Behavior of hindbrain neurons during the transition from rest to evoked locomotion in a newt.

Trains of electrical stimuli were delivered to the mesencephalic 'locomotor region' in the rough skin newt. The current (3-12 mcA) and the interstimulus interval (100 to 200 ms) were adjusted so that locomotion arose in approximately 10 s, or so that the train remained subthreshold for initiation of locomotion. Impulses of single neurons in the hindbrain were recorded during the transition period from rest to locomotion. Time-locked synaptic responses were bi- or unimodal with typical latencies close to 18, 23 or 28 ms, and weak irregular mode near 13 ms. Impulses that were not locked to the stimuli arose in some silent neurons, and the rate of firing of neurons with background discharge was sometimes enhanced. Composite responses consisted of both time-locked component and impulses distributed throughout the interstimulus interval. The data suggest that short-lived, wave-like propagation of the input volley ceases or is transformed into asynchronous activity after three or four translations. The latter variant could occur if the train reached the threshold for initiation of locomotion. The asynchronous activity persisted throughout interstimulus interval and could coexist with time-locked impulses. Some neurons generated only a few impulses, while others remained active from beginning to end of the train. These active neurons could either spike at a steady rate, or decrement or augment their rate of firing during the train. The time course of their activity was related to the initial rate of firing. The augmenting type of firing in a subset of neurons may arise due to the interaction of neurons with unstable, steady state and decrementing activity.

[Responses of medullary and spinal neurons to simultaneous stimulation of two locomotor points]. / Reaktsii neironov prodolgovatogo i spinnogo mozga na odnovremennoe razdrazhenie dvukh lokomotornykh tochek.

Responses of neurons in the medulla and cervical segments to simultaneous repetitive (50 pps) stimulation of two locomotor points (LPs) by currents in the range of 1 to 2 thresholds of locomotion were studied. In most cases a neuron responded to stimulation of only one LP. Stimulation of the second LP enhanced usually the firing index of that response, if it was low, diminished it, if it was high and did not influence it, if it was about 0.2. Part of neurons increased the background activity during stimulation of one of LPs though spikes were not locked to stimuli. A sign of influence of the second LP onto that elevation depended on its value. Data about the convergence of inputs from the ipsi- and contralateral midbrain and bulbar LPs on single neurons were obtained too. Possible mechanisms of summation of subthreshold excitation of two LPs during initiation of the locomotion are discussed.

[Synaptic responses of spinal cord neurons to rhythmic microstimulation of a stepping strip]. / Sinapticheskie otvety neironov spinnogo mozga na ritmicheskuiu mikrostimuliatsiiu shagatel'noi poloski.

Synaptic responses of neurons at the low thoracic level to microstimulation of a stepping strip in the dorsolateral funiculus with frequency of 20 and 40 pps were recorded in mesencephalic cats. Histograms of latencies of single neurons were usually monomodal, whereas two modes at 2-3 ms and 5-7 ms occurred in composed histograms. The data obtained show that decremental reverberation exists under these experimental conditions.

[The convergence of influences from the locomotor points of the midbrain and medulla oblongata and from the inhibitory point of the pons onto the bulbar neurons]. / Konvergentsiia vliianii iz lokomotornykh tochek srednego i prodolgovatogo mozga i tormoznoi tochki mosta na bul'barnykh neironakh.

Synaptic responses of medial and lateral medullary neurons to repetitive stimulation of locomotor points (LPs) of the midbrain and medulla and of an inhibitory pontine point were recorded in mesencephalic decerebellate cats. Excitatory postsynaptic potentials (PSPs) and discharges were observed usually in medial neurons as well as mixed PSPs when an inhibitory point was stimulated. Almost a half of lateral neurons and more than a quarter of medial ones changed the frequency of the background discharge giving no responses time-locked to stimuli. Medial neurons with responses time-locked to stimuli were equally susceptible to stimuli delivered to midbrain and medullary LPs and almost as often to stimuli applied to the inhibitory point. Medial neurons with responses time-unlocked to stimuli and lateral neurons were mostly susceptible to the input from the medullary LP, less affected by stimulation of the midbrain LP and responded rarely to stimulation of the inhibitory point. Convergence of influences from the midbrain and medullary LPs was the same onto neurons of all populations. Role of different neuronal populations for initiation and cessation of locomotion is discussed.

[2 types of neuronal reactions of the medulla oblongata to microstimulation of the locomotor and inhibitory points of the brain stem]. / Dva tipa reaktsii neironov prodolgovatogo mozga na mikrostimuliatsiiu lokomotornoi i tormoznoi tochek mozgovogo stvola.

Synaptic responses (postsynaptic potentials and action potentials) of medial and lateral bulbar neurons were evoked by stimulation of the medullary locomotor point (LP) and pontine inhibitory point (IP) by current up to 20 microA in mesencephalic decerebellated cats. Some neurons responded even to single (2-5/s) stimuli. Other neurons responded only to rhythmic (30-60/s) stimulation. Both groups of medial neurons were more susceptible to the input from IP. Lateral neurons which responded even to single stimuli were more reactive to the input from LP, whereas those which responded only to rhythmic stimulation were under the predominant influence from IP. Many neurons (both lateral and medial) with the background activity did not generate responses which were time-locked to a stimulus, however the background activity was either enhanced or inhibited under the rhythmic stimulation. These reactions were encountered more often during stimulation of LP. Partial redistribution of target neurons which accompanied the elevation of the frequency of the presynaptic input may be very significant in the motor control.

[The pathways necessary for eliciting walking by stimulation of the locomotor area of the brain stem]. / Puti, neobkhodimye dlia vyzova khod'by razdrazheniem lokomotornoi oblasti mozgovogo stvola.

Effects of bilateral lesions of the spinal cord at the upper cervical level on the walking elicited by stimulation of the midbrain or of the spinal cord, were studied in decerebrated cats. The locomotion could be elicited after lesions of the ventral funiculus together with the dorsal part of the lateral funiculus, or of the ventrolateral funiculus only, or of the grey matter. Following large lesions in the ventral half of the spinal cord, the midbrain stimulation elicited no walking, but low-amplitude alternative rhythmic movements could be observed during 1.5-2.0 hours. Nevertheless, the stimulation of the spinal cord (mostly bilateral) elicited the walking.

[Propagation of the activity along the "stepping strip" of the spinal cord in the cat]. / Rasprostranenie aktivnosti po "shagatel'noi poloske" spinnogo mozga u koshki.

Three stepping points (SPs) were found in one dorsolateral funiculus of the spinal cord at the low thoracic level in mesencephalic cats. The SPs were at a distance of about 8 mm from one another, and stimulation of each SP elicited stepping of the ipsilateral hind limb. Synaptic responses of single neurons to stimulation of the caudal and rostral SPs before and after electrolytic lesion of the intermediate SP were recorded 5-17 mm caudal to the caudal SP. Neurons excited by caudal SP stimulation encountered after the lesion as often as before it, whereas stimulation of the rostral SP (4 pps) evoked responses 5 times more seldom than before lesion. Even stimulation of the rostral SP with the frequency of 40-60 pps which increased essentially a lesion firing index before coagulation, excited only a few neurons. Thus, synaptic excitation of neurons becomes significantly more difficult after damage of the stepping strip between stimulation and recording sites.

[Frequency potentiation of neuronal synaptic reactions in the medial tegmentum of the medulla oblongata to microstimulation of the inhibitory point of the pons Varolii]. / Chastotnaia potentsiatsiia sinapticheskikh reaktsii neironov v medial'noi pokryshke prodolgovatogo mozga na mikrostimuliatsiiu tormozhnoi tochki varolieva mosta.

Synaptic responses of medial medullary neurons to single (2 pps) and repetitive (30-50 pps) stimuli delivered to the pontine inhibitory point were recorded in decerebrated cats. Firing index, inhibitory to the less extent, excitatory postsynaptic potentials usually increased when repetitive stimulation was applied. Suppression of the background impulse activity was observed in some neurons. Frequency potentiation makes a substantial contribution to functional effect of stimulation of the inhibitory point (termination of elicited locomotion).

[Activity of spinal cord neurons and their responses to stimulation of the "stepping" strip during spontaneous locomotor rhythmicity]. / Aktivnost' neironov spinnogo mozga i ikh otvety na razdrazhenie "shagatel'noi" poloski vo vremia spontannoi lokomotornoi ritmiki.

Spike activity of single neurons in the upper cervical segments C2-C3 or at the brachial enlargement C5-C6 was recorded in decerebrated cats during spontaneous locomotor rhythm after transection of the spinal cord at the low thoracic level. The shortest interspike intervals in 66% of cyclically active neurons at C5 were observed during electrical activity of extensor muscles of the ipsilateral forelimb. Only 8% of these neurons responded to stimulation of the stepping strip. The shortest interspike intervals in 65% of cyclically active neurons at C2 were observed during a silent period in extensor activity. 45% of C2 neurons responded to stimulation of the stepping strip. 87% of non-cyclic neurons responded to stimulation of the strip, were located at the C2 level.

[Responses of medullary neurons to stimulation of locomotor and inhibitory loci in the brain stem of the cat]. / Otvety medulliarnykh neironov na razdrazhenie lokomotornoi i tormoznoi tochek mozgovogo stvola u koshki.

The medullary locomotor point (L) and the pontine inhibitory point (I) were found in mesencephalic decerebellate cats. Repetitive (60/s) microstimulation of L elicited stepping of forelimbs which terminated during repetitive microstimulation of I. Responses of neurons were evoked applying 1.5 s-1 single or paired stimuli to L or a train of 2-4 stimuli to I, the interstimulus interval being 2 ms. Medial neurons (N-301) gave PSPs or action potentials of 1 stimulation three times as often as to L stimulation. Contrary, lateral neurons (N-166) responded two times as often as to L stimulation. IPSPs were recorded in both groups of neurons two times as seldom as EPSPs. In medial neurons IPSPs were produced mainly by I stimulation. I stimulation did not evoke usually IPSPs in neurons excited from L. Possible mechanisms of termination of stepping due to I stimulation are discussed.

[Synaptic responses of propriospinal neurons on stimulation of the locomotor strip of the dorsolateral funiculus of the cat]. / Sinapticheskie otvety propriospinal'nykh neironov na razdrazhenie shagatel'noi poloski dorsolateral'nogo kanatika u koshki.

Stepping points in the dorsolateral funiculus of thoracic and cervical divisions of the spinal cord were found in mesencephalic cats. Synaptic responses of single neurons at level Th12-Th13 to microstimulation of these points were recorded extracellularly. Latencies of these responses increase when the distance between the stepping point and the site of the recording is over 20 mm, and paired stimuli have now to be delivered to evoke responses. Neurons responding orthodromically to stimulation of the stepping strip send their axons into the ventrolateral funiculus near the grey matter. Occurrence of antidromic responses among neurons sending their axons in the caudal direction is 37% in the average till the distance between the recording and stimulation sites is up to 40 mm. Half of neurons with synaptic responses to stimulation of the stepping point can be also excited by a train of 3-6 stimuli applied to the midbrain locomotor point. Revealed neurons may contribute to propagation of the activity to the stepping generator of the hindlimb.

Medullary locomotor strip and column in the cat.

Responses of lateral medullary neurons to microstimulation of two points in the locomotor strip--rostral and caudal to the obex--were recorded intracellularly in mesencephalic decerebellated uncurarized cats. Excitatory and inhibitory postsynaptic potentials and orthodromic action potentials occurred up to 20 ms after a single stimulus. A number of cells responded to stimulation of a locomotor point by a repetitive discharge, and in some cells synaptic responses were evoked by contralateral stimulation. The responsive neurons were scattered among other cells in the lateral medullary tegmentum. At least one-third of neurons with synaptic responses to stimulation of the rostral locomotor point were antidromically invaded from the caudal one. The characteristic length of these descending axons was between 4 and 9 mm, although there were longer axons too. The lateral medullary cells which give synaptic responses to stimulation of the locomotor strip form the locomotor column located medial to the strip, and a portion of these cells send their axons to the strip. It is suggested that the activity is propagated polysynaptically along the column through axonal collaterals of its neurons. One can assume that when repetitive stimulation achieves a threshold for locomotion such as propagation occurs without decrement. As a result, spinal stepping generators are activated.

[Stepping movements caused by stimulation of the cat spinal cord dorsolateral funiculus]. / Shagatel'nye dvizheniia, vyzyvaemye razdrazheniem dorsolateral'nogo kanatika spinnogo mozga u koshki.

It has been demonstrated in experiments on mesencephalic cats that microstimulation of the dorsolateral funiculus (DLF) at any level (from C1 to L1) elicited stepping movements in the ipsilateral hind limb. The lesion of the DLF at the C2--C3 level interfered with provocation of the stepping from the more rostral part of the DLF. However, stimulation caudal to the lesion remained effective. Stimulation of the DLF at the C5--Th3 did not prevent the stepping from L1, whereas that at the Th13 level made L1 ineffective despite the fact that the stepping was provoked by stimulation at the Th12 level. After lesion of the DLF at the two sites (C2 and L1 levels) stimulation at any level (from C3 to Th13) continued provoking the stepping. However, after stimulation at the Th5 and L1 levels the lesion of an isolated DLF area appeared ineffective. The data obtained suggest that stimulation of the DLF can elicit the stepping of the hind limb through the propriospinal system, provided that the stimulated part of the DLF contains at least 8--12 intact segments.

[Responses of neurons of the upper cervical segments of the spinal cord in the cat to stimulation of the locomotor region of the brain stem with different frequencies]. / Otvety neironov verkhnikh sheinykh segmentov spinnogo mozga koshki na razdrazhenie lokomotornoi oblasti mozgovogo stvola s raznymi chastotami.

Synaptic responses of C2 and C3 neurons to stimulation of medullary or midbrain locomotor points were recorded extracellularly in mesencephalic cats. Neurons excited with a firing index of 0.4-0.6 at a stimulation frequency of 2 S-1 retained this index at frequencies of 20-60 S-1 too. Many neurons with the low firing index under the stimulation frequency of 2 S-1 increased it to 0.4-0.6 under high frequency. More than a half of neurons were excited by stimulation of both ipsi- and contralateral locomotor points; a quarter of units responded to stimulation of locomotor points both in the medulla and in the midbrain. The neurons under study were located at a distance of 1.8-4.2 mm from the dorsal surface of the spinal cord. Average latencies of responses with a firing index more than 0.5 were distributed between 2 and 30 ms with a mode of 2-8 ms. Long-latency responses had significant fluctuations of latency. A possible participation of the investigated neurons in the propagation of activity from the brain stem locomotor region to stepping generators in the spinal cord is discussed.

[Synaptic responses of neurons of the lateral medullary reticular formation to stimulation of the locomotor strip in the cat]. / Sinapticheskie reaktsii neironov lateral'noi medulliarnoi retikuliarnoi formatsii koshki na razdrazhenie lokomotornoi poloski.

Intracellular recording of neuronal responses in the lateral medullar reticular formation to stimulation of locomotor points located rostrally and caudally of the obex was made in mesencephalic uncurarized cats. Of 114 neurons with synaptic responses to the rostral point stimulation by current up to 30 microA 40 neurons were excited antidromically from the caudal point.

[Synaptic responses of medulla oblongata neurons to stimulation of the "locomotor area" in the turtle]. / Sinapticheskie reaktsii neironov prodolgovatogo mozga cherepakhi na razhdrazhenie "lokomotornoi oblasti".

Responses of medullary neurons to microstimulation of medullary locomotor region by current up to 30 micro A were recorded intracellularly in decerebrated turtles. Resting potential of these neurons varied from 22 to 42 mV. Depolarizing PSPs (EPSPs) were evoked in 43 neurons, hyperpolarizing ones (IPSPs) in 12 neurons, and both types of PSPs were observed in 36 neurons. Discharges of synaptic origin were observed in 29 neurons, including 11 which generated action potentials without any visible PSP. Latencies of PSPs were usually in the range from 2 to 8 ms. The delay from the beginning of EPSP to the onset of action potential was 1-3 ms when the firing index was high, but fluctuated significantly and increased under weaker stimulation. Unitary EPSPs in 15 neurons and IPSPs in 3 neurons were recorded. Their amplitudes were 0.6-0.8 mV, that was 2-12 times less than thresholds depolarization (1-7 mV). The data obtained as well as results of extracellular recordings from medullary neurons in turtles and cats are employed for discussion on possible mechanism of locomotor activity propagation in the lower brain stem.

[Responses of medulla oblongata neurons to microstimulation of the "locomotor strip" in cats]. / Otvety neironov prodolgovatogo mozga na mikrostimuliatsiiu "lokomotornoi poloski" u koshki.

Synaptic responses of medullary neurons to stimulation of the medullary "locomotor strip" by current of about 20 microA were recorded extracellularly in mesencephalic decerebellized cats. The mean latency of responses of 177 units was 3.2 ms. Synaptically driven units were located both in the lateral and in the medial reticular formation, however short-latent responses were observed predominantly in the lateral part. Single stimulus evoked discharge consisted of two or more spikes in 32% of units. Respiratory neurons were not activated by stimulation of the locomotor point. The data obtained show that neurons of the "locomotor strip" can excite both one another and neurons located more medially. Possible mechanisms of extending the activity to the upper cervical segments of the spinal cord are discussed.