mesencephalic cats with the afferent fibers from their hind limbs intact or divided. If the afferent fibers were intact, all neurons recorded showed modulation of activity during locomotion in the rhythm of stepping movements. In experiments on cats with afferent fibers from the hind limbs divided modulation was absent in 30% of neurons, while in the modulated neurons, the frequencies in the excitation phase were approximately the same as when the limb innervation was intact. Modulation of activity in some neurons occurred in response to stimulation of the locomotor region even before stepping movements began. The tuning of the spinal generator of stepping movements is discussed.M. V. Lomonosov Moscow State University. Institute of Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 4, No. 4, pp. 410–417, July–August, 1972.     

Role of the midbrain periaqueductal gray matter in conditioned reflexes

Unit activity in the midbrain periaqueductal gray matter (PGM) during an instrumental placing reflex, its extinction, differentiation, and conditioned inhibition, was studied in chronic experiments on cats. Spike responses 1–2 sec in duration in 69 (36.7%) of 182 neurons preceded by 400–800 msec the beginning of conditioned-reflex and voluntary intertrial movements. These advanced responses appeared 200 msec before the corresponding advance responses of motor cortical neurons. Fifty-eight neurons (30.9%) responded directly to acoustic stimulation with a latent period of 10–50 msec for 2–6 sec, 19 neurons (10.1%) generated double responses, linked with both the acoustic stimulus and subsequent conditioned-reflex movement, and 42 neurons (22.3%) did not respond to acoustic stimulation, although individual neurons of this group changed the level of their spontaneous activity in response to repeated conditioned stimulation, and this change was maintained for some tens of minutes. Extinction, differentiation, and conditioned inhibition all abolished conditioned-reflex movements, but each type of internal inhibition was accompanied by its own characteristic changes in the firing pattern of PGM neurons. Functional independence of neurons of the first and second groups was demonstrated during extinction and recovery of the conditioned-reflex. The results indicate the important role of PGM not only in the mechanism of the conditioned reflex, but also in the development of its internal inhibition.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 403–419, May–June, 1984.     

Responses of rabbit visual cortical neurons to intracortical electrical stimulation

Responses of rabbit visual cortical neurons to single and repetitive intracortical electrical stimulation were investigated. The stimulating electrode was located 0.7–1.2 mm away from the recording electrode. Response thresholds to single stimulation were as a rule 150–180 µA, whereas to series of stimuli they were 30–60 µA. The latent period to the first spike averaged 5–15 msec but the probability of the initial discharge was very low, namely 3–6%. With an increase in current intensity the duration of the initial inhibitory pause was increased in half of the neurons responding to it, whereas in the rest it was unchanged. After presentation of series of stimuli spontaneous activity was enhanced for a short time (4–6 sec). In about half of the cells the same kinds of discharge dynamics were observed in response to repetitive stimulation (frequency 0.25 Hz) as in responses to light, but more neurons with sensitization of discharge and fewer "habituating" neurons took part in responses to electrical stimulation. It is postulated that stimulation of a given point of the visual cortex evokes excitation of a local neuron hypercolumn and inhibition of neighboring cell columns.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 15, No. 4, pp. 412–419, July–August, 1983.